Overview:

The Minimum Design Metal Temperature (MDMT) is a critical parameter in pressure vessel design. It ensures that the selected material maintains adequate toughness at low temperatures, thereby preventing brittle fracture.

At reduced temperatures, carbon and low‑alloy steels may lose ductility and can fail suddenly without significant deformation. Therefore, proper MDMT evaluation is essential for safe and reliable operation.

Code compliance requirement:

AutoPIPE Vessel Report:

• Refer below report screenshot for your reference:

• Curve:
  • Impact test exemption curves depend on material type.
  • Refer to Figure UCS‑66 Notes (1) to (4) for curve assignment.
  • For Example: SA333 Gr 6 falls in Curve B

• Governing Thickness (Er):
  • It is nominal non corroded governing thickness defined under UCS-66(a)(1)(-a), (-b) & (-c).
  • Refer Figure UCS-66.3 for typical vessel details.
  • For Example pipe material (Er = Pipe nominal thickness - mill tolerance 12.5%)

• MDMTs:
  • Two temperature values are considered: MDMT as per Figure UCS-66 (Table UCS-66) and Impact test temperature defined by user.
  • These are initial values without any reduction.
• Ratio:
  • AutoPIPE Vessel calculates ratio as per Note 1 of Figure UCS-66.1 instead of calculating by formula trE*/(tn-c)

• Re:
  • Reduction in MDMT permitted based on Figure UCS-66.1 and UCS-68.2(a). 
• MDMTa:
  • Final MDMT after applying temperature reduction:
    • MDMTa​=MDMT (from UCS‑66)−Re​
  • If material is already impact tested per specification: Applies Clause UCS‑66(g)
    • Compare material test temperature with calculated MDMT
    • Impact testing can be exempted if:

• • • • MDMT≥(Test Temperature−3∘C)

 

See Also

Bentley AutoPIPE Vessel

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