Other methods of flow control for PD pumps
Pumps in parallel
PD pumps can generally be run in parallel without problems. This gives the ability to increase flow rate at the pressure rating of a single pump. The principal considerations are, the correct design of inlet and outlet pipework to avoid problems of NPIP/NPSH, overpressure in the discharge pipe and back flow through a non-running pump. Isolation valves for safe main tenance should also be considered.
Pumps in series
Rotary pumps in series can give increased pressure capability at the flow rate of a single pump, however it is not recommended that PD pumps be applied in this manner unless sophisticated speed control logic is used. Matching of the two pumps is extremely important and safety, in the event of failure of either unit, normally requires multiple overpressure protection devices and a means of ensuring the second pump is not underfed, creating low NPIP/NPSH and cavitation problems.
Safety of the second pump has to be taken seriously as the system pressure has to be catered for rather than differential operating pressure. The use of one drive for two pumps overcomes many of the starting/speed variation/synchronization issues and compensates for any fluctuations in electrical supply or motor loading.
Flow control valve
This is not an acceptable technique for flow control of a PD pump. Throttling the discharge is unsafe as these pumps have the potential to generate very high pressures.
Although an effective way of controlling flow rate for small flow variations, it is not preferred for major duties as the waste of energy and possible wear on by-pass control valves give rise to greater running costs. However, it can have the lowest initial cost.