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Re: Hot Water Boiler Minimum Relief Valve Capacity

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Posted by Bob D. on September 08, 2009 at 11:50:59:

In Reply to: Hot Water Boiler Minimum Relief Valve Capacity posted by Constant Heat on September 07, 2009 at 17:07:57:

It's been a while since I've pitched in on this forum, so here goes - Your questions cover a lot of territory, but I'll try to keep it simple. (1) Per HG-402.7(b): "To determine the discharge capacity of safety relief valves in terms of Btu, the releiving capacity in pounds for steam per hour is multiplied by 1000." This is pretty much what you've got in you question, so at least that one fairly straightforward. The actual calculation for steam capacity is covered in HG-402.3 and a few other places, and there's not enough time to walk you through that in this forum. Unless you are a valve manufacturer (in which case you wouldn't be asking these questions here) you need only go by the published data from the manufacturer's selection charts for capacities and set pressures.
(2) The selection of the proper safety relief valve has to be made with the knowledge of the MAWP, because the setpoint for the safety has to be within a certain percentage of the MAWP of the unit/vessel (That's why you're supposed to gag or remove safeties during a hydro test). You can take a 70 PSIG MAWP unit, fitted with a 73# set safety, down to 30 PSIG and not have to change the safety, but I can't think of a situation where a 30# MAWP unit, with a 35# set safety would be allowed to run up to 70# unless fully re-rated/re-stamped, and the safety replaced.
(3) For most people, look at the ASME stamp on the boiler for MAWP and gross input BTU rate, and select a safety from the cut sheets that is 5% above the MAWP and rated by the valve manufacturer for no less than the maximum input BTU rating of the burner. If you don't have either of these pieces of information, don't do anything until you can get them, from a reliable source. There are other ways to get the job done, but they can get a little involved if you've never done it before. Combination pressure & temperature (P&T) reliefs are a little different too.
(4) Never reduce the line size, or bush the fitting, between the vessel's safety fitting and the safety valve proper, and as a general rule, never reduce between the safety's discharge and the atmospheric outlet(the net back-pressure calculations get a little cumbersome). Piping of multiple safeties can also get a little complicated if you try to manifold them, just keep them seperate and you'll be done quicker and with less trouble from the Inspector.
Hope this is generally what you were looking for. If not, we'll try again later.

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