Principles of naval engineering - Historic Naval Ships Association

Chapter 15. -PUMPS AND FORCED DRAFT BLOWERSthe turbine. The same throttle valve is usuallyused to control the admission of steam to allblowers serving any one boiler. K only oneblower is to be operated, the root valve of thenonoperating blower must be kept closed.When admission of steam is controlled bythe four nozzle valve arrangement, no additionalnozzle area is required to bring the blower upto maximum speed. In the other type of installation,a special hand-operated nozzle valve isprovided for high speed operation. This nozzlevalve, which is sometimes called an overloadnozzle valve , is used whenever it is necessaryto increase the blower speed beyond that obtainablewith the fixed nozzles. As a rule, theuse of the overload nozzle valve is requiredonly when steam pressure is below normal.Checking Blower SpeedMany forced draft blowers are fitted withconstant-reading, permanently mounted tachometersfor checking on blower speed. Sometimesthe tachometer is mounted ontopof the governorand is driven by the governor spindle. The governorspindle is driven by the main shaft througha reduction gear, and therefore does not rotateat the same speed as the main shaft. However,the rpm of the governor spindle is proportionalto the rpm of the main shaft. The tachometeris calibrated to give readings that indicate thespeed of the main shaft rather than the speedof the governor spindle.Some blowers are equipped with a specialkind of tachometer called a pressure-gage tachometer. This instrument, which may be seenin figures 15-35 and 15-37, is actually a pressuregage which is calibrated in both psi andrpm. The pressure-gage tachometer dependsfor Its operation on the fact that the oil pressurebuilt up by the centrifugal lube oil pumphas a definite relation to the speed of the pumpimpeller; and the speed of the impeller, ofcourse, is determined by the speed of the mainshaft. Thus the instrument can be calibratedin both psi and rpm.Some forced draft blowers of recent designare equipped with electric tachometers whichhave indicating gages at the blower and at theboiler operating station. The electric tachometer(sometimes called a tachometer generator)consists of a stator and a permanent magnetrotor mounted at the bottom of the turbine shaft.The wire from the generator plugs into a connectorinside the sump. Another connector isprovided outside the sump for attaching the wirefrom the generator to the transformer box.Forced Draft Blower OperationForced draft blowers supply the air requiredfor combustion of the fuel oil. The amount ofair that enters the furnace is determined by theair pressure in the double casings. Althoughthe air pressure is affected by the number ofburners in use and by the amount that the airregisters are open, it is primarily determinedby the speed at which the forced draft blowersare operated. The speed of the forced draftblowers is controlled by manual adjustment ofthe blower throttle in all installations exceptthose having automatic combustion controlsystems.The forced draft blowers should be operatedin such a way as to furnish the required amountof air for the complete combustion of the fuelbeing burned. In actual practice, it is necessaryto supply just over 100 percent of theamount of air theoretically required, in orderto ensure the complete combustion of the fuel.Higher percentages of excess air are wastefulof fuel, since all air that does not actually enterinto a combustion reaction merely absorbs andcarries off heat.On the other hand, an insufficient quantity ofair for combustion is also detrimental to boilerefficiency. If there is not enough air for completecombustion, there may be a greater lossin efficiency. Or, if even less air is supplied,some of the carbon will not be burned at allbut will pass out the smokestack as black smoke.Insufficient air is also detrimental because itcauses the boiler to pant and vibrate; this is oneof the major causes of brick work failure inthe boilers.The air pressure in the double casing mustbe increased BEFORE the rate of combustionis increased, and must be decreased AFTERthe rate of combustion is decreased. There isusually little difficulty in teaching fireroompersonnel to increase the air pressure beforethe rate of combustion is increased, since failureto do so results in panting and vibrationof the boiler and in heavy smoke. It is moredifficult, however, to teach the men to decreasethe air pressure after the rate of combustionhas been decreased. However, operating theblower at a faster speed than is required forthe rate of combustion is definitely a poor419