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NPSHr Testing of Centrifugal Pumps

NPSHa of a system depends on the following variables for a liquid at a fixed temperature:-
Pressure imposed on the sump water level
  
Height of water level at sump from pump center-line
  
Frictional losses in suction pipe
 

According to the guidelines of Hydraulic Institute NPSHr is that value of NPSHA that caused total head to be reduced by 3% Reason -Flow blockage due to vapour formation & cavitation in the impeller internal passages

First test procedure
    The pump is supplied from a closed pressure tank
    The liquid level at the mark is help constant
    NPSHa is varied by varying the pressure on the liquid surface at the tank
 
  Several sets of readings are taken at different levels of NPSHA and constant Q to establish the total head of the pump.The procedure is continued till the total head drops by more than 3% The readings include the following
bullet
 Suction pressure from the compound gauge
bullet Discharge pressure shown by the pressure gauge fitted
bullet Capacity shown by the magnetic flow meter
bullet Ambient pressure
bullet Water temperature
The NPSHA value where total head drops by 3% is considered as NPSHr at the flow

The whole procedure is repeated 3-4 times at various flow points (generally left of b.e.p, at b.e.p & right of b.e.p)
NPSHa in this condition

= { Atmospheric Pressure (m) + (suction  gauge  reading  at that point (kgf/cm2) x 10) m + (Velocity Head at suction line of gauge connection)
m + (Level Difference Between  Gauge Connection Point & Impeller C/L ) m - (Liquid  Vapor Pressure at testing Temperature) m }
Typical NPSHr Test Arrangement sketch enclosed
Second Test Procedure
    The pump is supplied from a sump that's open to atmospheric pressure
    Liquid level is varied to determine the desired NPSHA
    Special care is taken when adjusting the liquid level to avoid vortex forming
Third test procedure
    At first the inlet valve is opened completely
    The pump is started with discharge valve closed
    After the motor attains full speed the discharge valve is gradually opened
    Next, the discharge valve is adjusted to achieve the desired constant flow
   

The suction control valve is throttled to reduce the effective pipe area.The suction pressure reduces due to increased friction losses in the valve.The discharge valve is adjusted suitably to keep the flow rate unchanged.

Precaution taken during NPSH test
  Testing  at low flow rate (a small fraction of normal capacity), low velocity through the pipeline, permits  more time to liberate the dissolved gas at low pressure zone.
  Operating pump near the shut-off causes most of the power wasted owing to low efficiency to heat-up the liquid running in closed loop.
  Pump pressure gauges should be located at least one pipe diameter from the pump flanges.
  There must not be any restriction between the pump flanges & pressure gauges.
  For systems with free surface, the approach of water to pump inlet should be uniform & free from any vortices or eddies that may result from valves, bends, pipe-joints, increaser, reducer or any other obstruction in the suction pipe-line.
  Air-leaks into the system must not be allowed.
  The discharge line diameter should be chosen as large as possible specially for pumps with low head when tested at low speed otherwise high velocity may result in lowering of absolute pressure causing the liquid to cavitate somewhere downstream of the pump.