/ GOVERNORS 253 



run continuously and the pumps are equipped with unloading 

 valves. Each unit has its own accumulator or pressure tank 

 situated close to the goveror to eliminate the effect of inertia in 

 the supply pipe, and unless the discharge piping is of liberal size, 

 each unit should have a local sump tank from which the oil or 

 water returns by gravity to the central reservoir supplying the 

 pumps. This latter method complicates the piping, and it is 

 better to use a large return pipe and only one sump tank. Both 

 oil and water systems are now generally of the open type; that is, 

 they are arranged to discharge under atmospheric pressure. The 

 closed or vacuum system, at one time commonly used, has been 

 discarded, even with individual pumping systems, because of the 

 tendency to break down the oil. 



The principal elements of the governing unit proper are: 

 One or two power cylinders or servo-motors, suitable mechanism 

 for transmitting the movement of the power piston to the gate 

 shaft, a main or relay valve, a pilot or regulating valve, a safety 

 stop, a centrifugal speed governor and a compensating device. 

 In addition they are also usually arranged so as to permit of hand 

 control as well. as remote control and when required a load limiting 

 device may be provided. 



The admission of the fluid from the pressure tank to the power 

 cylinder and back to the receiver tank is regulated by the main or 

 relay valve. This must, in most cases, be of such a large size as 

 to make it impossible to control directly from the centrifugal speed 

 elements, and for this reason an intermediate pilot or regulating 

 valve is provided. This is connected to the centrifugal mechanism 

 and regulates the admission of the pressure fluid to the main 

 relay valve, and this, in turn, to either end of the piston in the 

 power cylinder, which transmits its motive power to the gates or 

 nozzle mechanism of the turbine when a speed variation occurs. 

 The movement of the relay valve is always such as to return the 

 pilot valve to its neutral position after a load variation has oc- 

 curred, resulting in a movement of the governor piston. 



The rapid growth in size of units has brought about a corre- 

 sponding change not only in the size of governors, but also in the 

 arrangement. Standard governors were formerly self-contained; 

 that is, the control and power elements were combined in the gov- 

 ernor itself. It was necessary only to connect the centrifugal 

 element to the turbine shaft and the power element to the tur- 



