PRESSURE REGULATORS OR RELIEF VALVES 259 



must be provided to permit a slower governor action or a surge 

 tank must be used. If such a surge tank cannot be located close 

 to the power-house, relief valves must also be provided, and some 

 conditions may require all the devices. 



Relief valves are of two principal types, the synchronous by- 

 pass governor operated, and the direct-pressure operated. Their 

 design is essentially the same, except for the control mechanism. 

 The first becomes immediately operative with the closing gate 

 motion and this action continues until the gates stop moving. 

 The water rejected by the turbine as the gates close is discharged 

 through the regulator. Thus the penstock velocity instead of 

 being suddenly checked, resulting in waterhammer, remains 

 practically unchanged. The device is made water-saving by the 

 use of a dashpot, which permits of a relative motion of the con- 

 nection between the turbine gate mechanism and the by-pass 

 valve. The adjustment of the dashpot is made such that the 

 by-pass, after having been opened due to a sudden closing of the 

 turbine gates, is closed within a period which is sufficiently long 

 to prevent a dangerous pressure rise in the pipe line. If the load 

 goes off gradually and the gates are closed at a rate slower than 

 that produced by the dashpot, the pressure regulator remains 

 inactive. If the gates are again opened before the dashpot has 

 closed the pressure regulator, then it should close synchronously 

 with the gate motion, otherwise an excess quantity of water 

 is discharged, causing a drop of pressure in the pipe line. 



The second class of relief valves, or the direct-pressure operated 

 type, do not act until the pressure in the pipe line or turbine 

 casing has risen above normal and are, therefore, a more direct 

 means of protecting pipe lines against dangerous pressure rises, 

 such as caused by the clogging up of the gates, etc. Governor- 

 operated pressure regulators are, however, made which permit of 

 an automatic action independent of the gate motion to take care 

 of such emergencies. 



In order to obtain ideal results, the maximum capacity of 

 the regulator should be equal to the full-load discharge of the 

 turbine less the discharge required to run at synchronous speed 

 without load. Ordinarily some sacrifice is made to reduce the 

 size of the regulators. They are seldom installed in excess of 

 75 per cent of the maximum turbine discharge, and in many cases 

 not more than 40 per cent or 50 per cent is provided. In such 



