MDMT

Chapter from the Miscellaneous Design Guide.

MINIMUM DESIGN METAL TEMPERATURE (MDMT)

MDMT Fundamentals

MDMT is the lowest temperature at which a pressure vessel or piping component may be subjected to full design pressure without risk of brittle fracture.

Why It Matters:
• Carbon steel and low-alloy steels undergo ductile-to-brittle transition
• Below the transition temperature, steel fails by brittle (cleavage) fracture
• Brittle fracture is sudden, catastrophic, and occurs without warning
• MDMT ensures the material has adequate toughness at the lowest expected operating temperature

Key Standards:
• ASME VIII Div. 1, UCS-66: impact test exemption curves
• ASME B31.3: impact testing requirements for piping
• API 579: fitness-for-service assessment for MDMT issues

ASME VIII UCS-66 Impact Test Exemption

ASME provides exemption curves (A, B, C, D) that define the minimum temperature at which a material is exempt from Charpy impact testing, based on thickness.

Curve Assignment by Material Group:
• Curve A: SA-515 Gr 70 (>1"), SA-516 Gr 70 (not normalized), some SA-285
• Curve B: SA-516 Gr 55/60/65/70 (normalized or per UCS-66(b)(3)), SA-515 Gr 60
• Curve C: SA-516 Gr 65/70 (if impact tested), SA-537 Class 1
• Curve D: SA-203 (2.25Ni), SA-333 Gr 1 (piping)

Reading the Curves:
1. Determine governing thickness (lesser of nominal thickness or 1/4 of component diameter for flat plates)
2. Find curve for your material
3. At your thickness, read the minimum exemption temperature
4. If your MDMT < exemption temperature → impact testing required
5. OR apply thickness ratio reduction: if actual stress < allowable, MDMT can be reduced
Reduction per UCS-66.1(b): temperature credit = f(stress ratio)
This allows thinner (less stressed) components to operate at lower temperatures

Common MDMT Issues in Practice

Issue	Description	Resolution	Reference
Auto-refrigeration	Depressuring causes temperature drop below MDMT	Evaluate per API 521/UCS-66.1, may need impact testing	ASME VIII UCS-66
Startup in cold weather	Pressurizing equipment below MDMT	Warm up before pressurizing, or reduce startup pressure	Operating procedure
Piping branch connections	Small branches on large headers may have lower MDMT	Check each component individually, welded attachments too	ASME B31.3
Flange bolting	Bolts have separate MDMT requirement	Use SA-193 B7M or SA-320 L7 for cold service	ASME VIII UCS-66
Fitness-for-service	Existing equipment below original MDMT	API 579 Level 1/2/3 assessment	API 579 Part 3

Source: Miscellaneous_Design_Guide_v1.xlsx · sheet “MDMT”

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