Principles of naval engineering - Historic Naval Ships Association

Chapter 16. -AUXILIARY STEAM TURBINESgears, the governor also slows. Centrifugalweights on the governor move inward as thespeed decreases, and this causes the pilot valveto move upward, permitting oil to enter the operatingcylinder. The operating piston rises and,through the controlling valve lever, the liftingbeam is raised. The nozzle valves open andadmit additional steam to the turbines.The upward motion of the controlling valvelever causes the governor lever to rise, thusraising the bushing. Upward motion of the bushingtends to close the upper port, shutting off theflow of oil to the operating cylinder; this actionstops the upward motion of the operating piston.The purpose of this follow-up motion of thebushing is to regulate the governing action of thepilot valve. Without this feature, the pilot valvewould operate with each slight variation in turbinespeed and the nozzle valves would be alternatelyopened wide and closed completely.A reverse process occurs when the load onthe generator decreases. In this case, the turbinespeeds up, the governor speeds up, the centrifugalweights move outward, and the pilot valvemoves downward, opening the lower ports andallowing oil to flow out of the operating cylinder.The controlling valve lever lowers the liftingbeam and thereby reduces the amount of steamdelivered to the turbine.Constant-Pressure Pump GovernorsMany turbine-driven pumps are fitted withconstant-pressure pump governors. The functionof a constant pressure pump governor is tomaintain a constant pump discharge pressureunder conditions of varying flow. The governor,which is installed in the steam line to the pump,controls the pump discharge pressure by controllingthe amount of steam admitted to thedriving turbine.A constant-pressure pump governor for amain feed pump is shown in figure 16-10. Thegovernors used on fuel oil service pumps, lube oilservice pumps, fire and flushing pumps, andvarious other pumps are almost identical. Thechief difference between governors used for differentservices is in the size of the upper diaphragm.A governor used for a pump which operateswith a high discharge pressure has a smallerupper diaphragm than one for a pump which operateswith a low discharge pressure.Two opposing forces are involved in theoperation of a constant-pressure pump governor.Fluid from the pump discharge, at dischargepressure, is led through an actuating line to thespace below the upper diaphragm. The pump dischargepressure thus exerts an upward force onthe upper diaphragm. Opposing this, an adjustingspring exerts a downward force on the upper diaphragm.When the downward force of the adjustingspring is greater than the upward force of thepump discharge pressure, the spring forces theupper diaphragm and the upper crossheaddown.A pair of connecting rods connects the uppercrosshead rigidly to the lower crosshead, so theentire assembly of upper and lower crossheadsmoves together. When the crosshead assemblymoves down, it pushes the lower mushroom andthe lower diaphragm downward. The lower diaphragmis in contact with the controlling valve.When the lower diaphragm is moved down, thecontrolling valve is forced down and thus opened.The controlling valve is supplied with a smallamount of steam through a port from the inletside of the governor. When the controlling valveis open, steam passes to the top of the operatingpiston. The steam pressure acts on the top of theoperating piston, forcing the piston down andopening the main valve. The extent to which themain valve is open controls the amount of steamadmitted to the driving turbine. Increasing theopening of the main valve therefore increases thesupply of steam to the turbine and so increasesthe speed of the turbine.The increased speed of the turbine is reflectedin an increased discharge pressure from thepump. This pressure is exerted against the underside of the upper diaphragm. When the pump dischargepressure has increased to the point wherethe upward force acting on the under side of theupper diaphragm is greater than the downwardforce exerted by the adjusting spring, the upperdiaphragm is moved upward. This action allowsa spring to start closing the controlling valve,which in turn allows the main valve spring tostart closing the main valve against the now reducedpressure on the operating piston. When themain valve starts to close, the steam supply tothe turbine is reduced, the speed of the turbineis reduced, and the pump discharge pressure isreduced.At first glance, it might seem that the controllingvalve and the main valve would be constantlyopening and closing and that the pumpdischarge pressure would be continually varyingover a wide range. This does not happen, however,because the governor is designed with an431