Useful information on specific speed and suction specific speed
What is Specific Speed?
Specific Speed (sometimes called Pump Specific Speed) is a dimensionless rating for centrifugal pump performance. Although it is not actually a speed, it depends on shaft speed and is also a function of the flow rate and differential head at the pump’s Best Efficiency Point (BEP – see What is the Best Efficiency Point?). Specific Speed is an important factor in the design and selection of pumps and especially pump impellers.
How is Specific Speed calculated?
The formula for Specific Speed is:
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is the specific speed (often shown as ‘S’ when used with US units) |
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is the rotation or shaft speed of the impeller (rpm) |
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is the flow rate, per impeller eye, at the BEP (US units: gpm; SI units: m3/hr); flow is taken for the largest impeller diameter available for a pump and should be halved for double inlet pumps) |
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is the differential head at the BEP (US units: ft; SI units: m) |
When doing calculations, it is important to ensure that compatible units are used. To convert values between US units (gpm) and SI units (m3/h, m3/s, l/s) the following factors can be used:
• S (US: gpm) = 1.63 Ns (SI: l/s) = 0.86 Ns (SI: m3/h)
• S (US: gpm) = 51.64 Ns (SI: m3/s)
• Ns (SI: m3/h) = 1.169 S (US: gpm)
• Ns (SI: l/s) = 0.614 S (US: gpm)
What is Specific Speed used for?
When selecting a pump for a particular application, and knowing the shaft speed, flow and differential head, a calculation of specific speed will identify the impeller shape most suited to the task (Figure 1).
If the specific speed is less than 40 (US units: 2000), radial impellers are suitable for the application. These produce high head through centrifugal action as the fluid is expelled perpendicular to the pump shaft. Compared with other impeller designs, however, flow rates are low. Efficiencies are also low because of high friction and mechanical losses. At the other end of the scale, axial flow impellers are preferred if the specific speed is greater than 175 (US units: 9000). With this design, fluid enters and is discharged parallel to the rotating shaft. This gives a high flow rate but low head.
With intermediate values, a mixed-flow type impeller, sharing elements of both radial and axial designs, is more suitable.
What is Suction Specific Speed?
Suction Specific Speed is a special case of the same equation with the value of NPSHR (Net Positive Suction Head Required) substituted for the differential head. Whereas Specific Speed relates to a pump’s discharge performance, Suction Specific Speed defines its suction characteristics. In particular, a value indicates the tendency of a pump to become unstable as a result of suction recirculation and cavitation. It can also be used to assess the safe operating range around the BEP.
The formula for Suction Specific Speed is:
Again, it is important to ensure that compatible units are used for the calculation and the same conversion factors apply.
What is the Best Efficiency Point?
As the pressure a centrifugal pump has to overcome increases, the discharge flow decreases until, at a certain head, the output drops to zero. Conversely, with no head to work against, a centrifugal pump can achieve the maximum possible output allowed by its design, impeller selection and rotational speed. The range of performance between these two points is specified in a pump curve (Figure 2).
A centrifugal pump has a best efficiency point (BEP) somewhere on its pump curve. These are the precise conditions, determined by the manufacturer, at which the pump operates with greatest efficiency and can be expected to have maximum working life and experience reduced maintenance. For a particular pump design, the performance can be modified by fitting a different impeller, by operating it at a different rotational speed, or by changing the suction side pipe diameter.
The pump curve describes how a centrifugal pump performs in isolation from plant equipment. How it operates in practice is determined by the resistance of the system it is installed in: restrictions in the pipework and downstream frictional losses as well as static inlet or outlet pressures. A graphical representation of these factors is called the system curve. This shows how the head pressure (at the location to be occupied by the pump) increases with increasing throughput.
By plotting the pump and system curves on the same graph (Figure 2), the intersection of the lines identifies the flow rate you can expect from the pump in this configuration. This intersection is called the operating point. Ideally, the operating point and best efficiency points should coincide. Over the lifetime of the pump this can have considerable cost benefits. The calculation of Suction Specific Speed allows you to identify ‘safe’ operating margins around the BEP.
A pump operating at a lower capacity is often said to be “operating to the left of the curve” and at a higher capacity is termed “operating to the right of the curve” (as is the case in Figure 2). These correspond to the relative positions on the pump curve on either side of the BEP.
To the left of the BEP, a pump’s throughput is lower than its design specification and the fluid may not flow correctly through the system. There is a danger of recirculation in the pump’s inlet and the impeller (particularly in the eye). This can lead to cavitation, vibration and seal wear.
At the right side of the BEP, a pump’s throughput is higher than its design specification and this may result in vibration and noise in the pump, placing greater strain on its drive shaft and other components, and also in downstream pipework. This can lead to greater maintenance costs and a higher incidence of pump failures.
What is Suction Specific Speed used for?
Industry experience has shown that a pump with a suction specific speed less than 120 (SI units) or 6000 (in US units) is in danger of recirculation and cavitation. Values above 210 (SI units) or 11,000 (US units) may lead to an increase in pump wear and maintenance. Studies have shown that the operating ranges around the BEP are narrower at these higher Suction Specific Speeds. However, there is no consensus in the industry regarding the ‘safe’ range of Suction Specific Speed values and each industry, pump manufacturer and operator may work to different limits.
Knowing the characteristics of a pump: flow rate and speed, and taking the minimum acceptable Suction Specific Speed, the equation can be rearranged to calculate NPSHR (Net Positive Suction Head Required) for the pump.
Summary
Specific Speed is a dimensionless rating of pump discharge performance derived from an equation involving shaft speed, flow rate and differential head at a pump’s Best Efficiency Point. It is an important factor used in the design and selection of pumps and impellers. Suction Specific Speed is a special case of the same equation used for assessing the safe range of flow rates around a pump’s BEP or, when working to a specific minimum Suction Specific Speed requirement, to calculate NPSHR for the pump.
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