Minimum design metal temperature - concept 2

[Allcciinneeii]

What is the concept / definition of Minimum Design Metal Temperature? How can we define it for a specific design of a pressure vessel which will be installed in such a location where the minimum environment temperature is around -40ºC? My understanding is that we don't need to adopt such a low temperature as the Minimum design metal temperature of that pressure vessel in case the Minimum operating temperature will never reach it. What is the general understanding under the definitions of ASME VIII div.1 Code and EN 13.445 Code?

 

[XL83NL]

ASME VIII-1: see UG-20(b)

(b) Minimum. The minimum metal temperature used in
design shall be the lowest expected in service except
when lower temperatures are permitted by the rules of
this Division10 (see UCS-66, UCS-160, and UG-116). The
minimum mean metal temperature shall be determined
by the principles described in (a) above. Consideration
shall include the lowest operating temperature, operational
upsets, autorefrigeration, atmospheric temperature,
and any other sources of cooling [except as
permitted in (f)(3) below for vessels meeting the requirements
of (f) below]. The MDMT marked on the nameplate
shall correspond to a coincident pressure equal to the
MAWP. When there are multiple MAWP’s, the largest value
shall be used to establish the MDMT marked on the
nameplate. Additional MDMT’s corresponding with other
MAWP’s may also be marked on the nameplate.

EN 13445: see EN 13445-2 para 3.1.1:

minimum metal temperature T_M
the lowest temperature determined for any of the following conditions (also see 3.1.2, 3.1.3):
- normal operations;
- start up and shut down procedures;
- possible process upsets, such as flashings of fluid, which have an atmospheric boiling point below 0 °C;
- during pressure or leak testing.

Now, as to your question

My understanding is that we don't need to adopt such a low temperature as the Minimum design metal temperature of that pressure vessel in case the Minimum operating temperature will never reach it

An important aspect of the MDMT, or whatever you want to call it, is that it does not mean that temperature which the vessel will ever see in it's life as being the lowest temperature. However, it is that temperature that the vessel will ever see in its life, coincidental with pressure. That difference is defined very well in UG-20(b), first sentence, second line (service).

 

[weldstan]

A star for you.