Answer / mohamed

NPSH Available should be more than The Rquired NPSH.
I mean that the pressure at the suction eye of the impeller
should be more than the saturated pressure for the pumped
liquid otherwise the pumped liquid will start to evaporate
and cavitation problems will start.
i will mension here some reasons for cavitation:
- pump selection is not correct
-suction tank is not in a proper elevation.
- isolation valves are partially closed
- mesh size of pump strainer is not correct
- pump strainer is partially blocked
- the system is not vented in aproper way .
- tempertaure of pumoed liquid is deviated form normal
values.
still many reasons are there , but finally you should keep
the pressure at pump suction more than saturated pressur.

Answer / mazher

For avoiding cavitation NPSH(Available) should be always
greater than NPSH(Required).

Answer / rajesh

Cavitation takes place when liquid pressure falls below its
Vapour Pressure forming bubbles of liquid.
To avoid cavitation (NPSH)available > (NPSH)required.
We know that...
NPSH is Net Positive Suction Head.
Therefore,
NPSH = (H)atm - (H)vapour - (H) manometric
the above equation must come Positive value to avoid
cavitation!!!

SIR plz send me old exam papers for RRB chennai

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