Chapter 16. -AUXILIARY STEAM TURBINES 



gears, the governor also slows. Centrifugal 

 weights on the governor move inward as the 

 speed decreases, and this causes the pilot valve 

 to move upward, permitting oil to enter the oper- 

 ating cylinder. The operating piston rises and, 

 through the controlling valve lever, the lifting 

 beam is raised. The nozzle valves open and 

 admit additional steam to the turbines. 



The upward motion of the controlling valve 

 lever causes the governor lever to rise, thus 

 raising the bushing. Upward motion of the bush- 

 ing tends to close the upper port, shutting off the 

 flow of oil to the operating cylinder; this action 

 stops the upward motion of the operating piston. 

 The purpose of this follow-up motion of the 

 bushing is to regulate the governing action of the 

 pilot valve. Without this feature, the pilot valve 

 would operate with each slight variation in tur- 

 bine speed and the nozzle valves would be alter- 

 nately opened wide and closed completely. 



A reverse process occurs when the load on 

 the generator decreases. In this case, the turbine 

 speeds up, the governor speeds up, the cen- 

 trifugal weights move outward, and the pilot valve 

 moves downward, opening the lower ports and 

 allowing oil to flow out of the operating cylinder. 

 The controlling valve lever lowers the lifting 

 beam and thereby reduces the amount of steam 

 delivered to the turbine. 



Constant-Pressure Pump Governors 



Many turbine-driven pumps are fitted with 

 constant-pressure pump governors. The function 

 of a constant pressure pump governor is to 

 maintain a constant pump discharge pressure 

 under conditions of varying flow. The governor, 

 which is installed in the steam line to the pump, 

 controls the pump discharge pressure by con- 

 trolling the amount of steam admitted to the 

 driving turbine. 



A constant-pressure pump governor for a 

 main feed pump is shown in figure 16-10. The 

 governors used on fuel oil service pumps, lube oil 

 service pumps, fire and flushing pumps, and 

 various other pumps are almost identical. The 

 chief difference between governors used for dif- 

 ferent services is in the size of the upper dia- 

 phragm. A governor used for a pump which oper- 

 ates with a high discharge pressure has a smaller 

 upper diaphragm than one for a pump which oper- 

 ates with a low discharge pressure. 



Two opposing forces are involved in the 

 operation of a constant-pressure pump governor. 



Fluid from the pump discharge, at discharge 

 pressure, is led through an actuating line to the 

 space below the upper diaphragm. The pump dis- 

 charge pressure thus exerts an upward force on 

 the upper diaphragm. Opposing this, an adjusting 

 spring exerts a downward force on the upper dia- 

 phragm. 



When the downward force of the adjusting 

 spring is greater than the upward force of the 

 pump discharge pressure, the spring forces the 

 upper diaphragm and the upper crossheaddown. 

 A pair of connecting rods connects the upper 

 crosshead rigidly to the lower crosshead, so the 

 entire assembly of upper and lower crossheads 

 moves together. When the crosshead assembly 

 moves down, it pushes the lower mushroom and 

 the lower diaphragm downward. The lower dia- 

 phragm is in contact with the controlling valve. 

 When the lower diaphragm is moved down, the 

 controlling valve is forced down and thus opened. 



The controlling valve is supplied with a small 

 amount of steam through a port from the inlet 

 side of the governor. When the controlling valve 

 is open, steam passes to the top of the operating 

 piston. The steam pressure acts on the top of the 

 operating piston, forcing the piston down and 

 opening the main valve. The extent to which the 

 main valve is open controls the amount of steam 

 admitted to the driving turbine. Increasing the 

 opening of the main valve therefore increases the 

 supply of steam to the turbine and so increases 

 the speed of the turbine. 



The increased speed of the turbine is reflec- 

 ted in an increased discharge pressure from the 

 pump. This pressure is exerted against the under 

 side of the upper diaphragm. When the pump dis- 

 charge pressure has increased to the point where 

 the upward force acting on the under side of the 

 upper diaphragm is greater than the downward 

 force exerted by the adjusting spring, the upper 

 diaphragm is moved upward. This action allows 

 a spring to start closing the controlling valve, 

 which in turn allows the main valve spring to 

 start closing the main valve against the now re- 

 duced pressure on the operating piston. When the 

 main valve starts to close, the steam supply to 

 the turbine is reduced, the speed of the turbine 

 is reduced, and the pump discharge pressure is 

 reduced. 



At first glance, it might seem that the con- 

 trolling valve and the main valve would be con- 

 stantly opening and closing and that the pump 

 discharge pressure would be continually varying 

 over a wide range. This does not happen, how- 

 ever, because the governor is designed with an 



431 



