Double Action / Dual Action
A reciprocating pump design which produces a pressure differential across the pump on both the in and out stroke of each drive rod.
Dual action pumps use two separate pumping chambers per drive rod. On the forward stroke, it pushes fluid out of the first chamber, and draws fluid into the other. On the back stroke, it the draws water into the first chamber and pushes fluid out of the other.
Dual Action Plunger Pump Concepts
Dual action pumps use either a swab on a piston rod, a piston, or two plungers. When two separate plungers or pistons are used, both of them move in unison, in the same direction as the drive rod. Although different designs allow for different configurations, the basic concept is shown below.
Dual Action Piston Pump Operation
In this instance, packing is used to seal the pumping chamber around the piston rod. Additional sealing is required around the piston to prevent fluid from flowing around the piston.
Note that a double action piston pump with the above configuration shares one of the pumping chambers with the drive rod. The shared volume reduces the output volume and pressure of this chamber. In many designs, it is considered negligible and factored into the pump's overall maximum pressure and flow rating. In a double action plunger pump with the configuration shown below, both chambers are able to reach equal pressures and flow rates.
Double Action Plunger Pump Operation
Since double acting pumps are designed to produce pressure on both a "push" and a "pull" stroke, wear is different than in single acting pumps. The forces variations seen within the power end increase per revolution of the crankshaft. Power ends of dual acting pumps are generally designed slightly larger than single acting pumps of the same horsepower rating. This larger size is able to withstand wear and increase the life expectancy of the pump.
Dual acting pumps are most commonly found as a duplex pump design.They can also be found in other reciprocating pump sizes, but are generally less common.