Vaporization Cavitation 1-3.1
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.
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
- 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 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 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 often practical.
- Insulate the piping from the sun’s rays.
- Be careful of discharge recirculation lines. They can heat the suction fluid.
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 it.