VARIATION OF PRESSURE IN PUMP 



609 



from C C' as base line, give the effective pressures in the suction chamber, 

 expressed in feet of water. 



If the suction head be increased so that D represents 34 h s feet, 

 separation will occur at the commencement of the stroke, since the 

 available head is now insufficient to give the necessary acceleration. At 

 D" the head becomes sufficient both for this purpose and to overcome 

 frictional losses, while at some point X, the acceleration of the water 

 remaining approximately constant while that of the piston is continually 

 diminishing, the water overtakes the piston and knocking occurs. Up to 



FIG. 290. Theoretical Diagram of Piston Pressure during Suction Stroke 

 of Reciprocating Pump. 



this point the pressure behind the piston is constant, and has a negative 

 value equal to D in feet of water. At the shock, violent oscillations of 

 pressure are set up as represented by the dotted lines and the most that 

 theory can do in this case is to indicate the maximum pressure to be 

 expected and the means of reducing this. 



Under normal conditions of working, the maximum pressure attained 

 on the suction stroke is less than the delivery pressure F. 



If, however, this latter pressure be reduced, as for example to E t the 

 suction pressure becomes equal to this at the point E", and for the 

 remainder of the stroke discharge takes place through the delivery valves, 

 the pressure remaining constant and following the line E" E'. 



A. R, R 



