CENTRIFUGAL PUMPS - Modern Design and Practices

Suction specific speed

Suction specific speed (Nss) is a dimensionless number, or index, that defines the suction hydraulics, or suction characteristics, of a centrifugal pump. The term Nss is a misnomer because it is not a unit of speed. It is calculated from the equation:

Nss = [ (N x Q^0.5) / (NPSHR3)^0.75 ]

where:

N = speed of rotation in RPM

Q = capacity in GPM at maximum impeller BEP (for double suction impeller, Q=GPM/2)

NPSHR3 = the NPSHR in feet based on 3% head loss at BEP

BEP is defined as the best efficiency point, or the capacity with the highest efficiency on the pump curve, at maximum impeller diameter (not at the rated diameter).

In a two-stage, or multistage pump, NPSHR3 is based on a 3% head loss of the first stage impeller. The NPSHR of the series stage impeller does not affect NPSHR3. In pumps with dissimilar first stage and series stage impellers it is acceptable to estimate the head of the first stage impeller as it may be impractical to isolate and measure the actual first stage head such as in double barrel pumps.

NPSHR3 is the default basis for determining NPSHR and it allows for an apple-to-apple comparison of the suction characteristics of similar pumps. Aside from NPSHR3, and in specialized high-energy service, some end users may require the NPSHR to be based on one percent or zero percent head loss.

Example:

What is the suction specific speed of a double suction pump operating at 3560 RPM with 18 FT of NPSHR3 at its BEP of 800 GPM?

Solution: 

Nss = [3560 x (800/2)^0.50 / (18)^0.75] = 8148

Some practical applications of NSS are:

1. It  is commonly used as a basis for estimating the allowable operating      range (AOR) for a pump. The higher  the NSS is, the narrower is its AOR as a percentage flow range relative to BEP. Many users prefer their pumps to have an NSS in the range of 8000 to 11000 for improved MTBF or trouble-free operation.
2. It is commonly used as indicator if a pump were susceptible to develop      internal suction flow recirculation problem. The higher the NSS number,      the higher is its susceptibility.
3. It      is a good indicator if a hydraulic re-rate to reduce NPSHR3 is feasible. A      system NPSHA may be reduced over time due to increased friction loss as      pipes and fittings aged over time. In such instances the pump NPSHR3 may      have to be reduced.
4. It is used as index to compare the suction characteristics of pumps under      consideration by an end user to evaluate whether an assessment is to be      added or deducted to the pumps, in a similar manner that an assessment is      made based on the pumps' shaft flexibility factors (SFF).

As the term implies, the suction specific speed of a pump is mainly influenced by its suction design geometry its suction nozzle diameter, the casing suction bay area development, the impeller suction eye area, eye inlet diameter, and suction vane angle

The second part of this article includes discussions on:

1. The two types of suction specific speeds (NSS) that are distinct from one      another and why it is important to understand their differences and      implications.
2. Why the 11,000 NSS limit is misguided and can be very costly to pump end      users.
3. How  pumps with NSS of over 11,000 can be selected for safe and reliable      operation.
4. Why an unusually low NSS may actually be an indication of poor suction      hydraulic design and should be avoided.