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Rated MDMT

Rated MDMT

Rated MDMT

Hello Respected Members ,
I am trying to run calculation in COMPRESS , for NPS18 Sch120 - SA-106 C smls pipe as cylinder and 2:1Ellip Head 1.25'' Thk SA-516 Gr.70 . Used MDMT is -20deg.F. Design Pressure 2500psi @350degF .
So in result : In Deficiencies report, it gives me that one note : '' for Cylinder Pipe The Rated MDMT of 4.65 deg F is warmer than the design MDMT of -20 deg.F'' & for the Head rated MDMT 0.95 deg F warmer than -20deg.F ''
My request to you , can you please kindly guide me how to remove this deficiencies ? do I need to go with Impact testing check option on it ? Additionally do I need to put full complete vessel for PWHT or just Head and Shell only? I guess as per SecVIII div.1 for SA-516 Gr70 thickness above 1.5'' Normalize is mandatory . but Is it mandatory to put complete vessel as PWHT option on it too?
And after running all different calculation , If I will put / check mark all these 3 option Impact tested , Material Normalize , and PWHT performed , than only I can be able to get report without deficiencies. Do I really need to put all these 3 condition on complete Vessel ?

Thank you .
- AB53Mech.

RE: Rated MDMT

Paragraph UCS-66 of ASME B&PV Code, Section VIII, Division 1, is a very complex section.

By specifying the material as normalized and/or produced to fine grain practice you effectively change the material curve in Figure UCS-66 (the four curves A, B, C, and D), which then changes the material's basic impact test exemption temperature as based on the governing thickness.

See Figure UCS-66.2 "Diagram of UCS-66 Rules for Determining Lowest Minimum Design Metal Temperature (MDMT) Without Impact Testing". COMPRESS follows the method outlined in this flowchart. Further, UCS-66(i) indicates that the provisions of UCS-66 may be applied to material that has been impact tested in order to obtain a colder rated MDMT.

The use of PWHT, when not otherwise required by Code rules, will get you a 30° F reduction (colder) in the rated MDMT per paragraph UCS-68(c).

Note that COMPRESS calculates the MDMT rating using the pressure chamber's MAWP. The MDMT may vary with the pressure via the rules of Figure UCS-66.1 and related paragraphs. COMPRESS provides functions to limit the chamber MAWP in order to "maximize" the MDMT rating, if desired. Otherwise, the MAWP is maximized.

Tom Barsh

RE: Rated MDMT


Please do not cross-post on multiple threads - pick one.

See my response to your thread in Boiler and Pressure Vessel engineering thread.


RE: Rated MDMT


Thank you Tom & Marty for your detail helpful guidelines on it.

- Yes , Marty , sorry for posting it in two different post about same question. next time I will keep it in my mind.

Regards ,
- AB

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