Micro Blog For Chemical & Process Technology: January 2009

Sunday, January 11, 2009

Pump Suction Specific Speed (Nss)

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

N_ss=( N* Q ^0.5 ) / (NPSHr)^0.75

Where

N_ss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm)
NPSHr : Net Positive Suction Head required (ft)

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

To achieve lower NPRHr;
- reduce pump speed (N)
- reduce flow i.e. double suction (Q)
- increase Suction Specific Speed (Nss)

Example :
Flow 2,500 gpm, head 800 ft, pump rotational speed 2950 rpm.
What NPSHr will be required?

N_ss = ( N* Q ^0.5 ) / (NPSHr)^0.75

10000 = (2950 * 2500 ^0.5 ) / (NPSHr)^0.75
NPSHr = 36.17 ft

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