CAVITATION, HOW TO STOP IT. C022
Cavitation is about the forming and collapsing of bubbles. Bubbles will form when the fluid temperature gets too high or the fluid pressure becomes to low.
To cure vaporization problems you must either increase the suction head, lower the fluid temperature, decrease the fluid velocity, or decrease the net positive suction head required (NPSHR). We shall look at each possibility:
How to increase the suction head
- Raise the liquid level in the tank
- Elevate the supply tank.
- Put the pump in a pit.
- Reduce the piping losses. These losses occur for a variety of reasons that include :
- The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.
- A pipe liner has collapsed.
- Solids have built up on the inside of the pipe.
- The suction pipe collapsed when it was run over by a heavy vehicle.
- A suction strainer is clogged
- Something is stuck in the pipe. It either grew there or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe.
- The inside of the pipe, or a fitting has corroded.
- bigger pump has been installed and the existing system has too much loss for the increased capacity.
- A globe valve was used to replace a broken gate valve.
- A heating jacket has frozen and collapsed the pipe.
- A gasket is protruding into the piping.
- The pump rpm has increased.
- Retrofit the pump with a higher specific speed impeller.
- Install a booster pump or inducer.
- Pressurize the tank.
- Be sure the tank vent is open and not obstructed. Some vents can freeze in cold weather.
Lower the fluid inlet temperature
- Injecting a small amount of cooler fluid at the suction is often practical.
- Insulate the suction piping from the sun’s rays.
- Be careful of discharge re-circulation and vent lines re-circulated to the pump suction; they can heat up the suction fluid.
Decrease the fluid velocity
- Remove obstructions in the suction piping
- Do not run the impeller too close to the pump cutwater.
- Reduce the speed of the pump.
- Reduce the capacity of the pump.
- Do not install an elbow too close to the pump suction.
Reduce the net positive suction head required (NPSHR)
- Use a double suction pump. Double suction designs can reduce the net positive suction head required (NPSHR) by as much as 27%, or in some cases it will allow you to raise the pump speed by 41%
- Use a lower speed pump.
- Use a pump with a larger impeller eye opening.
- If possible install an inducer. These inducers can cut net positive suction head required (NPSHR) by almost 50%.
- Use several smaller pumps. Three half-capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter loads.
It is a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water net positive suction head required (NPSHR) requirements or 10 feet (3 meters), whichever is smaller.
I would suggest you use this as a safety margin rather than design for it.
We recognize four separate types of cavitation when dealing with centrifugal pumps:
- Vaporization cavitation.
- Internal recirculation cavitation.
- Flow turbulence cavitation.
- Vane Passing Syndrome cavitation.