MDMT (minimum design metal temperature) is the coldest metal temperature a pressure vessel is designed to safely see in service, and it is the temperature [ELI-REVIEW: ASME Section VIII, Division 1] uses to decide whether a material needs impact (Charpy) testing to guard against brittle fracture. For a refrigeration vessel, MDMT is usually set at or below the coldest saturated suction temperature the vessel will ever operate at, since metal at low temperature can fracture in a brittle rather than ductile manner if it is not shown to be tough enough at that temperature.
Why MDMT matters for refrigeration vessels
Refrigeration vessels are a common place MDMT becomes a real design driver, because they routinely operate colder than typical process equipment. [ELI-REVIEW: ASME VIII-1] provides a set of exemption curves (by material category) that state, for a given governing thickness, how cold a carbon or low-alloy steel vessel can go before impact testing is required. Thicker material and colder MDMT both push a design toward the exemption curve’s limit; cross it, and the material either needs impact testing or the vessel needs a different treatment path (normalizing, or normalizing plus post-weld heat treatment) to regain an exemption.
| Factor | Effect on impact-test exemption |
|---|---|
| Colder required MDMT | Makes exemption harder to achieve |
| Thicker governing plate | Makes exemption harder to achieve |
| Normalizing / PWHT | [ELI-REVIEW: can shift the applicable exemption curve and extend the exemption to a colder temperature] |
| Lower stress ratio at the design condition | [ELI-REVIEW: an under-stressed component (operating well below its full allowable stress) can qualify for a colder exemption] |
MDMT vs. austenitic stainless
Austenitic stainless steels behave very differently at low temperature than carbon steel and are generally exempt from impact testing down to a much colder threshold ([ELI-REVIEW: commonly cited around −320°F]), which is one reason stainless is sometimes chosen for very cold ammonia or CO₂ service specifically to avoid the MDMT problem carbon steel would otherwise create.
Run it
PRELIM screens your vessel design against the MDMT exemption curves and reports whether impact testing, normalizing, or PWHT is indicated, on screen, free.
Open PRELIM →FAQ
Is MDMT the same as the coldest operating temperature?
They're related but not identical. MDMT is a design basis chosen to bound the coldest metal temperature the vessel could see, including startup, upset, and standstill conditions, not just the normal steady-state operating temperature.
What happens if impact testing is required?
The plate material has to be tested (or purchased pre-qualified) to demonstrate adequate toughness at the required MDMT, which adds cost and lead time. Many designs are deliberately arranged (material choice, thickness, treatment) specifically to stay inside an exemption and avoid that requirement.
Does corrosion allowance affect the MDMT check?
The governing thickness used for the exemption-curve lookup is typically the vessel's actual (often corroded) thickness at the joint, so corrosion allowance and plate rounding both factor into which point on the exemption curve applies.
Can PWHT help avoid impact testing?
Post-weld heat treatment can, in some cases, extend how cold a design can go before impact testing is triggered, as part of a broader treatment/exemption strategy alongside normalizing and stress-ratio credit. Whether it helps a specific design is a case-by-case code determination.