Hello.

In general vessels have a MDMT (usually specified by client), and a MAWP (calculated by designer). Vessels may have multiple MAWP’s. My post is not concerned about multiple MAWP’s for the time being.

Coming back to my question. In UG-20 (Design Temperature), the MDMT marked on the nameplate shall correspond to a coincident pressure equal to the MAWP. What is the relation of the MAWP to the MDMT, and how can one arrive at that?

Thanks. Appreciate it.

Sharjeel

First of all vessel will not have multiple MAWP, MAWP may be different for each component of the vessel but final MAWP for the vessel shall be the lowest MAWP of any of the component.

Client specified MDMT and MAWP will not have impact on each other but calculated/ rated MDMT of the component may vary based on the stress level/ MAWP of the component.

Regards

The vessel MAWP and MDMT form a "pair" of related numbers. The vessel MAWP will be the least value of those calculated for each component of the vessel, using the allowable stress at the given MDMT.

Vessels to ASME Code may have more than one combination of MAWP and MDMT; see footnote 37 to UG-116. Providing for multiple MAWP/MDMT ratings allows for vessels to "start up" at colder temperatures.

MDMT for carbon steel components is determined from rules of UCS-66. The MDMT rating of a particular component is based on its governing thickness and its metallurgy. eg: SA-516 gr 70 at 1" thick is rated to +31° F based on curve B (Fig UCS-66), but is rated to -30° F if the material is normalized.

Pressure will affect the MDMT rating when the reduction in MDMT based on coincident stress ratio can be used to determine the rated MDMT (see Figures UCS-66.1 and UCS-66.2). For a given component, as the pressure decreases its stress and coincident stress ratio also decrease, consequently the reduction in basic impact test exemption temperature can be further reduced by use of Figure UCS-66.1.

The vessel MAWP and MDMT form a "pair" of related numbers. The vessel MAWP will be the least value of those calculated for each component of the vessel, using the allowable stress at the given MDMT.

Should not the MAWP be calculated at the designated coincident pressure (i.e. hot and corroded condition) as per definition given in Mandatory Appendix 3.

Thanks again.

AL

Yes, MAWP to be calculated at the designated coincident temperature/s (in hot and corroded condition).


Appears to be a typo in Tom's post.

MAWP is the maximum gauge pressure permissible at the top of the completed vessel in its normal opearting positon at the designated coincident temperature (ug-20 b). The designated coincident temperature is basically MDMT that is defined by client, according to the their service. The calculated MDMT, as per code, is not meant for deignated temperature.

.

Seems another typing error.
Quote:
<The designated coincident temperature is basically MDMT that is defined by client>
Unquote.
It is not the MDMT but it is the design temperature (max.). The MDMT is defined by the User or may be calculated. Please read definitions in UG-20.
Mauro