Answer / phani

After some period, churning effect will takes place, which means dry run. This is due to continuous water circulation within the casing. After some period, due to friction between pump casing and flowing water, heat will be generated, slowly water getting heated up and further evaporation takes place, then pump will be under dry run condition.
Internal circulation means, discharged water can't escape from casing, so only the way to escape is passes through wear ring clearance and came to suction, again from suction to discharge water will flow.

Due to this dry run, shaft and casing will be over heated and thermal expansion takes place, shaft may brake at time.

If bluish or burning marks are found on shaft/impeller, it means that "pump runs on shut off head or Churning effect", it is confirm.

Answer / hameedwahab

Pump will run with Shut off Head. Pump will not be healthy, if run with Shut off Head for loner duration.

Answer / dineshkumar

It will be okay for certain amount of time. Centrifugal pump will increase fluid pressure to its maximum pressure, as shown in characteristic curve( P_max at Q_min = 0). However this will not last long, if the condition continue to happen, then fluid will heating up until vaporization occur and will induce failure to the pump and piping system. That's way there is minimum flow capacity of any centrifugal pump, so although delivery valve closed, the pump can still running well.

Respected Sir, Please send me the technical question paper of ONGC.I am in urgent need of it.

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