Cavitation issues

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

  1. Bob Rohr

    Bob Rohr
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    For return protection the goal is to stop or divert flow when the boiler is running to cool (cold:)

    In your unique case we are trying to come up with the least problematic method. Put the sensor wherever it gets a sample of the boiler conditions. In your boiler the design may be fine with cool return if it blends quickly with all the volume inside and prevents condensing conditions.

    It will be obvious quickly in wood fired appliances as you will see condensation in the flue, and creosote forming quickly. Monitoring flue temperature is also and indicator, although it looks like you don't have low temperature issues there?

    In some cases manufacturers build sloppy, inefficient HX on purpose. A typical gas fired WH has a fairly inefficient HX design so it can handle low incoming water temperatures on a continuous basis, hence the high flue gas temperatures and low efficiency numbers.
     
  2. warno

    warno
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    @Bob Rohr I only get creosote if my boiler goes into idle mode. Other than that it's just soot and fly ash.

    I just threw this idea together. What about a simple manual bypass I could throttle and possibly find a happy medium for return temps throughout the burn.

    I know you said the idea is to not have any devices on the suction side but I think we are all in some agreement that 90 plate doesn't have alot of loss. Here's to drawings one with the circ on return one with the circ on supply. Both with the manual bypass idea. All large looking piping would be 1.25" and all smaller would be 1".

    Option #1

    20180126_151300.jpg

    Option #2

    20180126_151459.jpg
     
  3. BoiledOver

    BoiledOver
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    Mounted to the top of the boiler would be my choice. Definitely not on the return.

    You have a good sense of reasoning. Definitely not on the return.

    Wish I knew more about heat exchangers but hadn't had the need to know. As I stated in another post, maybe your current FPHX is up to your task, I do not know. You have already crunched the numbers concerning an appropriate size.
     
  4. Bob Rohr

    Bob Rohr
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    The top of the boiler will or should be the hottest point, it may not respond accurately to cool return.
    The valve you have now watches and responds to return temperature to assure you stay above the condensation setpoint, probably has a 140F sensor in it. You would need circulation of course for a return sensor to be accurate.

    All the tekmar controls show return protection sensor on return piping. As do boiler installation manuals, Taco ∆T circulators, etc.
     

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  5. warno

    warno
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    Yes it is the 140° sensor in my protection valve. In a drawing you posted earlier it didn't show the protection valve at all. Do you think i really need it?

    Ever year at the start of the season I get condensation in my boiler with the first fire of the season. And ever year the condensation stops when the water jacket gets to 110-115°. Am I to believe that as long as my water jacket doesn't dip below 120 that it should be a non issue?
     
  6. Bob Rohr

    Bob Rohr
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    Skip the return protection valve. IF you had a high tech gasification style it would be a must have.

    I think your design is closer to an OWF and I've never seen the need to protect them. That combined with the operating method you describes make it sound like it will never have a chance to cool down.

    I'd run the boiler pump any time you are firing. If needed, just drop off the secondary side circulator, to allow the boiler to catch up.

    A simple aquastat, setpoint control or snap disc stat could be added to pulse that B side circ without getting in to the piping again.

    Keep the goal in mind, remember we are trying to simplify and address the two basic problems you are having.
     
  7. warno

    warno
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    Ok I will leave out the protection valve. Worst case is I do see the need for it and can put it in later. At season start up I'll just shut off the storage pump until the boiler is hot, which is how I've been doing it since adding storage.

    I agree with the thought of the boiler pump running at all times during a burn cycle. I will add an aquastat to cut the storage side pump if the boiler water jacket gets to low.
     
  8. warno

    warno
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    So I've done another drawing I'd like some feed back on. After measuring my plumbing closet in the garage my only option to get the circ where I want it before hitting the pex to go out to the boiler is I have to go up with the piping first then go into the circ. The storage side is pressurized so if I'm understanding correctly it shouldn't matter on this side. It is about a 24" rise then horizontal then into the pex underground. There will be at least 10" of 1" straight pipe before the pump suction port. Will this work?

    20180130_064300.jpg

    On a side note, I did draw in a strainer simply because I already have one and like Bob Rohr said I could just pull the screen after a few weeks of running.
     
  9. maple1

    maple1
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    Yes should be OK if pressurized. I would plumb in a vent on that high spot, or some kind of bleeder valve - it would likely make for a place for air to get hung up in. Same for all high spots, actually.
     
  10. warno

    warno
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    Ok Thanks.

    All valves in that drawing will be webstone pro pals and I'll be using the garden hose for purging so it should clear out pretty easy.

    I've been meaning to ask this for awhile now but what pressure does everyone run their systems at? I've seen mine up to 10 psi at 170° but I'm wondering if I should bump it up alittle more.
     
  11. maple1

    maple1
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    10 should be OK. It comes down to where exactly that is measured at, and how high the system goes above that point. Static 10 at the circ should be good for 15' or so above that of system height. But I would assess that, at cold system temp - i.e., if that 10 drops to say 5 when cold, that would only be good for maybe 8' of system height. (roughly roundly talking).

    BTW, is that expansion tank a bladder?(I feel like that has been talked about before). It should likely be T'd in, on the suction side of the circ - by rights and popular hydronic heating thinking. Although practically speaking, with adequate system pressure, may not be an issue.
     
  12. warno

    warno
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    That 10 psi was measured on my upper temp/pressure gauge that's in my supply header for my tanks. That's why I was wondering about it. I'll watch it to see how far it drops at a cooler temperature.

    My expansion tank is a non bladder that sits up in the rafters of my garage. The top storage tank in the stack has a fitting on its top that goes up to the expansion tanks. It seems to work pretty well so far.
     
  13. maple1

    maple1
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    I'd just go with it then.

    The basic principle is, that if you tie it in at the circ inlet, then all the pressure diff that the circ makes, will show up at the outlet. I.e., if your circ can generate 5 psi of dP, the inlet pressure will stay the same and the outlet will rise that 5 psi. So good for cavitation prevention since inlet pressure won't drop. By contrast, if it was tied in at the outlet, all the dP would show at the inlet - the outlet pressure would stay the same and inlet would drop the 5 psi. Which could be bad for cavitation. Yours might give an in-between result, each side moving a couple pounds in opposite directions - likely no concern with adequate system pressure. I don't know if bladder vs. non-bladder plays any kind of part in all that, but thinking not.

    Carry on. :)
     
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  14. warno

    warno
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    I just realized that my 10 psi reading was on my bottom pressure gauge of my tank. So that was basically the weight of the water. My top gauge was only reading a few pounds at 150 degrees today. Top gauge has about 12" of water above it. Should I add some pressure to my storage tanks?
     
  15. maple1

    maple1
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    Should be ok if there's only a foot of water above it. But is there any part of the system that goes higher than that? Dump circuit?
     
  16. warno

    warno
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    Nothing gets above that pressure as far as I know. The dump circuit is tied into the open side of the system right now.

    Here's a picture of my top gauge. It shows temp and pressure. As you can see its barely, if at all, reading any pressure.

    20180204_184750.jpg
     
  17. maple1

    maple1
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    What I meant was higher in elevation. Tank top is highest system point? What's the elevation diff between bottom gauge and very top of system? I take my readings with a grain of salt when they are at the lowest limit of the gauge.
     
  18. warno

    warno
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    Ok, the highest point in the system is the water/air HXs in the garage rafters. They are about 9ft off the floor. The lowest gauge is about 6" off the floor. And this is all in the closed side of the system. The top of the tank stack is 7ft 10in off the floor. 8 ft ceilings in the garage.
     
  19. maple1

    maple1
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    10 psi at your bottom gauge should be around 6-7 at the top then. Which should be good.
     
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  20. Bob Rohr

    Bob Rohr
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    To measure the pressure at the bottom of your open system boiler you will need a very low scale, accurate gauge. A 5' column of water exerts 2.1 psi. Not many of those inexpensive tridicators will even read that low.

    A manometer reading inches of water volume would be accurate. Or just do the math .433 psi to lift water 1 foot.

    On the pressurized side ideally you want 5 psi at the highest point. So if the high point is 12' X .433= 5.2, add 5 for a 10 psi gauge reading.

    Depending of course on where the gauge is located in the piping.
     
  21. warno

    warno
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    The open side I know now I can only raise the pressure by added water height. But my closed storage side is what I was mainly asking about.

    Here's a picture of my gauge locations. You can see the top gauge installed and the red arrowed circle is the port my bottom gauge is in. The top of the top tank is at 7ft 10in.

    Screenshot_20180206-193148.jpg
     
  22. maple1

    maple1
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    The top of the top tank is at 7ft 10in.

    Plus you would also add on how far up the piping thru the ceiling or whatever else is hooked to the tanks also goes from there to get total dH. Then x 0.4psi/foot.
     
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  23. warno

    warno
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    So any plumbing at all above the ceiling adds pressure down on the tanks? Even if it's the heating loops?
     
  24. maple1

    maple1
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    Yes. As long as it's connected.
     
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  25. Marshy

    Marshy
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    Looks like I have a lot to catch up on.
     
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