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Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

(OP)
I have a system where the predicted metal temperatures of carbon steel flare piping are lower than the MMDT of -29°C. I'm trying to figure out the allowances given in ASME B31.3 Chapter III to see if I can reduce the allowable minimum temperature.

The material is type 3 in Table 323.2.2 (carbon steel), therefore it seems that the material must be heat treated as per A333 then impact tested, however:
ASME B31.3 then goes on to say in Section 323.2.2 paragraph d that impact testing is not required provided that the stress ratio as defined in fig. 323.2.2B is <1. OK, I get that, in this case stress ratio is <1, as the MAWP is far higher than the operating pressure at the minimum temperature predicted.

Question is, does the material still have to be heat treated, even though impact testing is not required? The code doesn't specifically say whether this is the case.

ASME is a bit confusing around this point. Any help much appreciated!

Thanks!

Jim

RE: Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

General industry interpretation of this rule is to include in the total stress on piping, that due to localised stresses from piping supports to the coincident max internal pressure. There are no exact methods to quantify these localised stresses, from what I've read.

However, it is possible to use a higher fidelity simulation program that may tell you what the mean metal temp is on this flare piping for this transient (?) flaring event.

RE: Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

(OP)
Hi George,

Thanks for the reply.

I know the exact metal temperatures during depressuring, and they are below the MMDT of the metal. My question is really whether the material needs to be heat treated and impact tested, either or neither as the minimum predicted temperature is only a couple of degrees below the MMDT, and the stress ratio is only around 0.5.

Thanks

James

RE: Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

The risk associated with the unknown additional stresses induced by restraining piping supports remains unaccounted for in your case so far.

It sounds like you've got this exact metal temp from a skin temp sensor or a temp gun during an actual plant depressure run.

RE: Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

If this is a new system design then, where the predicted temperature is lower than -29 deg C, the MDT is also lower than -29 deg C by definition. -29 deg C is the default threshold for a Charpy testing requirement, it should not be confused with the MDT.

The heat treatment referred to will be specified by the particular material manufacturing standard, not necessarily A333.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

RE: Reduction Of Minimum Metal Design Temperature As Per Asme B31.3

As EmmanuelTop alluded, why ask a mechanical or metallurgical question in a forum dominated by process engineers? You would get a more focused audience in the Boiler and Vessel forum or, for B31.3, at forum378: Pipelines, Piping and Fluid Mechanics engineering.

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