The most obvious difference with Duplex pumps is that they have two cylinders instead of three for the triplex, but also there is an important dissimilarity that you most take into consideration when doing pump calculations. In a duplex pump, the fluid ends are arranged in a way that each cylinder pumps in both directions. Therefore, with no regards of the direction in which the piston is moving, fluid is being simultaneously pressurized on one side and input/filled on the other side of the piston.
This process of pumping in both directions is the reason the pump is called “double-acting” and is said to produce smoother pump pressure (without smoothing accumulators).
