Specific Speed and Suction Specific Speed for Centrifugal Pumps

10 Mar

Specific Speed and Suction Specific Speed for Centrifugal Pumps

Specific speed and suction specific speed are two ways to characterize speed with turbomachinery. Both have applications for centrifugal pumps with PIPE-FLO software.

Here we define each of these terms and determine their relationship with centrifugal pumps.

Specific Speed (N_s) Formula and Application

Specific speed is a dimensionless index that numerically describes the geometry of a pump’s impeller and classifies the pump as to its type. The equation for specific speed (designated N_s) is:

Where:

N = pump rotational speed (rpm)
Q = pump capacity at BEP, full diameter (gpm)
H = pump head per stage at BEP, full diameter (ft)

The most common types of centrifugal pumps develop head by redirecting the liquid flow radially away from the centerline of the shaft. These are referred to as radial flow pumps, and typically have low specific speeds of about 2000 or less. Common characteristics of radial flow pumps are relatively low flow and high head.

At the other end of the spectrum are axial flow pumps which develop head through axial forces. These are also referred to as propeller pumps, and typically have high specific speeds of about 8000 or more. Common characteristics of axial flow pumps are high flow and low head.

In between these two types of pumps are the mixed flow pumps which are neither pure radial flow nor pure axial flow. Instead, they are some combination of the two and include Francis vane impellers. The specific speeds for mixed flow pumps range between 2000 and 8000.

Suction Specific Speed (Nss) Formula and Application

Suction specific speed is an index that describes the characteristics of the suction side of the impeller. It’s calculated using the NPSHr in feet, speed (N) in rpm, and flow rate (Q) in gpm at the pump’s best efficiency point and maximum impeller diameter. The formula for suction specific speed (designated N_ss) is:

From the equation we can see that the lower the NPSHr for a pump the higher the N_ss. N_ss values for many standard impellers typically range from 7000 to 9000, but some designs may have an N_ss as high as 18,000 to 20,000.

For the pump user, it is important to consider that increasing the N_ss of a pump has been shown to shift the onset of suction or discharge recirculation closer to the best efficiency point (BEP) flow of the pump. This effectively decreases the window of stable operation for the pump. Suction recirculation is a reversal of flow in the impeller eye that can lead to increased noise, surges, and cavitation-like damage on the pressure side of the impeller vane. Discharge recirculation is a similar reversal of flow occurring at the discharge of the impeller vane. All centrifugal pumps will exhibit recirculation if the flow rate is sufficiently reduced.

Studies on Suction Specific Speed and Recirculation

Many studies on recirculation have produced graphs and equations that indicate the minimum continuous flow or stable operating region (as a percentage of BEP flow) based on the N_ss of a pump. There are a number of researchers, companies, and organizations that specify maximum N_ss recommendations for pump selections.

For instance, an article by Karassik recommended an N_ss of no more than 9,000 for water and 11,000 for hydrocarbons. A number of resources also suggest that suction and discharge recirculation is not a concern for lower horsepower (<100 hp) pumps.

Overall, there is not a consensus on N_ss recommendations for pumps and there is some debate over when the onset of recirculation occurs and what factors predict it.

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