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OT Centrifugal pump cavitation

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  • OT Centrifugal pump cavitation

    Here on the farm we get our water from a spring located in a wooded ravine. It takes about 90-100 psi to force the water up out of this canyon to the existing 10,000 gallon cistern (reservoir) from which a pressure system takes over and distributes to the various farm buildings.

    Over the years there have been a number of different types of pumps used to pump the water into the cistern, I can think of twin cylinder piston pumps, gear pumps and the latest, a style of centrifugal pump that Grainger wants to call a "turbine pump".

    I was somewhat disappointed in the life I was getting from these "turbine pumps" so I called the factory to see if they had any sympathy for my plight.

    Talked to a very nice and seemingly knowledgeable fellow who suggested the problem could be cavitation due to air in the suction piping.

    Given the way the suction piping was laid out, I could see where bubbles of air could exist despite the scavenging effects of the relatively low flow (2-3 gpm) of water in a 1-1/4" galvanized suction pipe. These bubbles would not be in or at the pump but approximately 6"-24" away from the pump in the suction piping. I should also mention that the suction head or lift can vary from 2-6 feet.

    My question is this; would the above mentioned bubbles tend to promote long term cavitation effects even though short term pumping performance seems unaffected?

    Dave

  • #2
    What is happening to the pumps?

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    • #3
      ... Cavitation is produced from either too much suction pressure, or leaks in your suction lines that let air in. Or air bubbles getting into the intake (say intake near a water fall?)

      An air bubble in the system should not cause it as any air that gets carried into the pump will be pumped out and then its no longer a problem.

      Cavitation results in lowered efficiency while it occurs and mechanical erosion of the pumps working surfaces.
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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      • #4
        BM,

        Isn't cavitation caused by too little pressure in the suction line? Increasing NPSH is the most usual way that I've seen to stop it, either by using a suction booster pump, less lift height or larger diameter suction piping.

        Dave,

        As Macona asked - what happened to the pumps - were the impellers carved up by cavitation, or did something else go wrong with them?

        You probably can't decrease the lift height much without going for a submersible pump. First thing I'd probably do is to fit larger suction piping, try to eliminate bends & other restrictions, and check it for air leaks.

        Ian
        All of the gear, no idea...

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        • #5
          Another cause of the low pressure might be a restrictive pick-up strainer.

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          • #6
            i believe cavitation in centrifugal pumps is primarily caused by using too much pump for not enough water.
            ie- trying to pump more than can be supplied.

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            • #7
              Also cavitation is caused by too low an output pressure...
              Precision takes time.

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              • #8
                Originally posted by Ian B
                BM,

                Isn't cavitation caused by too little pressure in the suction line? Increasing NPSH is the most usual way that I've seen to stop it, either by using a suction booster pump, less lift height or larger diameter suction piping.

                Ian
                Err, Yes sorry, Too little pressure (Too much vacuum)
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  Originally posted by Black_Moons
                  An air bubble in the system should not cause it as any air that gets carried into the pump will be pumped out and then its no longer a problem.

                  Cavitation results in lowered efficiency while it occurs and mechanical erosion of the pumps working surfaces.
                  That is what I was trying to get at. I can see where there ARE places in the suction piping where a bubble is not in the direct flow, thus is able to remain there even though the pump seems to be pumping correctly.

                  As an example, imagine a capped riser used for filling the pump case and associated suction piping to achieve initial prime. If air was to get into this riser it really can't get out because there is no flow through the riser, only across the end of it. Or so I imagine!

                  I didn't think this trapped bubble would cause any problems once the pump was initially primed but the manufacture's rep I talked to seemed to imply that any air, anywhere in the suction piping, would tend to promote cavitation.

                  As for how the pump quits working, it is a gradual degradation of performance. Initially a new pump will fill the reservoir in 4-5 days running 24 hrs. a day. At some point (2000-3000 ? hrs. of operation) the pump will fail to develop sufficient pressure to fill the reservoir all the way to the top, even when running 24-7 for 2 weeks or more. Disassembly reveals subtle dimensional differences as compared to new parts, but no obvious signs of erosion of either the bronze impeller or volute. These worn surfaces have don't have an "eroded" appearance, more of a matte or specular color.

                  Dave

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                  • #10
                    I think the significance of a bubble has not been understood. A minute bubble intrained in the fluid will suddenly collapse when it is pressurized. That action will erode the surface of the impeller. If the fluid has any foreign materials such as sand the effect is magnified
                    Cavitation erosion can happen on marine propellers and even on the outside of cylinder liners on diesel engines.
                    If you have ANY bubbles in a pump you will get some cavitation. In some cases the bubbles are created simply by extreme vacuum causing the liquid to vapourize then collapse when pressurized. This is the case with flexing cylinder liners and the engine coolant.

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                    • #11
                      What RPM does the impeller rotate at? What diameter is it?

                      .

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                      • #12
                        Here's a good site about cavitation http://www.mcnallyinstitute.com/01-h...ir%20ingestion

                        BTW, as the site points out air ingestion (AKA pseudo cavitation) is not true cavitation, but results in the same problems.

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                        • #13
                          Think of the cavitation problem at an outboard motor propellor. It is usually cured by reducing RPMs. The higher speeds cause vaporization of the water at the outer edges of the blades. The collapse of those "bubbles" cause erosion or deformation of the blades. If deformation takes place it can be either permanent or temporary. In either of those cases, the prop loses effectiveness and no longer moves water as well as it should.

                          http://en.wikipedia.org/wiki/Cavitation

                          Look at the section describing "discharge cavitation."

                          Pops
                          Last edited by armedandsafe; 04-13-2012, 02:56 PM.

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                          • #14
                            Originally posted by EddyCurr
                            What RPM does the impeller rotate at? What diameter is it?
                            .
                            Good question!

                            The impeller is directly mounted on a 3450 rpm motor shaft, the motor being 1/3 hp capacitor start unit. The impeller is about 3"-3.5" dia.

                            Some of the responses deal with "entrained air" or "air ingestion" which is not the case here. I am wondering about air that is present in the suction piping and does not go through the pump.

                            Thanks to all those who responded!!

                            Dave

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                            • #15
                              If the pump is like this, I think it may not be the best choice for the job you describe.
                              http://m.grainger.com/mobile/details...rgav06?R=4UP50

                              Since it's not a submerged motor, you could be getting air through the shaft seal.
                              Any products mentioned in my posts have been endorsed by their manufacturer.

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