2000 Hook-up Book - Spirax Sarco

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SYSTEM DESIGN 8 Draining Steam Mains Steam main drainage is one of the most common applications for steam traps. It is important that water is removed from steam mains as quickly as possible, for reasons of safety and to permit greater plant efficiency. A build-up of water can lead to waterhammer, capable of fracturing pipes and fittings. When carried into the steam spaces of heat exchangers, it simply adds to the thickness of the condensate film and reduces heat transfer. Inadequate drainage leads to leaking joints, and is a potential cause of wiredrawing of control valve seats. Waterhammer Waterhammer occurs when a slug of water, pushed by steam pressure along a pipe instead of draining away at the low points, is suddenly stopped by impact on a valve or fitting such as a pipe bend or tee. The velocities which such slugs of water can achieve are not often appreciated. They can be much higher than the normal steam velocity in the pipe, especially when the waterhammer is occurring at startup. When these velocities are destroyed, the kinetic energy in the water is converted into pressure energy and a pressure shock is applied to the obstruction. In mild cases, there is noise and perhaps movement of the pipe. More severe cases lead to fracture of the pipe or fittings with almost explosive effect, and consequent escape of live steam at the fracture. Waterhammer is avoided completely if steps are taken to ensure that water is drained away before it accumulates in sufficient quantity to be picked up by the steam. Careful consideration of steam main drainage can avoid damage to the steam main and possible injury or even loss of life. It offers a better alternative than an acceptance of waterhammer and an attempt to contain it by choice of materials, or pressure rating of equipment. Efficient Steam Main Drainage Proper drainage of lines, and some care in start up methods, not only prevent damage by waterhammer, but help improve steam quality, so that equipment output can be maximized and maintenance of control valves reduced. The use of oversized steam traps giving very generous “safety factors” does not necessarily ensure safe and effective steam main drainage. A number of points must be kept in mind, for a satisfactory installation. 1) The heat up method employed. 2) Provision of suitable collecting legs or reservoirs for the condensate. 3) Provision of a minimum pressure differential across the steam trap. 4) Choice of steam trap type and size. 5) Proper trap installation. Heat Up Method The choice of steam trap depends on the heat up method adopted to bring the steam main up to full pressure and temperature. The two most usual methods are: Separator (a) supervised start up and (b) automatic start up. A) Supervised Start Up In this case, at each drain point in the steam system, a manual drain valve is fitted, bypassing the steam trap and discharging to atmosphere. These drain valves are opened fully before any steam is admitted to the system. When the “heat up” condensate has been discharged and as the pressure in the main begins to rise, the valves are closed. The condensate formed under operating conditions is then discharged through the traps. Clearly, the traps need only be sized to handle the losses from the lines under operating conditions, given in Table 5 (page 10). This heat up procedure is most often used in large installations where start up of the system is an infrequent, perhaps even an annual, occurrence. Large heating systems and chemical processing plants are typical examples. Trap Set Steam Supply Figure 4 Trap Boiler header or takeoff separator and size for maximum carryover. On heavy demand this could be 10% of generating capacity
B) Automatic Start Up One traditional method of achieving automatic start up is simply to allow the steam boiler to be fired and brought up to pressure with the steam take off valve (crown valve) wide open. Thus the steam main and branch lines come up to pressure and temperature without supervision, and the steam traps are relied on to automatically discharge the condensate as it is formed. This method is generally confined to small installations that are regularly and frequently shut down and started up again. For example, the boilers in many laundry and drycleaning plants are often shut down at night and restarted the next morning. In anything but the smallest plants, the flow of steam from the boiler into the cold pipes at start up, while the boiler pressure is still only a few psi, will lead to excessive carryover of boiler water with the steam. Such carryover can be enough to overload separators in the steam takeoff, where these are fitted. Great care, and even good fortune, are needed if waterhammer is to be avoided. For these reasons, modern practice calls for an automatic valve to be fitted in the steam supply line, arranged so that the valve stays closed until a reasonable pressure is attained in the boiler. The valve can then be made to open over a timed period so that steam is admitted only slowly into the distribution pipework. The pressure with the boiler may be climbing at a fast rate, of course, but the slow opening valve protects the pipework. Where these valves are used, the time available to warm up the pipework will be known, as it is set on the valve control. In other cases it is necessary to know the details of the boiler start up procedure so that the time can be estimated. Boilers started from cold are often fired for a short time and then shut off while temperatures equalize. The boilers are protected from undue stress by these short bursts of firing, which extend the warmup time and reduce the rate at which condensation in the mains is to be discharged at the traps. Determining Condensate Loads As previously discussed there are two methods for bringing a steam main “on line”. The supervised start up bypasses the traps thus avoiding the large warm up loads. The traps are then sized based on the running conditions found in Table 5 (page 10). A safety factor of 2:1 and a differential pressure of inlet minus condensate return pressure. Systems employing automatic start up procedures requires estimation of the amount of condensate produced in bringing up the main to working temperature and pressure within the time available. The amount of condensate being formed and the pressure available to discharge it are both varying continually and at any given moment are indeterminate due to many unknown variables. Table 4 (page 10) indicates the warm up loads per 100 feet of steam main during a one Draining Steam Mains hour start up. If the start up time is different, the new load can be calculated as follows: lbs. of Condensate (Table 4) x 60 Warm up time in minutes = Actual warm-up load. Apply a safety factor of 2:1 and size the trap at a differential pressure of working steam pressure minus condensate return line presure. Since most drip traps see running loads much more often than start up loads, care must be taken when sizing them for start up conditions. If the start up load forces the selection of a trap exceeding the capability of the “running load trap,” then the warm up time needs to be increased and/or the length of pipe decreased. Warm Up Load Example Consider a length of 8" main which is to carry steam at 125 psig. Drip points are to be 150 ft. apart and outside ambient conditions can be as low as 0°F. Warm-up time is to be 30 minutes. From Table 4, Warm Up Load is 107 lb./100 ft. For a 150 ft run, load is 107 x 1.5 = 160.5 lb/150 ft. Correction Factor for 0°F (see Table 4) 1.25 x 160.5 = 200.6 lb/150 ft. A 30 minute warm up time increases the load by 200.6 x 60 = 401 lb/h 30 total load Applying a safety factor of 2:1, the trap sizing load is 802 lb/h. If the back pressure in the condensate return is 0 psig, the trap would be sized for a 125 psi differential pressure. This would result in an oversized trap during running conditions, calculated at 94 lb/h using Tabe 5 (page 10). Either increase the warm up time to one hour or decrease the distance between drip traps. 9 SYSTEM DESIGN
Page 1 and 2: DESIGN OF FLUID SYSTEMS HOOK-UPS
Page 3 and 4: Spirax Sarco Spirax Sarco is the re
Page 5: Section 1: System Design Informatio
Page 8 and 9: SYSTEM DESIGN 2 The Working Pressur
Page 10 and 11: SYSTEM DESIGN 4 Sizing Steam Lines
Page 12 and 13: SYSTEM DESIGN 6 Sizing Superheated
Page 16 and 17: SYSTEM DESIGN 10 Draining Steam Mai
Page 18 and 19: SYSTEM DESIGN 12 Steam Tracing The
Page 20 and 21: SYSTEM DESIGN 14 Steam Tracing Sizi
Page 22 and 23: SYSTEM DESIGN 16 Steam Tracing Trac
Page 24 and 25: SYSTEM DESIGN 18 Steam Tracing A si
Page 26 and 27: SYSTEM DESIGN 20 Pressure Reducing
Page 28 and 29: SYSTEM DESIGN 22 Parallel and Serie
Page 30 and 31: SYSTEM DESIGN 24 How to Size Temper
Page 32 and 33: SYSTEM DESIGN 26 Temperature Contro
Page 38 and 39: SYSTEM DESIGN 32 Makeup Air Heating
Page 40 and 41: SYSTEM DESIGN 34 Draining Temperatu
Page 42 and 43: SYSTEM DESIGN 36 Draining Temperatu
Page 44 and 45: SYSTEM DESIGN 38 Steam Trap Selecti
Page 46 and 47: SYSTEM DESIGN 40 Steam Trap Selecti
Page 48 and 49: SYSTEM DESIGN 42 Flash Steam Flash
Page 50 and 51: SYSTEM DESIGN 44 Flash Steam Flash
Page 52 and 53: SYSTEM DESIGN 46 Condensate Recover
Page 54 and 55: SYSTEM DESIGN 48 Condensate Pumping
Page 56 and 57: SYSTEM DESIGN 50 Clean Steam Printi
Page 58 and 59: SYSTEM DESIGN 52 Clean Steam Overal
Page 60 and 61: SYSTEM DESIGN 54 Clean Steam Figure
Page 62 and 63: SYSTEM DESIGN 56 Testing Steam Trap
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SYSTEM DESIGN 58 Spira-tec Trap Lea
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SYSTEM DESIGN 60 Steam Meters Densi
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SYSTEM DESIGN 62 Compressed Air Sys
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SYSTEM DESIGN 64 Compressed Air Sys
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SYSTEM DESIGN 66 Compressed Air Lin
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SYSTEM DESIGN 68 Typical Steam Cons
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SYSTEM DESIGN 70 Specific Heats and
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SYSTEM DESIGN 72 Specific Heats and
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SYSTEM DESIGN 74 Conversions To Con
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SYSTEM DESIGN 76 Flow of Water thro
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SYSTEM DESIGN 78 Moisture Content o
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SYSTEM DESIGN 80 ANSI Flange Standa
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HOOK-UP APPLICATION DIAGRAMS Sectio
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Figure II-3 Draining and Air Ventin
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Figure II-8 Draining Steam Main whe
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Figure II-12 Typical Pressure Reduc
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Figure II-16 Installation of Pressu
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Figure II-20 Typical Pneumatic Sing
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Condensate Return Steam Supply Stea
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Steam Supply Steam Supply Figure II
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Figure II-31 Temperature Control of
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Elevated or Pressurized Return P 2
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Figure II-39 Controlling Temperatur
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Page 113 and 114:
Page 115 and 116:
Figure II-50 Trapping Small Utensil
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Figure II-55 Draining and Air Venti
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Figure II-59 Air Venting and Conden
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Evaporator Pressure Figure II-64 Dr
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Figure II-68 Installation of Pump/T
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Figure II-73 Draining Small Condens
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Figure II-78 Flash Steam Recovery a
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Figure II-80 Heating Water using Re
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Figure II-82 Recovery of Flash Stea
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Figure II-84 Flash Steam Condensing
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Figure II-87 Tank Sterilization Cle
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Steam Supply Figure II-93 Typical S
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Figure II-97 Hand Operated Rotary F
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Figure II-102 Freeze Proof Safety S
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Figure II-106 Continuous Boiler Blo
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Figure II-111 Controlling Temperatu
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141
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PRODUCT INFORMATION Section 3
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Steam Traps • Balanced Pressure T
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147
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Subject Index Safety...............
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Subject Index Radiation—Baseboard
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Argentina Spirax Sarco S.A. Ruta Pa
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