SUBJECT : Cavitation 1-3

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 form for five basic reasons and it's 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 :

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.

We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:

Increase the suction head

Lower the pumping fluid temperature

Reduce the N.P.S.H. Required

It's a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water N.P.S.H. requirements, or 10 feet (3 meters), whichever is smaller. I would suggest you use this as a safety margin, rather than design for it.

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 :

Both vaporization and air ingestion have an adverse 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.

Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

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.

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 speed

Capacity = 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

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.

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 :

The metric numbers are :

FLOW
MINIMUM SUBMERGENCE

4,500 M3/HR

1.2 METERS

22,500 M3/HR

2.5 METERS

40,000 M3/HR

3.0 METERS

45,000 M3/HR

3.4 METERS

55,000 M3/HR

3.7 METERS

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.

For information about my CD with over 600 Seal & Pump Subjects explained, click here  

 Link to Mc Nally home page www.mcnallyinstitute.com