Most of the flow change occurs near shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage if used to regulate flow.
A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps.
The intention here is to obtain a well-defined seat between two narrow mating surfaces, giving firm shutoff of the flow from only a light float pressure.
