THE HYDKAULIC BAM 



689 



will transform up to 75 per cent, of the energy in the supply reservoir 

 into useful work. 



1 TT 



Rankine gives the efficiency in terms of the ratio d . as being equal to 



1-12 - -2 



and while this cannot be looked upon as being generally true, it indicates 

 how rapidly the efficiency falls off as 



^ increases. The curves of Fig. 332 



also bring out this fact very clearly. 



The simple type of ram already 

 described gives excellent results where 

 the diameter of supply pipe does not 

 exceed about 4 inches. With larger 

 sizes the shock caused by the sudden 

 closing of the waste valve becomes 

 excessive, and though various devices 

 have been adopted to prevent this, 

 none of them have proved satisfactory 

 as applied to the ordinary ram. One 

 such device is illustrated in Fig. 330 b. 

 Here an air cushion, regulated by the 

 air-cock K, is provided for the waste 

 valve, but although this effectively 

 prevents shock, it also prevents any 

 high degree of efficiency being ob- 

 tained. This is clear if it is remem- 

 bered that the velocity of efflux of the waste water is increasing the 

 whole of the time that the valve is closing and has its maximum value 

 immediately before the valve comes to its seat, so that leakage during 

 this portion of the cycle is more important than at any other time. 

 Slowness of closing is thus particularly detrimental as the valve approaches 

 its seat, and in fact the more quickly the valve reaches its seat after once 

 beginning to close, the less will be the consequent loss of energy in the 

 waste water expressed as a proportion of the whole kinetic energy of the 

 column, A further drawback to the device lies in the fact that because of 

 this leakage it becomes impossible to pump against a head greater than 

 about six times the supply head. 



In the above example the upward pressure of the water on the valve 



FIG. 333. 



H.A. 



Y Y 



