The best way to describe the shape of an impeller is to use its specific speed number. This is a dimensionless number that was generated by the formula :
Ns = Specific speed
N = Pump shaft speed
Q = Capacity in GPM.
H = Total head in feet
The following chart gives you a graphic picture of the impeller shape represented by this number:
The major use of the specific speed number is to help you specify pumps that are more efficient.
- The maximum pump efficiency is obtained in the specific speed range of 2000 to 3000.
- Pumps for high head low capacity occupy the range 500 to 1000. While low head high capacity pumps may have a specific speed of 15,000 or larger.
- For a given head and capacity the good news is that the pump having the highest specific speed, that will meet the requirements, probably will be the smallest size and the least expensive. The bad news it that it will run at the highest speed where abrasive wear and cavitation damage become a problem.
- Efficiencies start dropping drastically at specific speeds below 1000. Also smaller capacities exhibit lower efficiencies than higher capacities at all specific speeds.
- In propeller and other high specific speed impellers (axial flow) it is not practical to use a volute casing. Instead, the impeller is enclosed in a pipe like casing.
- The lower the specific speed number, the higher the power loss you get with wear ring clearance.
The clearance between the impeller and the tongue of the volute has a bearing on efficiency, pressure pulsations and cavitation. For high efficiency you would want a small clearance, but this produces larger pressure pulsations and the increased flow in this area can reduce the fluid pressure enough to cause flashing of the product and a type of cavitation known as ” The vane passing syndrome”.
For impellers up to fourteen inches in diameter (355 mm) this clearance should be a minimum of four percent of the impeller diameter. If you are using greater than fourteen inch diameter impellers the clearance should be at least six percent of the impeller diameter. Also remember that as this clearance increases the impeller experiences some slippage. That is the major reason that we do not like to remove more than ten percent of the impeller diameter when trimming is called for.
If you work in both metric and imperial units as I do, the subject of specific speed becomes very confusing because both systems use the same specific speed numbers to describe the impeller shape. They do this even though they use a different set of units to arrive at the same number.
In the metric system the capacity is calculated in liters/ minute and the head in meters. Knowledgeable people in this area feel that if the calculations are done in imperial or other metric units the final number should be reduced by the following amount:
U.S. Gallons/ minute and feet divide the result by 1.63
U.K. Imperial gallons and feet divide the result by 1.93
M3/hour and meters divide the result by 1.50
SUCTION SPECIFIC SPEED is another number that we use in pump selection. The formula looks the same as the regular specific speed formula, but in this formula we use the NPSH required number rather than the total head produced by the pump.
Ns = Specific speed
N = Pump shaft speed
Q = Capacity in GPM.
NPSH = Net positive suction head required to prevent cavitation. Remember that this number is for sixty eight degree F. (20°C) fresh water. You are going to have to add the vapor pressure of you product to this number to get the real number that you will be using.
We use this number to predict cavitation problems with your impeller selection.
- The flow angle of the inlet vanes and the number of vanes affect this number.
- A desired value would be below 8500 with impellers having a flow angle of about seventeen degrees and five to seven vanes. The higher the flow angle number, the faster the liquid will travel and the lower suction head (pressure) we will get.
- Boiler feed and condensate pumps often require suction specific speed numbers as high as 12,000 to 18,000 because of the temperature and pressure of the water. To get to these values the impeller inlet flow angle is reduced to a low as ten degrees and the number of vanes reduced to as little as four . Fewer and thinner vanes help to reduce the blockage in the impeller inlet. A disadvantage to these low flow angles is that the pump will probably run very rough at below fifty percent of capacity.
- Water applications can run at these higher numbers because the amount of fluid expansion is very low for hot water. Mixed hydro-carbons have this same advantage because unlike a single product, the flashing of the mixed hydro-carbons does not take place all at the same time.
- The higher the suction specific speed number the narrower the stable window of operation.
- Inducers have been used successfully with suction specific speed numbers of approximately 24, 000
- Should the available NPSH be so low that a suction specific speed number of more than 18,000 is required, then a separate axial flow impeller (an inducer) can be used ahead of the centrifugal impeller to prevent cavitation. Its flow angle is some where between five and ten degrees with typically two vanes and no more than four. In other instances a booster pump can be installed between the pump and the source.
- In their desire to quote a low NPSH required some manufacturers will cut away the impeller inlet vanes to reduce fluid drag and thereby lower the NPSH required. If this has been done with your application, you must insure that the impeller to volute clearance is adjusted correctly with open impeller designs and the wear ring clearance meets the manufacturers specifications with closed impeller designs, or you will experience internal recirculation problems and cavitation at the impeller outlet vane tips. Keep the suction specific speed number below 8500 and this problem should never comes up.
A pump’s suction specific speed (SSS) number is a constant. You can re-arrange the formula to calculate a new NPSHR: