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
- 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.
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
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
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
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
- Raise the liquid level in the tank
- Raise the tank
- Pressurize the tank
- Place 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
- 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 formed 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 heating jacket has frozen and collapsed the pipe.
- A gasket is protruding into the piping.
- The pump speed has increased.
- Install a booster pump
Lower the pumping fluid temperature
- Injecting a small amount of cooler fluid at the suction is
- Insulate the piping from the sun's rays.
- Be careful of discharge recirculation lines. They can heat the
Reduce the N.P.S.H. Required
- Use a double suction pump. This can reduce the N.P.S.H.R. by
as much as 25%, or in some cases it will allow you to raise the
pump speed by 40%
- Use a slower 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.
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
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