Essential Variables to Decide MDMT

For proceeding any process design for a pressure piping system, it is necessary to decide -at the 1st stage of design- the Design Temperature, which represents the max. (worst) expected temp. which can be attained during an operation. And this temp. shall be determined by a process designer.

The same thing occurred with MDMT, which represents the lower (worst) temp. can be attained during the operation and we have to protect our piping system to withstand the lowering in the temp. Also, the process designer who decided the Design Temp. is requested to define the MDMT to adapt with the process design requirements.

By defining the MDMT based on the process design, we can select the type of material, its type of heat treatment, and its thickness, then you can select the Curve A, B, C or D of Fig. 323.2.2a of ASME B31.3, from which you can find the Design Min. Temp. without impact test.

Example: Suppose you have a pipe in a system with an MDMT -20 ^oC, pipe wall nominal thickness 15 mm, and type of pipe material belong to Curve B. From Fig. 323.2.2a of ASME B31.3 the min. design temp. without impact test shall be approx. -16 ^oC (higher than -20 ^oC).

So, we have to proceed an impact test to verify the soundness of pipe material at -20 ^oC, otherwise you have to select another piping material with a lower Min. Design Temp. (belong to lower curves C or D).

For the same situation, if you select another pipe material which belongs to Curve C, the Design Min. Temp. for the same thickness 15 mm shall be approx. -33 ^oC which is lower than the MDMT -20 ^oC, hence your selection is OK without an impact test.

Fig. 323.2.2A Minimum Temperatures Without Impact Testing for Carbon Steel Materials (See Table A-1 for Designated Curve for a Listed Material; see Table 323.2.2A for Tabular Values)

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NOTES:

(1) Any carbon steel material may be used to a minimum temperature of −29°C (−20°F) for Category D Fluid Service.

(2) X Grades of API 5L, and ASTM A 381 materials, may be used in accordance with Curve B if normalized or quenched and tempered.

(3) The following materials may be used in accordance with Curve D if normalized:

(a) ASTM A 516 Plate, all grades

(b) ASTM A 671 Pipe, Grades CE55, CE60, and all grades made with A 516 plate

(c) ASTM A 672 Pipe, Grades E55, E60, and all grades made with A 516 plate

(4) A welding procedure for the manufacture of pipe or components shall include impact testing of welds and HAZ for any design minimum temperature below −29°C (−20°F), except as provided in Table 323.2.2, A-3(b).

(5) Impact testing in accordance with para. 323.3 is required for any design minimum temperature below −48°C (−55°F), except as permitted by Note (3) in Table 323.2.2.

(6) For blind flanges and blanks, T shall be 1⁄4 of the flange thickness.