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Sizing TMV for Boiler Protection

Santilal
Santilal Member Posts: 42
Hi

I am trying to size the TMV for protecting a conventional coal boiler. The boiler is rated at 30kW (102,000BTU/hr). I have calculated that the flow required through the boiler is 28.6l/min (7.7gpm) for a 15*C (27*F) delta T. In the attached sketch I show the arrangement (I have omitted the pressure vessel and make-up water system for clarity). I have read that the TMV should be chosen to give a 1PSI pressure drop at full flow. Am I right using the flow calculated above for the Cv. This means the Cv = 7.7. I have been quoted $NZ800 for such a valve and this seems far too expensive. Have I got the calculation wildly wrong?

Thanks.

Santilal

Comments

  • Zman
    Zman Member Posts: 7,328
    What model valve are you looking at? What are your options? Is that a cast iron boiler? What circ are you using for that loop?
    I don't think there is anything magical about the 1 psi in your application. You do need to make sure that your circulator can overcome the resistance in whatever valve you choose.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEI
    SWEI Member Posts: 7,356
    I suspect you won't find a valve smaller than that regardless.
  • Santilal
    Santilal Member Posts: 42
    Hi
    Thanks for the input. I am trying to modify the existing system which does not work so well, so I am trying to re-use what I can. The valve I have been quoted is a Caleffi TV523160. But I do have the exisiting one which is a Taco Nova MT52 6003.104.0210. It has a Cv of 2.2. If I have calculated correctly this will suffer a pressure drop of 12PSI at a flow of 7.7GPM. I have not explored all the options available, but I get the feeling that they are not a comon commodity in NZ. The existing boiler pump is a Wilo Star-RS25/6 which can only produce 6PSI at 7.7GPM. The boiler is a NZ made boiler, steel, I think. It is called an Ecomax.

    I understand what you say about nothing magical about the 1PSI drop, but I suppose you want to keep this as low as possible to keep power consumption low.

    I hope I have answered all the questions. Thanks.
    Santilal
  • Zman
    Zman Member Posts: 7,328
    With some configurations you actually want a specific CV value in order the give the valve proper "authority" over the flow. I think that is what you probably read about.
    What I meant was, in your case I don't think that is critical.
    As you determined, a CV of 2.2 is not going to work well for your design.
    In the US market a valve like this is what I would be looking at. www.caleffi.com/sites/default/files/file/01223_na.pdf
    A nice valve at a reasonable price. CV of 10 in the 1" size.

    I don't know if you have access to that kind of product.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 16,286
    In a perfect world every valve would be sized with the flow requirement meeting the Cv number. I doubt that will ever happen :) It is best to get as close as possible.

    A big mistake is trying to move a lot of gpm thru an undersized valve. We see that in radiant applications here a lot, a 2 or 3 Cv
    thermostatic mixer running the entire home. Or trying to.

    In addition to pressure drop, you may get noise and undue wear of the insides.

    If boiler protection is the goal, I would look for a high flow mixer or a non-adjustable valve designed for boiler protection. The circ pump you have would probably be adequate.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream