Due to reciprocating motion of piston, the flow through a single cylinder and single acting pump is intermittent and uneven. For the suction pipe, it fluctuates from zero at the beginning of suction stroke to a maximum value at about mid stroke and back to zero at the end of the suction stroke Pundai Blogspot. Evidently then an additional suction head is required to accelerate the flow at the beginning of the suction and an opposing effect is required to retard the flow towards the end of suction stroke. Likewise, there will be an acceleration head Pundai Blogspot at the start of delivery stroke and a corresponding retardation head at its termination.

Thus reciprocating motion of the piston causes acceleration during first half of each stroke, and retardation during the late half of each stroke. That transmits a corresponding acceleration and retardation to liquid in the suction and delivery pipes. Work input to pump can be calculated directly from the indicator diagram which depicts the pressure of liquid in the pump cylinder corresponding to any crank position during the suction and delivery strokes. In an indicator diagram, the stroke length is represented by abscissa and the pressure head is plotted the ordinate. Line abs represents the pressure head in the cylinder during the suction stroke; this pressure is below the atmospheric pressure by an amount equal to the Pundai Blogspot suction head. Line cad represents the pressure head in the cylinder during the delivery stroke; this pressure is above the atmospheric pressure by an amount equal to the delivery head. Now consider a pump with long suction pipe and a short delivery pipe operating at high speeds. Inertia force in the suction pipe would then be large in comparison with the pressure of the outside of the delivery valve. Naturally, the delivery valve would open before completion of the suction stroke and some liquid would be displaced to the delivery pipe even before the commencement of the delivery stroke.

The Pundai Blogspot would result in the actual discharge to be more than the theoretical discharge; the pump would then be functioning with negative slip. Whereas in single acting pump the liquid is intermittently fed into discharge, a double acting pump gives comparatively a more uniform discharge because of the continuity of suction and delivery strokes. Quite often, a multi cylinder arrangement having two or more cylinders is also employed to get a still more uniform feed.