Cavitation issues

warno Posted By warno, Jan 11, 2018 at 8:54 AM

  1. jwise87

    jwise87
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    The way I understand it is that there is already the maximum amount of water that can get to the impeller at 1 bar pressure right now, so increasing the intake side pipeing size won't help. Following that line woundn't throttleing the output side of the pump help? It would at least move the cavitation to the other side of the throttleing valve.
     
  2. Marshy

    Marshy
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    What is NPSH?

    The margin of pressure over vapor pressure, at the pump suction nozzle, is Net Positive Suction Head (NPSH). NPSH is the difference between suction pressure (stagnation) and vapor pressure.

    https://www.pumpsandsystems.com/topics/understanding-npsh-npsh-definitions

    You need to understand that in order for the water to remain liquid and not turn to vapor, you need to remain above the required NPSH for the pump. The pump manufacture specifies the NPSHR (R=REQUIRED) and draws it on the pump curve. You can reduce the NPSHR by throttling the pump back on its curve (add restriction on the discharge) or by increasing the pressure at the inlet (ie pressurize the boiler waterjacket or lower the pump in relation to the waterjacket) or lower the temperature of the fluid. Increasing the size of the supply piping will not be sufficient to eliminate the cavitation.
    images.jpeg
     
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  3. tom in maine

    tom in maine
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    Pipe diameter might have an impact on pressure around the pump. I do not know for sure.
    I do know that if you can install what amounts to a small expansion tank that is still open to the atmosphere over the pump, the system is going to operate at a higher pressure, even though it is open, and the
    pump impeller will cavitate less.
    For safety sake, there would be no shutoffs between the boiler and that opening on the tank. The opening can be small.
    And I would still have a pressure relief valve on the boiler shell.
     
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  4. Marshy

    Marshy
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    So basically you have available NPSH which is determined strictly by the pressure of the boiler water jacket in pressurized systems plus the elevation difference between the pump and boiler, or by just the height difference between the pump and the top of the water level in the boilers jacket for unpressurized minus thread loss from the piping. The the length, size, roughness of and number of elbows in the pipe causes a reduction in the available npsh. What your left with is the systems total available NPSH. Increasing the size will increase the available NPSH but the difference in head loss is very minor between say 1-1/4" and 1-1/2" pipe. It may be enough to keep it from cavitating but not likely enough if he wants to incresasue the water temp up to 190 or even 200.
     
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  5. warno

    warno
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    There is no room in the boiler shed to go upwards any higher. The bubble is basically rubbing the roof of the shed already. And there is no way to pressurize the boiler. Even with a safety pop off valve, there is far too much flat surface on my boiler too hold any pressure what so ever.

    so it sounds like the only option left for me to try is pull supply from the bottom port of the boiler and run return back in the top. With this circ I'm running I think I'll still get good boiler mixing so that shouldn't be a problem. Where my lower port is located from the bottom I should have roughly 100 gallons of water above it. Would that be enough to help my issue? I know this was mentioned earlier in this thread. I wasn't ruling it out, just wanted to see if other options were available.
     
  6. TCaldwell

    TCaldwell
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    Are the supply and return lines where they go to your garage lower than your boiler elevation, if so could you move the circ to that location?
     
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  7. warno

    warno
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    The boiler lines only flow to the plate HX. The HX is about half way up the boiler height, I'm guessing. I do have room in the garage for the supply circ, if that would work.
     
  8. tom in maine

    tom in maine
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    The pressure created by a reservoir positioned above the rest of the heating system can be located remotely from the boiler. It just has to be higher than the rest of the system. The pressure developed by the water column is not really affected by the tubing diameter, I believe.
     
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  9. Marshy

    Marshy
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    How many inches of water are above the port that the pump suction is hooked to?

    You could mount the pump horizontally down near the floor. That will provide a very small increase in available NPSH.

    You could also convert to 1-1/4" or 1-1/2" pipe too the suction of the pump in an effort to reduce head loss to the pump suction. Use large radius elbows out of the boiler and to the pump every where possible.

    If you have a 3 speed pump toy could try a slower speed.

    You could throttle the discharge flow some to get the pump run ball on its curve.

    Simply drawing from the bottom of your tank might not be helpful if the temp at the bottom of the boiler isn't much cooler than where your currently dreading from but it might be your best bet. Temperature is your enemy in this situation.
     
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  10. Marshy

    Marshy
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    This. You need to lower the pump and reduce the temp of the inlet. Enough of either one or a mix of both will solve the issue.
     
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  11. warno

    warno
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    In a vented system it can't be above the vent line though, correct? Gravity would simply level it out, would it not?
     
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  12. maple1

    maple1
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    You could T in a high open expansion tank in your boiler loop, if that goes to your garage. Not sure where your hx is.
     
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  13. maple1

    maple1
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    Yes true. Which means you'd need to seal up the boiler.
     
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  14. OT_Ducati

    OT_Ducati
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    What if you place the circulator next to the floor changing the orientation 90*?
    Looks like you might gain some head and only ad a couple fittings. ?
     
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  15. Marshy

    Marshy
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    Guys, the only way to prevent cavitation is to lower the temp of the water or increase the pressure of the water at the inlet of the pump.

    The latter can be achieved by lowering the pump relative to the boiler or raising the boiler relative to the pump.

    The things like increasing the inlet piping size, use of long radius elbows, adding some restriction at the oulet and running the pump are going to make very little difference because the pipe from the boiler to the pump are so short. Adding restriction at the outlet and running the pump slower will have the most effect.

    Recommending changes to the piping between the boiler and pump like adding some sort of tank is pointless and will not work.
     
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  16. warno

    warno
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    Theres about 8-9" of water above the supply port.

    I'm running a bell and gossett NRF-36 on the boiler. I was hoping to run it on speed 3 next year to help push btus through my plate HX. Also why I don't want to throttle the discharge much. Although that does work because the first time I heard the cavitation happening I throttled the discharge and it quit.

    The temperature problem sucks because I was hoping to run higher temps next year.
     
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  17. Marshy

    Marshy
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    You would get the best bang for your buck of you mmmkoved the pump to just after the heat exchanger. The reduced temp would be a huge benefit. The down side is you would have to modify your system if you don't want constant flow through your htx.
     
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  18. maple1

    maple1
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    But- adding a tank, higher up, would effectively be the same as raising the boiler. Wouldn't it? In order to add that tank you would have to seal the boiler up so water couldn't come out the top, though.
     
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  19. Marshy

    Marshy
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    Technicality yes, as long as the boiler becomes sealed and you make the new high point open to atmosphere. Unfortunately he had no overhead room. Its more practical to plumb the pump after the htx.
     
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  20. jwise87

    jwise87
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    If the only reason you need to run the high temps is to charge your storage, and you already have a second port your not using, you could buy a small cheap pump (Taco 007) that won't flow enough to cavitate and use it move water into the storage tank. I know that wouldn't be ideal and might require some creative plumbing, but it might be another option.
     
  21. brenndatomu

    brenndatomu
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    That's precious little head pressure on the suction port. As stated previously, the water pressure will be .485 PSI per foot of water column above the suction port...doesn't matter if its a gallon, or 100 gallons, its all about the height above the pump.
    Larger port and/or line would help some, lowering the pump would help, slowing it down would help, lowering the water temp would help.
    Lowering the pump would probably help the most out of the choices here that you are likely going to be willing to actually try.
    One thing you could do is to throttle a valve on the discharge side of the pump a bit...if it doesn't take much throttling to stop the cavitation, then it won't take much of a change to fix the issue permanently...until you raise the system temp anyways...you may be dealing with it again then...
     
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  22. warno

    warno
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    I just went out to lite a fire and I grabbed some pictures of my current plumbing.

    The boiler is located just outside my garage there's 22 feet of 1" thermopex in the ground. My Supply port is a 1" port and is 8" from the water level surface. The boiler is an open system. And I do batch burns to heat storage. The only time the boiler circ is running is during a firing cycle.

    I'm getting conflicting ideas of things that will or won't work. If moving the circ down the the ground with a 1.5" suction side will get it done then I'm good with that. The only thing I can't change at this time is my underground lines. For 2 reasons, cost and a massive PITA to dig up and replace with bigger lines.

    Here's a picture of the boiler side of things.

    20180112_212126.jpg

    20180112_212333.jpg


    And here's inside the garage in the "plumbing closet". You can see the thermopex lines flow into the diverter valve then you the HX. The diverter valve flows to either the HX or the heat dump zone.

    20180112_201823.jpg
     
  23. TCaldwell

    TCaldwell
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    The Garn is the same idea just bigger in all respects, ports too however the manual states the circ needs to be as low as possible for the npsh factor. A little simplistic analogy but the same idea
     
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  24. Marshy

    Marshy
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    The reason it seems like you are getting conflicting ideas is because no one is 100% certain what will resolve it.

    If it was my system I would start with relocating the pump as low as I physically could. Reorient it so that it's horizontal and near the floor in the boiler cabinet. At the same time I would change the inlet piping to 1-1/4", including the flanged isolation valve at the inlet. Not much you can do for the welded outlet through the boiler jacket.
     
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  25. TCaldwell

    TCaldwell
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    Oct 26, 2007
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    I agree with above but possibly not all that con flicking if rising a water column above the boiler ( not feasible) is really the same as lowering the circ.
     
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