Cavitation issues

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

  1. warno

    warno
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    When my boiler reaches anything over 195°F my circ begins to cavitate really bad. I don't believe it's pulling air down into the supply because the port is 8-9" below the surface of the water. I've been told it's called "net pressure suction head", is that what I'm experiencing? What is the best way to alleviate the problem?

    Here's a picture of my supply port locations. I'm only using the one to the left of the flue exit. It's a 1" port.

    20150713_043559.jpg

    And here is where my circ is located. Never mind the mess this was during the summer when I was changing things.

    20160905_161245.jpg



    Any help would be great thank you.
     
  2. hondaracer2oo4

    hondaracer2oo4
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    Unfortunately I think you fix is swapping the supply and return and pull the supply off the bottom. Your pump is cavitating I believe because as you reduce the inlet pressure on the intake side of the pump your actually reduce the boiling point of water. So I think being so close to 195 you are causing the water to flash steam as it enters the eye of the impeller and thus cavitate.
     
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  3. maple1

    maple1
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    I was going to say try to mount the circ lower - but with 195 supply, a couple of feet likely might not make a difference.

    Maybe you could T in that bottom left port too, and pull from both left ports? I was also going to suggest a mixing valve at the T, so it wouldn't pull too much cold in - but that might present enough head to also add some cavitating effect. So maybe T that port in but leave the ability to partly throttle it with a ball valve?
     
  4. Golovkin

    Golovkin
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    I would mount the pump lower and T the other port into the inlet using a larger inlet size. I'm not well educated on this so, don't take my word on it. I just read somewhere when setting up my garn that I should mount the pump as low as possible and have my supply line larger than my return to reduce cavatation at high temps. - don't take my word for it, but its something for you to look into.
     
  5. warno

    warno
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    Would making the supply port significantly bigger help any? Being it's on a homemade unit there's no warrantee to void if I change things up. If i run a 1.25" supply line or 1.5" line then bush it down to 1.25" just before the circ inlet. And also put the circ as low as possible?
     
  6. 3fordasho

    3fordasho
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    Ran across this in the Garn system design manual, may be useful.
     

    Attached Files:

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  7. Fred61

    Fred61
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    I Know next to nothing about this so I have no suggestions. The guys in the know that have posted here have had several good suggestions but none have suggested pumping toward > the boiler. Can that not be done?
     
  8. warno

    warno
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    You mean move the pump to the return side? If so, it absolutely can be done I just thought the "pumping away" method was the preferred method.
     
  9. Fred61

    Fred61
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    That's what I meant. Seems like several years back. To me that's more than 55 years back all the systems pumped toward the boiler. That's when circs were more susceptible to damage from heat so they were put on the cooler return side. Hopefully someone with more knowledge about this will chime in.
     
  10. hondaracer2oo4

    hondaracer2oo4
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    Fred, on a closed pressurizzed system that works fine because water is just being spun in a loop and the pump can’t cavitate from lack of water on the suction side because the pump can only push out what it can pull in. On an open system if you had a 5 inch line on the discharge side and a 1 inch on the supply side you would cavitate all day long because you would never be able to supply the pump with enough water compared to the head pressure presented to it on the discharge side.

    Yes warno if you can increase the size of the supply port to the pump you should be able to stop the cavitation. Bottom line is the vacuum bearing created on the supply side of the pump because of restricted flow is lowering the boiling point of water to at or below 195 causing the water to flash steam as soon as it enters the vacuum st the eye of the impeller.
     
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  11. warno

    warno
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    How big of supply port would you suggest? I really want to get this figured out because next year I'm planing on pushing 200 degrees in the boiler to help charge my storage. So if i can avoid this caviation problem that'd be great.
     
  12. hondaracer2oo4

    hondaracer2oo4
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    What size is the supply pipe off the boiler currently? I think I would go with 1 1/2. I would honestly try pulling off the bottom first and see if you solve the issue. I would just use 1 inch truck heater hose to temporally hook it up and see if the cavitation goes away. This would be cheap and easy.
     
  13. warno

    warno
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    The ports are currently 1".

    In order to hook into the bottom port I'll have to break everything down. And if I'm going that far then I'm god with just putting on a bigger port. Unless you think there's a chance it would fix the issue.

    If you think going 1.5" supply line will do it then I'm good with that. Pipe and fittings are cheap to me, to the tune of scrap costs. And the TIG welder in the garage is happy to glue it all together.
     
  14. heaterman

    heaterman
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    I take it your boiler is not a sealed type you can maintain any pressure in?
    If so, you're going to find 180-185* is the maximum temperature you can run without destroying pumps.
    Also keep in mind that the higher the "head" is in the piping the more NPSH you need to maintain on the inlet of the circ.
    Larger diameter tube on the discharge side is the only thing that will accomplish that all other factors being equal.

    Think of head this way... 1' of head is developed by a column of water obviously 1' above the point of measurement. In your case, the suction inlet of the pump.
    Until you reach the point where the piping on the suction side cannot flow enough to satisfy the pump, inlet pipe size is immaterial.
    Whether you use 1" pipe or 4" pipe on the inlet side, a vertical foot of height above the pump still equals only 1' of head pressure.
     
  15. warno

    warno
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    It is an open system boiler.

    So you're saying that even with a bigger supply line to the suction side of the circ I'll probably still have this issue? I'm running alittle under 20ft of head for this pump on my boiler. Do you think there's any way to over come this issue at all?

    Edit: I guess you did say a larger pipe on the discharge would help.
     
  16. jwise87

    jwise87
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    On my last homemade system I pulled off the bottom on a open system with 3/4 ports and didn't have cavitation issues even when my water would occasionally get above 200*, so I would put it at the bottom and see if that helps. If I recall correctly you run a rather small water jacket, so if your pump is running all the time your temp differences should be negligible from top to bottom, as long as you have good circulation in the jacket.
     
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  17. tom in maine

    tom in maine
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    The usual way on a closed system, to prevent this, would be to increase the pressure at the pump. This suppresses the cavitation bubbles from forming in the impeller, where localized boiling is occuring.
    In an unpressurized system, only the standing head does this. You might try installing an open (small opening) pipe and/or reservoir higher than the high point on the system. This would still keep your system open and relatively safe (if that is what you were going for) but increase the pressure at the pump.

    BTW, nice welding!
     
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  18. maple1

    maple1
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    Where and what is the vent to atmosphere on your boiler? Can you extend that up? Pipe in an open higher up 'expansion' tank? Wouldn't have to be all that big.
     
  19. Hydronics

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    I'm assuming this is an outdoor unit with a long loop between buildings, correct? Bear in mind that the major pumping loss is the friction loss in the underground piping which looks to be only 1". The suction side losses cause the NPSH issue. Upsizing a few feet at the boiler alone will do little to improve NPSH.
    If I understand your setup, I'd say you have the following options to reduce cavitation:
    1. increase underground pex size to boiler to reduce friction losses
    2. pressurize the loop (would require a HX if you can't pressurize the boiler)
    3. decrease temperature (opposite of your intent to max storage)
    4. decrease flow (smaller circulator)
    5. a combination of all 4 above
    Sorry to be the bearer of bad news...

    Are you destroying impellers? If not excessive, it may last a long time this way.
     
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  20. warno

    warno
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    In this picture you can see the square box sitting on top of my water jacket. I call this "the bubble".

    20150902_142433.jpg

    And inside that opening is a stand pipe that runs down and out the bottom. that pipe is my vent line.

    20150902_142529.jpg

    I built it this way so the water could expand up into the bubble and if needed overflow out the stand pipe. Also this helps with evaporating loses. As far as making it higher, I could maybe go a few more inches before it gets too close to the cover.

    My boiler has 22 feet of thermopex under ground before entering my garage. Then it goes through a plate HX and back. With fittings and valves inbetween.

    This pump has been running for 2 seasons in it's current setup. It doesn't always get to the point of cavitation but if I over load the boiler then it does. It's not very often i do that but i know these pumps hate air in them so I'm trying to fix the problem now.



    Also I want to thank everyone for trying to help me out.
     
  21. hondaracer2oo4

    hondaracer2oo4
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    So I confused by some of the previous comments. Can you guys explain why reducing the head presented to the pump on discharge side would cause the pump to not cavitate? Reducing the head on the pump would cause it to
    Be able to move more water which would further starve the pump on the intake side for water which would increase the vacuum and drop the boiling point even lower. The pump has atmospheric pressure 14 psi pushing the water into the inlet.
     
  22. tom in maine

    tom in maine
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    A vacuum forms at the impeller when the pump is operating. As the pressure drops below atmospheric, the boiling point drops.
    Higher pressure suppresses this.
     
  23. hondaracer2oo4

    hondaracer2oo4
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    Tom, yes I agree, that's what I posted above. But it was stated that increasing the pipe size on the discharge side of the pump would fix this but I don't follow how that would increase the pressure on the intake side of the pump. If the pump has less head loss on the discharge side then it is going to decrease the pressure even further on the intake side. Increasing the intake side of the pump size will allow more water to flow to the intake side of the pump under atmospheric pressure.
     
  24. maple1

    maple1
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    I can't really explain why I am picturing this - but what I am picturing is me putting my hand over the outlet of Warnos circ, and that resulting in a swarm of bubbles appearing around his impeller.
     
  25. maple1

    maple1
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    How hard or impractical would it be for you to make that box cover water tight, and plug that pipe. Then/and put an outlet on the cover going up (or on another convenient boiler tapping, if you have one - one of your unused ones on the back might work), connected to a small open tank up as high as you can practically get it. Thereby raising your water height & circ inlet pressure? It would increase about 0.4psi for every foot your water height goes up.
     

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