In a double acting reciprocating pump, the suction and delivery strokes occur simultaneously. When the crank rotates from IDC to ODC, a vacuum is created on the left side of the piston and the liquid is sucked from the sump through suction valve S1. At the same time, the liquid on the right side of the piston is pressed and a high pressure causes the delivery valve D2 to open and the liquid is passed to the discharge tank. This operation continuous till the crank reaches ODC.
With further rotation of the crank, the liquid is sucked from the sump through suction valve D2 and delivered to the discharge tank through valve D1. When the crank reaches IDC, the piston is in the extreme left position. Thus one cycle is completed.
Double acting reciprocating pump gives more uniform discharge because of continuous of delivery strokes. Quite often, a multi cylinder arrangement having two or more cylinders is also employed to get uniform feed.