3.3.3 Flow and pressure pulsation in PD pump

Flow and pressure pulsation in PD pump

Flow and pressure pulsation in PD (positive displacement) pump is a natural consequence of the pump design and function. For many displacement pumps the delivered volume flow varies during the course of the working cycle. A drastic example of this is a single cylinder piston pump, see figure 3.33a.

Figure 3.33a Volume flow Q from a single cylinder single action piston pump as a function of time.

During a working cycle the displacement varies, to a greater or lesser extent, for most positive displacement pump types. In order to reduce variations in flow it is possible to arrange built-in accumulators on the suction and delivery side. These can be filled with air or equipped with air cushions or springs separated from the fluid by an elastic membrane. The size of the flow variation is a feature of quality, which unfortunately is only occasionally specified by the manufacturer. A knowledge of the size of the variation expressed, for example, as flow amplitude at various frequencies, is important when sizing and specifying both suction and delivery piping, especially for reciprocating pumps.

As an alternative to flow variations, pressure variations or pressure pulsations may be stated. These values, Flow and pressure pulsation in PD (positive displacement) pump, are usually obtained by measurements made in special test rigs and can only with difficulty be transposed to practical piping installations. Flow resistance in a test rig consists largely of throttling losses in control valves i.e. with a resistance varying as the square of the flow. Resonance and dampening in, tor example, rubber hoses also have considerable influence. The size of pressure pulses are, in principle, independent of the mean pressure. The practice of expressing pressure pulsations as a percentage of the mean pressure can therefore be misleading.

The fluctuations in volume rate of flow vary from about ±50 percent for the piston pump shown, to a few percent for screw pumps. Variations in flow, together with pockets of trapped fluid, is the primary cause of noise and vibration in displacement pumps. In the case of gear pumps for example, it is very easy for a small volume of fluid to become trapped in a pocket at the moment of gear contact, the volume of which changes during the course of rotation. This gives rise to harsh running and high noise levels. The problem can be considerably reduced by introducing a slight helix angle on the gear; by providing ventilation grooves in the side plates or by increasing the side clearance. The use of higher speeds or number of strokes is limited for displacement pumps by these trapped volumes of fluids and by unsteady flow when filling the working space.