Subsubsection By Displacement Characteristics
A positive displacement pump is one in which a definite volume of liquid is delivered for each cycle of pump operation, regardless of the discharge pressure the pump operates against, and provided the capacity of the power unit driving the pump is not exceeded.
A non-positive displacement pump is one in which the volume of liquid delivered for each operational pump cycle depends on the resistance to flow in the discharge line. This type of pump produces a force on the liquid that is constant for each particular operating speed of the pump. Pressure in the discharge line produces a force in the opposite direction. When these two forces are equal, the liquid is in a state of equilibrium and does not flow.
If the discharge valve of a positive displacement pump is completely closed, the discharge pressure will rise to a point where either the unit driving the pump will stall or parts of the pump exposed to discharge pressure will fail. This situation should be avoided. On the other hand, closing the discharge valve on a non-positive displacement pump will cause the discharge pressure to rise to a maximum for that type of pump operating at a specific speed. Nothing more will happen except the pump will churn the liquid trapped in the pump chamber and produce frictional heat in the liquid.
Most positive displacement pumps deliver liquid in slugs, with no delivery in between, and this causes a rise then a drop in discharge pressure and flow. Pumps equipped with many small pumping chambers generally have overlapping delivery that minimizes this effect. Non-positive displacement pumps deliver a practically continuous even flow for a given operating speed and discharge pressure.
