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Re: Hot Water Boiler Minimum Relief Valve Capacity

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Posted by Bob D. on September 10, 2009 at 10:39:40:

In Reply to: Re: Hot Water Boiler Minimum Relief Valve Capacity posted by Constant Heat on September 09, 2009 at 23:59:41:

: Thanks for the reply!

: However, I may not be explaining myself properly.

: Low pressure steam boilers (ASME Section IV) are generally designed for 15 psi steam/30 psi hot water. However the same size boiler can be designed for a higher pressure (hot water only) application by means of increased thicknesses of the parts making up the pressure vessel of the boiler, etc. For example, the boiler has an output rating of 2,350 lbs/hr steam, but when designed for a hot water application, the "Minimun Relief Valve Capacity" rating on the ASME plate must be in MBH.

: 1. I am looking for the proper conversion equation from lbs/hr to MBH?

RD - (Lbs/hr) x 1,000 = XXX MBH ; per HG-402.7(a)

: 2. I have noticed that the discharge capacity (MBH) of pressure relief valve ratings, increase slightly as the setting of the pressure relief valve increases (i.e. a 40 psi valve rating might be 1,194,000 BTUH but the same size valve, when set for 70 psi, is rated at 1,865,000 BTUH. Since an ASME boiler must list the "Minimum Relief Valve Capacity", would a hot water boiler that is designed for 40 psi have a different MVRC than a hot water boiler designed for 70 psi?

RD - Yes. There are many reasons.

: 3. What is the calculation used for determining the pressure relief valve opening in the boiler (i.e. what size valve)? Outlet diameter for the Water?

RD - Buy the ASME B&PV Code, Section IV. Look at Article 4 in it's entirety and HG-512. There are several ways to do it - Coefficient Method, Slope Method, Three Valve Method, and Accumulation Tests. All involve bench testing by qualified people under standardized conditions. You and I do not determine which valve, the boiler Manufacturer does based on the valve Manufacturer's certified ratings resulting from these tests.

: Thanks.

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