Boiler Relief Valve Discharge Piping

[bmeeks]

Hello,

I'm currently working to verify relief valve sizes and I'm wondering if the 3%/10% suggestion from API for inlet/ outlet piping also is assumed to apply to the relief valves on a boiler. When reading the section 1 code from 2007 (latest I have available) it seems as though that rule does not apply. It states the following:

"PG-71.3 ...It shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves"

I don't see any reference to API code which makes me think the 3/10% rule does not apply to boiler valves. I'm looking at around 22-25% drop on "rated flow" and 12-14% on "required flow"

I would appreciate any suggestions or references.

BMeeks

 

[EnergyMix]

What code are you working to? ASME has requirements for relief valves.

 

[bmeeks]

I have ASME Section 1 and Section 8 code for relief valves. I don't see anything referring to using rated flow of orifice/ valve size to calculate pressure drop in outlet piping. It essentially says keep it short. I think I have enough information to go forward from here.

Thanks,

BMeeks

 

[davefitz]

The relief valve exhaust stacks are under the admin and tech jurisdiction of B31.1 for power boilers.

B31.1 app II discusses the recommended ( but not required) design of inlet and outelt piping, pipe supports, etc.

The size and rating of the relief valve orifice is defined insection I and is certified by the valve mfr. The pressure drop calc for the outlet piping is presented in B31.1 app II ( based on the Bechtel- Liao method, which in turn is based on Fanno curves).

There are also precautions as to avoiding flow induced vibration, etc.

If the outlet pipe is too long or has too many bends, then the B31.1 curves might not be useful- you can extend those curves usig Fanno flow theory, but it remains essential to prevent the pressure at the relief valve outlet from exceeding 50% of the lift pressure, to ensure choked flow , for these spring loaded releif valves.

 

[chicopee]

As davefitz point out ANSI/ASME B31.1 is the approach to the discharge piping. ASME codes describe the sizing requirements of safety and relief valves.

 

[bmeeks]

Awesome. Thanks for the help! I'll check that out Monday!