19-03 Cavitation

We recognize four separate types of cavitation when dealing with centrifugal pumps:

Symptoms

Why a noise?

Why does it do damage?

Why not at ninety degrees?

Types:

VAPORIZAION CAVITATION,

Bubbles will form when the fluid temperature gets too high or the fluid pressure becomes to low. Pump cavitation occurs at the suction side of the pump.

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

Lower the fluid inlet temperature

Decrease the fluid velocity

 Reduce the net positive suction head required (NPSHR)

Just as a matter of note: We also see evidence of cavitation in valves caused by the fluid accelerating through the valve, lowering its pressure causing bubbles, and the bubbles then collapsing on the downstream of the valve

SUCTION SPECIFIC SPEED

Suction specific speed problems are recognized by a random crackling noise around the pump suction, accompanied by high intensity knocks.

The main function of the suction specific speed number is to predict a special cavitation problem. The formula looks the same as the specific speed formula, but in this formula we use the net positive suction head required (NPSHR) number rather than the total head produced by the pump.

Ns = Specific speed

N = Pump shaft speed

Q = Capacity in GPM.

NPSH = Net positive suction head required to prevent cavitation. Remember that this number is for 68°F. (20°C.) fresh water. You are going to have to add the vapor pressure of you product to this number to get the real number that you will be using.

As mentioned in the above paragraph, we use this number to predict cavitation problems with your impeller selection.

VANE PASSING SYNDROME

 

N = Pump shaft speed
Q = Capacity in gpm.
NPSH = is the net positive suction head required (NPSHR) by the pump in feet.

FLOW TURBULENCE.

Good piping layouts would include:

AIR INGESTION is not cavitation, but it resembles it.

The effect of air sucked into the pump is twofold.

A centrifugal pump can handle 0.5% air by volume. At 6.0% air, the results can be disastrous. Air gets into a piping system several ways that include:

Some things you can do:

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