VAPORIZATION CAVITATION PT007
A fluid vaporizes when its pressure gets 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 net positive suction head required (NPSHR) is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (20 degrees Centigrade) is the fluid being pumped.
Since there are losses in the piping 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 (NPSHR) to prevent the fluid from vaporizing.
We take the net positive suction head available (NPSHA), subtract the vapor pressure of the product we are pumping, along with the losses in the suction lines and this number must be equal to or greater than the net positive suction head required (NPSHR)
If you find this somewhat confusing, please look in the alphabetical section for an article on how to calculate net positive suction head available (NPSHA)
To cure vaporization problems you must increase the suction head, lower the fluid temperature, or decrease the net positive suction head required (NPSHR). We shall look at each possibility:
How to increase the suction head
- Raise the liquid level in the tank
- Elevate the supply tank.
- Put the pump in a pit.
- Retrofit the pump with a higher specific speed impeller. This will lower the NPSH required.
- Install a booster pump between the suction tank and the pump
- Pressurize the suction tank
- 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
- Something is stuck in the pipe. It either grew there or was left 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.
Lower the fluid inlet temperature
- Injecting a small amount of cooler fluid at the suction is sometimes practical.
- If the system is located outside in the weather, you can insulate the piping from the sun’s rays.
- Be careful of discharge recirculation and vent lines recirculated to the pump suction. These lines can heat up the fluid at the pump’s suction.
Reduce the net positive suction head required (NPSHR)
- Use a double suction pump. This can reduce the net positive suction head required by as much as 27%, or in some cases it will allow you to raise the pump speed by 41%
- Use a bigger lower speed pump.
- Use a pump with a larger impeller eye opening.
- If possible install an inducer. These inducers can cut net positive suction head required 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 is a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water net positive suction head requirements or 10 feet (3 meters), whichever is smaller. This is because mixed hydrocarbons do not all vaporize at the same time and hot water expands to a lower volume than cold water. I would suggest you use this as a safety margin rather than design for it.
- Air ingestion (acts like cavitation).
- Internal recirculation cavitation.
- Flow turbulence cavitation.
- Vane Passing Syndrome cavitation.