What is the reason that the pump discharge pressure is zero when the discharge valve is closed?

Question

The fire engine is get dry when close the discharge valve during the pump is opeating. Normally the discharge pressure should equal to the maximu pump head. But in this case the discha pressure come down to 0 bar. What are the possible reasons for this? The suction is negative.

Answer 1

When the discharge valve in a centrifugal pump is closed, still pump can work without any damage to the pump impeller as the liquid will be circulating through the impeller and avoids creating a resistance against the impeller. While in the gear type pumps, closing the discharge valve can cause a damage to the gears as the liquid cannot circulate through the gears and creates a resistance. When a centrifugal pump operates in a system with closing the discharge valve, the impeller can freely rotates inside the pump case in liquid but does not generate any pressure therefore the pressure gage on the discharge line shows the pressure at zero.