POSITIVE ROTARY PUMP 



627 



Since a large proportion of the loss is due to mechanical friction and is 

 therefore approximately independent of the pressure, this efficiency falls 

 off rapidly as the working head is reduced, so that in general, for heads 

 below about 100 feet, the centrifugal pump becomes the more efficient. 

 The piston pump, however, has the advantage of being positive in action, 

 is not so liable as the centrifugal pump to lose its water, and has an 

 efficiency which, to a larger extent than in the case of the latter type, is 

 independent of speed. 



AET. 171. POSITIVE ROTARY PUMPS. 



This type of pump forms the connecting link between the centrifugal 

 and the reciprocating piston pump. Like the former, its motion is rotary 



FIG. 300. Drum Pump. 



and its delivery practically continuous and free from vibration, while, like 

 the latter, its action is positive and it will work well over a large range of 

 speeds. Its great drawback lies in the difficulty of keeping the rotating 

 pistons tight against each other and against the pump casing, the wear 

 which invariably occurs leading to considerable leakage and loss of 

 efficiency. 



In spite of this, the convenience of the method of driving, the fact that 

 no valves are required, and the steadiness of working, render the pump 

 valuable in many instances, and more particularly where viscid liquid is 



S 3 2 



