MDMT and Material Selection Under API 650: Where the Two Quietly Fight Each Other

What MDMT Is and Why It Isn't a Simple Lookup

Minimum Design Metal Temperature is the lowest metal temperature a tank component can experience in service — usually driven by the coldest ambient condition expected at the site, adjusted for any process heating or refrigeration effect on the stored product. Below a material's ductile-to-brittle transition, steel that would normally yield and deform under overload instead fractures suddenly with little or no warning. MDMT exists to keep every plate, nozzle neck, and structural member on the safe side of that transition for the coldest condition the tank will actually see.

The part that trips engineers up is that MDMT is not a property you look up once for a material grade and apply everywhere. API 650's impact-test exemption curves relate three things simultaneously: the material grade, the governing thickness of the specific component, and the actual stress the component carries relative to its allowable. Change any one of the three, and the exemption temperature for that component changes with it. A shell course, a nozzle neck, and a structural clip on the same tank can each have a different MDMT exemption temperature, because each has a different governing thickness and stress ratio — there is no single "the MDMT" for the tank as a whole, only a lowest-common-denominator design MDMT that every component must individually satisfy.

How the Impact-Test Exemption Curves Work

API 650's exemption curves plot governing thickness against a temperature below which Charpy V-notch impact testing is required for that material group, unless the component is thin enough or the stress ratio low enough to fall in the exempt region. The logic runs in the opposite direction from how most engineers first read it:

• Thinner plates get more exemption headroom. A thin plate has less material through which a brittle crack can propagate, and the curves reflect that — the same material grade can be exempt from impact testing at a lower MDMT when the governing thickness is smaller.
• Thicker plates need either a tougher grade or actual impact testing. As governing thickness increases, the exemption temperature rises — meaning the same steel grade becomes exempt only at progressively warmer minimum temperatures. Past some thickness for a given grade, the design MDMT falls below the exemption curve entirely, and the plate must either be impact tested to a specified energy value, or a tougher material grade with better low-temperature notch toughness must be substituted.
• The stress ratio matters too. A component operating well below its allowable stress gets a more favourable exemption temperature than one operating close to full allowable stress, because the reduced stress ratio reduces the driving force for brittle fracture propagation.

This is why "governing thickness" is the term to focus on, not nominal ordered thickness. It typically means the greater of the actual plate thickness or some fraction of it depending on the detail (butt-welded plate vs. as-welded nozzle attachment), and it is calculated per component, not once for the whole tank.

The Collision Nobody Plans For: Corrosion Allowance vs. MDMT

Here is the interaction that causes real project rework. Corrosion allowance is added to increase the ordered plate thickness for a completely separate reason — service life against metal loss. But governing thickness is exactly the input that drives MDMT exemption in the other direction. Increase CA late in a project — a client tightens the corrosion spec, or a revised process stream is more aggressive than assumed — and the ordered thickness of every affected course goes up. That same increase can push a previously-exempt plate below the exemption curve for the site's design MDMT, at the same material grade.

The failure pattern in practice: the shell design was checked for MDMT exemption once, early, using the original CA and an early-stage plate thickness. CA or thickness is revised later for an unrelated reason — a corrosion study update, a hydrotest thickness governing after a specific gravity change, a nozzle schedule upgrade — and nobody re-runs the MDMT exemption check against the new governing thickness. The tank proceeds to fabrication with plates that are, on paper, no longer exempt from impact testing at the design MDMT, and the gap surfaces during document review, or worse, during a third-party audit after fabrication is complete.

The same collision happens in reverse with nozzle necks: a schedule upgrade specified to satisfy reinforcement area after CA is deducted (see the nozzle reinforcement guide) increases the neck's governing thickness, which can independently push the nozzle material's MDMT exemption in the wrong direction — a second-order effect that is easy to miss because the schedule change was made to solve a completely different problem.

Any change to plate thickness or corrosion allowance — for any reason — should trigger a re-check of MDMT exemption on every affected component, not just the check that motivated the change. The two calculations use the same governing thickness input but move in opposite directions.

Material Selection: Trading Grade for Thickness Headroom

When a component's governing thickness pushes below the exemption curve for its current grade at the site's design MDMT, there are only three ways forward, and each has a different cost profile:

• Impact test the plate. Charpy V-notch testing at the specified temperature, to a minimum absorbed energy value, qualifies the material for service below the exemption curve. This adds testing cost and lead time from the mill, and it is a per-heat qualification — a mill certificate covering the specific heat and thickness must be tracked through fabrication.
• Substitute a tougher material grade. Grades with better inherent low-temperature notch toughness (typically achieved through fine-grain practice, normalizing, or a different chemistry) have exemption curves shifted to colder temperatures at the same thickness. This avoids impact testing but usually costs more per tonne and may affect weldability procedures.
• Reduce the governing thickness where possible. If the thickness increase was driven by an avoidable design margin rather than a hard requirement, reducing it back toward the structural minimum can restore exemption without a grade or testing change — but this only works if the thickness increase wasn't actually required for CA or structural reasons in the first place.

The decision is a cost and schedule trade, not a purely technical one, and it belongs in front of the client early — discovering it during fabrication document review, when mill order lead times are already committed, is the expensive version of this conversation.

Practical Tips

• Establish the site design MDMT before selecting any material grade. It should come from the lowest one-day mean ambient temperature at the site (adjusted per the project's basis of design), not an assumed value carried over from a previous project in a different climate.
• Check MDMT exemption per component, not once for the tank. Shell courses, roof plates, nozzle necks, and structural attachments each have their own governing thickness and may not share the same exemption outcome even on the same tank.
• Re-run the MDMT check whenever thickness or CA changes — for any reason. Treat it as a mandatory downstream check alongside the weight and foundation load updates, not an optional one.
• Track exemption status on the design register. A simple pass/fail column per component against the site design MDMT, updated whenever a thickness input changes, catches the collision before fabrication rather than during document review.
• Don't assume a "cold climate" material choice made early stays valid. A grade selected for MDMT exemption at the project's original thickness assumptions can silently stop being exempt after later revisions — verify, don't assume it still holds.

Related reading: Continue with Corrosion Allowance Strategy, Shell Course Thickness Design, and Nozzle Reinforcement Under API 650 to keep the full API 650 design workflow connected.