HYDRAULIC RAM 687 



Whenever the efficiency of a ram is given, it should be stated which 

 formula is used. 



Sources of Loss. These are due to : 



(1) Leakage at waste valve. 



(2) Resistance of valves and of supply pipe. 



(3) Eddy production due to sudden changes of section. 



(4) Loss of resilient energy. 



Leakage from the waste valve increases with the time during which it 

 is off its seat ; with the lift and area of the valve ; and with the velocity 

 of efflux ; all of which, with the exception of the valve area, must be 

 increased with an increase in the delivery head. 



The area of the valve is usually made from two to four times that of the 

 supply pipe, an increased area with a correspondingly reduced lift tending 

 to efficiency in working. 



This is the most serious of all the sources of loss, and usually accounts 

 for between 15 and 25 per cent, of the total energy received. 



To enable it to be reduced as far as possible, the weight and travel of 

 the waste valve should be adjustable to suit any given conditions of 

 working. It should be noted that this loss is not necessarily least when 

 the time during which the valve is open is reduced to a minimum, for 

 with a given supply and delivery head there is a certain valve travel below 

 which the pressure will not exceed the delivery head. Increasing the 

 travel increases the velocity of efflux and the time of opening, and there- 

 fore the leakage, but at the same time increases the pressure and therefore 

 the proportion of energy entering the air chamber. This goes on up to a 

 certain point, which can only be determined experimentally, where the 

 increased leakage losses counterbalance the proportional gain of energy, 

 and which gives the most efficient working lift. 



This point is brought out in Fig. 332 which shows a series of efficiency 

 curves obtained by the author from a small hydraulic ram of the type 

 shown in Fig. 330 a, having a supply pipe 4 feet 6 inches long- and 

 1J inches diameter, and working under a uniform head of 4 feet 6 inches, 

 the waste valve being 1 j inches diameter. From these curves it is evident 

 that although with low delivery heads the efficiency increased with an 

 increase in the number of beats of the waste valve per minute, as the 

 delivery head was increased the speed for maximum efficiency rapidly 

 diminished. 



(2) and (3) Valve resistances are approximately independent of head, 

 while loss by shock and frictional losses in supply and delivery pipes 

 increase as the velocity, and therefore as the delivery head, increases. 



