Hot Water Boiler - Min Relief Valve Capacity

[ConstatnHeat]

Could someone please clarify the proper method for calculating "Min. Relief Valve Capacity" in MBH for an ASME (Section IV) Hot Water Boiler. As I understand it, using the greater of 8 x Heating Surface or Gross Output in LBS/Hr. Divide that amount by 34.5. Multiply that number by 33,475? Secondly, does the working pressure of the hot water boiler have any relationship to the "Min. Relief Valve Capacity" (MBH) stamped on the ASME plate (i.e. does a hot water boiler designed at 30 psi working pressure have a different Min. Relief Valve Capacity" than a 70 psi? Having established that, How do I determine the proper size for inlet/outlet of the valve. Thank you!

 

[GenB]

relief v.c for any steam or h.w. ; lb/hr steam mbtuh for h.w.
at any pressure,
of course steam max 15psi.
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[ConstatnHeat]

Thanks for the reply. But, can someone be a bit more specific to the post (i.e. Are the calculations correct? What about the working pressure of the boiler,, does it affect the "stamped" Min. Relief Valve Capacity on the A.S.M.E nameplate? Proper sizing for relief valve inlet/outlet?)

 

[GenB]

reads minimum valve relief capacity MVRC
the valve you install must excede that #
the stamped MAWP SV must be at or below the stamed ressure
for hot water you will have 30PSI or low pressure and 160 si models
steam can only be max 15 psig
I do not have more info on this , this will close my input in this one thread.
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[vpl]

I'm not a Section IV expert; however, my read of the standard (specifically Article 4 Sections HG-400 through 405) indicates that the minimum capacity is determined through testing.

I could not find the formula given by the OP anywhere in Section IV. I'm not sure if the values given are converting from one unit to another, or just what is being done.

Patricia Lougheed

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[ConstatnHeat]

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.

If a steam/hot water boiler has an output rating of 2,350 lbs/hr steam, when it is designed for a hot water application (for example, at 40 psi), 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?

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?

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?

Thanks.

 

[vpl]

1. Perhaps it would help if you defined what "MBH" stood for. That abbreviation isn't in the standard.

2. Probably, but it would depend. <ducking>. Ok, assuming that the boilers are the same size, and the only difference is the pressure, then "yes." But again, I'm not an expert in this Code, so don't presume my answer is absolutely correct. I'm basing my answer on review of Section HG-402.3 of the Code.

3. There's a nice table in Section HG-400.1 which gives the minimum capacity. The accompanying words appear to describe how to determine the relief valve capacity.

I'm presuming that you have a copy of the Code ...?

Patricia Lougheed

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[steve99f]

To determine RV minimum requirements look at HG400.2(e). It is the greater of the two cases specified. With regard to sizing the inlet/outlet of the RV, you probably don't need to do that since almost all relief valves in this service are stock items from the valve manufacturer.