Low temperature

[PPVE]

It's a long time since I used the ASME VIII, Div.1 routine for low temperature service and I've fogotten how the code treats the Test pressure (at ambient temperature) for a Design temperature below ambient. Also, I don't have a copy of ASME VII to hand.

Can someone remind me of the rules, please?

Thanks,

PPVE

 

[bvi]

Except for the recently added rules that permit higher allowable stresses in certain cryogenic services (which I am assuming don't apply in your case), the allowable stress does not change below 100F, even though the material gets stronger as it gets colder. Since the allowable stress is the same at design and test conditions, the stress ratio correction is 1.0, so the hydrotest pressure is not affected by the fact that the operating temperature is colder than ambient.

 

[JohnGP]

I am not a frequent user of ASME VIII, but as I see it hydrostatic test pressure is derived by multiplying MAWP (or Design Pressure) by 1.3 and then by the ratio of the stress value at Test Temperature compared to stress value at Design Temperature (UG-99)

ASME II Part D stress values at ambient are actually applicable for the temperature range -20 to 100degF, so your stress ratio at test temperature should be 1.0.

A general note to Table 1A of ASME II Part D also states: "Stress values for -20 to 100degF are applicable for colder temperatures when toughness requirements of Section III or Section VIII are met."

John

 

[PPVE]

Thanks for your reply cb4. Yes, I am interested in cryogenic temperatures and yes, my chosen materials do indeed have enhanced mechanical properties at the design temperature.

What do the new rules say, in brief, of course?

Can I take advantage of the low temperature properties?

Any help greatly appreciated.

Thanks,

PPVE

 

[bvi]

It is Part ULT of Section VIII, Div 1. It provides higher allowable stresses at very low temperatures. It only covers a handfull of specific alloys, such as 304, some nickel steels, and some aluminum. There are of course additional rules to be complied with in ULT.

 

[martintw]

You have probably got this point covered but you must be careful that none of your materials (including supports etc) that are subject to low temp will experience britttle fracture. This normally only applies to the carbon steel components but at least once every twenty years there has been a major catastrophe becuase of brittle fracture in low temp equipment.