SUCTION AIR VESSELS 617 



the pump. The effect of such an air vessel will now be considered. Its 

 action is as follows : During the first part of the stroke the pressure 

 behind the piston is reduced and water flows out of the air vessel. The 

 flow along the suction pipe is thus reduced, as is the acceleration of the 

 whole mass of water. This reduces the frictional resistance in the suction 

 pipe, while the pressure behind the piston is increased both on this 

 account and because of the reduced acceleration. At the same time 

 tendency to separation and to water hammer at the end of the stroke is 

 reduced, if not entirely eliminated ; the discharge coefficient becomes 

 unity, or slightly less than unity ; and the pump diagram approximates 



Atmos , ^"" y L ine. 



FlG. 295. Pressure Diagram from Suction Air Vessel of 

 Reciprocating Pump. 



more and more nearly to the rectangle given by a perfect pump, as the 

 sizes of the air vessels are increased. 



A typical indicator diagram taken from the suction air vessel of a 

 double-acting pump is shown in Fig. 295. 



ART. 166. AIR VESSEL APPLIED TO A SINGLE OR DOUBLE-ACTING SINGLE 



CYLINDER PUMP. 



Effect of Air Vessel placed on Suction Side of Pump. 

 Let a s , VM and a s represent the area of suction pipe between supply 

 reservoir and air vessel, the velocity and the acceleration in this 

 pipe respectively. 



v> v v> a v represent ditto, in the air vessel itself. 

 A, V, a represent ditto, in the pump cylinder. 

 h a represent the pressure of air in air vessel, in feet of water. 

 TT represent the atmospheric pressure, in feet of water. 

 h v = height in feet, of water level in air vessel above centre line of 



pump. 



During the first part of suction stroke water will flow out of air vessel 

 into pump, and for continuity of flow : 



v s a s + v v a v = V A 



.*. Differentiating, we have a s a s -f- a v a v = A a (1) 



If, as is usual, the air vessel is placed near to the pump, so that the 



pressure at the junction of air vessel and suction pipe may be taken as 



