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LABOUR DEPARTMENT HONG KONG - HKU Libraries

LABOUR DEPARTMENT HONG KONG


UNIVERSITY OF HONG KONGLIBRARYHong Kong CollectionGift from:Information Services Dept.,Hong Kong


A GUIDE FOR BOILER OPERATORSBYR. COLACOLABOUR DEPARTMENTHONG KONGFebruary 1975PRINTED AND PUBLISHED BY THE GOVERNMENT PRINTERHONG KONG


PREFACEThis text book is offered as a guide to those studying for theexamination leading to the issue of a Certificate of Competency underthe Boilers and Pressure Receivers Ordinance Cap. 56 of the Laws ofHong Kong. It should ensure success for the seriously minded student.It will also help the boiler operator in the day-to-day operation andmaintenance of steam boilers and their auxiliary equipment.Because there are a few electrically heated boilers in use in HongKong, a separate chapter on electrode boilers has been included.As boiler auxiliaries are usually motor driven, a chapter on electricityhas been added. The fundamentals of electricity are discussed and hintsgiven on the care and maintenance of electric motors.Further, as fire is a serious hazard in Hong Kong, a brief chapter onfire fighting has been included. A few types of fire extinguisher commonlyused in boiler room fires are described.Every effort has been made to keep the text sufficiently non-technicalfor the beginner, and yet contain information that will be of interestand value to the more experienced boiler operator.The generous co-operation of the Naval Architectural Draughtsmenof the Marine Department who aided in the preparation of the sketchesis gratefully acknowledged.Finally, grateful thanks are expressed to Mr. CHENG Tsan-son,Translator in the Marine Department, the Inspectors of the PressureEquipment Unit, particularly Mr. LI Wood-fan, and the Translatorsof the Labour Department, for their assistance in rendering the text intoChinese.R. COLACOLabour DepartmentHong Kong.in


CONTENTSCHAPTER ICHAPTER IICHAPTER III'CHAPTER IVCHAPTER V-VARIOUS TYPES OF BOILERS—ELEMENTS OF BOILER CONSTRUCTIONRiveting — Welding — Boiler Stays —Boiler Tubes—Manholes, Mudholes AndCoversPageDefinition — Fire Tube Boilers — WaterTube Boilers 1-5—BOILER MOUNTINGSSafety Valves — Water Gauges — TestCocks — Steam Outlet Valves — FeedWater Valves And Piping — Blow-DownAnd Drain — Pressure Gauges — FusiblePlugs —Scum Valves . . . .9-15—COMBUSTIONOil Fuel And Combustion — Oil BurningInstallations — Atomizing Burners —Furnace Fittings — Lighting BurnersManually—Automatic Controls—Draught—Smoke . 16-28—BOILER OPERATIONOperating Procedure — Raising Steam —Oil Fuel System — Oil Temperature atBurners — Opening Up Steam From ABoiler To A Pipe Range —Taking ABoiler Out Of Service — Practical OperationAnd Periodic Inspection — CleaningA Fire Tube Boiler And Preparing ItFor Inspection — Precautions To BeObserved Before Entering A Boiler —Laying Up A Boiler . . . . 29 - 37


CHAPTER VI—PROBLEMS OF PLANT OPERATIONGeneral — Boiler Explosions — ScaleFormation — Scale Removal — Oil InBoilers — Internal Corrosion — ExternalCorrosion — Erosion — Caustic EmbrittlementOf Boiler Plates — Grooving OfBoilers — Bulges — Priming And Foaming—Tube Troubles — Flame Impingement— Refractory Troubles — WaterHammer . . . . . . .Page38 - 48CHAPTER VII —PUMPS AND OTHER AUXILIARIESFeed Water Pumps And Steam Injectors— Oil Fuel Pressure Pumps — FeedWater Heaters — Superheaters — Economizers— Air Preheaters — Feed WaterRegulators — Pressure Reducing Valves— Steam Separators — Steam TrapsCHAPTER VIII —ELECTRODE BOILERS . . . .49 - 5758 - 59CHAPTER IXCHAPTER X—FUNDAMENTALS OF ELECTRICITYElectric Flow — Units of Measurement— Some Definitions — Generators AndMotors — Why Motors Will Not Start —Dangers, Remedies And Care Of Motors—FIRE PRECAUTIONS, FIRE FIGHTING ANDEQUIPMENTLiquid Fuel — Precautions — Fire Fighting—Fire Fighting Equipment60 - 6667 - 70Diagrams 71 - 107


DefinitionVARIOUS TYPES OFA Steam Boiler is a closed vessel in which steam is generated underpressure greater than atmospheric. It is constructed of good qualitysteel and is partially filled with water when in use. Fuel is burned inthe furnace and the heat thus generated is transferred to the water inthe boiler, thus converting it into steam. This steam is used for avariety of purposes, viz. power generation, process, or heating service.Internally, the boiler is divided into two parts, viz. the water space,which contains water from which steam is generated, and, the steamspace, which is the space above the water space. The steam spaceprovides a storage space for the steam produced, and to a limited extent,allows the separation of steam and water.Externally, the furnace absorbs radiant heat from the burning fueland the small diameter tubes extract further heat from the hot gasesafter combustion has been completed.In general, there are two main types of boiler—(a) The Shell or Fire Tube type, in which the burning fuel andproducts of combustion are inside the furnace and tubes, withwater on the outside.(b) The Water Tube type, where the water is inside drums and tubes,with burning fuel and products of combustion on the outside.Fire Tube BoilersFire Tube boilers in common use are—A. Vertical Tubular Boilers. There are various designs and makesof this type of boiler but the following are commonly encountered.(i) Vertical Cross Tube Boiler—In its simplest from the boiler(Fig. 1) consists of a cylindrical shell surrounding a nearlycylindrical fire-box. A tube, called an uptake, passes from thecrown of the fire-box to the crown of the shell, where it isI


connected to a chimney. To increase the amount of heatingsurface and improve circulation of the water, and also toincrease the strength of the fire-box, the fire-box is fittedwith one or more cross-tubes.(ii) Cochran Boiler—As can be seen from Fig. 2, the crown ofthe fire-box and external shell of the Cochran boiler arehemispherical in shape. The hot gases of combustion passfrom the fire-box through the flue-pipe into the combustionchamber, and from there through numerous tubes to thesmoke-box and thence to the chimney.(iii) Vertical Dry Top Boiler—This consists of a vertical cylindricalshell with flat ends as seen in Fig. 3A. The cylindricalshell surrounds a fire-box, and numerous tubes connect thetop of the fire-box to the top end plate of the boiler. Thehot gases pass from the fire-box through the vertical tubesto the smoke-box and chimney.Fig. 3B illustrates a vertical wet top boiler.In general, the advantages of vertical boilers are—(1) The cost of construction is comparatively low.(2) A minimum area of floor space is required.(3) The tubes are the same size; one spare can replace any tubein the boiler.(4) These boilers are self-contained and require little or nobrickwork.(5) These boilers are semi-portable and can be moved and setup in various locations.The disadvantages are—(1) The interior is difficult of access and therefore hard to clean.(2) The water capacity is small and there is a tendency to prime.(3) Efficiency is inclined to be low and therefore they are notvery economical.(4) They are prone to corrosion on the outside.(5) A comparatively high headroom is required.


B. Scotch Marine Boiler—This type (Fig, 4) has a cylindrical shellwith flat ends. The shell contains one or more corrugated furnaceswhich lead into combustion chambers which are more orless rectangular in shape. From the combustion chambers a largenumber of tubes lead the products of combustion to the front ofthe boiler and into the smoke-box.The advantages of Scotch boilers are—(1) A minimum space is required.(2) A minimum amount of brickwork is required.(3) The headroom required is comparatively low.(4) The tubes are the same size, so one spare can replace anytube in the boiler.The disadvantages are—(1) Because of the large diameter, circulation difficulties sometimesoccur when starting up from cold.(2) The tubes are inclined to leak because of expansion andcontraction.C. Horizontal Return Tube Boiler—This type (Fig. 5) has a cylindricalshell with flat end plates which are connected together bynumerous tubes. The shell is set in brickwork which forms thefire-box and combustion chamber. The hot gases of combustionpass from the fire-box to the combustion chamber and throughthe tubes to the front of the boiler and smoke-box.The advantages of horizontal return tube boilers are—(1) The cost of construction is comparatively low.(2) The required headroom is comparatively low.(3) Circulation is simple and positive.(4) Tube replacement is fairly easy.(5) The tubes are the same Size, so one spare tube can replaceany tube in the boiler.The disadvantages are—(1) Needs much more brickwork than other types of fire tubeboiler.(2) Has a limited capacity and pressure.


D. Packaged Boilers—These boilers are usually Scotch marine typeunits. They are factory assembled, tested and adjusted. Eachcomplete unit consists of boiler assembly, electrically-driven feedand fuel pumps, forced-draught fan, combustion equipment, also,operating and safety control equipment for either semi-automaticor fully automatic operation after initial start-up. The packagedboiler is therefore a completely self contained unit.The advantages of packaged boilers are—(1) The boiler is portable and can be readily set up in variouslocations.(2) It is compact and therefore takes up a minimum of floorspace.(3) It is easy to operate.(4) It is very efficient(5) These boilers are either semi-automatic or fully automatic.The disadvantages are—(1) It cannot be operated with power off.(2) There is a danger of furnace explosion in intermittentoperating.(3) Excess feed water leads to scale increase.Water Tube BoilersWater tube boilers may be classified into two divisions, viz. (i) thosewith straight tubes, and (ii) those with bent tubes. There are severalstandard types of both straight-tube or 'header' boilers and bent-tubeboilers. One of each type is illustrated here.(i) The Straight-tube or Header Boiler—As shown in Fig. 6, theboiler consists of inclined tubes forming a tubular heating surface,boxes or headers to which the tubes are attached, ahorizontal steam and water drum, a mud-box and a furnace ofsuitable capacity immediately beneath the tubes. The inclinedtubes are divided into vertical sections and to ensure continuouscirculation of the water in one direction they are inclined fromthe horizontal. Baffles are placed in the path of the hot gaseswhich deflect the gases back and forth between the tubes a numberof times so that more heat may be absorbed by the boiler


tubes. Fig. 6 does not include a superheater but one can befitted amongst the inclined tubes, five or six rows of tubes abovethe furnace, if super-heated steam is required.(ii) The Bent-tube Boiler—The boiler illustrated in Fig. 7 consistsof one steam drum and one water drum connected together bya main tube bank and also by a few rows of D-shaped tubes.The space between the main tube bank and the D-shaped tubesprovides furnace volume. The superheater is placed within themain tube bank and baffles are fitted in the gas path so that moreheat may be absorbed by the boiler tubes. On leaving theboiler the combustion gases pass through an economiser whichabsorbs waste heat from the gases.The advantages of water tube boilers are—(1) The heating surface offered is much greater than in the fire tubetype.(2) Circulation is rapid.(3) Steam generating capacity is great.(4) Access is easy and therefore cleaning and inspection are easy.(5) Pressures encountered are generally high.(6) The quantity of water is much less compared with the fire tubetype.The disadvantages are—(1) The initial cost is high.(2) A lot of brickwork is generally required.(3) Baffles are needed.(4) A high headroom is required.


CHAPTER IIELEMENTS OF BOILER CONSTRUCTIONRivetingRiveting used to be extensively employed in boiler construction untila few years ago. Holes for the rivets were drilled in the plates or partsto be joined and riveting was performed either by hand or machine.Some types of rivets are shown in Fig. 8.Riveted joints most commonly used in boiler shell connections weresingle- and double-riveted lap joints and treble-riveted double-buttstrap joints. Some riveted joints are shown in Fig. 9 9 while Figs. 10Aand 10B show how concave and convex heads were jointed.WeldingWith the advent of electric welding riveting has been almost entirelysuperseded for all forms of boiler construction.In electric welding, the plate edges are machined or flame cut into aVee and the plates to be joined are butted and welded on both sides.Molten metal from the electrode is added to the Vee, which in fusingwith the plate edges forms a good efficient joint. Test pieces from thesame boiler plates are welded at the same time by the same welderunder similar conditions, and very severe tests are applied to these testspecimens.Good welded joints are better than riveted joints. A welded jointoffers less resistance to water circulation and does not allow scale tolodge; this is a disadvantage with riveted joints. Further, the amountof metal used is less and therefore the weight of the welded joint isless than a riveted one.Boiler StaysIn steam boilers the flat surfaces subjected to pressure must besupported and this support is provided by stays. The principal kindsof stays in use are—(a) Direct stays: round bars placed at right angles to the flat surfacessupported by them.(b) Diagonal and Gusset stays: used for supporting a flat surface bytying it to another surface inclined to the first.(c) Girder stays: placed edgewise to the flat surface to be supported.


Brief description of various types of stays advantages and disadvantages—(a) Screw Stays (Fig. 11) are used to stay flat surfaces that are closetogether, for example, the flat parallel surfaces of combustionchambers in Scotch boilers. Properly spaced, these stays willgive efficient service and should not interfere with the circulationof the water.(b) Through Stays (Fig. 12) are the best and most direct form ofstay for supporting parallel flat surfaces that are a considerabledistance apart. They do not interfere with free movement withinthe boiler when cleaning or carrying out repairs.(c) Diagonal Stays (Fig. 13) are used when it is not possible ordesirable to carry a bar stay direct from one flat surface toanother surface parallel to it. The stay may be taken in adiagonal direction and be secured to a surface at right angles tothe first. Such stays take up very little room and leave morespace for movement and the use of tools within the boiler.(d) Gusset Stays (Fig. 14) are diagonal stays in which a flat plate isused instead of a bar or rod. They are very rigid and liable tocause grooving of the plates along the edges of the angle barfasteners if used where heat expansion and contraction changesare frequent. They also take up too much space and interferewith the free circulation of the water. They are most often used inCochran boilers.(e) Girder Stays (Fig. 15) are generally used in staying the flat crownsof the combustion chambers in Scotch boilers. It consists of twoplates riveted together with a space between them. It is supportedat its ends on the vertical plates forming two oppositesides of the combustion chamber, and the flat crown is suspendedfrom the girder at intervals by bolts. This is a good form of stayif properly made and fitted.Boiler TubesThree common methods of boiler-tube fabrication are used. (1) Theseamless tube is pierced hot and drawn to size. (2) The lap-welded tubeconsists of metal strip curved to tubular shape with the longitudinaledges over-lapping. Heat is applied and the joint is forge-welded. (3)The electric-resistance butt-welded tube is formed like the second type,but as its name implies the joint is butt-welded.


Manholes, Mudholes and CoversEvery boiler should be provided with suitable manholes, mudholesand sightholes. These are required to enable the boiler to be properlycleaned and inspected.When a boiler is large enough, manholes not less than 12" X 16"(30.48 cm X 40.64 cm) or 11" X 15" (27.94 cm X 38.10 cm) shouldbe provided, so as to allow the boiler to be entered. Fig. 16 illustrates amanhole and cover.Mudholes not less than 2J" X 3£" (6.98 cm X 8.89 cm) are providedin smaller boilers for cleaning and inspection.The covers are made oval so that they can be manipulated into theoval openings, and gaskets of asbestos rope are used on the seatingsurface. Yokes or 'dogs' are used to secure the cover in place.


Safety ValvesCHAPTER IIIBOILER MOUNTINGSA safety valve is the most Important safety device on the boiler. Itsfunction is to prevent excessive pressure from building up in the boilerand it is set at or below the maximum safe working pressure for theboiler it protects. The safety of the plant, the buildings, and the operators,depends largely on its efficiency.All boilers must be fitted with an approved type of safety valve ofsufficient capacity to discharge all the steam that the boiler can generatewithout permitting the pressure to rise more than 10 per cent above thepermissible working pressure. The safety valve should be connecteddirectly to an independent steam outlet on the boiler, and no valve ofany description should be placed between the safety valve and theboiler, nor on the waste steam pipe between the safety valve and theatmosphere.The main constructional requirements are that the valve lid and seatshould be of non-corrosive material. The seat should be fastened tothe body so that it cannot lift with the valve lid. All parts should be soconstructed that failure of any part will not interfere with the fulldischarge capacity of the valve. Twin valves are used when the safetyvalve area is greater than the area of a 3J" (8.89 cm) diameter valve.Safety valves must be of the direct spring-loaded type. Weight andlever or dead-weight safety valves are not permitted because the adjustmentof such valves is very easily tampered with. The safety valvespring is usually of square section for maximum clearance between thecoils—for if the coils come in contact the valve cannot lift.It is important that the steam opening to, and the waste steam pipefrom, the safety valve should be at least as large as the safety valveconnection.A lifting lever is required in order to lift the valve from its seat. Itis used mainly for testing purposes.Most spring-loaded valves make use of a lip on the periphery of theactual valve lid for the purpose of giving them additional uplift oncethey are raised from their seats by steam pressure. This additional up-


lift helps to counteract the increase in spring load as the spring iscompressed by the valve lifting. Fig. 17 illustrates this point.Safety valves should be placed in a vertical position and should betested at regular intervals. The lifting lever should be pulled by handonce a day and the valve blown by steam pressure once a week.Water GaugesEvery boiler must be fitted with at least one water gauge to show thewater level in the boiler. In low and medium pressure boilers i.e. up toabout 300 lb/in 2 (21.09 kg/cm 2 ) pressure, the water gauge consists of around glass tube held in place by cock or valve mountings and stuffingboxes as shown in Fig. 18. The gauge glass is usually 12" (30.48 cm)long and is connected by its fittings either directly to the boiler as in thecase of vertical boilers, or indirectly to the boiler through a watercolumn. The gauge glass connections between the glass and the boiler,or the water column if this is included, 'should be at least |- inch.(1.27 cm). The water column is a casting and is connected by pipes atthe top and bottom to cocks or valves on the steam and water spacesof the boiler. The water column serves to eliminate excessive fluctuationsof water level indication in the glass due to rapid boiler circulation,and so it acts as a steadying medium.Referring to Fig. 18, when both cocks A and B are open and clearthe level of water showing in the glass will be a true indication of thelevel of water in the boiler. However, if the top cock A is either chokedor closed, the pressure in the boiler will cause the water in the glass torise higher than the true level of water in the boiler and a false readingwill be obtained. Also, if the bottom cock B is either choked orclosed, the steam will condense at the top of the glass and fill the glass,indicating a higher level than really exists in the boiler and a falsereading will again be obtained. This will also happen if either of thecocks D or E, or associated piping become choked.When a water glass is broken, shut cocks A and B, remove thebroken pieces and slowly open the cocks to blow out any remainingpieces. Before inserting the new glass, open the drain cock C and ensurethat the glass is the exact length and that the connections are inline. Insert the glass with packing rings and tighten the stuffing boxnuts by hand pressure. Then warm the glass by opening the top cockA slightly, allowing a small amount of steam to flow through the glass.10


When the glass is warmed sufficiently, shut the drain cock C, open thebottom cock B slightly, and when the level of the water in the glasshas become stable, open cock B fully and then open cock A fully.Water gauges should be frequently tested, particularly when takingcharge after a previous shift. The safe operation of the boiler dependslargely on the accuracy and condition of the gauge glass.Testing Water Gauges—Referring to Fig. 18(0), the procedure to beadopted when verifying the water level in the gauge glass is as follows—-First shut both steam and water cocks A and B, and open drain C,thus proving that the gauge cocks are in order. Then, with the drainstill open blow through the steam cock A and then the water cockB separately to prove a clear way through both cocks and the gaugeglass.Referring to Fig. 18(6), the addition of pipes and boiler-shell shut-offcocks brings in additional possibilities of faulty level indication andthorough verification of this type is a little more involved. To test awater-column water gauge glass thoroughly, the following proceduremay be adopted—First prove the bottom connections are in order by shutting cocksD and A, leaving E, B and drain C open; if water blows freely outof the drain C, the bottom connections are in order. Then open cocksD and A, and shut E and B; if steam blows freely out of the drainC, the top connections are in order.In the event of either end not blowing freely a cross test can beused which will show up the cock that is faulty. To cross test, closecocks D and B, leaving E, A and C open; then close cocks E and A,leaving D, B and C open. This procedure is known as a cross blow.Water Tube Boiler Water Gauges—As mentioned previously, for pressuresover 300 lb/in 2 (21.09 kg/cm 2 ) the round water-gauge glass is notsuitable and is replaced by a built-up rectangular-section box havingthick glass in front and back (see Fig. 19). It is illuminated from the rearby an ordinary electric lamp. The inner surface of the two plate glassesused are protected against any etching action of the steam by the fittingof thin sheet mica.Test CocksOn certain sizes of fire tube boiler a set of test cocks may be requiredin addition to the water gauge glass as a second means of determining11


the water level in the boiler. Usually three test cocks are fitted, butsmall boilers of less' than 50 square feet (4.64 sq. m) of heating surfacemay have two test cocks. Test cocks may be placed on the boiler shell,in which case the lowest cock is placed level with the bottom of theglass. The others measured vertically, are placed two or three inchesapart. They may also be attached to the water column, in which caseone cock is fitted half way up the column and the top and bottom onesthree inches above and below the middle cock.If the top test cock is opened and steam issues, a sharper sound isheard and the steam comes out in a narrow straight line. If steam andwater is shown at the middle cock, the issuing mixture will be more ofan umbrella shape. The bottom test cock should always show water.Test cocks should always be kept in an efficient condition.Steam Outlet ValvesEvery steam outlet from a boiler must be fitted with a stop valveattached directly to the boiler shell or drum, or, as near as practicable.In the case of a boiler with a superheater, the stop valve is located asnear the outlet from the superheater header as is convenient andpracticable.The spindles of all valves over 1-|" (3.81 cm) diameter should haveoutside screws and the covers should be secured by bolts or stud's andnuts. All valves are arranged to shut with a clockwise motion of thehand-wheel.When two or more boilers are connected to a common header orsteam manifold, the steam connection to each boiler should be providedwith one stop valve and one screw-down non-return valve capableof being locked in the closed position. This is to ensure that if one ofthe two boilers is opened up for inspection and cleaning, steam cannotbe inadvertently turned on. A stop valve capable of being locked in theclosed position and a separate automatic non-return valve may besubstituted for the screw-down non-return valve.Feed Water Valves and PipingEach boiler should be fitted with a feed stop valve and a nonreturnvalve. Alternatively, a combined screw-down non-return valvemay be fitted.12


When both, a stop valve and non-return valve are fitted, the stopvalve is placed nearest to the boiler. The non-return valve ensures theflow of water in one direction only, viz. into the boiler; the valve willclose because of steam pressure in the boiler if the flow tends to reverse.The purpose of the stop valve is for inspecting or repairing the nonreturnvalve with the boiler in service.The feed connection is made at the coolest part of the boiler and ifan internal pipe is fitted, care should be taken to ensure that the coldfeed water does not discharge directly on any part of the heating surface.Two separate means of supplying feed water should be installed.Slow-down and BrainEvery boiler should be fitted with a suitable blow-down valve orcock placed at or as near as practicable to the lowest point of theboiler, so that all the water can be drained off. Blow-down valves areof extra-heavy construction and should be of special design to give astraight through blow off.If exposed to furnace heat, the blow-down pipe should be protectedby brickwork or other heat-resisting material. It should be so arrangedthat the pipe may be inspected and can slide freely when the boiler isexpanding and contracting.In some installations, the blow-down pipe from the boiler is led intoa blow off tank which in turn discharges into the sewer. The purposeof this blow off tank is to prevent damage to the sewer from the steamand water discharged from the boiler.Pressure GaugesEvery boiler must be fitted with a pressure gauge in order to measurethe pressure of steam in the boiler in pounds per square inch aboveatmospheric pressure.The Bourdon pressure gauge (Fig. 20} was invented by a Frenchman,Eugene Bourdon. It consists of a flattened brass tube bent in the formof a circle, one end being anchored and open to the pressure to bemeasured, the other end being closed and free to move. The end thatis free to move is attached to a toothed quadrant which operates apinion to which a pointer is secured. This pointer moves on a dialmarked off in pounds per square inch. The gauge depends for its work-13


ing on the principle that a flattened, curved tube tends to straighten outwhen subjected to internal pressure, since there is more area exposedto pressure on the outside of the curve than on the inside.If steam entered the Bourdon tube direct it would cause it to expandor lengthen and the pointer movement would be due to a combinationof pressure and heat effects. This would mean that the indicated pressurewould be more than the actual pressure. For this reason the gauge isconnected to the boiler through a pig-tail siphon (Fig. 21) which actsas a condenser and retains water, Boiler steam acts on this water toshow correct pressure. For the same reason, the gauge should not beconnected to any part of the boiler which will cause it to becomeheated.A I" (0.63 cm) threaded connection should be provided in thepressure gauge piping between the pig-tail siphon and the gauge toserve as a test connection. This permits attaching an inspector's gaugewith the boiler under pressure.Fusible PlugsA fusible plug is another safety device for giving a warning of lowwater. It consists of a threaded bronze plug having a tapered holethrough its centre and this hole is filled with practically pure tin witha melting temperature of 400°F to 500°F (204.44°C to 260°C). Theplug is screwed into the boiler plate at a location just above the fireline so that one end is exposed to flame or hot gases and the other iscovered by water. The large end of the tapered filling is always underboiler pressure during operation. If the water level in the boiler fallsbelow the plug, it will heat up and the fusible core will melt and fallout, allowing escaping steam to sound a warning of low water.The value of any type of fusible plug, particularly in boilers of thewater tube type, is questionable. They are unreliable and little dependenceshould be placed on them. If used, they should be kept in goodcondition and renewed at least annually. When cleaning the boiler,scrape and clean the exposed surfaces of the fusible metal and thesurface of the boiler surrounding the plug.In the case of oil-fired boilers low water alarms are usually fitted inpreference to fusible plugs.14


Seem ValvesSome boilers are fitted with a surface blow off or scum valve. Thescum valve is usually located slightly below the working water level ofthe boiler, the outlet from it being connected to the blow-down dischargepipe. It is used for getting rid of surplus water in case the waterlevel in the boiler is too high, or for removal of scum which may befloating on the surface of the water, or for reducing the concentrationof solids in the water. By judicious use of the scum valve prior toblowing down a boiler, any floating impurities can be discharged, thuspreventing them from adhering to tubes and plates when blowingdown.15


Oil Fuel and CombustionCHAPTER IVCOMBUSTIONBefore the Second World War coal was extensively used as a fuel.Since then however, oil has superseded coal and today it is almostuniversally used. The reason for this is because oil is much moreconsistent and has a higher heating value.The heat-producing constituents in oil are carbon, hydrogen andsulphur in the following proportions—CarbonHydrogenSulphur84-87 per cent11 -14 per cent0.5-1 percentWhen two substances having a chemical affinity for one another arebrought together under favourable conditions, they combine to forma new compound. During this combination heat is generated, which ifthe chemical affinity is sufficiently strong, will raise the temperature ofthe substances to such a degree that they grow luminous. This is calledcombustion. In the case of fuels the heat-producing constituents combinewith oxygen in the air to generate heat, with temperature rangingfrom 1 9 850°F (1,010°C) to over 3,000°F (1,648°C).Carbon in the fuel if completely burned will form carbon dioxide.Hydrogen on complete combustion produces moisture, while sulphurforms sulphur dioxide. If the carbon is not completely burned due tolack of air, carbon monoxide will be formed; this means that only onethird of the heat available will be generated.The various constituents of fuel on combustion will show in thefurnace as follows—Carbon burns with a white luminous flame.Carbon monoxide burns with a light blue flame.Hydrogen burns with a colourless flame.Sulphur tends to colour the flame yellow.16


To ensure complete combustion, sufficient air must be supplied andto ensure that there is sufficient air, a controlled excess is supplied.This excess air depends on the type of draught and furnace. With aweak draught such as natural draught, about twice as much air willbe required, while with mechanical draught only about one and onehalf times as much will be needed. Also, the amount of draught requiredvaries with the rate of combustion.As oil is in general use in Hong Kong, only oil-burning installationswill be discussed in this chapter.Oil-burning InstallationsA normal oil-burning installation (Fig. 22) consists essentially of asettling tank and a fuel oil unit comprising suction filter, pressure pump,discharge filter and heater. The fuel oil unit is designed to supplysufficient fuel for generating all the steam that may be required fromthe boiler. The hot fuel is delivered to the boiler front through a pressureline which is fitted with a circulating valve and return line to thesuction side of the fuel unit.The settling tank usually has sufficient capacity for about twelve hourssteaming. The tank is so named because in it any water in the oil isallowed to settle to the bottom to be drawn off at regular intervals. Thetank is fitted with a filling pipe, an outlet pipe near the bottom of thetank leading to the pump suction, an air pipe and a level indicator. Adrain valve is fitted near the bottom to drain off any water andsediment.From the settling tank oil is drawn by the pressure pump through asuction filter and then forced through the heater and discharge filter tothe pressure line for the burners. An adjustable spring-loaded reliefvalve is fitted between the discharge and suction ends of the pump, solimiting the discharge to any set pressure.Oils in general use for fuel purposes in boilers are of a low gradeand quite thick or viscous. In order to reduce the viscosity sufficientlyto ensure efficient atomisation, oil is heated in a heater where its temperatureis raised to between 150°F (65.5°C) and 240°F (115.5°C),depending on the quality of the oil. The oil heater may be heated byeither steam, hot water, or electricity.17


Atomising BurnersHeated oil delivered to the boiler front is sprayed into the furnace byatomising burners. The main types of atomising burners are—(a) Pressure Atomising Burners. In this type the pressure head ofthe oil fuel is converted into velocity head as it passes throughsmall tangential holes in the atomiser tip. In addition, the holesimpart a swirling motion to the oil, the discharge from the nozzlebeing thus broken up into a fine spray. Fig. 23 shows details ofpressure atomising burner tips.(b) Steam Atomising Burners. Low pressure steam h used in thistype to increase the effectiveness of fuel pressure as a meansof obtaining atomisation. A typical steam atomiser burner isshown in Fig. 24.(c) Air Atomising Burners—The Rotary Cup Type. Fig. 25 illustratesa horizontal rotary cup burner. In this method of atomisationthe fuel oil is delivered through a tube to the back endof a cup which is rotating at high speed (4,600 to 4,700 rpm.).The oil film is spread evenly by centrifugal force over the cupsurface until it reaches the rim where it meets swirl air, which isdelivered there in the opposite direction of rotation. The swirlair breaks down the oil into a stream of very fine droplets. Anadjustable air guide enables the shape of the flame to be varied.Furnace FittingsFurnace front fittings vary but in the main these consist of burner,air director for giving the air a conical swirling motion, a master airsupply check, a secondary air supply check regulating the supply ofstraight unswirled air around the burner (which controls the angle andlength of the flame) and a blue glass window for observation purposeswhen making flame adjustments. A typical arrangement is shown inFig. 26.Lighting Burners ManuallyTo start up any oil burning installation, close all burner valves andcirculate the oil line. Then, check to see that the damper, if fitted, iswide open. The furnace should be checked to ensure that there is noaccumulation of oil due to oil drippings. If the installation works onnatural draught, sufficient time should be allowed for air to flow18


through the furnace to carry any combustible gases up the chimney.If the installation works on mechanical draught, the furnace should bethoroughly purged by mechaninal draught before lighting the burner.The torch should then be lit and placed close to the burner tip, the oilbeing gradually allowed to flow until ignition occurs, and then slowlyincreased. When lighting the burner, stand well clear of sight holes andtorch holes in furnace fronts in case of a blow back, and extinguish thetorch used for lighting the burner immediately after use.If a steam atomising burner is used, the steam should be allowed toflow through the burner and clear the steam lines of condensate. Oncethe burner has been lit, the atomising steam should be regulated to givea good flame. To keep the consumption to a minimum steam shouldbe gradually reduced until sparks show at the flow. This indicates thatthere is insufficient atomising steam and the flow should be turned onagain enough to stop these sparks on the furnace floor.In starting oil installations where the furnace is cold, it is essentialthat plenty of excess air is furnished. This can be gradually reduced asthe furnace comes up to normal working temperature. If a burner isshut down for fifteen minutes or more, the furnace should be wellpurged before lighting up again.When shutting off an oil burner, the oil supply to the burner shouldbe gradually reduced before the air supply is reduced. This ensures theclearance of dangerous gases from the furnace.Automatic ControlsMost new industrial boilers are fitted with automatic water level andfiring controls. However, experience has shown that the incidence ofdamage or explosion caused by low water conditions has been relativelyhigher with boilers having fully automatic water level and firing controlsthan with those which are manually controlled. Investigation ofthese accidents shows the main causes to be—(a) Lack of testing and maintenance of controls and alarms, leadingto malfunction.(b) Isolation of control chambers.(c) Occasional inadequate standard of controls.The following paragraphs form the basis of discussion of each of theabove listed causes.19


Periodic Testing of Automatic Controls. The safe operation of anautomatically controlled boiler depends on the correct functioning ofits water level and firing controls, and such controls should be testedregularly. A suitable test procedure for externally mounted float controlsfitted with sequencing blow down valves is given below. When controlsare not of this type the procedure will require to be modified andexpert advice of a Boiler Inspector should be sought.Daily Operating Test—The following tests should be carried out atleast once a day, or once a shift, by a competent boiler operator familiarwith boiler controls.(a)(b)Water Level Control(i) Close the water isolating valve to the control chamber anddrain the chamber.(ii) Check that the feed water is being automatically supplied tothe boiler.(iii) Return valve to operating position.Firing Controls(i) With the burner operating, close the water isolating valveto the control chamber and drain the chamber.(ii) This should automatically cause the alarm to sound and thefuel and/or air supply to be cut off.(iii) Return valve to operating position.(c) Independent Overriding Control(i) With the burner operating, close the water isolating valve tothe independent overriding control and drain the chamber.(ii) This should automatically cause the alarm to sound and thefuel and/or air supply to be cut off and locked out to safety.(iii) Return valve to operating position.Items (a) and (b) are often in the same chamber, and in such casesthe two will be checked simultaneously.Weekly Tests—At least once a week the water controls should bechecked by manually interrupting the feed water supply and loweringthe level of water in the boiler by evaporation until the alarm soundsand the fuel and/or air supply locks out.20


After carrying out the daily and weekly tests, the boiler operatorshould ensure that the water level is restored to normal and all valvesare in the operating position. He should not leave the boiler until he issatisfied that it is operating normally. He should remain at least afurther 20 minutes.Maintenance of Controls—Automatic controls should be regularlyserviced and maintained by persons having the necessary competenceand facilities for maintaining the particular type of control. Regularmaintenance should be carried out at least once every three months.Records—It is strongly recommended that a record should be keptof all periodic tests and quarterly servicing and maintenance of controls.Isolation of Control Chambers—Isolation of the control chambercaused by the boiler operator closing and leaving closed either the wateror steam isolating valves or both, after closing the drain, has resulted inmany cases of damage and explosion from overheating of the boilerbrought about by the resulting low water level. Therefore, to preventisolation of the control chambers it is essential that the water isolatingvalve cannot be closed unless the drain valve is open and towards thisend, valves known as 'Sequencing blow down valves', which performthis function and allow steam and water connections to be blownindependently are usually fitted. This ensures that with the drain valveopen, the float will be at the bottom of the chamber thereby cuttingoff the fuel supply to the boiler, and, it will not be possible to relightthe boiler under these conditions.Standards for Automatic Controls—Automatic water level controlsare of two basic standards, viz. controls intended to assist the boileroperator who constantly supervises the boiler, and, controls intendedto replace continuous supervision with occasional supervision.When boilers are not continuously supervised, the minimum requirementsshould be—(a) Automatic water level controls should be so arranged as topositively control the boiler feed pumps. Alternatively, theyshould regulate the water supply to the boilers and effectivelymaintain the level of water in the boiler between certain predeterminedlimits.21


(6) Automatic firing controls should be so arranged as to effectivelycontrol the supply of fuel to the burners, and to shut off thesupply in the event of any one or more of the following conditionsarising—(i) Flame/pilot flame failure. The control should be of thelock-out type requiring manual resetting,(ii) Failure to ignite the fuel within a predetermined time. Thecontrol should be of the lock-out type requiring manualresetting,(iii) When a predetermined high pressure at or below the safetyvalve set pressure is reached.(iv) When the water level falls to a predetermined point belowthe operating level. This control should also cause anaudible alarm to sound.(v) Failure of forced or induced draught fans, or any automaticflue damper, when these are provided.(c) Independent overriding control. This control should cut off thefuel supply to the burners and cause an audible alarm to soundwhen the water level in the boiler falls to a predetermined lowwater level. The control or its electrical circuit should be soarranged so that it has to be reset by hand before the boiler canbe brought back into operation.(d)Electrical failure. All electrical equipment for water level andfiring controls should be so designed that faults in the circuitscause the fuel and air supply to the boiler to be automaticallysliut off. Positive means requiring manual resetting should beprovided to cut off the fuel and air supplies to the boiler shouldthere be a failure of electrical supply to water level and firingcontrol equipment.Automatic Controls may include the following—(d)(b)A Blower Damper—This may be actuated by a control motorwith an adjustable linkage interlocking the blower damper withthe oil metering pump, proportioning the air and oil supply forhigh and low fire.Steam Pressure Controls—These are mounted next to the columnand may consist of high-low control operating limit control andline (auxiliary) control.22


(c) Burner and Flame Failure Controller—An electronic programmingrelay actuated by the steam pressure controls provides propercycling of start, stop, ignition, and safety shut down.(d) Gas Pilot—Premix type with internal spark ignition. An electrodetype flame rod protects the pilot flame and permits thefuel valve to open only after the pilot flame is established. Apressure regulator and solenoid valve control the gas flow in thepilot line.(e) Low Water Control—This control may be composed of twomercury tube switches operated by a metal ball float that risesand falls with changes in boiler water level. The float operatesthe switches through a mechanical linkage which includes ametal bellows that acts as a seal between the float chamber andthe switches. Figs. 27 and 28 illustrate fundamental types of lowwater cut-offs.Control of Oil Burners—Automatic Start-up—The control systemused in industrial packaged boilers are mostly electric, and a burnercontrol panel contains most of the components and gear. Most oilburners are fully automatic requiring only a push of a button to startfurnace purging, ignition, proof of ignition, and normal burner operationresponsive to the pressure or temperature of the boiler. Fail-safeprinciples are built into most of the control functions which briefly areas follows—The push of a button or a signal from a time-clock starts a motordriving a cam shaft control. Closure of the first switch starts up theforced draught fan which purges the furnace of any accumulationof combustible gas. At the same time the burner oil heater is switchedon. After the fan has run for a predetermined time, the ignitionsystem, consisting of a high tension electric spark, with or withouta supplementary gas supply, is switched on.The oil is turned on provided that—(d) the oil temperature iscorrect, monitored by a thermostat in the oil heater, and (b) there issufficient water in the boiler, monitored by a float switch measuringboiler water level.If the flame has not ignited within a predetermined period (a fewseconds only) a photocell or a lead sulphide cell scanner shuts off thefuel and returns the cam shaft to the start position. One automaticrestart may be given, and if this fails, the burner locks out* requiringa manual restart. If however, as is normally the case, the scanner23


egisters 'flame present', the burner continues to operate under thecontrol of boiler pressure, load, or temperature. Shortage of waterin the boiler, flame failure, or failure of electric supply will causelockout. Fig. 29 illustrates by means of a line diagram the principleof oil burner control.Some Typical Equipment Used to Control Semi-Automatic andAutomatic Oil Burners—With automatic oil burners it is necessary todesign controls to shut off the fuel and air supply if the burner fails toignite during starting or if the flame goes out while the burner is inoperation. Without flame failure protection, the furnace will fill withan explosive mixture of fuel and air within seconds after the flame hasfailed.Photocells—To detect flame failure, photo electric devices or photocellswere developed. Photocells have the property of converting variationsin light intensity into corresponding variations in an electriccircuit. Thus, the light of the fire actuates the photocell and energizesa relay permitting oil to flow to the burner. Should the fire be extinguished,the flow of current in the photocell will be interrupted, therelay will be de-energised and oil flow to the burners shut off.Lead Sulphide Cells—Another development for detecting flame failureis the Lead Sulphide Cell or photo scanner eye conductive cell that issensitive to infra red radiation given off by a fire. The lead sulphidereacts to the infra red rays which envelope every flame—a match, a gasflame or an exhaust. The practicability of this cell rests on an electroniccircuit which receives voltage variations based on the varying resistanceof the lead sulphide cell when exposed to a flame. The electronic circuitwill accept and amplify only those voltage variations that correspondto the flicker frequency of the flame. The lead sulphide cell is probablyone of the best types of flame failure safeguard for oil burners, especiallyif an orifice is used to screen out the reflected rays from the refractory(furnace brickwork). A later development uses an electronic controloperated by the ultra violet rays of the flame. Fig. 30(A) is a sketch ofa lead sulphide cell scanner, while Fig, 30(B) illustrates the applicationof the scanner.Steam Pressure Switch—A steam pressure switch is shown in Fig. 31.It is actuated by a change in pressure operating on a bellows. It maybe designed to open electrical contacts on a rise in pressure, or to closethem under a similar condition. At predetermined pressures, it stopsor starts the flow of steam.24


Fuel Pressure Switch—A fuel pressure switch is shown in Fig. 32. Itis used to ensure sufficient pressure for proper atomisation of the fueloil before allowing the fuel valve to open.Solenoid Oil Valve—A solenoid oil valve (Fig. 33) makes use of asolenoid, which when energised will open the valve. A solenoid is anelectric magnet consisting of several layers of insulated wire, and havinga movable Iron plunger arranged to move in and out of the middle ofthe coil. When the coil is energised the plunger will be pulled in and theconnecting linkage will open the valve. The pressure usually associatedwith solenoid valves is 85 Ib/in 2 (5.97 kg/cm 2 ).These valves may fail to operate for one or more of the followingreasons—(a) Valve parts or linkage fouled with dirt or foreign matter.(b) Valve parts worn out or bent.(c) Valve or linkage damaged due to rough handling.(d) Coil burned out or electrical connections broken.DraughtDraught is required to supply the air which contains the oxygennecessary for the complete combustion of the fuel. It is also necessaryto carry away the products of combustion, viz. the flue gases. Draughtis obtained in two ways—natural draught and mechanical draught.Natural Draught—In natural draught, the hot gases being lighter, riseinto the chimney leaving an area of negative pressure, or a weakdraught, in the furnace into which cold air enters. In other words, naturaldraught is caused,by the difference of weight in the heated gases in thechimney and the cold air entering the furnace. To obtain a gooddraught the chimney temperature should be between 600°F (315.5°Qand 700°F (371.1°Q, this temperature being necessary to bring aboutthe required difference in weight. The draught can also be improvedby increasing the length of the chimney as by this means, the columnof heated and therefore lighter air, is made less in weight, against thesame volume of cold and heavy air.Mechanical Draught—This can be produced by blower fans and themethods can be classified as follows—(a) Forced Draught, which is produced by a blower fan, wherebythe air is forced into the furnace. The forced draught air is25


(b)usually partly heated by the waste gases before entering thefurnace.Induced Draught, which is produced by an exhaust fan placedbetween the boiler and chimney. This exhaust fan draws thegases from the furnace and discharges them into the chimney.The advantages of forced draught over natural draught are—(a) For the same power a smaller boiler would be suitable as morefuel is burned in the furnace.(b) Heated air enters the furnace instead of cold air.(c) The boiler steams better.(d) Better control of fires, as the draught is independent of weatherconditions.Draught and Damper Regulation—With natural draught the draughtis regulated by opening or closing a damper in the chimney and controllingthe air admitted to the furnace.A damper is a steel plate pivoted in the chimney and controlled bylevers for opening and closing. A hand controlled damper is opened andclosed according to the load and judgment of the boiler operator. Whennecessary, the position of the damper should be changed gradually.Automatic damper regulation is accomplished by a mechanismoperated either by steam, air, or hydraulic pressure and connected bylevers to the damper.Draught Gauge—A simple draught gauge is illustrated in Fig, 34. Theforce or intensity of the draught is measured by a U-shaped glass tubecontaining water, one end of the tube being connected to the forceddraught air trunk and the other end left open to atmosphere. The airpressure in the trunk forces the water, which is usually tinted withsome colouring fluid for ease of reading, higher in the leg of the tubewhich is open to the atmosphere and lower in the leg of the tube opento the trunk. The difference in the two water levels is called the airpressure, and is expressed in inches of water.Purpose of a Chimney—A chimney is necessary for the followingreasons—(a)To create a draught when no induced or forced draught fans areused.26


(b)To lead the waste gases or products of combustion to a suitableheight so that they will not be a nuisance to the surroundingcommunity.When mechanical draught is used, the second reason, viz. (6), is themain one.A chimney of 75 to 100 feet (22.86 to 30.48 metres) in height shouldcreate a draught of f to one inch (1.90 to 2.54 centimetres) water gauge.SmokeThe products of combustion constitute smoke. Therefore, smokeconsists of carbon dioxide, carbon monoxide, moisture, sulphur dioxide,nitrogen, unburnt particles of carbon, and free air.Black smoke is a definite indication of bad combustion and thecolour is caused by unburnt particles of carbon. These particles beingvery tiny are easily carried by the moving gas stream. There will be ahigher percentage than normal of carbon monoxide and the percentageof carbon dioxide will be less than normal.Causes of black smoke and methods of prevention are—(a) Too little air—increase forced draught pressure.(b) Carbon deposits on furnaces or combustion chambers—furnacesand combustion chambers should be cleaned at regularintervals.(c) Bad atomisation causing poor penetration and bad distributionof the oil fuel spray—burner nozzles should be cleaned frequently.(d) Oil fuel temperature either too low or too high—correct temperatureshould be maintained.(e) Oil fuel pressure too low or too high—correct pressure shouldbe maintained.White Smoke—If too much air is being put through the furnace theefficiency of the boiler may be seriously lowered, as the excess air iscarrying away heat up the chimney. This condition usually produceswhite vapour. If the air supply exceeds the correct amount actuallyrequired for complete combustion, the furnace temperature is loweredas heat is lost in heating the surplus air.27


In the absence of apparatus to analyse the constituents of the productsof combustion, it is considered good practice to reduce the excessair from the smokeless-chimney state until a light-brown haze isobtained at the mouth of the chimney.28


Operation ProcedureCHAPTER VBOILER OPERATIONThe first duty of a boiler operator when taking over a shift is to checkwith the out-going operator to see if anything out of the normal occuredon the previous shift. Next, he should test the water gauge glass,column, cocks or valves, and piping. This ensures that the water levelin the glass is correct. The blow-down valve should also be checked tosee that it is properly closed and not leaking. The pumps and auxiliaryequipment should then be inspected to ensure that they are in goodworking order.All routine duties such as testing the water gauge glass, blowing down,soot blowing and cleaning burners should be arranged on a regularschedule. These duties could be recorded and hung up as a notice sothat each operator knows what to do and when to do it.An important aspect while generating steam, is the regulation of thefeed water. The water should be maintained at a definite level. Enoughwater should be fed into the boiler to make up for the steam that isbeing used. This is a fairly easy matter when feed water regulators areused or with a steady load, but it still possible with hand control offeed pumps if the operator is reasonably alert.With a straight heating load, the water might be maintained at halfglass level. Where engines are involved, the level should be lower,preferably about three inches, so that carry-over or excess moisturecan be minimised. In the latter case, greater care will have to be exercised,by the operator because if the pump stopped, a drop of threeinches in the water level would mean the possibility of shutting off fuelto the burners.Raising SteamInspection—If the boiler is new, the operator should examine theCertificate of Fitness issued for the boiler. If it cannot be found heshould notify the owner to request a Boiler Inspector to conduct aninternal and external inspection of the boiler for the issue of a Certificateof Fitness. If an inspection has been made, he could request aduplicate Certificate for display in the boiler room. If the boiler is aused one, the current Certificate of Fitness should be displayed.29


Preparation in the case of a new boiler—When a new boiler is tobe put into service for the first time, it should be thoroughly cleanedinternally and any protective coating of oil on shell and tubes shouldbe removed. Before closing up the boiler a careful examination shouldbe made internally to ensure that all tools, waste or any other materialhas been removed, and that all pipe openings are clear. The lowermanhole or handhole doors are re-jointed and the doors set centrallyin place and tightened up. The boiler can now be filled with fresh waterto half glass level, making certain beforehand that the blow-down valveis shut and that the water gauge cocks are open and drain closed. Forthe initial 'boiling out' of the boiler, an alkaline detergent should beused, e.g. one containing sodium carbonate, sodium phosphate or amixture of these chemicals, together with twice their weight of anhydroussodium sulphate to avoid intergranular cracking. Two to fourpounds of such detergent per ton of water in the boiler should beadequate. The top manhole or handhole can now be jointed up. Allstop valves should be eased off their seats and closed down hand-tight.To allow for escape of air during flashing up, the air cock on top ofthe boiler should be left open; if no air cock is fitted then the steamspace test cock should be left open for this purpose. Start a fire witha small nozzle if such a nozzle is provided, and gradually bring thepressure up to 5 lb/in 2 (0.35 kg/cm 2 ). Continue boiling for about twodays. Thereafter, empty the boiler and wash thoroughly with freshwater.Preparation in case of an old boiler—Thoroughly clean and removeall mud and scale by scraping, chipping and washing. Where thenecessary facilities exist, the use of air pressure or vacuum for removalof dust is advantageous before washing out.The shell and tubes should be inspected for signs of corrosion, crackingor leakage. Also examine the stays and internal pipes for looseconnections, broken bolts or rivets, cracks, or scaled up pipes. Cleanor replace the fusible plug if one is fitted. Before closing up the boilerensure that no tools, waste or any other material has been left inside,and see that all pipe openings are clear. If dampers are fitted, operateboth inlet and outlet dampers and make sure that they are free andwill remain in any desired position; leave the inlet and outlet dampersopen. Check to see that the blow-down valve is shut and that the watergauge cocks are open with drain closed. Check if the feed valves arein good order. All stop valves should be eased off their seats and closeddown handtight30


Preparation of new and old boilers—Rejoint the lower manhole orhandhole doors, manipulate the doors into the boiler, position centrallyand tighten on dogs. Now fill the boiler with fresh water so that a quarterglass of water is showing in the water gauge and then replace the topmanhole or handhole door. Open the cock to the steam pressure gaugeand examine safety valves as far as is practicable to see that they arein working order. Examine the blow-down valve and piping and allother parts and fittings of the boiler before starting a fire. It is easierto rectify a fault at this stage than to have to take the boiler off rangelater on.In lighting up, a smaller nozzle than normally used in the workingburner should be used, if one is provided. Maintain a light fireuntil the furnace brickwork is dried out thoroughly. If the entire brickworkis new, the drying out may require several days. If only the liningof the combustion chamber has been renewed, about 48 hours will besufficient time for drying out. Uneven heating of the brickwork willresult in cracking of the lining and brickwork, particularly in the caseof new brickwork, thereby destroying its value as an insulator andsupport.When the boiler is beginning to show a good heat, the nuts on manholeand handhole doors should be nipped up and any new joints thathave been made similarly treated.When the boiler has been uniformly heated and after steam hasescaped through the air cock, the air cock should be shut and thesteam pressure allowed to rise slowly to the working pressure. Thewater gauge should be tested to prove that it is in proper workingorder. The blow-down valve should be checked for leaks.It is essential that the boiler is evenly heated when raising steam.Uneven heating will cause unequal expansion resulting in distortedtubes and opening of the boiler seams (riveted joints), especially wherethe circulation of the water may be sluggish.After the usual week-end shut down, a light fire should be maintainedfor about one hour.Oil Fuel SystemIf the system is new or has been out of service for a considerableperiod of time, first clean and examine the strainers, clean and examinenozzles and burners. If compressed air is available, blow out the oil31


supply lines with air. Work the air registers to see that they are In goodworking order. Ensure that the oil valves to individual burners areshut. Remove any spilled oil about the burners, fronts, and boiler roomfloor, and see that there is no oil on the furnace floor. Ventilate theboiler by opening dampers.If steam is required for the oil pump and no steam is available, ahand-pump or auxiliary power pump (if such is available), is employedto charge up the oil heater and pipe line system and to circulateoil in same, between the heater and boiler front manifold.Open the drain valve on the settling tank and drain off any waterand sediment that may have settled. Then open the outlet valve on thetank, the hand-pump suction valve, inlet valve to the heater, outlet valvefrom heater and valve to discharge and circulating valve on boilerfront manifold. The burner shut-off valves require to be tightly closed.Examine oil lines and equipment for leaks.Light the centre fire by holding a hand torch near and just below thetip of the burner. Then turn on the oil and close the circulating valveto raise the oil pressure to about 50 lb/in 2 (3.51 kg/cm 2 ) or more.Stand well clear of sight holes and torch holes to avoid possible flashback.If the torch is snuffed out before the oil is lighted, shut off theoil and relight the torch. Always use the torch for lighting burners.Next, light the adjacent burners, but be sure that there is an excessof draught before lighting additional burners. Do not allow the oil toimpinge excessively on the brickwork or parts of the boiler.Oil Temperature at BurnersThe oil temperature at the heaters is seldom more than 240°F(115.5°C). The temperature regulates the viscosity or fluidity of the oil,and the higher the specific gravity the higher the temperature required.Excessive temperature produces carbonisation of the oil in the heatertubes and furnace 'sprayers. Generally, pressure of oil regulates quantityor output through burners while temperature regulates the fluidity.Purity of oil is essential, otherwise dirt or water present may result inchoking up of burner nozzles, sputtering or extinction of burners, orchoked filters.For efficiency and safe working of oil fuel, the following points areof importance—(a) Correct temperature of oil.32


(b) Lowest pressure which will give the output required.(c) General cleanliness of heaters, filters, burners, furnace andboiler room.Opening op Steam from a Boiler to a Pipe RangeIn opening steam from a boiler to any pipe range, it is important tosee that the drains are opened before turning on steam. The steam stopvalve should afterwards be eased off the face and sufficient timeallowed for the pipe to thoroughly drain itself and warm up before thevalve is slowly opened up fully. As steam will condense in cold pipes,care should be taken to drain these lines as thoroughly as possible andto open any steam valve slowly and carefully.When two or more boilers comprise an installation and are connectedto the same pipe range, each must be fitted with an automatic nonreturnvalve directly at the boiler and a main stop valve between thenon-return valve and the pipe range.When the pressure on the incoming boiler is nearing thepressure on the line, open the drain cock on the non-return valve andcrack open the main stop valve until the pressure in the line betweenthe pipe range and the non-return valve has been equalised. Then openfully the main stop valve leaving the drain on the non-return valveopen. When the pressure in the in-coming boiler is about 5 lb/in 2(0.35 kg/cm 2 ) below that of the range, open the non-return valve slowly.In order to ensure that the non-return valve functions properly, alwaysuse it automatically for putting on or taking off a boiler from range,provided the pipe range is filled with steam at working pressure.Where a superheater is installed, the normal practice is to open allheader drains before lighting a burner. When steam is showing, thesteam-to-superheater valves (if fitted) are opened, and after the superheaterhas been well blown through, the drains on the superheater inletheader are shut, leaving the outlet header drains open to create acirculation through the tubes. When the boiler has been coupled tothe steam line the outlet header drains should be shut.Taking a Boiler out of ServiceClose the oil valve and the air register to each burner, one by one*until all burners are shut off. Then secure the main oil valve. Removeatomisers at once and drain, immersing the tips in kerosene. Keep all33


openings to the furnace tightly closed to avoid quick cooling and therebycausing uneven strains in the boiler. If the boiler is a single unit themain steam stop valve can be left open. If it is one of a battery ofboilers, allow the water level to fall to two inches in the glass, thenclose the main steam valve, feed water valve, etc., after the pressurehas fallen a little due to the non-return valve closing off. If the pressurestarts to rise again due to radiation from brickwork, additional watercan be added to keep the pressure down.Where a superheater is installed, open the superheater outlet drainsto prevent moisture from collecting in headers and tubes when thesteam flow from the boiler is stopped. It is very important that alldrains and vents on the superheater be kept open when there is no flowof steam through it.Practical Operation and Periodic InspectionCleanliness—Great care must be taken to see that no oil is allowedto accumulate in the air boxes, furnace bottoms, boiler room floor,etc. If a leakage from the oil system to the boiler room occurs at anytime, shut off the oil 'supply to that part of the system immediately.Place oil tight trays under all fittings from which liquid fuel may escapewhen the fitting is opened out. Keep a sand box in a readily accessibleplace in the boiler room.Occasional examination of oil filters and strainers is necessary toensure that they are in proper working order. Pressure gauges areusually fitted on each side of the filters so that the pressure differencecan be frequently read for indications of clogging.Smoke observation windows and mirrors should be cleaned at leastonce every twenty-four hours and used constantly to see that there isno smoke.The feed water supply should be as uniform as possible and thewater level in the boiler should never be allowed to get above the topof the gauge glass due to the possibility of priming if a sudden increasein steam demand occurs. If difficulty is experienced in maintaining thecorrect water level, the steam output of the boiler should be reducedand the cause ascertained.Low water is the most dangerous condition experienced in theoperation of boilers and is generally due to inattention on the part of34


the boiler operator. If the loss is gradual and noticed by the operator,the following action should be taken—(a) Increase the rate of feed.(b) Check feed line for leaks or closed valves.(c) Check blow-down for leaks or open valve.(d) Start auxiliary feed system.If at any time the water level falls out of sight in the gauge glass, theboiler should be immediately taken out of service. Do not feed waterinto the boiler, since some parts may be very hot and application ofcold water will cause sudden contraction of the material and possiblyeven an explosion. Ease the safety valves off their seats and let theboiler cool down. When all pressure is off, open and examine theboiler to see if any damage has been done; this means a very carefulexamination for signs of overheating. If any are found, the boiler inspectorshould be requested to make an expert examination.Boiler tubes should be kept reasonably free from soot. When a sootblower is used, drain the steam or air supply connections so that thesteam or air is practically dry.Corrosion in air heaters, sometimes of a rather severe nature, is dueto the deposition of sulphuric acid originating from the sulphur in thefuel, when the temperature of the gases in the heater falls below theirdew point. It is therefore essential when starting up the boiler to operatethe damper, by-passing the air heater until the boiler is heated throughoutand there is no danger of the gases in the heater being cooled tobelow their dew point. A reasonably safe precaution is to by-pass theair when the temperature of the gases leaving the air heater falls tobelow 220°F. (104.4°C).In mechanical draught installations the operating efficiency of thefan will depend to a great extent upon the condition of the blades ofthe fan wheels. Due to conditions under which the fans usually operatedust will gradually accumulate on the blades and form a scale whichwill lower the efficiency of the blades considerably. Accumulation andwear may cause the fan wheel to be thrown out of balance. The fanwheels should be cleaned at intervals. If the fan shows any signs ofbeing out of balance, shut it down immediately, inspect it and rebalanceit. Fans are usually motor driven and all operating instructions andmaintenance recommendations made by the manufacturer should bestrictly observed.35


Blowing down a boiler serves three objects—(a) It is a rapid means of lowering the boiler water level when itaccidentally rises too high, thus reducing the possibility of waterhammer or slugs of water passing over with the steam.(b) It permits the removal of sediment or sludge while the boileris in service.(c) It serves to control the concentration of dissolved solids andsuspended matter in the boiler water.Except where the amount and frequency of blowing down is determinedby chemical analysis, blow down freely at least once every eighthours. Blow down the boiler at a time when steam production is lowest.Where it is necessary to blow down a large amount of water, openthe blow-down valve until it is about half open and leave in thatposition until the water is lowered about one half inch in the gaugeglass; then open wide until the blowing down is completed and thereaftershut the valve. Repair leaky blow-down valves or cocks as soonas practicable. If a surface blow-off or scum valve is fitted, use it untilthe undesirable conditions for which it is employed are corrected. Incases where the gauge glass is not in view of the operator blowing downa boiler, another operator should be stationed where he can see thegauge glass and signal to the operator blowing down the boiler.Cleaning a Fire Tube Boiler and Preparing it for InspectionHaving reduced the pressure to zero allow the boiler to cool slowly.When it is cool, open the blow-down valve and let the water drain,leaving the air vent cock or a valve on top of the boiler if no air ventcock is fitted, open, so as to allow air to enter the boiler and preventthe formation of a vacuum. Soot should then be blown and swept clearof all tubes, shell plates, heads, and seams, and every accessibleexternal surface.The blow-down valve should be shut if any other boilers feed thisline. The manhole and handhole covers, and any inspection plugsshould then be removed. All loose deposits of sludge or other sedimentshould be washed out. The blow-down valve should be opened onlywhen it is certain that there is no pressure in the line and no one insidethe boiler. Attached scale or oil deposits should be left for the boilerinspector to see.36


A boiler properly prepared for inspection should be cool, clean anddry. It is advisable to attach a red tag marked 'Man In Boiler' to thesteam, blow-down, feed and fuel valves, and also to the manhole coverwhenever anyone is in the boiler.Precautions to be Observed before Entering a BoilerIf the blow-down enters a common line with other boilers in operation,ensure that all valves on the line to the open boiler are closed.If other boilers are operating on the same steam line, both stop valveand non-retufn valve must be closed and the drain between them open.Any other valves on lines under pressure leading to the boiler must bechecked. The boiler operator on duty must be told that someone isgoing into the boiler. A responsible person should be stationed at themanhole door while someone is inside the boiler.Laying up a BoilerWhen a boiler is to be put out of service for a considerable length oftime it should be entirely emptied of water and every accessible part,outside and inside, thoroughly cleaned. All soot should be removedfrom external surfaces and tubes, as soot is likely to absorb moisturefrom the air and cause corrosion. Soot should be removed from thefurnace and combustion chamber. Scale and mud should be cleanedoff the boiler and the boiler thoroughly washed out. Use airing-stovesto make sure that the boiler is perfectly dry inside. All steam and waterconnections should be tightly closed so that no water can enter theboiler. Then place trays of silica gel or calcium oxide (quicklime) insidethe boiler. Place the manhole and handhole doors in position andensure all other openings are tightly closed. The silica gel or calcimoxide absorbs the oxygen thus preventing oxidation.If the boiler is to be laid up for a period exceeding three months, theboiler should be opened after three months for inspection. Againplace trays of silica gel or calcium oxide inside the boiler before closing.The external parts of the boiler coming in contact with the products ofcombustion should be coated with red lead to prevent externalcorrosion.When a boiler is to be laid off for only a few days, it is not necessaryto blow it down. But if layoff is to last more than a week, the watershould be brought to the boiling point with a vent open on top todischarge all gases. The water should be made alkaline by adding about2 Ibs. (0.90 kg) of caustic soda per ton of water.37


GeneralCHAPTER VIPROBLEMS OF PLANT OPERATIONSteam boiler operation places safety, efficiency and control in thehands of the boiler operator. Many automatic devices have beendeveloped to make this control easier, safer and more efficient. However,if automatic equipment is not understood and maintained it willfail If it should fail the boiler operator must be capable of picking upmanual control of many operations on a second's notice.Boiler ExplosionsBoiler explosions are usually the result of one of three faults—(a) A defect in the boiler—Defects contributing to an explosion includecracking, improper design or construction to withstand theoperating pressure, and defective material used in construction.Most of these defects are a result of lack of proper inspectionprocedures and should be brought to light by a competentinspection.(b) A defective appliance—Appliance defects most likely to causean explosion are safety valves of defective design, or impropersetting or condition of these valves. Defective installation,design, or condition of water-level-indicating equipment constitutesa similar hazard. Pressure gauge inaccuracy may lead toan explosion, as may failure of feed-water equipment, Defectiveblow-down equipment resulting in fouled internal surfaces of theboiler may cause explosions. Internal surfaces may be fouledfrom other sources to such an extent that explosions may occur.(c) Improper operation—This is usually due to an incompetent orcareless operator. Improper operation includes fouling ofinternal surfaces by neglecting water cleanliness control, orpermitting low water conditions to arise. In fact, negligence orincompetence of a boiler operator may lead to any amount oftrouble, from an explosion down.The results of a boiler explosion can be devastating and even a smallboiler may cause tremendous damage. Small boilers, indeed, mustoften be considered more hazardous than large units, because of the38


mistaken belief of some owners and operators that small boilers donot have a potential for destruction, and therefore, do not need competentattention.Scale FormationScale formation on the water side of boiler heating surfaces is due,in general, to the combined effects of heat, pressure, and concentrationof certain impurities in boiler feed water. The most common and mosttroublesome of these impurities are Calcium Carbonate and MagnesiumCarbonate which tend to form soft scale, and Calcium Sulphate andMagnesium Sulphate which form a hard scale that is difficult to remove.Silica is another scale former.Scale forming water is said to be 'hard'. This hardness is either'temporary' or 'permanent' or both. The temporary hardness may beeliminated by heating the feed water to about 212°F (100°C) in an openheater where the salts causing temporary hardness are precipitated. Thepermanent hardness must be controlled either by treatment in watersofteners or by treatment in the boiler.The two main objections to scale on boiler heating surfaces are—(a)(b)Scale is a poor conductor of heat and its presence in appreciablethickness means that less heat is absorbed by the boiler water,and the boiler efficiency is thereby reduced.Because scale is a bad conductor of heat, the heating surfacesinsulated by scale from the boiler water on one side and exposedto hot gases on the other, may soon reach dangerously hightemperatures. Serious damage, rupture of tubes and even boilershells may result.Scale formation often increases with the rate of evaporation. Thus,scale deposits will often be heavier where the gas temperatures arehighest.Heavy scale deposits are usually an indication of neglect, for scalecan be prevented in most cases by proper treatment of the water.Where scale has formed to an appreciable thickness, it should beremoved, and once the boiler is clean, steps should be taken to preventits recurrence.39


Scale RemovalScale removal is effected by one or both of the following methods—(a)mechanical removal, and (i) water treatment.(a) Mechanical removal of scale is effected while the boiler is idleand empty. When soft scale is encountered it is easily washedout with a strong jet of water. Hard scale is removed with scalingtools operated by hand or power. There are two general typesof mechanical tube cleaners on the market, viz. hammers andcutters.The hammers are operated by'steam or air. The hammer (Fig.35) is inserted in the tube and strikes a hard blow first on oneside and then on the other. Scale is dislodged by vibrations ofthe tube. Great care should be exercised in the handling of thecleaner as otherwise tubes may be deformed.Cutters used in removing scale are known as turbine tubecleaners. A cutter is operated by a small water turbine inside thecasing, and may be purchased in any size to fit the various sizesof tubes. Water is led into the water turbine through a flexiblehose and the cutters are thus made to revolve rapidly. By passingthe cleaner back and forth through the tube, the scale is cut,and the water discharged from the turbine carries the loose scalewith it. Care should be taken not to operate the turbine tubecleaner too long in one place or to force it unduly, as damageto the tube may result.The accessible parts of shells, drums, and heads are chippedwith a dull chisel or scaling hammer. Care should be taken notto score the metal.(b) Water Treatment—Feed water contains varying amounts ofscale forming substances. It should therefore be purified beforeit enters the boiler. However, the methods by which this maybe accomplished comprise too large a subject for detailed discussionin this chapter.Scale-forming substances composed of Carbonate compoundscan to a large extent be removed from the water-by first heatingthe water in a feed water heater and then pumping it into theboiler. Sulphate compounds cannot be removed so easily andchemicals known as boiler compounds are commonly fed intothe boiler along with the feed water while it is in operation. Thecompounds are composed mostly of soda ash, but as different40


scale-forming substances are found in various localities it isnecessary to have the water analysed. The selection of a compoundfor a particular feed water should be done by a skilledchemist or engineer who can determine the chemical and theamount of it needed to neutralise the scale forming propertiesof the impurities in the water. Alternatively, guidance could beobtained from any industrial organisation which specialises inthe supply of chemicals, testing equipment, etc., for use in watertreatment. In most cases, these organisations have an efficientsystem of technical service whereby advice and information canbe provided for any particular set of conditions.Generally, the amount of the chemical needed is governed byperiodic tests. The chemical is fed into the boiler along with thefeed water once a day, and in time, helps in loosening any scalepresent and in causing the scale forming substances in the waterto precipitate as mud. This can be blown out through the blowdownvalve. The amount of blow down will depend on the dailytests of the water, but conscientious and regular blowing down isimportant.Overtreatment with chemical compounds may cause causticembrittlement over a period of time, while on the other hand,undertreatment will allow scale to form. Thus the importanceof daily tests cannot be over emphasised.Hard scale is difficult to remove by mechanical means. Thiswork can often be materially lessened by inserting a quantity ofcaustic soda into the boiler and allowing the solution to boilfor 24 hours on a slow fire with the boiler vented to atmosphere.At the end of this period, the boiler should be cooled and emptiedand all deposits removed. This treatment often will loosenscale so that it may be washed off with a high-pressure jet ofwater.All water scales are soluble in acid and modern practicefavours removal of scale by acid cleaning. Dilute hydrochloric,sulphuric, and other common mineral acids used with inhibitorshave proved effective in removal of water scale. As strong concentrationsof acid solution could prove extremely harmful tothe boiler, acid cleaning should be done only under the directsupervision of a skilled consultant in^the field. Industrial organisationsspecialising in boiler water treatment usually provideinformation and service for acid cleaning'of boilers.41


Oil in BoilersThis is a dangerous condition. Oil is an excellent heat insulator, andits presence on heating surfaces exposed to high temperatures maycause serious overheating and damage to the boiler.Every precaution should be taken to avoid oil from entering a boiler.Not only will oil cause the boiler to foam badly, but it also has atendency to mix with other impurities in the water, which when theboiler is not in use will settle on the metal in a spongy mass. Thisgrease prevents the water from coming in contact with the metal. As aresult the metal under operating conditions may be seriously overheated.This could lead to explosion.A common cause of the presence of oil in a boiler is the use ofreciprocating steam machinery exhaust containing cylinder oil forcondensate return to the boiler feed system. Also, oil fuel heatingequipment may leak oil into the steam system and create this problem,if the condensate is returned to the boiler. To prevent the conditionarising from these two sources, (a) a minimum amount of high-gradeproperly compounded cylinder oil should be used for lubrication ofsteam engines and pumps where the condensate is returned, and, anefficient type of oil filter should be used in the exhaust system, (b) condensatefrom oil fuel heating equipment may be trapped to wasteor fed through an observation tank.Oil deposits should be removed from a boiler by scraping all partswithin reach and then boiling out with a caustic solution.Internal CorrosionInternal corrosion is generally caused by the presence of a free acidin the feed water. The free acid may result from the splitting up ofcertain salts in the water, or the water supply being contaminated, orby adulterants in the cylinder oil (used in lubricating steam machinery)which find their way into the boilers, The presence of air also causescorrosion, especially in the steam space of a boiler. All wat£r containssome dissolved air. When it is heated in a boiler the air will be releasedand the metallic surfaces attacked.Corrosion in a boiler is also due to electrolytic action. Minute strayelectrolytic currents flow in the boiler-water solution between someparts of the boiler steel and a fitting of non-ferrous metal. This causeswastage of the steel, usually around non-ferrous fittings.42


To prevent internal corrosion due to an acid condition of the boilerwater, an analysis of the water should be made and suitable chemicaltreatment applied.External CorrosionBoilers in service may be exposed to external leaks of different kindswhich tend to corrode the shell. The boiler operator should guardagainst leaky, safety valves and steam mains which drip water onto theboiler and cause external corrosion. Special attention should be given toareas where the water runs under protective coverings. Leaky manholesand handhoies are especially dangerous as they corrode the shellrapidly; such places should be kept clean and tight. Leaky tubes shouldbe rolled, or if necessary replaced promptly, to avoid corrosion of thetube plate.ErosionErosion is closely allied with external corrosion in its effect. But it ispurely a mechanical action, a wearing away of external surfaces bysand, steam, or water, or any similar agent.Erosion by improperly adjusted soot blowers is not uncommon.Steain soot blowers should be supplied with practically dry steam andthey should be well drained before use.Caustic Embrittlemeat of Boiler PlatesCaustic embrittlement is a condition which sometimes develops inriveted boilers during operation and which may cause cracking leadingto a dangerous condition.Laboratory workers have found that cracking due to caustic embrittlementonly takes place when the metal is under stress, and at thesame time exposed to high concentrations of caustic soda. In rivetedboilers such conditions could occur if parts of the boiler plate areunder stress due to say heavily-worked rivet holes. Small pockets areformed between the plates of the riveted joint which could collect anexcessive quantity of caustic soda from the water-softening agents. Fig.36 illustrates caustic cracking.43


Extremely high concentrations of caustic soda are necessary beforeembrittlement can take place, and such concentrations may be foundunder butt straps or under laps of the plate in riveted seams.So far no cases of caustic embrittlement have been found in welded,stress-relieved or solid forged drums.Grooving in BoilersGrooving takes place in parts of the boiler material that are subjectedto repeated bending action due to changes of pressure andtemperature. Repetitive slight movements loosen particles of the protectivefilm of rust on the surfaces of the steel plate as soon as they areformed, thus exposing a fresh surface for the formation of rust. Wherethe corrosion thus induced is confined to a particular place instead ofbeing spread over a wide area, grooving is the result. Therefore, groovingis a form of deterioration of boiler plate by a combination ofcorrosion and mechanical action.Grooving is most likely to occur at the bottom of furnace necks, Fig.37(0), and in front end plates which are flanged inwards to take thefurnace, especially if the radius of the flanging is small, Fig. 37(Z>). Invertical type boilers, grooving is sometimes encountered at the junctionof the firebox and shell, Fig. 38(a), in the firebox crown-plate flanging tothe uptake, Fig. 38(6), and in the longitudinal lap-joints of the boilershell, Fig. 38(c). In the case of longitudinal lap-joints of boiler shells,grooving may occur due to the plates straining to become a perfectcircle; such grooves do not occur where single-butt or double-buttstrap joints are used.The boiler operator can be instrumental in preventing grooving byensuring that—(a)•(b)The boiler water is properly circulated during steam raising.Steam is not raised rapidly.(c) The boiler is not forced continually.(d)(e)Firing is regular.Feed-water is of good quality and is suitably treated withchemicals.44


BulgesIf an accumulation of scale, oil, or dirt is allowed to collect on themetal surfaces exposed to flame or hot gases, it acts as a heat insulatorand keeps the water away from contact with the metal. The metal istherefore exposed to excessive heat, and may become overheated. Inits overheated condition the metal will become soft and pliable. Beingunable to withstand the pressure within the boiler, it is pushed outforming a bulge. Only a small distortion will take place at the first time,but this creates a pocket for more scale to lodge and increases thedanger of further distortion. If the distortion is great the plate is thinnedand weakened to such an extent that rupture takes place and an explosionmay occur.Priming and FoamingPriming is the lifting of boiler water by the steam flow and is causedby carrying too high a water level for the demands for steam flow. Thewater may be lifted as a spray or in a small body, and as it enters thesteam line its weight and velocity may cause severe damage to equipment.To remedy this situation the firing rate should be reduced andthe surface blow-down valve opened until the water level drops toslightly below normal. The water level should be kept a couple ofinches lower than normal if the demand for steam fluctuates greatly,because sudden demand for steam sometimes tends to pick up waterfrom the surface directly below the steam stop valve.Foaming is more a chemical than a mechanical problem. Highsurface tension of the boiler water causes many of the steam bubbles tobe encased by a water film. These film-encased bubbles rise and passout in the steam flow. The cause of high surface tension is usually a highconcentration of suspended matter in the boiler water, a high boilerwater density, or the presence of oil.Priming and foaming are factors usually controllable by a competentboiler operator.Tube TroublesTube troubles usually follow overheating due to scale, oil, or flameimpingement. If the overheating is serious, a rupture may occur andthe only remedy is a new tube.45


Flame ImpingementFlame impingement is a source of damage to boilers and refractory(brickwork). If the flame impinges directly on the boiler shell, excessiveevaporation of water will result on the water surface over that point.The high temperatures may cause damage through local scale formation,or the temperature may be high enough to cause serious damage byoverheating of the plate.Direct impingement of flame on tubes of water tube boilers may resultin excessive evaporation, and the resultant circulation upwards in thesetubes may be more rapid than the rate at which cooler water can besupplied from its lower end; thus steam pockets are formed. A steampocket, serious overheating and failure of the tube usually result.Refractory TroublesRefractories are used mainly in water tube boilers, and in general,serve the following purposes—(a) They form the envelope of the furnace or combustion chamberand assist in maintaining the high temperatures necessary forcomplete combustion of the fuel under all conditions.(b) They protect the furnace casing from overheating, burning, andpossible escape of gases into the boiler room.(c) They ensure even distribution of heat throughout the furnace,(d) They serve to protect exposed parts of drums which otherwisecould be overheated.Burner openings are usually formed by specially shaped blockscalled quarl blocks. The quarls around the burners are subject to veryhigh temperatures when the burners are in use. If the air registers arenot closed tightly, or if they leak when the burners are cut out, thequarls are swept by cool air. This will bring about rapid change intemperature and cause cracking. If the quarls are not kept to theiroriginal shape and size, they will cause poor combustion, and in extremecases, may result in serious damage to registers, furnace frontsand the remaining brick in the furnace. Dirty burners will cause dripping,improper combustion, and flame impingement on the side wallsand quarls, resulting in brick failure in a very short time. These conditionsshould be watched for and corrected immediately, as badbrickwork may lead to serious casing damage as well as an increase infuel consumption.46


Brick and insulation behind boiler tubes, water wall tubes, floor androof tubes, etc., should be inspected at frequent intervals. If at any timeit is found cracked or otherwise defective, it should be repaired as soonas possible to reduce radiation losses and gas leaks.Water HammerThere is a definite hazard from water hammer in every steam plant.This may occur when steam Is admitted into a pipe where there is coldwater of condensation. As steam enters the pipe it disturbs the surfaceof the water and causes the formation of waves upon it. The turbulenceincreases rapidly, and in a very short time one of the waves may breakin such a way that its crest, in pitching forward, momentarily forms abubble-like enclosure containing steam. By this time there is a certainamount of pressure in the pipe, and as the steam enclosed in the bubblecondenses, the pressure will cause the bubble to collapse with somenoise and force. This will greatly increases the agitation of the waterin the immediate vicinity. Similar action will take place in other partsof the pipe and the end result is that the water in the pipe will be rollingabout with great agitation. A surging wave may be big enough to blockthe pipe. Steam which is trapped on that side of the wave away fromthe steam inlet will rapidly condense creating a partial vacuum. Theincoming steam behind the water wave will therefore drive it to theend of the pipe, or against an obstruction, where it will strike withgreat force. Pipes have often been pulled out of their fittings in thisway, often with the fittings broken. In accidents of this nature, steamunder pressure can be liberated in quantity with grave consequences.To summarise, the condition's favourable to water hammer are—(a) Water in contact with steam.(b) Rapid condensation of steam in pipes or valve chests.(c) Agitated water surfaces in pipes.(d) Steam pressure at one part of the pipe and partial vacuum atanother part.To eliminate or reduce the risk of water hammer action, open thedrains provided in the steam pipe line and drain away any water ascompletely as possible before opening the steam valve. These drainsare usually located at the lowest point in a pipe system. After the pipeline has been thoroughly drained, the steam stop valve can be cracked47


open and the line warmed through. When the pipe line is well warmed—usually in a few minutes—the steam valve may be opened a littlemore, and then gradually to the full open position.Sometimes a vacuum in the steam pipe will not allow the condensedwater to run out freely, as the vacuum tends to draw air into the pipe.Under such conditions the drain should be left open, and after the airhas filled the vacuum, the steam stop valve should be cracked open topermit only a small amount of steam to enter the pipe to warm the air.When the air in the pipe has been warmed, a considerable amount ofwater should drain out if the drain is free. It is important to note thatwater should be expelled by the expansion of air rather than by steampressure.Where the water in a pipe system is discharged by way of steamtraps, additional hand operated drain valves should be provided so thatdrainage can be checked and completed.48


PUMPS ANDVIIAUXILIARIESFeed Water Pumps and Steam InjectorsFeed water pumps In general use may be divided into two classes,reciprocating and centrifugalReciprocating Pumps—One form of the reciprocating type of pumpconsists of two single acting pumps with two steam cylinders and twowater cylinders. The steam cylinders are side by side at one end andthe water cylinders are at the other end. The cylinders are connectedby an open frame. The steam pistons are connected by rods to thebuckets (water pistons). Stuffing boxes suitably packed at the steam andwater ends keeps leakage to a minimum. In the middle of the rods arespools which actuate rocker arms for each steam valve assembly. Thepiston rod of one pump actuates the steam valve of the other throughthe rocker arm. The pistons move alternately, and since one or the otheris always in motion the flow of water is practically continuous. Thispump is known as a 'duplex' pump as there are two cylinders inparallel. Fig. 39 illustrates such a pump.Many reciprocating pumps have domes connected to the discharge.The reason for this is to absorb shock and give an even flow of waterin the pipe.Owing to their simplicity and low first cost, reciprocating pumps aremostly found in plants using small boilers (rated at up to 17,000 Ib(7711.2 kg)/hour from and at 212°F (100°C)). Larger plants usuallyinstall centrifugal-type feed pumps in spite of the high price, becauseof the resulting increase in their operating efficiency.Centrifugal Pumps—The principle of the centrifugal pump is that ittakes the suction at the centre or fi eye* of the rotating member knownas the 'impeller'. Vanes curved from the impeller eye to the peripheryare curved in a volute shape, and therefore, when the impeller rotates athigh speed, the velocity of the water increases rapidly. The dischargechamberconstruction changes this velocity to pressure. Fig. 40 showsthe arrangement of a centrifugal pump.A number of impellers may be mounted on the same shaft, the dischargebeing fed from the periphery of each impeller to the suction, oreye, of the following impeller or 'stage'. This arrangement serves toboost the discharge pressure. Depending on the number of stages, pressure'sof 1,500 lb/in 2 (105.48 kg/cm 2 ) and above may be attained.49


Steam Injectors—By means of a steam injector, feed water can beforced into a boiler by the steam pressure carried in the boiler. Theprinciple involved is that when a body of water meets a high velocitysteam jet, it acquires a velocity approximately equal to that of thesteam, and the energy thus generated is sufficient to overcome theboiler pressure. Steam injectors are commonly used for feeding waterto small boilers.Pressure energy in the steam from the boiler is changed into kineticenergy by passing it through the coverging nozzle C (Fig. 41), fromwhich it emerges at a high velocity. The steam in passing the openingof the pipe connection B to the feed tank, creates a vacuum in the pipeand the atmospheric pressure forces the water from the feed tank A upthe pipe. The water is entrained and acted on by the steam jet. Onentering the converging nozzle D, the steam condenses and gives thefeed water velocity. The feed water, on entering the diverging nozzle E,gradually looses velocity, and the mass of water attains sufficientmomentum to force open the feed check valve against boiler pressure.Water is thus fed to the boiler. When starting, an overflow allows forexcess of steam or water or air.Oil Fuel Pressure PumpsThese are usually of the rotary type and a typical gear pump isillustrated in Fig. 42. The pump consists of two small-toothed wheelsgearing with one another and fitting exactly into a casing. When running,the oil is urged round between the wheels and case, filling thespaces between adjacent teeth. As there are no suction and deliveryvalves the pump is practically immune from breakdown.Rotary pumps are positive displacement pumps. They developdangerously high pressures if operated against a closed discharge. Theyare particularly suitable for small capacities. At each revolution of theshaft a given amount of fuel oil in a steady flow is discharged.Feed Water HeatersFeed-water heaters are used to bring the feed water nearer to thetemperature of the boiler water. This serves two purposes, not onlydoes it increase the over-all boiler efficiency but it also reduces temperaturestresses in.the boiler by feeding water into it at higher temperatures.50


Two general types of feed-water heater are used—the open andclosed types. The open heater is sometimes called a 'direct-contact'heater because in this type the water and steam mix. The closed heateris sometimes called the 'indirect' heater because the water and steamare separated by tubes and the water is heated by conduction.Open Heaters—The open heater makes use of exhaust steam for feedwater heating and is essentially a low-pressure heater. It is alwayslocated on the suction side of the feed pump and at least five feet abovethe pump. In this case the hot water will flow by gravity to the pumpsuction when the water is pumped to the boiler.The principle of the open heater is to pass the cold condensate fromthe top, over a series of perforated metal trays, so that it leaves themin the form of rain. Low pressure steam enters between these trays,condensing and mixing with the water thereby heating it.There is always a certain amount of loss in return water due toleaks, blowing down, etc. A certain amount of make-up water is thereforenecessary. As can be seen in Fig. 43, a make-up water pipe is fittednear the top of the heater and on this pipe is fitted a valve, controlledby a lever mechanism which is attached to a float that floats on thesurface of the water in the heater. This arrangement always keeps thewater in the heater at a constant predetermined level. To prevent theheater from becoming flooded an overflow is provided. Also incorporatedin the heater is a filter chamber located near the bottom, thepurpose of which is to extract any impurities from the water before itleaves the heater for the pump suction.As steam supply to most heaters is often the exhaust from reciprocatingengines and pumps, the steam may contain a certain amount oflubricating oil, which will prove harmful if allowed to enter the boiler.Therefore, open heaters usually have some arrangement for extractingoil from exhaust steam. The shell is vented to atmosphere through asmall'line, and is protected against overpressure by a relief valve.Besides raising the temperature of the feed water, the open heaterperforms the following important functions—(a) Deposits solids causing 'temporary*'hardness .in the water.(b) Removes a considerable proportion of free oxygen by bringingthe water to near boiling point and venting the gases toatmosphere.51


Item (a) will help reduce scale formation in the boiler, and item (b) willhelp reduce corrosion and pitting in the boiler, which are acceleratedby the presence of free oxygen.Closed Heaters—The closed feed-water heater was originally developedto overcome the problem of oil contaminated exhaust steam. Inmodern steam plant, closed heaters are operated under high steampressure and high feed water temperatures are attained by its use. Itis usually located between the feed pump and the boiler.Closed heaters are designed for passing the heating steam eitherthrough the tubes, or over the tubes. In the former case the shell hasto be of substantial construction to withstand boiler pressure (as thefeed pumps discharge through the heater), while in the latter case theshell may be of lighter construction as it has to withstand heating steampressure only. Closed heaters have to be provided with drains to takeaway the condensate of the exhaust steam. A typical closed heater isshown diagrammatically in Fig. 44.SuperheatersIn land boiler practice, superheaters are associated mainly withwater tube boilers. They can be divided into two classes, convectionand radiant. In the former the tubes are heated by the convectioncurrents of gases passing over them, and in the latter they are heatedby direct radiation from flame and hot brickwork. Steam from theboiler passes through the superheater and attains a higher temperaturethan would be possible otherwise.Besides having a greater volume, superheated steam because ofhigher temperature, contains more heat per pound than saturated steamat the same pressure. It can therefore work more efficiently. Superheatingalso reduces the loss of efficiency through condensation.Superheaters encountered in water tube boilers are generally of theheader type. The headers, two in number, are usually of forged orfabricated steel construction, circular or rectangular in section and areconnected together by U-tubes. Modern practice in the case of highlyrated boilers is to weld the U-tube ends to the numerous inlet andoutlet stubs fitted to the headers for that purpose during construction.It is common to fit welded-in division plates so that the steam is forcedto make several passes through the superheater. The inlet and outletbranches are located on the same header.52


Water tube boiler superheaters' are invariably considered as part ofthe boiler. They are directly connected to the steam drum withoutpassing through any stop valve. On this account the boiler safety valvesare fitted on the superheater outlet header, so that in the event of asudden stoppage of machinery the safety valves will lift and ensure agood passage of steam through the superheater. When additionalsaturated safety valves are fitted (on the steam drum), they are usuallyloaded in excess of the superheater valves. In this case the superheatervalves will lift first so as to ensure a good flow of steam through thesuperheater.EconomisersEconomises are encountered mainly in the larger water tube boilerinstallation. Operating and maintenance problems do not make themfinancially attractive for installation in fire tube boiler plant. Feedwater is pumped through the economiser on its way to the boiler inorder to absorb waste heat from flue gases, thereby increasing theefficiency of boiler plant.A typical economiser consists of a number of mild steel tubes onwhich are shrunk on gilled rings of cast-iron. The cast-iron gills providea much extended surface for heat absorption, and at the same timeprotect the mild steel tubes from the acid effects of furnace gases andcondensation. The flue gases pass over the tubes and the feed waterpump discharges the water to the boiler through the tubes. Soot blowersare usually fitted to economizers.The casing of the independent type of economiser is located betweenthe boiler and the chimney with the flue gases passing between thetubes. However, the trend of modern practice is to consider the economiser,when fitted, as part of the boiler and to have the outlet coupledto the boiler without any intervening fittings.Air PreheatersThe air preheater is a device that uses the flue gases as a medium toheat air for combustion purposes in the boiler furnace. They arenormally used is large water tube boiler plant and are installed afterthe economiser. Thus, further heat is recovered from the flue gases andthe boiler efficiency is thereby increased.53


The air preheater most frequently encountered Is the tubular type,in which thin mild steel tubes are expanded at their ends into steel tubeplates. The flue gases pass on one side of the tube wall and the combustionair on the other. A lot of trouble can be experienced withcorrosion and fouling of the air preheater surfaces, if steps are nottaken to prevent the condensation of sulphuric acid from the fluegases. Such condensation could occur when lighting the boiler, oroperating it at reduced power. To avoid trouble an air by-pass is fittedon the heater for use on such occasions. This prevents undue coolingof the heater surfaces. Normal practice is to fit soot blowers to airpreheaters.Feed Water RegulatorsIt is difficult to regulate the supply of feed water by hand, especiallywhere the demand for steam, fluctuates. Regulators have been designedto take care of this function automatically. A common type of feedwater regulator is the Copes, shown in Fig. 45.The regulator consists of two stainless-steel expansion tubes inclinedat 45° and mounted in a rigid steel frame. The upper ends jointtogether in a yoke anchored to the frame, and are connected by aheavily lagged steam pipe to the steam space of the boiler drum. Thelower ends of the tubes are pin jointed to levers, and from there leadback to the water space of the drum. The regulator is installed so thatthe upper halves of the expansion tubes are filled with steam and thelower halves with water.In operation, as the steam connection to the drum is heavily lagged,the steam temperature in the tubes will be the same as in the boiler.But the water temperature in the tubes will be appreciably lower as thewater connection is not lagged. The levers A and B magnify the motioncaused by the expansion of the tubes T, the motion of levers A and Bbeing added and further increased by being linked together and to afurther lever C. The lever C directly operates the feed water controlvalve, which works with very little friction.As the water level falls in the boiler drum, the level in the stainlesssteelexpansion tubes falls correspondingly. More steam and less waterin the tubes causes the tubes to expand, resulting in a right-hand movementof lever B and a left-hand movement of lever A. The result ofthese two movements is a downward movement of lever C. The down-54


ward movement of lever C opens the feed control valve, and so increasesthe feed to the boiler as the water level falls. As the water levelrises, the expansion tubes contract. This has the reverse effect and leverC is lifted, thus decreasing the feed.Pressure Reducing ValvesConditions sometimes exist in steam plants where it is found necessaryto reduce the boiler pressure of the steam before it is used tosupply either manufacturing processes, low-pressure feed water heaters,or other auxiliaries, as it is not economical to use high pressure steam.To achieve this, pressure reducing valves are used to supply steam at adesired constant pressure lower than that of the supply.A weight and lever type of reducing valve is illustrated in Fig. 46. Thelever and weight keep the valve open while the high pressure steamenters the high pressure chamber passing through the two valve seatsof the balanced valve to the outlet side at a reduced pressure. On thelow pressure side, about fifteen feet away from the valve, a one-halfinch pipe is taken off and connected to the underside of the diaphragmchamber. This is the control pipe which is fitted at a distance of fifteenfeet in order to obtain an average pressure. To ensure that the rubberdiaphragm is protected by a water seal, the point at which the controlpipe enters the diaphragm chamber must be below the point at whichit is taken off the low pressure line.When the pressure in the low pressure line is equalised it presses upon the diaphragm, through the spindle and closes the valve against theweight and lever. The area of the diaphragm is such as to ensure instantresponse of the valve to any slight fluctuations in pressure.Steam must flow in the direction indicated by the arrow cast on thebody, as otherwise the valve will not function. The reducing valve mustbe placed in a horizontal position with the diaphragm down, so thatthe water of condensation which forms in the diaphragm chamber willprotect the rubber diaphragm from the heat of the steam.After the reducing valve has been installed, turn on the steam, makingsure that the weight on the lever is as close to the fulcrum as it will go.When the valve has warmed up, slide the weight along the lever untilthe desired pressure is obtained on the low pressure side. The fartherout the weight is placed on the lever the greater will be the deliverypressure, and, the closer the weight to the fulcrum the lower thepressure.55


In well designed installations a pressure gauge and safety valve arefitted on the low pressure side of the reducing valve. The former isrequired to check the operation of the regulator, the latter to protectthe low-pressure equipment against excessive pressure should theregulator fail.Steam SeparatorsSaturated steam always contains some moisture. The amount ofmoisture is determined by a number of factors such as the design of aboiler, the manner in which it is operated, and faulty or poor insulation.As the presence of moisture in steam interferes with the lubricationof engines, scores valves and cylinders, absorbs heat from the steam byre-evaporating during the expansion of the steam, and if in sufficientquantity can cause damage to the engine, efforts are made to separatemoisture from steam. This separation is achieved either by superheatingthe steam sufficiently to evaporate all moisture, or, by the useof a separator which reduces the moisture content to acceptable limits.The normal design of separator depends upon the effectiveness of anabrupt change of direction of the flow of steam and water to separatesubstances of different densities. Fig. 47 illustrates one form of steamseparator, in which, the steam is subjected to a sudden change of directionof travel. Steam being lighter than the moisture contained in it willmake the change, but the moisture being heavier has more momentumand will continue travelling in the original direction until it strikesagainst the baffle. Water droplets are formed and fall to the bottom ofthe separator and are drained to a steam trap. Any particles of grease,oil or dirt are likewise thrown off by the steam.Steam TrapsFloat Traps—The principle of operation of both float type andbucket type traps are practically the same. In the float trap (Fig. 48)the hollow ball floats on the return condensate water which flows bygravity from the steam line drains to the trap. The float, which actuatesa valve on the trap outlet through a lever mechanism, rises when thelevel of water in the trap rises and this action opens the outlet valveallowing the water to be discharged. When the level of water drops, thefloat is lowered and closes the valve. A vent is fitted on top of the trapto allow for the escape of any air which may collect.56


Bucket Traps—Fig. 49 illustrates a bucket trap. The water of condensationenters at A and fills the spaces S between the bucket B andthe walls of the trap. This causes the bucket to float and close valve V.The water rises in the chamber until it overflows the edges of thebucket causing it to sink and open valve V. Steam pressure acting onthe surface of the water forces it up through ring R and through dischargeopening D. When the bucket is emptied it again floats closingvalve V. The cycle is then repeated.A good boiler operator always ensures that trap outlet valves are ingood condition as otherwise steam would escape from the trap and goto waste, resulting in inefficiency.57


CHAPTER VIIIELECTRODE BOILERSThere are several distinct types of electrically heated steam generators.In the floating-electrode type, water boils at the surface and the levelof the electrode falls as the water evaporates. In another type, controlis exercised by partial immersion of the electrodes. In both cases, wateris heated directly by passing alternating current through it. Boilers formedium-voltage supplies are made with loadings from 10 kW to 3,000kW, and full pressure, up to 300 lb/in 2 (21.09 kg/cm 2 ) is available inthree to five minutes after switching on. Very small generators mayhave working pressures as high as 1,000 lb/in 2 (70.32 kg/cm 2 ), suchpressures usually being required for pressure testing of equipment.A typical electrode boiler (Fig. 50) comprises a steel shell containingcurrent carrying electrodes. Usually three in number, the electrodes areinsulated from each other and from the shell. When starting, water ispumped into the boiler and as soon as it reaches the electrode tips,current flows between them. The principle applied here is that thepassage of current through any resistance causes a rise in temperaturewithin the material of the resistance. In the boiler, the passage of currentthrough the water causes a rise in temperature in the water. Heat isgenerated within the water and is not transmitted from an externalsource at a higher temperature.As the water level rises around the electrodes, more current flowsuntil the amount of steam generated exactly matches the demand. Theheat generated is equivalent to the electrical energy expended, and theboiler operates at 100 per cent efficiency, less the slight loss due toradiation.When closed down the boiler shell is empty and no current flows.The electrode boiler is therefore absolutely safe when dry.Electrode boilers are controlled in two ways—(a) by pressure, and(b) by load. A load selector switch enables the electrical load of theboiler to be selected in 20 per cent steps up to full load, and in no casedoes the boiler take a greater proportion of the load than selected.Operation is. completely automatic. Since the controls regulate theload taken by the boiler to meet the steam demand, a constant workingpressure is maintained.58


Electrode boilers being small and compact can be installed in placeswhich would be useless for any other purpose e.g. under stairways,on roofs of buildings, often enabling the boiler to be located at theequipment using the steam.Electrode boilers can be used only on alternating current supplies.For with direct current the two elements which constitute water, viz.hydrogen and oxygen, would be liberated due to electrolysis. Withalternating current at frequencies of 10 cycles per second and above,no electrolysis taken place. The majority of electrode boilers run onthree-phase systems.As the cost of electricity is generally higher than the cost of oil fuel(even with special rates for supply), the operating cost of an electrodeboiler is usually high. However, it does not require a chimney and isnot susceptible to variations in draught. It may be installed where mostconvenient, and is in fact portable.59


CHAPTER IXFUNDAMENTALS OF ELECTRICITYElectric FlowElectricity when in motion is a form of energy. Its precise nature isnot yet clearly understood, its presence is rarely obvious, and its resultsare often sudden and tremendous. It is convenient to think of electricityas a fluid flowing around a path, without loss.The electron theory supposes that atoms which constitute all knownphysical matter are composed of Electrons having a negative charge, andProtons having a positive charge. The electrons, which are capable ofmovement, normally circulate the positive protons called the Nucleusof the atom. Atoms of different substances differ only in the numberand grouping of their electrons.Under the action of a force and with movement restricted to a definitepath, such as along a wire, electrons will flow in a stream. Electricity isthus easily transmitted from a central supply and distributed to anynumber of places where it is to be used. The force which sets the electronsin motion outside the confines of their atoms is called the ElectroMotive Force.Therefore, Electro Motive Force (E.M.F.) is that force or pressurewhich causes a flow of electricity in a circuit. A difference of E.M.F.is called Potential Difference (P.D.) and may be set up by chemicalaction, by heat, or by mechanical means. It is usual to regard one sideof this difference as positive and the other as negative, and to call thepositive the high pressure and the negative the low pressure. The unitof electro motive force is the Volt.As long as a potential difference exists in a circuit a Current will flowthrough it. The current is assumed to flow from the positive to thenegative.Units of MeasurementA Volt is an electrical unit of pressure, and is the force required tosend one ampere of current through a resistance of one ohm.An Ampere is an electrical unit of current, and is the current producedby an E.M.F. of one volt in a circuit with a resistance of oneohm.60


An Ohm. Resistance is offered by all substances to the flow ofcurrent through them and this resistance is measured in ohms. An ohmis therefore an electrical unit of resistance, and is the resistance offeredby a circuit to the passage of one ampere of current under a potentialdifference of one volt.A Watt is the electrical unit of power, and is the power expanded ina circuit when a current of one ampere flows between two points at apotential difference of one volt.A Kilowatt. The watt is too small a unit for practical purposes, soa larger unit called a kilowatt and equal to one thousand watts, isgenerally used.A Watt Hour is the electrical unit of energy and is the energy suppliedby one watt for one hour.A Kilowatt Hour is the large, practical unit of electrical energy andrepresents the energy supplied by one kilowatt for one hour.Some DefinitionsAn Electrical Circuit is a system of electric conductors providing acontinuous path for the purpose of carrying current. An example iscurrent flowing from its source through a system of motors or Hghts,back to its source by the necessary wiring.A Circuit Breaker is a special type of switch designed for openingautomatically a current carrying circuit when under excess current conditions,such as overloads or short circuit currents, thus safe-guardingthe circuit. An ordinary switch, unlike a circuit breaker, is not designedfor the interruption of short circuit currents.An Electric Switch is a device for closing and/or opening a circuitunder the conditions of load for which it is rated. Switches are made ina variety of forms, from the simple lighting 'tumbler' or link switch tothe larger knife switches. It can also be a single, double, triple or fourpole type or a single or double throw switch.A Fuse is a device for the purpose of protecting a circuit againstdamage and acts as an automatic circuit-breaker. It consists in mostcases of a strip of lead or tin, placed in line with the main wire whichis cut to allow of the fuse being fitted in its place. The ends of the fuseare connected to small terminals or screws.61


Fuses are arranged so that should an excess current, caused by somedefect or breakdown, attempt to pass, the strip of tin or lead wouldmelt and break the circuit automatically, thus preventing further damageby burning out the rest of the circuit beyond where the fuse is placed.Fuses are fitted on switchboards, distribution boxes, and for generalsafety at various other places on lamp circuits.When renewing fuses, the main switch should always be opened, ifpossible, and a regular fuse cartridge used for fuses carrying highcurrents or voltages. Fuses should not be handled unless it is ascertainedthat the line is dead, and, the person carrying out the work shouldalways stand on a rubber mat or a dry piece of wood, if one side of theswitch is still alive.A Relay is a device for opening or closing an auxiliary circuit, or forreleasing circuit breakers or switches. Other types actuate for timecontrol, for counting, sorting, or measuring. These release devicesmay be of the thermal, magnetic, or induction types. Overload relaysoperate when an overload occurs, and, no-voltage relays operate onlow or no voltage. These two types are generally used with startingequipment.D.C. means Direct Current and refers to an electric current flowingthrough a conductor in one direction only, i.e. the axial drift of theelectrons is in one direction only.A.C. means Alternating Current and refers to an electric current flowingthrough a conductor the direction of which is reversed at regularintervals, i.e. the axial motion of the electrons is a backward andforward motion. A generator in which the current reverses in strengthand direction 50 times per second, is called a 50 cycle generator.An Electric Generator is a machine that converts mechanical energyinto electrical energy. There are D.C. generators and A.C. generatorsor alternators.An Electric Motor is a machine that converts electrical energy intomechanical energy. There are D.C. motors and A.C. motors.A Rheostat, To prevent over-current damage to motors when starting,and, in some cases to vary the speed of motors, rheostats are used.A rheostat is a variable resistance which generally consists of a seriesof brass contacts arranged in various forms and which are connectedto one another by a resistance coil. A contact arm slides over the brasscontacts, cutting out each resistance in turn.62


An Electrical Conductor is a substance which permits the flow ofelectricity through it without offering much resistance. The best conductorsof electricity are metals, silver being the best of all but tooexpensive for common use. Copper and aluminium are next best andare in general use.An Electrical Insulator is a substance which offers considerableopposition to the passage of electricity. Such substances are called nonconductorsor dielectrics, the most common being air, indiarubber,mica, glass, porcelain, plastic materials, and textile fabrics.Generators and MotorsAn electric current is generated when a coil of wire is passed throughor rotated in the magnetic field of a magnet, the magnetic field beingthe invisible lines of force given off by the magnet. Generators makeuse of this principle, and consist of a number of conductors continuallycutting across magnetic fields.In direct current generators, the field magnets are made to producea magnetic field by passing either all or part of the total currentgenerated through coils of wire wound on the magnet. In alternatingcurrent generators, the magnets are energized from an external sourceof direct current; this is because alternating current is fluctuating andtherefore cannot be used for such a purpose. This external source isusually a 'small D.C. machine called an exiter, and the D.C exitingcurrent is led into the rotor of the A.C. generator by means of brushesand slip rings, generally two in number.In D.C. generators and motors, the field (field poles of iron laminationsand coils) is always stationary and the armature revolves within theinfluence of the field. In A.C. generators and motors, the field polesare generally the revolving member, the armature being stationary. Theconductors of the armature are wound in the slots of the iron laminationsof the frame. The revolving part of the A.C. generator is calledthe rotor and the stationary part the stator.All generators generate alternating current, but in the D.C. machinea copper commutator and carbon brushes are povided to convert theA.C. current into D.C. current before it leaves the machine.Why Motors Will Not StartSome of the reasons why a motor will not start are—63


Switch not closedLow voltageOverloadBlown fuse or fusesBroken wireRheostat burned out (D.C.)Contacts burned outPoor brush contact (D.C.)Windings burned outShort circuit in windingsRelays out of contactDamp insulationDefective insulationExcessive grease or oilExcessive dustExcessive bearing frictionWorn out bearingsJournals too tightWrong frequencyUneven air gapImproper alignment of motorwith the machine it isdrivingDangers, Remedies and Care of MotorsDanger to Personnel—The physical danger from an electric motorwould be an electric shock or even electrocution resulting in death inthe case of an earth or short circuit. These conditions could arisethrough carelessness, defective or worn out insulation, or excessivelydamp conditions.Faults in Motors—Motors could be damaged due to sudden surgesof current in the line, moisture, or excessive temperature causingdamage to the insulation or materially reducing its normal life. Excessivetemperature is usually attributed to overloading, blocking up ofthe ventilation passages with dirt and fluff, or accumulation of foreignmatter on the windings and core acting as insulation and thereby preventingdissipation of heat.A water hose played at or near a motor is not only dangerous to themotor, but also dangerous to the man with the hose since water is agood conductor of electricity. Overflowing of liquid tanks, filters, ordying tubs should be guarded against. Danger can also arise fromwater leakage at the packing glands of motor driven pumps.Excessive vibration should not be allowed. Oil or grease should beprevented from entering the motor and should be used only in properamount in the bearings. A motor will normally last for twenty years ifserviced correctly. It should then be rewound if any doubt exists regardingthe quality of the insulation.64


Care and Maintenance—Lubrication and care of insulation are themain factors governing the life and efficiency of a motor and both areoften neglected. Lubrication, of course, would apply to the bearings,whether sleeve, ball, or roller, and modern types of bearings are veryefficient. However, too much oil added to the sleeve type bearing, ortoo much grease applied to ball or roller bearings, may cause the"surplusto be carried into the motor where it traps dirt and aids in breakingdown the insulation. Often, grease fittings are inserted into the pipethread openings on the ball bearing housings and a shot or two ofgrease Is added whenever someone feels like adding it. Some housingshave been so packed with grease in this way that bearing friction hasincreased sufficiently to affect the running of the motor.In modern design, bearings are usually dust proof, and in the sleeveor journal type, oil levels should be checked daily and a little oil addedweekly if necessary. If an oil gauge is used in pedestal bearings, checkoil level by the line in the gauge glass. It is best to add oil when themotor is stopped. If oil is found to be leaking or creeping along theshaft, check oil level and ascertain cause of leakage. Bearing oil shouldbe changed every three months if the motor is in a dirty location, andin any case should be changed every six months, the oil wells beingflushed and cleaned before refilling.Ball or roller bearings should be checked every week for vibrationwhile the motor is running and the temperature checked daily by feelingwith the hand. If the motor is running continuously and on hard service,grease should be flushed out with kerosene or trichloroethylene if in ahorizontal motor, but in the vertical type the bearings should bestripped and re-assembled after cleaning. In most motors, a pipe plug isplaced top and bottom of the bearing housing. After the bearing andhousing are flushed out, leave top and bottom plugs out and pack thehousing with a grease gun. Allow surplus grease to work out throughthe pipe opening and then seal with pipe plugs. In the case of speciallocations where extreme heat is present, the type of grease is changedto suit the condition. Sleeve bearings are generally used for horizontalmotors and for D.C. machines while ball or roller bearings are usedfor tilted, vertical or horizontal positions.Insulation is best cared for by keeping the motor clean and free fromdust, damp, and oil. Under normal operating conditions motors shouldbe cleaned once a week and this can be done with dry, low pressure air.65


At least once a year the motor should be thoroughly cleaned and overhauled.The condition of the insulation should also be tested by aresistance testing instrument such as a megohmmeter.During the yearly cleaning of the motor, the dust could be blownoff using dry, low pressure compressed air (about 25 lb/in 2 or 1.75 kg/cm 2 ), and the dirt and grease removed by brush or cloth, using acleaning fluid such as trichloroethylene. When clean, the motor shouldbe dried thoroughly, preferably in an electric oven, and then a coat ofinsulating varnish applied to the windings by brush or spray gun. Thestator frame should be dried using electric heaters. When re-as'sembled,the motor clearance or air gap should be checked.66


CHAPTER XFIRE PRECAUTIONS,FIGHTING AND EQUIPMENTLiquid FuelsLiquid fuels all evaporate at rates varying with the temperature, themore volatile fuels being those which give off vapour more readily atlower temperatures. With appropriate quantities of air, these vapoursform mixtures which will flash or explode if ignited. If ignition takesplace inside a compartment there will be an explosion with destructiveresults. The destructive ability of vapour mixtures exceeds that of manysolid explosives; a cupfull of gasoline has the potential explosive powerof 5 Ibs (2.26 kg) of dynamite.PrecautionsPrecautions relating to the storage of liquid fuel generally aim atachieving—(a)(b)The elimination of either liquid or vapour accumulations outsidethe oil fuel tank or pipe system in use.The exclusion of all sources of ignition from the neighbourhoodof any position where vapour-air mixture may have developed.Air vent pipes to oil fuel tanks should be fitted with flame arrestersconsisting of double wire gauze of fine mesh. They must be kept clean,especially from paint, to allow them to fulfil their purpose.In the boiler room no oil should be allowed to accumulate in the airboxes, furnace bottoms, or boiler room floor. If leakage from the oilfuel system to the boiler room occurs at any time, the oil supply to thatpart of the system should be shut off immediately. Oil-tight trays shouldbe placed under all fittings from which liquid fuel may spill when thefitting is opened. Savealls should be frequently examined for the presenceof oil A box filled with sand should be kept in a readily accessibleplace in the boiler room.Oily waste can set light to itself without any external application ofheat such as from a flame or spark; this is called spontaneous ignition.Therefore, until it can be disposed off or burnt, oily waste should bekept in a metal receptacle partially filled with water to prevent spontaneousignition.67


In general, the best safeguard against fire is a proper attitude towardscleanliness, the disposal of inflammable refuse in all its forms, and anintelligent suspicion of unknown lurking danger. Many explosions haveoccured when opening boilers merely on account of lack of suspicion.Fire FightingIn case of a fire in the boiler room, the competent boiler operatorshould—(a)(b)Raise the alarm.Attack the fire using fire extinguishers.(c) Shut off air by closing windows and doors leading to the boilerroom.(d)Shut off the fuel supply to the burners.Oil Fires—If water is used in fighting an oil fire,-it should be sprayedon the oil using a special spray nozzle. Water has the effect of loweringthe temperature of the oil below its fire point, and the fire will thereforego out. However, care should be taken not to allow too much waterto accumulate, as oil being lighter than water it will float on top of thewater, and may cause what started as a small local fire to become a largegeneral one. Foam is a better fire extinguishing agent to use in the caseof oil fires and at least one 2-gallon (9 litres) foam extinguisher isnormally provided in each boiler room. Foam floats on the surface ofthe oil and acts as a blanket thereby starving the fire of oxygen, whichis necessary for combustion. Dry sand may be used as a method ofconfining the oil to a small area thus preventing the oil from spreading.In case of a fire in the boiler room, the oil fuel supply to the burnersshould be shut off, and for this purpose, a master shut-off valve isusually fitted in the oil fuel supply line and located outside the boilerroom.Electrical Fires—In the case of electrical fires or fires in the closevicinity of electrical appliances, a fire extinguishing medium which is anon-conductor of electricity should be used, as otherwise, the firefighter would experience electric shock. Dry powder extinguishers andcarbon dioxide (CO 2 ) extinguishers are suitable for use on electricalfires. All fuses, switches, etc., necessary to isolate the affected sectionfrom the source of electrical supply should be withdrawn or opened.68


Fire Fighting EquipmentSome commonly encountered types of portable fire extinguishers usedin combating oil and electrical fires are described in the followingparagraphs.Foam Extinguishers—A typical 2-gallon (9 litres) foam extinguisher isillustrated in Fig. 51. It is made in two parts, an inner container and anouter casing. The outer casing is of lead-coated steel, lead-plated afterriveting. The inner container is made of copper. The foam makingcontents are a solution of aluminium sulphate in the inner container andbicarbonate of soda in the outer container. This model is operated bymerely turning it upside down. Other similar models may have doublesealing valves which are released by a T-handle or lever before theextinguisher is inverted.Foam is emitted to a distance of from 20 ft (6.09 m) to 30 ft (9.14 m),and once started the extinguisher will empty and eject about 20 gallons(90 litres) of foam. The foam should be directed to fall upon the fire,if need be, by deflecting it from another surface.Foam extinguishers are suitable for oil fires. They should not beused in fires involving electrical eqiupment as electrical shock, whichcould prove fatal, might be experienced.Dry Powder Extinguishers—These extinguishers are suitable for oilfires as well as electrical fires. In the extinguisher illustrated in Fig. 52,the extinguishing contents are 30 Ibs (13.62 kg) of finely processedbicarbonate of soda, pressurised with 11 ozs (311.85 gms) of CO 2 ata pressure of 300 lb/in 2 (21.09 kg/cm 2 ). The dry powder is a nonconductorof electricity, is non-corrosive, non-abrasive 9 and non-poisonous.The CCh pressure charge may be checked instantly by means of aflush-fitting pressure gauge fitted to the body.To operate the extinguisher, pull'out the safety clip and strike theknob on top of the extinguisher. This causes a stainless-steel piercerto perforate a metal seal and release the contents. A horizontal fanshaped cloud of powder is discharged, 25 ft (7.62 in) long, 6 ft (1.82 m)wide and 4 ft (1.21 m) deep. The duration of discharge is 28 secondsand the discharge may be prolonged by interrupting the flow with thehand lever provided at the hose end.69


Carbon Dioxide Extinguishers—In the extinguisher shown in Fig. 53,the charge is controlled by a valve and lever so that part of the chargemay be conserved if not fully used. The extinguisher contains 5 Ibs(2.26 kg) of CO and the pressure inside the bottle under normal temperatureconditions is about 850 lb/in 2 (59.77 kg/cm 2 ). The period ofdischarge is 8 seconds. Being sealed with a domed nickle diaphragmwhich requires piercing with the striker to discharge the contents, theextinguisher is virtually leak-proof. As carbon dixoide is a nonconductorof electricity these extinguishers may be used on firesinvolving electrical appliances.70


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