There is a multitude of pump designs that are available for any given task. Pump designers have needed a way to compare the efficiency of their designs across a large range of pump model and types. Pump users also would like to know what efficiency can be expected from a particular pump design. For that purpose pump have been tested and compared using a number or criteria called the specific speed (NS) which helps to do these comparisons. The efficiency of pumps with the same specific speed can be compared providing the user or the designer a starting point for comparison or as a benchmark for improving the design and increase the efficiency. The next equation gives the value for the pump specific speed, H is the pump total head, N the speed of the impeller and Q the flow rate.
Pumps are traditionally divided into 3 types, radial flow, mixed flow and axial flow. There is a continuous change from the radial flow impeller, which develops pressure principally from the action of centrifugal force, to the axial flow impeller, which develops most of its head by the propelling or lifting action of the vanes on the liquid.
Specific speed has also been used as a criteria for evaluating the efficiency of standard volute pumps (see next Figure). Notice that larger pumps are inherently more efficient and that efficiency drops rapidly at specific speeds of 1000 or less.
SUCTION SPECIFIC SPEED
Suction specific speed is a number that is dimensionally similar to the pump specific speed and is used as a guide to prevent cavitation.
Instead of using the total head of the pump H, the N.P.S.H.A (Net Positive Suction Head available) is used. Also if the pump is a double suction pump then the flow value to be used is one half the total pump output.
From the previous article on cavitation, the N.P.S.H.A at the pump suction is :
where HA and Hva are in feet of fluid. The above equation requires that the piping (HF1-S) friction loss and equipment friction loss (HEQ1-S) be calculated. The meaning of some of the variables in the above equation is shown in the next Figure.
We can avoid doing the calculations in the above equation by measuring the N.P.S.H.. The value for the N.P.S.H.A can be deduced by taking a pressure measurement at the pump inlet and using the next equation
We may be considering an increase in the pump’s speed to increase the flow rate. If so, be aware that an increase in speed will also require an increase in N.P.S.H. required. The suction specific speed value give us an indication of what the impeller speed limitation will be for a given N.P.S.H.A . The Hydraulic Institute recommends that the suction specific speed be limited to 8500 to avoid cavitation.
When a pump has a high suction specific speed value, it can mean that the impeller inlet area is large reducing the inlet velocity which is needed to enable a low NPSHR. However, if you continue to increase the impeller inlet area (to reduce NPSHR), you will reach a point where the inlet area is too large resulting in suction recirculation (hydraulically unstable causing vibration, cavitation, erosion etc..). The recommended cap on the S value is to avoid reaching that point. (paragraph contributed by Mike Tan of the pump forum group).
Keeping the suction specific speed below 8500 is also a way of determining the maximum speed of a pump and avoiding cavitation.
According to the Hydraulic Institute the efficiency of the pump is maximum when the suction specific speed is between 2000 and 4000. When S lies outside this range the efficiency must be de-rated according to the following figure.
The following graph represents the value of the Thoma cavitation parameter (sigma) vs. the pump specific speed and the suction specific speed. This chart can be found in the Pump Handbook published by McGraw Hill. It can predict the onset of cavitation and you can use it to help you diagnose if your pump is cavitating. You will find an article on specific speed and suction specific speed as well as many other related articles on the web site of Light my pump at www.pumpfundamentals.com/downloads-free.htm
The value of the Thoma sigma number is given in this image from the Pump Handbook.
The applet allows you to navigate this graph and displays the values of the Thoma sigma number, the specific speed and the suction specific speed right above the image. This way you don't have to strain your eyes. You can compare the values that you obtain from this graph with the values that you have entered on the right which are pertinent to your system. Before you use the Calculate button you must enter the flow rate, the head, the rpm and the N.P.S.H.A. of your pump. I have put in some typical values as a start. If you change these values, and you press the Calculate button, you will obtain new values for the specific speed and the suction specific speed.
As you can see there is a safe region in the upper left corner of the graph. If your calculations for the specific speed and the suction specific speed indicate that you are in that region then everything should be fine. The lower right region is unsafe and if you are in that region there is no doubt that the pump will cavitate. In the middle is a gray zone where you may or may not cavitate.