Temperature derating factor

[rizpet23467]

HI,

Why "Temperature derating factor" not considered in calculating linepipe wall thickness as per ASME B31.4?

Regards,
rizpet23467

 

[LittleInch]

section 401.2.3.7 effectively limits the design in B 31.4 to a temperature range of -30C to + 120C. At these sorts of ranges, pipeline steel is not affected to the extent that the design factor on a global de-rating is required. Of course the actual SMYS of the material at your design temperature might be affected, but this is not the same thing as a global de-rating regardless of material.

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[BigInch]

I'll echo what LittleInch says, with a few more code references...

See 400.1.2 is a little bit more specific stating that the code was not developed for use outside the temperatures range of -20F (-30C) to 250F (120C), however actual temperature limits of the code are not specifically stated.

In the above and in Table 403.2.1-1 you see that B31.4 does not set allowable stresses at various temperatures as does B31.3. The temperatures encountered in pipeline work seldom reach outside the codes stated applicability range 4012.3.7. Inside the B31.4 stated temperature range, no adjustment of material properties, ie. yield stress for temperatures different from those given at 100F is necessary (404.1.2). When (if) designing for temperatures outside of the above range you would first adjust yield stress for the higher temperature, then calculate the allowable stress from that. At lower temperatures you would take caution in regard to potential for brittle fracture.

 

[racookpe1978]

What is the common practice then for the very few feet of pipeline above ground (very cold air temperatures) that can go below -30 deg C?

I know the valve stations, pig stations, etc are short, but they would go lower in the colder climes.

 

[BigInch]

Making LACS for lower temperatures is normally possible, so just require your steel to have adequate Charpy impact energy and to demonstrate at least the specified energy from tests conducted at the low temperature (often minus an additional 5F).

 

[LittleInch]

Are you guys working in the same design? -

http://www.eng-tips.com/viewthread.cfm?qid=386937

The materials listed are all basically carbon steels of one form or another and I would have no issue with that, but beware temperature yield de-rating would apply for other steels such as stainless or duplex.

For pipe exposed to metal temperatures below -30, then as the notes in the attached thread say, you really need low temperature pipe material and test it accordingly.

31.4 isn't very clear about what you do outside of -30 to +120 or how to use different pipe, at least as far as I can see other than to say in note a of table 403.3.1-1 that Sy = SMYS of the pipe material. The inference is that outside of the temperature range or the materials listed, you use SMYS at the design temperatures you have.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.