Cavitation cheat sheet

Cavitation 19-03 

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

  • Vaporization cavitation.
  • Internal re-circulation cavitation (Suction specific speed).
  • Flow turbulence cavitation.
  • Vane Passing Syndrome cavitation.
  • Air ingestion resembles cavitation, but it is not as damaging

Symptoms

  • A reduction in pump capacity.
  • A reduction in the head of the pump.
  • The formation of bubbles in a low-pressure area of the pump volute.
  • A noise that can be heard when the pump is running.
  • Damaged that can be seen on the pump impeller and volute.

Why a noise?

  • The bubble is collapsing at the speed of sound in the medium you are pumping.
    • 1090 feet/sec in air or 743 miles/hour
    • 4800 feet/sec in water or 3,300 miles/hr

Why does it do damage?

  • Bubble implodes, but if against something solid, it collapses in from opposite side

Why not at ninety degrees?

  • Inclusions cause weak spots in the bubble

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

  • 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.
  • The pump suction is too far away from the supply tank.
  • 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.
  • 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 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 re-circulation and vent lines re-circulated to the pump suction; they can heat up the suction fluid.

Decrease the fluid velocity

  • Remove obstructions in the suction piping
  • Do not run the impeller too close to the pump cutwater.
  • Reduce the speed of the pump.
  • Reduce the capacity of the pump.
  • Do not install an elbow too close to the pump suction.

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.

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 68В°F. (20В°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.

  • The flow angle of the inlet vanes and the number of vanes will affect this number.
  • A desired value would be below 8500 with impellers having a flow angle of about seventeen degrees and five to seven vanes. The higher the flow angle number, the faster the liquid will travel and the lower suction head (pressure) we will get.
  • Boiler feed and condensate pumps often require suction specific speed numbers as high as 12,000 to 18,000 because of the temperature and pressure of the water. To get to these values the impeller inlet flow angle is reduced to a low as ten degrees and the number of vanes reduced to as little as four. Fewer and thinner vanes help to reduce the blockage in the impeller inlet. A disadvantage to these low flow angles is that the pump will probably run very rough at below fifty percent of capacity.
  • Water applications can run at these higher numbers because the amount of fluid expansion is very low for hot water. Mixed hydrocarbons have this same advantage because unlike a single product, the flashing of the mixed hydrocarbons does not take place all at the same time.
  • The higher the suction specific speed number, the narrower the stable window of operation.
  • Inducers have been used successfully with suction specific speed numbers of approximately 24,000
  • Should the available NPSH be so low that a suction specific speed number of more than 18,000 is required, then a separate axial flow impeller (an inducer) can be used ahead of the centrifugal impeller to prevent cavitation. Its flow angle is some where between five and ten degrees with typically two vanes and no more than four.  In other instances a booster pump can be installed between the pump and the source.
  • In their desire to quote a low net positive suction head required (NPSHR) some manufacturers will cut away the impeller inlet vanes to reduce fluid drag and thereby lower the net positive suction head required. If this has been done with your application, you must insure that the impeller to volute clearance is adjusted correctly with open impeller designs, and the wear ring clearance meets the manufacturers specifications with closed impeller designs, or you will experience internal recirculation problems and cavitation at the impeller outlet vane tips. Keep the suction specific speed number below 8500 and this problem should never comes up.
  • In the metric system we calculate the capacity in liters/sec and the NPSH in meters. You should try to keep the final SSS number below 5200. Above 7800 you are going to have trouble with internal recirculation and cavitation.

VANE PASSING SYNDROME

  • The bubbles collapse just beyond the cutwater and there is where you should look for volute damage. You will need a flashlight and mirror to see the damage unless it has penetrated to the outside of the volute.
  • The damage is limited to the center of the tip of the impeller vane and does not extend into the shrouds. You can prevent this problem if you keep a minimum impeller tip to cutwater clearance of 4% of the impeller diameter in the smaller impeller sizes (less than 14″ or 355 mm.), and 6% in the larger impeller sizes (greater than 14″ or 355 mm).
  • To prevent excessive shaft movement bulkhead rings can be installed in the suction eye. At the discharge, rings can be manufactured to extend from the walls to the impeller shrouds Internal recirculation (Suction specific speed)
  • Visible on the leading edge of the impeller and will usually be found at the discharge tip working its way back to the suction. It can also be found at the suction eye of the pump.
  • The fluid re-circulates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low net positive suction head required (NPSHR) pumps and the term “suction specific speed” was coined to give you a guide in determining how close you have to operate to the best efficiency point (BEP) of a pump to prevent the problem.
  • The higher the numbers, the smaller the windows in which you have to operate. The numbers range between 3,000 and 20,000 (1,800 to 12,000 metric). Water pumps should stay between 3,000 and 12,000 (1,800 and 7,400 metric).

 

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

  • The flow angle of the inlet vanes and the number of vanes affect this number.
  • A desired value would be below 8500 with impellers having a flow angle of about seventeen degrees and five to seven vanes. The higher the flow angle number, the faster the liquid will travel and the lower suction head (pressure) we will get.
  • Boiler feed and condensate pumps often require suction specific speed numbers as high as 12,000) to 18,000 because of the temperature and pressure of the water. To get to these values the impeller inlet flow angle is reduced to a low as ten degrees and the number of vanes reduced to as little as four. Fewer and thinner vanes help to reduce the blockage in the impeller inlet. A disadvantage to these low flow angles is that the pump will probably run very rough at below fifty percent of capacity.
  • Water applications can run at these higher numbers because the amount of fluid expansion is very low for hot water. Mixed hydrocarbons have this same advantage because unlike a single product, the flashing of the mixed hydrocarbons does not take place all at the same time.
  • The higher the suction specific speed numbers the narrower the stable window of operation.
  • Should the net positive suction head available (NPSHA) be so low that a suction specific speed number of more than 18,000 is required, then a separate axial flow impeller (an inducer) can be used ahead of the centrifugal impeller to prevent cavitation. An inducer has a flow angle some where between five and ten degrees with typically two vanes and no more than four. Inducers have been used successfully with suction specific speed numbers of approximately 24,000. In other instances a booster pump can be installed between the pump and the source.
  • In their desire to quote a low net positive suction head required (NPSHR) some manufacturers will cut away the impeller inlet vanes to reduce fluid drag and thereby lower the net positive suction head required (NPSHR). If this has been done with your application you must insure that the impeller to volute clearance is adjusted correctly with open impeller designs, and the wear ring clearance meets the manufacturers specifications with closed impeller designs, or you will experience internal recirculation problems and cavitation at the impeller outlet vane tips. Keep the suction specific speed number below 8500 and this problem should never come up.

FLOW TURBULENCE.

Good piping layouts would include:

  • Ten diameters of pipe between the pump suction and the first elbow.
  • In multiple pump arrangements we would prefer to have the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical, a number of units can be installed in a single large sump provided that:
    • The pumps are located in a line perpendicular to the approaching flow.
    • There must be a minimum spacing of at least two suction diameters between pump centerlines.
    • When all pumps are running.
      • The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform flow to the suction bells.
      • Each pump capacity must be less than 15,000 gpm.
      • Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction diameter.
      • Bottom clearance should be approximately 0.30 of the suction diameter
        • The minimum submergence should be as follows:
        • 20,000 gpm 4 feet
        • 100,000 gpm 8 feet
        • 180,000 gpm 10 feet
        • 200,000 gpm 11 feet
        • 250,000 gpm 12 feet

AIR INGESTION is not cavitation, but it resembles it.

The effect of air sucked into the pump is twofold.

  • It mixes with the liquid to form a two-phase mixture, which then has a lower specific gravity resulting in a lower discharge pressure from the pump.  Experience has shown that as much as six percent of the liquid pumped can be handled before the pump stops pumping liquid. This limit assumes that the pump is operating near the best efficiency flow.
  • Air binding of the pump. The impeller also acts like a centrifuge, causing the air to gravitate to the center of the impeller eye. If the flow is high enough, the velocity will carry the mixture through the impeller. As the flow is reduced, the air will accumulate in the impeller eye, thereby blocking any further flow. If the pump is operating below the BEP, the percent of air that can be handled will be decreased

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:

  • Through the pump stuffing box. This problem occurs in any packed pump that lifts liquid or pumps from a condenser, evaporator or any piece of equipment that runs in vacuum.
  • Some pumps are equipped with a repeller that will lower the pressure in the stuffing box
  • Through valves above the water line.
  • Through leaking flanges.
  • Any vortexing fluid.
  • A pump discharge bypass line that has been installed too close to the pump suctions.
  • The suction inlet pipe is out of fluid. This can occur when the tank level gets too low or there is a false reading on the gauge because the float is stuck on a corroded rod.

Some things you can do:

  • Increase the size of the suction pipe to reduce the suction pipe losses, thereby increasing the absolute suction pressure.
  • Vent the air from the suction pipe back to the liquid source by a continuous slope in the suction pipe or a separate vent pipe from the top of the suction pipe close to the pump.
  • Check all pipe joints, valve stems, and other sources of air leakage.
  • Change the impeller configuration to put discharge pressure on the shaft seal, thereby reducing air leakage into the pump at that point.
  • Use a self-priming pump design which can handle more air.

Posted

  • On February 13, 2018