Sizing TMV for Boiler Protection

Santilal

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

Zman

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.

SWEI

I suspect you won't find a valve smaller than that regardless.

Santilal

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

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

hot_rod

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.