ASME VIII Div. 1 - Definition of pressurization and MDMT 1

[VGianini]

I'm facing a problem with a div 1 vessel and I need your help.

If I'm right a vessel can be at a temperature below MDMT if it's not pressurized. Can you please help me with the definition of pressurization? I found on an old thread that it's defined as 35% of MAWP but I can't find this on the code.

Thank you in advance.

 

[moltenmetal]

I'd define "pressurization" as any pressure which would put the vessel into the scope of the code, i.e. any pressure above 15 psig.

This is a common misunderstanding. MDMT is the minimum temperature which the vessel will experience during actual pressurized service, in the same way that MAWT is the maximum temperature anticipated in pressurized service. You can have non-code operating cases at temperatures above MAWT or below MDMT and they do not need to be reflected on the nameplate.

For example, it is idiotic to design water-conveying piping or vessels for -40 F, using killed carbon steel etc., merely because in the installation location, ambient temperatures may drop to -40 F in winter. Those units MUST be heat traced to keep water above freezing, and if they aren't, they're going to rupture due to the expansion of freezing water and there's no pressure relief device which will prevent that rupture. But a CO2 system which may auto-refrigerate in service to a very low temperature? Different matter entirely.

 

[TGS4]

I'd use 30% of MAWP because a stress ratio of 0.3 in UCS-66 puts any material below the concern for brittle fracture. Otherwise, I'd agree with moltenmetal.

 

[david339933]

I believe ratio is .35 in UCS-66

 

[TGS4]

I stand corrected. Figure USC-66.1 uses a cut-off ratio of 0.35.

 

[moltenmetal]

Good to know- proving the usefulness of Eng-Tips yet again!

 

[SnTMan]

As if needed

The problem with sloppy work is that the supply FAR EXCEEDS the demand