Internal Recirculation Cavitation 1-3.3
Cavitation means that cavities or bubbles are forming in the liquid that we’re pumping. These cavities form at the low pressure or suction side of the pump, causing several things to happen all at once:
- The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration, and damage to many of the components.
- We experience a loss in capacity.
- The pump can no longer build the same head (pressure)
- The pump’s efficiency drops.
The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration, and damage to many of the components.
This condition is visible on the leading edge of the impeller, close to the outside diameter, working its way back to the middle of the vane. It can also be found at the suction eye of the pump.
As the name implies, the fluid recirculates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low NPSH pumps and the term Suction Specific Speed was coined to guide you in determining how close you have to operate to the B.E.P. of a pump to prevent the problem.
The higher the number the smaller the window in which you can operate. The numbers range between 3,000 and 20,000. Water pumps should stay between 3,000 and 12,000. Here is the formula to determine the suction specific speed number of your pump:
rpm = Pump speedCapacity = Gallons per minute, or liters per second of the largest impeller at its BEP
Head= Net positive suction head required (feet or meters) at that rpm
- For a double suction pump the flow is divided by 2 since there are 2 impeller eyes
- Try to buy pumps with a suction specific speed number lower than 8500.(5200 metric ) forget those over 12000 (8000 metric) except for extreme circumstances.
- Mixed hydrocarbons and hot water at 9000 to 12000 (5500 to 7300 metric) or higher, can probably operate satisfactorily.
- High specific speed indicates the impeller eye is larger than normal, and efficiency may be compromised to obtain a low NPSH required.
- Higher values of specific speed may require special designs, and operate with some cavitation.
- Normally a pump operating 50% below its best efficiency point (B.E.P.) is less reliable.
With an open impeller pump you can usually correct the internal recirculation problem by adjusting the impeller clearance to the manufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution seems to be to contact the manufacturer for an evaluation of the impeller design and a possible change in the design of the impeller or the wear ring clearances.