NPSH and Suction Specific Speed

In designing a pumping system, it is essential to provide adequate NPSH available for proper pump operation. Insufficient NPSH available may seriously restrict pump selection, or even force an expensive system redesign. On the other hand, providing excessive NPSH available may needlessly increase system cost.

Suction specific speed may provide help in this situation.

Suction specific speed (S) is defined as:

Where
N = Pump speed RPM
GPM = Pump flow at best efficiency point at impeller inlet (for double suction impellers divide total pump flow by two).
NPSHR = Pump NPSH required at best efficiency point. For a given pump, the suction specific speed is generally a constant - it does not change when the pump speed is changed. Experience has shown that 9000 is a reasonable value of suction specific speed. Pumps with a minimum suction specific speed of 9000 are readily available, and are not normally subject to severe operating restrictions, unless the pump speed pushes the pump into high or very high suction energy.

An example:
Flow 2,000 GPM; head 600 ft. What NPSHA will be required?

Assume: at 600 ft., 3500 RPM operation will be required.

A related problem is in selecting a new pump, especially at higher flow, for an existing system. Suction specific speed will highlight applications where NPSHA may restrict pump selection. An example:

Existing system: Flow 2000 GPM; head 600 ft.; NPSHA 30 ft.; Specific Gravity 1.0; Suction Nozzle 6 in. - What is the maximum speed at which a pump can be run without exceeding NPSH available? (NPSHMargin Ratio = 1.5 from above @ S.E. = 173 x 106)

Running a pump at this speed would require a gear and at this speed, the pump might not develop the required head. At a mini-mum, existing NPSH A is constraining pump selection.

Same system as 1. Is a double suction pump practical?
For a double suction pump De = .75 x 6" = 4.5
S.E. = 4.5 x 3550 x 9000 x 1.0
S.E. = 136 x 106 (High S.E.)

For a double suction pump, flow is divided by two.

Using a double suction pump is one way of meeting system NPSH and obtaining a higher head.

The amount of energy in a pumped fluid, that flashes into vapor and then collapses back to a liquid in the higher pressure area of the impeller inlet, determines the extent of the noise and/or damage from cavitation. Suction Energy is defined as:

Suction Energy = De x N x S x Sg

Where D e = Impeller eye diameter (inches)
Sg = Specific gravity of liquid (Sg - 1.0 for cold water)

High Suction Energy starts at 160 x 10 6 for end suctabtion pumps and 120 x 10 6 for horizontal split case pumps. Very high suction energy starts at 1.5 times the High Suction Energy values. For estimating purposes you can normally assume that the impeller eye diameter is approximately 90% of the suction nozzle size, for an end suction pump, and 75% of the suction size for a double suction split case pump.

According to the Hydraulic Institute, ans NPSH margin is required above the NPSHR of the pump to supress incipient cavitation. The amount of margin is a function of Suction Energy and the critical nature of the application as follows:

 Suction Energy NPSHMargin Ratio (NPSHA/NPSHR) Low 1.1 - 1.3 High 1.2 - 1.7 Very High 1.7 - 2.5
Suction specific speed 9,000, pump speed 3550 RPM, suction nozzle size 6 inch, specific gravity 1.0, and the pump type is end suction.

De ~ .9 x 6" = 5.4"
Suction Energy = De x N x S x Sg
= 5.4 x 3550 x 9,000 x 1.0
= 173 x 106

Since 173 x 106 > 160 x 106 , this is a High Suction Energy pump.