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A centrifugal pump is fully primed, but on starting it fails to deliver fluid. The probable reasons are listed below:

1. Leaky foot valve or suction line

2. Suction head is very low

3. Insufficient motor speed

4. Damaged or closed delivery valve

Which of these reasons are correct?


1. 1, 2, 3 and 4
2. 1, 2 and 3 only
3. 2, 3 and 4 only
4. 1, 3 and 4 only
5.

1 Answer

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Best answer
Correct Answer - Option 1 : 1, 2, 3 and 4

Concept:

The working of a centrifugal pump starts with its priming.

  • Priming is the process of filling the suction pipe, casing and the delivery pipe up to the delivery valve by the liquid which is to be pumped.
  • Priming is required in order to drive out the air voids present, which otherwise would make the operation of the pump ineffective.
  • Because the pressure energy imparted by the centrifugal pump is proportional to the density of the liquid and if any air pocket exists in the casing, then only negligible amount of pressure would be generated.
  • This pressure might not be sufficient for lifting of the liquid. Hence, priming becomes essential.


If the pump fails to prime or loses prime i.e. stops pumping check for the following possible causes:

  • Insufficient liquid in the pump
  • Improper priming: Pump has not been primed properly or it has lost prime.
  • Air leak in the suction line due to lose connections or pinholes in the hose
  • Collapsed suction line or clogged strainer
  • Seal worn and sucking air
  • Worn impeller – impeller gap too large
  • Pump not running fast enough (Speed too low)
  • Suction lift too high
  • Priming against high head or obstruction such as check valve in discharge line
  • Low available suction head: When the pump is installed, the available NPSH is fixed. In order to have cavitation free operation, the available NPSH should be greater than the required NPSH.

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