4 multistage slurry pumps

Proressive Cavity Pump

Progressive Cavity Pump

A progressive cavity pump is basically a Positive Displacement (PD) type of device. In operation, progressive cavity pumps are fundamentally fixed flow rate pumps – such as piston pumps. This type of pump needs a fundamentally different understanding to the types of pumps to which people are more commonly first introduced - namely ones that are thought as generating a pressure (i.e., centrifugal pumps). This can lead to the mistaken assumption that all pumps can have their flow rates adjusted by using a throttle valve attached to their outlet. However, with PD pump this assumption is a serious and potentially hazardous problem, since a throttling discharge valve will have practically no effect on the flow rate and completely closing it will involve very high, probably damaging, pressures being generated. In order to prevent this, PD pumps’ discharges are fitted with cut-off pressure switches, pressure relief valves, bursting disks (deliberately weak and easily replaced points), or a recycle valve that allows an amount of discharged fluid to return back to the pump’s suction source. With a recycle flow fitted, a fixed flow rate type of pump is effectively converted to a variable net flow rate producer.

Flow rate of a PD pump is commonly controlled by recycling flow. It is also common to control PD pump flow using a variable frequency drive (VFD). The flow rate of the pump will be proportional to the pump speed (RPM), which is in turn proportional to power frequency (Hz).

An example of PD pump recycling can be seen on the diagram. It concerns a rotary pump transferring (heavy) fuel oil to a boiler. Of course recycling does not substitute PRV, having to be placed at the immediate discharge of the PD pump.