Cavitation issues

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

  1. warno

    warno
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    So back on track here with the caviation issue.

    I ,badly, drew up some piping plans. which would be best or if someone has something better let me know. All of them put the pump on the floor of the boiler shed. And all supply piping will be 1.5" pipe and all fittings will be butt weld fittings. All return piping is 1" unless you guys think i should step that up too. Either way all fittings are butt weld fittings.

    #1 basically just keep the same supply and return ports just bigger piping

    Screenshot_20180122-144539.jpg

    #2 use both supply and return ports. Return feeding into the current return port mainly.

    Screenshot_20180122-143932.jpg

    #3 use both supply and return. Return feeds into a tee to help split the return between ports.

    Screenshot_20180122-144309.jpg


    And if you guys want to draw your own piping if you think a different way is better here's the shot I used.

    20150713_043559.jpg
     
  2. warno

    warno
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    @Marshy @Bob Rohr @maple1 You guy's still around?

    Anyone have a chance to look at the piping options i drew up? Anyone have a better way to plumb it?
     
  3. Marshy

    Marshy
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    I'm in favor of #1 but slightly modified. Keep the pump vertical and move the 90 degree elbow to transition to horizontal on the discharge of the pump. The extra few inches difference will likely not be worth the added restriction of the elbow right in front of the inlet. It will also add turbulence right before the pump and could exacerbate the cavitation. Not for nothing but, if that don't fix it your looking at having to reduce the pump inlet temp.
     
  4. warno

    warno
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  5. maple1

    maple1
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    Maybe. You might do some numbers, on equivalent feet of pipe to an elbow. And also work in what height difference you would gain by going horizontal. Also the more straight pipe you can get behind the circ, if the circ is horizontal, the less chance for that turbulence effect. Kind of hair splitting stuff on a fuzzy 'maybe' situation.

    As far as your piping choices - since you won't gain any more height no matter what you do there, it might not make any difference cavitation-wise, on its own. I would likely decide, based on where your coldest & hottest spots are. Pull from hottest, return to coldest. Since your main goal is to sent hottest water to storage. Which might also help internal boiler flows. If, say, the top RH outlet has water just as hot as top LH, or hotter, I might T them together. But if top RH is cooler, I likely wouldn't. Yes it would reduce cavitation potential, but just by decreasing circ inlet temp. Which would in turn send cooler water to the HX. The old Catch-22. Similarly, if the bottom LH inlet location is as cool as the bottom RH, I might T those together. But likely wouldn't if bottom RH is consistently the cooler.
     
  6. warno

    warno
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    We have a pipe bender at work that can put a long sweep 90 in the 1.5" if that would make the turbulence factor less. What's the rule of thumb on minimum length of pipe after a elbow to a circ?
     
  7. Marshy

    Marshy
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    The general rule of thumb is there should be a straight section 5-10 times the pipe diameter between pump inlet and any obstruction (valves, tee's, elbows etc.).

    Another general rule of thumb is the piping diameter should be equal or one size larger than the pump inlet in an effort to limit suction piping velocities to 7 to 8 ft per second or less.

    20 gpm through a 1" pipe is 8.17 ft/sec.
    15 gpm through a 1" pipe is 6.13 ft/sec

    20 gpm through a 1.5" pipe is 3.63 ft/sec.
     
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  8. warno

    warno
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    I measured the boiler tonight. From the top of the water jacket to the floor of the shed is 60". So at best I'm getting a useable 58" of water column.

    If I plumb like this would it gain me anything at all? It's similar to the earlier posted illustration but doesn't go above the water line.

    20180123_162151.jpg
     
  9. maple1

    maple1
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    What aspect of that are you wondering about?
     
  10. warno

    warno
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    If it would gain me any extra water column to add those few precious psi at the pump inlet I'm chasing.
     
  11. maple1

    maple1
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    Still not sure what you mean by 'like this'. If you mean the short section that goes down into the water that won't do anything. If the pic shows greater vertical separation between circ and water level then probably?
     
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  12. warno

    warno
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    If I just pull straight off the water surface it will draw air into the pump. The little drop down piece is a dip tube to insure the pump doesn't suck air in.

    Will the added height of the pipe headed to the pump benefit any from doing this? I mean As far as the water column pressure goes.
     
  13. maple1

    maple1
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    Still not completely grasping. Are you talking about raising your boiler outlet up higher - to the water height? Your outlet isn't up to water level now, is it? If not and you aren't increasing vertical separation I don't think you'd gain. But I still might be misunderstanding.
     
  14. warno

    warno
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    I just realized my drawing makes literally no difference other then more parts. The water level above the port is pushing down on the pump regardless where the port is in the water jacket.

    Disregard my above 3 posts. <> ;em <> ;em
     
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  15. maple1

    maple1
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    We're in too deep to start disregarding, lol...
     
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  16. warno

    warno
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    Lol. very true.
     
  17. warno

    warno
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    I drew up a piping plan for this project, let me know if you guys think I should change anything.

    I went with the circ horizontal to gain about 6" more height to the center line. The 48" length of pipe coming out of the supply is using my current port location. With that there will be a total of 56" of water above the pump center line.

    20180124_091805.jpg
     
  18. BoiledOver

    BoiledOver
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    Did you consider placing the new heat exchanger in the boiler room? That would make for a very short boiler loop (lower head) and using a lower head circulator. The Grundfos 15-58 here pushes 2 gpm on low, 7 gpm on medium, and 13 gpm on high. It sees no more than a difference of 9-1/2 feet of head as per the pressure gauged bypass loop. It is 15 feet each way of 1 inch copper with 20 fittings, a strainer, air eliminator and Danfoss valve. I did not include the various isolation valves as they are all full port. It would mean adding another circulator, the 15-58 is $82 US or four for $315. The boiler loop circ after the plate exchanger for it to see cooler temps.
     
  19. warno

    warno
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    The only problem with putting the FPHX in the boiler shed is heat dump reasons. If I have the HX out there I can't use heat dump to heat my garage. Essentially any dumped heat is wasted in the boiler shed which doesn't need anymore heat in it. Ambient air in the shed is right around 130° while burning. I really like the idea of the heat dump being in the garage where i could use heat. Even though i know the dump zone won't run all the time.
     
  20. maple1

    maple1
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    I was also wondering about the heat dump. How much does it actually get used? Theoretically a dump zone should only come into play in an overheat situation. So for max overheat protection, it should likely go up high right above the boiler (if possible). Maybe drawing from the other top boiler tapping. With a NO zone valve in between & separate aquastat & controller to control it. Have you ever had a power outage issue when burning? That might be the most common cause of an overheat - and I don't think your setup would offer any protection in that situation. That would also do away with the diverting valve & possible head loss from that. Then if you wanted to heat the garage, just pull from storage for that. Then again - I don't think I have seen many OWBs with any kind of heat dump circuit. They just kind of seem to bubble for a bit. So maybe it's not needed at all if you have a decent depth of water above the firebox?
     
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  21. BoiledOver

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    Just some thoughts from an alternate perspective.

    Yeah, I see how the added heat in the boiler shed is a concern. I do not have experience with a FPHX but would think they can be insulated. Rigid polyiso can be cut to size, has good r-values, and is heat rated for use around hydronics. Easily assembled and dis-assembled.

    The dump zone theory wouldn't change since it is over at the garage and could remain. If the dump zone is drawing heat away from the FPHX, it truly does not matter the location of the FPHX, does it? The piping is the pathway and will still exist.
     
  22. warno

    warno
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    My heat dump is for an over fire situation. The way it works is, there's a temp sensor strapped onto the boiler supply side of the FPHX. When it is reading 195 the diverter valve opens to dump zone air/water heat exchanger where that fan also kicks on. When boiler cools down enough the diverter closes the dump port and switches back to the FPHX side for storage charging. Its a NO valve to the FPHX, NC to the dump zone.

    Take this for what's it's worth, I'm doing my damndest to not be the "typical" OWB owner. I'm doing batch burns to heat storage. I hate to see my boiler go into idle mode. If it's not burning full out it's not what i want. So if I overload the box for the amount of heat into storage then I want my boiler to just lose heat to my garage instead of idling.

    We very very rarely have power loss in my area. If, big if, power goes out a damper kills the fire via idling.
     
  23. maple1

    maple1
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    Do you get over 195 very often though? With other changes being made - that might tend to happen even less, if those changes get more heat moving into your storage. It likely won't hurt anything to just leave it in place as-is. Aside I guess from moving it lower for reasons already mentioned somewhere else in the last 9 pages. But if for some reason you end up looking for a place where you could get just a little more pressure drop out of the circuit, that would be one place to look.
     
  24. BoiledOver

    BoiledOver
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    Yes, this can still happen with the FPHX in the boiler shed. The storage loop is cooling the boiler loop. Your existing diverter valve for the dump will do the same with the storage loop as it has with the now long boiler loop. Making a much shorter boiler loop should result in less pressure and lowering the boiler circ should cause reduced pressure. Combined even better. Your storage will become additional dump zone.
     
  25. warno

    warno
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    I'm hoping that with the new addition of the bigger HX it won't reach those temps. But if it does I'm wanting to use the heat appropriately. I think boiled over is on to something.

    I'll draw up another plumbing plan and post it in a little while. I think you are right. If the dump is cooling the HX it is in fact cooling the boiler.
     

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