How does adjusting mud pump speed in drilling affect flow and pressure?
Mud pumps are piston, that is, positive displacement pumps and typically turn in the 100-200 rpm range. In this type of pump the flow is directly proportional to speed; pressure arises from the resistance of the line. Increasing the speed increases the flow, which indirectly raises the pressure; however, speed adjustment never takes the place of a relief valve.
The relationship is established as follows:
- Flow: a fixed volume is delivered at each stroke, so flow is directly proportional to the number of strokes, that is, to speed. When you increase the speed by 20%, the flow also increases by about 20%.
- Pressure: the pump does not produce pressure, the resistance of the line does. Passing the same flow through a narrower or longer line raises the pressure.
In practice what is wanted in drilling is a certain rate of penetration and cleaning capacity; that is determined by flow. If the discharge pressure is approaching the safe limit, reducing the speed is a correct response, but this is not a protection system; a relief valve must always be present and in working order in the line.
On the engine side the speed is reduced to the pump speed by the gear or belt ratio; take this ratio into account when setting the upper limit. On the GS740-3P there is a separate potentiometer for each pump engine and the channels are independent of each other; in this way the speeds of the three pumps can be adjusted separately without affecting one another.
Full topic: Speed control of three pumps in drilling