SUCTION SPECIFIC SPEED S105
Suction specific speed problems are recognized by a random crackling noise around the pump suction, accompanied by high intensity knocks.
The main function of the suction specific speed number is to predict a special cavitation problem. The formula looks the same as the specific speed formula, but in this formula we use the net positive suction head required (NPSHR) number rather than the total head produced by the pump.
- Q = Capacity in GPM with largest impeller at the BEP.
- NPSH = Net positive suction head required to prevent cavitation. Remember that this number is for 68°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.
- To raise a number to the 3/4 power, cube the number and then take the square root twice.
As mentioned in the above paragraph, we use this number to predict cavitation problems with your impeller selection.
- The flow angle of the inlet vanes and the number of vanes will 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 hydrocarbons have this same advantage because unlike a single product, the flashing of the mixed hydrocarbons 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 net positive suction head required (NPSHR) some manufacturers will cut away the impeller inlet vanes to reduce fluid drag and thereby lower the net positive suction head 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.
- In the metric system we calculate the capacity in liters/sec and the NPSH in meters. You should try to keep the final SSS number below 5200. Above 7800 you are going to have trouble with internal recirculation and cavitation.