Flow–Speed Relationship in a Positive Displacement Pump
In positive displacement pumps of the piston or gear type, flow is directly proportional to speed. Because a fixed volume is moved on every revolution, doubling the speed roughly doubles the flow. Pressure, on the other hand, is a result of the resistance of the line, not of the pump; it is not set directly by speed.
A positive displacement pump forces a defined volume out on every stroke. That makes the flow calculation simple: stroke volume times the number of strokes per minute, minus some volumetric efficiency loss. Speed sits directly inside that product.
Pressure is a different quantity. The pump forces flow; that flow builds a pressure because of the resistance of the line, the drill nozzle and the depth. If the line becomes blocked, the pump tries to produce a much higher pressure at the same speed. Therefore:
- Setting flow from speed is correct.
- Setting pressure from speed is indirect and does not replace the relief valve.
The situation is different with centrifugal pumps; there both flow and pressure change markedly with speed. When setting up speed control, knowing which pump type you are working with is a precondition for setting expectations correctly.
Context in which this term is used: Speed control of three pumps in drilling