INTERNAL RECIRCULATION CAVITATION PT009
This condition is visible on the leading edge of the impeller and will usually be found at the discharge tip working its way back to the suction. It can also be found at the suction eye of the pump.
As the name implies the fluid re-circulates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low net positive suction head required (NPSHR) pumps and the term “suction specific speed” was coined to give you a guide in determining how close you have to operate to the best efficiency point (BEP) of a pump to prevent the problem.
The higher the numbers, the smaller the windows in which you have to operate. The numbers range between 3,000 and 20,000 (1,800 to 12,000 metric). Water pumps should stay between 3,000 and 12,000 (1,800 and 7,400 metric). .
- Ns = Specific speed
- N = Pump shaft speed
- Q = Capacity in gpm. or liters per second
- NPSH = is the net positive suction head required (NPSHR) by the pump to prevent cavitation. The measurement is either in feet or meters.
We use this number to predict possible 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 (5200 in metric) 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 (7400 metric) to 18,000 (11,000 to 12,000 metric) 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 numbers the narrower the stable window of operation.
- Should the net positive suction head available (NPSHA) be so low that a suction specific speed number of more than 18,000 (11,000 metric) is required, then a separate axial flow impeller (an inducer) can be used ahead of the centrifugal impeller to prevent cavitation. An inducer has a flow angle some where between five and ten degrees with typically two vanes and no more than four. Inducers have been used successfully with suction specific speed numbers of approximately 24,000 (14,400 metric). 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 (NPSHR). 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 come up.