Introduction: 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 form for five basic reasons and its common practice to lump all of them into the general classification of cavitation. This is an error because we'll learn that to correct each of these conditions; we must understand why they occur and how to fix them. Here they are in no particular order:
- Air ingestion (Not really cavitation, but has similar symptoms)
- Internal recirculation
- Flow turbulence
- The Vane Passing Syndrome
Vaporization: A fluid vaporizes when its pressure becomes too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (Twenty degrees Centigrade) is the fluid being pumped. Since there are losses in the piping leading from the source to the suction of the pump, we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing
Air ingestion (Not really cavitation, but acts like it): A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into as system in several ways that include:
- Through the packing stuffing box. This occurs in any packed pump that lifts liquid, pumps from a condenser, evaporator, or any piece of equipment that runs in vacuum.
- Valves located above the water line.
- Through leaking flanges.
- Pulling air through a vortexing fluid.
Internal Recirculation: 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 to guide you in determining how close you have to operate to the B.E.P. of a pump to prevent the problem.
Turbulence: We always prefer to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions can change the velocity of this liquid, and any time you change the velocity of a liquid, you change its pressure. Good piping layouts would include:
Ten diameters of pipe between the pump suction and the first elbow.
In multiple pump arrangements locate the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical, a number of units can be installed in a single large sump provided that :
- The pumps should be positioned in a line perpendicular to the approaching flow.
- There must be a minimum spacing of at least two suction diameters between pump center lines.
- All pumps are running.
The Vane Passing Syndrome: This type of cavitation damage is caused when the OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, lowering the fluid pressure and causing local vaporization. The bubbles then collapse at the higher pressure just beyond the cutwater. This is where you should look for volute damage. You'll need a flashlight and mirror to see the damage, unless it has penetrated to the outside of the volute. The damage is limited to the center of the impeller vane. If it's a closed impeller, the damage will not extend into the shrouds. You can prevent this problem, if you keep a minimum impeller tip to cutwater clearance of 4% of the impeller diameter in the smaller impeller sizes (less than 14' or 355 mm.) and a 6% clearance in the larger impeller sizes (greater than 14" or 355 mm.).
To prevent excessive shaft movement, some manufacturers install bulkhead rings in the suction eye. At the discharge side, rings can be manufactured to extend from the walls to the impeller shrouds.