VAPORIZATION CAVITATION V009
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 head requirement 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 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 (NPSHR) to prevent the fluid from vaporizing.
We take the net positive suction head available (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)
To cure vaporization problems you must either 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.
- 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 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 broken gate valve. Globe valves have a higher K factor than gate valves and present more fluid resistance.
- A heating jacket has frozen and collapsed the pipe.
- A gasket is protruding into the piping.
- The pump rpm has increased. Retrofit the pump with a higher specific speed impeller.
- Install a booster pump or inducer.
- Pressurize the suction tank.
- Be sure the tank vent is open and not obstructed. Some vents can freeze in cold weather.
Lower the fluid inlet temperature
- Injecting a small amount of cooler fluid at the suction is often practical.
- Insulate the suction piping from the sun’s rays.
- Be careful of discharge recirculation and vent lines recirculated to the pump suction; they can heat up the suction fluid.
Reduce the net positive suction head required (NPSHR)
- Use a double suction pump. Double suction designs can reduce the net positive suction head required (NPSHR) by as much as 27%, or in some cases it will allow you to raise the pump speed by 41%
- Use a 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 (NPSHR) 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 required (NPSHR) requirements or 10 feet (3 meters), whichever is smaller. I would suggest you use this as a safety margin rather than design for it.