Flow Control – Positive Displacement Pump Part 1

Positive displacement pumps may be used when it is necessary address flow requirements of 2GPM or less. The pump design ( e.g. vane, gear, peristaltic) selected for a particular application should be based on properties of the liquid and the expected operating condition. One characteristic of a positive displacement pump is that when it is operated at fixed speed then a fixed volume of liquid will be pumped independent of the downstream restriction/pressure, as illustrated below.

Thus, a motor provided with a positive displacement pump is often equipped with a variable speed drive. By adjusting the speed of the pump it is possible to regulate flow through the pump. To address the case where the flow path downstream of the pump is blocked (e.g. blocking valve accidentally shut) it is necessary to include a safety relief valve to avoid excessive pressure buildup that could damage the pump and potentially rupturing the downstream line. In some cases this protecting is built into the positive displacement pump or a relief valve may be installed at the pump discharge as illustrated below.

The manufacturer of the variable speed drive limits the minimum speed of motor operation to prevent the motor from overheating. Thus, this minimum speed limit establishes the minimum flow rate that may be achieved using a variable speed drive. To allow automatic operation below this flow limit it is necessary to structure the flow control in a different manner – as addressed in next week’s blog.