A positive displacement (PD) pump operates precisely according to its title: it positively displaces liquid entering its chamber – regardless of anything else as long as its driver continues to input motive energy. Anything done externally by the operator (with the exception of decreasing the driver’s rpm) does nothing to lessen the pump’s liquid displacement. This simply means that the pump will continue to pump the same flow rate whether the operator throttles the discharge or not. HOWEVER, if you throttle the pump’s discharge (or block it off) you will force the pump’s discharge pressure to increase almost exponentially due to the liquid’s relative incompressibility. This is a hazardous situation and that is why all PD pumps require a mandatory pressure relief valve in their discharge piping upstream of any blocking device. That is the reason you should not try to force a portion of the discharge fluid to recirculate back to the suction side of a PD pump by throttling. With a PD pump, it is like the song,“All or nothing at all” as far as volumetric capacity is concerned – unless you can vary the pump’s speed.
A centrifugal pump, being a dynamic device, has a dead head with a maximum discharge pressure. Hence, at 100% blocked discharge, a portion of its discharge can be diverted back to its source and thereby maintain at least a small, minimum flow through the pump that ensures its seal lubrication and self-cooling. A PD pump can’t do that. If you can’t accept its flow rate downstream during a part of its operation, you either shut it down, reduce its speed, or divert the discharge through some instrumented procedure.