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Sunday, February 17, 2008

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Why centrifugal pump Net Positive Suction Head required (NPSHr) increases with flow ?

There is a simple answer. NPRHr is a parameter fixed during design and fabrication of a centrifugal pump. The NPSHr is increased with flow. Main reason to this increase is increase in frictional lose at entrances nozzle and turbulence friction lose at vane tip.


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Let's look at the following image.


The pressure at different point location from entrance pipe to impeller is indicated as red line in the curve for normal flow. From point A-C, the friction lose cause by nozzle whilst turbulence friction lose from point C-D at impeller center. The pressure increases from D-E as impeller is adding head into the liquid. As the flow increases from normal flow and the physical configuration is fixed, the frictional lose and turbulence friction lose will increase with flow. The pressure profile from entrance nozzle to pump impeller center and outlet will be shifted downward as indicated in blue line. The lowest pressure at the impeller center is getting closer or even cross the vapor pressure of the fluid. This potentially results cavitation.

Pump manufacturer would needs to design a pump with the Suction specific speed (Nss) below 10000 (normally) to **ensure a healthy life span as high Nss would leads to rough operation and shorten life span. Some manufacturer may even design for 8000-8500 .

Suction specific speed ( Nss) of a pump is a dimensionless number expressed as

Nss=( N* Q 0.5 ) / (NPSHr)0.75

Where
Nss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm)
NPSHr : Net Positive Suction Head required (ft)
Since the normal pump speed is 1450 rpm for low speed pump and 2950 rpm for high speed pump, it is obvious that the NPSHr is increased as flow rate increased.

Updated on Feb 21, 2008 : ***

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posted by Webworm, 1:51 PM

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