520 HYDRAULICS AND ITS APPLICATIONS 



speed falls, the gates commence to reopen, and the state of hunting, whict 

 is here outlined, may not die out for some considerable time. To prevenl 

 this some form of relay return device should be fitted, this being sc 

 arranged that all parts of the governor connections, including the 

 regulating valve, return to a normal mid position as soon as the action ol 

 the governor ceases. This tends to prevent over-regulation and hunting 

 Where, in the case of a long penstock, the quick closing of the turbiin 

 gates may lead to excessive water hammer, a pressure regulator shoulc 

 also be fitted, this consisting of an automatic relief valve which is throwr 

 open by the closing of the turbine gates, and which is then slowh 

 returned to its seat. One such pressure regulator is shown in Fig. 204 

 while a second type, as described on p. 451, is shown applied at P to f 

 Thomson vortex turbine in Fig. 248. 



Types of relay return devices are shown in Figs. 202 and 206, anc 

 a further type, similar in general principle to those fitted to the Niagara 

 turbines (Art. 135), is illustrated in Fig. 249. Here a force of 50 tons 

 is available on the relay piston for operating the governing mechanisn 

 and gates. Hunting is prevented by the wedge W, which, as the pistoi 

 moves out, lowers the fulcrum of the governor lever, and thus closes th< 

 relay valve until a fresh movement of the governor reopens it. 



Two other devices which also give considerable assistance in specia 

 cases are the stand pipe and the relief valve. 



The stand pipe (Art. 152) consists of a vertical open pipe, its lower em 

 being connected to the penstock near to its connection with the turbines 

 and of such a height that when exposed to the statical head in the suppb 

 reservoir the water level is within a short distance of the top. An> 

 increase in pressure at the turbine, due to a sudden closing of the gates 

 then produces a flow up the stand pipe, the water escaping at the top, am 

 the maximum possible pressure in the penstock becomes that due to th 

 statical head in the supply reservoir together with that necessary t 

 produce flow up the stand pipe. Obviously the stand pipe cannot 

 applied where the supply head is very great, although in one mode 

 plant 1 such an open pipe 235 feet in height has been fitted. 



A sudden demand for power is also responded to more easily where 

 stand pipe is provided, the level in the pipe falling, and energy thus boil 

 supplied to the wheel while the supply column is being accelerated (Art. 



The relief valve, as its name implies, consists simply of a valve plac 

 on the penstock near to the turbine casing and arranged so as to op< 

 outwards. This is adjusted so as to open directly the pressure excee 



1 At the St. Louis Hydro- Electric Plant, See The Engineer, Februarj' 15, 1907. 



