CAVITATION

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Cavitation

Cavitation means different things to different people. It has been described as:

• A reduction in pump capacity.
• A reduction in the head of the pump.
• The formation of bubbles in a low pressure area of the pump volute.
• A noise that can be heard when the pump is running.
• Damaged that can be seen on the pump impeller and volute.

The capacity of the pump is reduced

• This happens because bubbles take up space and you cannot have bubbles and liquid in the same place at the same time.
• If the bubble gets big enough at the eye of the impeller, the pump will lose its suction and will require priming.

The head is often reduced

• Bubbles unlike liquid are compressible. It is this compression that can change the head.

The bubbles form in a lower pressure area because they cannot form in a high pressure area.

• As the velocity of a fluid increase, the pressure of the fluid decreases. This means that high velocity liquid is by definition a lower pressure area. This can be a problem any time a liquid flows through a restriction in the piping, volute, or changes direction suddenly. The fluid will accelerate as it changes direction. The same acceleration takes place as the fluid flows in the small area between the tip of the impeller and the volute cut water.

A noise is heard

• Any time a fluid moves faster than the speed of sound, in the medium you are pumping, a sonic boom will be heard. The speed of sound in water is 4800 feet per second (1480 meters/sec) or 3,273 miles per hour (5,267 kilometers per hour).

Pump parts show damage

• The bubble tries to collapse on its self. This is called imploding, the opposite of exploding. The bubble is trying to collapse from all sides, but if the bubble is laying against a piece of metal such as the impeller or volute it cannot collapse from that side, so the fluid comes in from the opposite side at this high velocity proceeded by a shock wave that can cause all kinds of damage. There is a very characteristic round shape to the liquid as it bangs against the metal creating the impression that the metal was hit with a "ball peen hammer".
• This damage would normally occur at right angles to the metal, but experience shows that the high velocity liquid seems to come at the metal from a variety of angles. This can be explained by the fact that dirt particles get stuck on the surface of the bubble and are held there by the surface tension of the fluid. Since the dirt particle has weakened the surface tension of the bubble it becomes the weakest part and the section where the collapse will probably take place.
• The pressure of the implosion has been calculated to be in the region of 1000 bar, with velocities around 4000 m/s.

To cure cavitation problems you must either increase the suction head, lower the fluid temperature or decrease the N.P.S.H. Required.

Increase the suction head

• Raise the liquid level in the tank
• Raise the tank
• Put the pump in a well
• 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.
• Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather
• 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.
• A 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 gate valve.
• A gasket is protruding into the piping.
• The pump speed has increased.
• Install a booster pump
• Pressurise the tank

Lower the fluid temperature

• Injecting a small amount of cooler fluid at the suction is often practical.
• Insulate the piping from local heat sources.
• Minimise discharge recirculation lines, they can heat up the suction fluid.

Reduce the N.P.S.H. Required

• Use a double suction pump. Use a lower speed pump
• Use a pump with a larger impeller eye opening.
• If possible install an Inducer. These inducers can cut N.P.S.H.R. 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.

Both cavitation and air ingestion have an affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.