Pump Discharge Pressure
It is the pressure measured at the pump outlet, arising from the total resistance the flow meets along the line. A pump does not produce pressure, it produces flow; pressure is the system's response to that flow. For that reason a sudden rise in discharge pressure is usually a sign that the line or the well, not the pump, has changed state.
Understanding the source of the pressure correctly changes the diagnosis fundamentally. If the pressure rises while running at the same speed, something in the line has narrowed: a blockage, a collapse, a valve left closed or a change in the properties of the mud. If the pressure falls there is a loss in the opposite direction: air at the suction, a worn piston seal or a leak.
Its relation to speed control is as follows. Lowering the speed reduces the flow, and as the flow drops the pressure arising from the line resistance drops as well. That is, you affect the pressure indirectly through the speed. However this is not pressure regulation; when the line is blocked, lowering the speed does not guarantee bringing the pressure down into the safe region.
In this chain the GS740-3P only drives the throttle lever position; it does not measure pressure and does not decide according to pressure. Protection against overpressure is provided by the line's relief valve, and this element must have been set correctly independently of the speed control.
Context in which this term is used: Speed control of three pumps in drilling