This article highlights a long-standing trap in the ASME Boiler & Pressure Vessel Code – one that has persisted for years and continues to catch many designers off guard during pressure vessel calculations. In ASME VIII, Div. 1 (and Div. 2), the Minimum Design Metal Temperature (MDMT) can be determined for certain Code materials (UCS & UHA) by applying a stress percentage rule, commonly expressed as the coincident ratio. This ratio can be written in several equivalent ways:
v Thickness-based: Required thickness at MDMT ÷ Analysis thickness
v Pressure-based: Operating pressure at MDMT ÷ Maximum allowable pressure at MDMT
v Stress-based: Coincident tensile stress at MDMT ÷ Allowable stress at MDMT
The code clearly depicts internal pressure only for the formation of the coincident ratio. However, in practice there are situations where relying on this coincident stress ratio, formed only based on internal pressure, may be misleading or even unsafe.
Situations Where the Approach Becomes Risky
When global loads (UG-22) govern
In tall towers, for example, stability under combined loads may control design rather than internal pressure. It is not always clear whether such stability checks would influence tensile stresses, compressive stresses, or both.
When local nozzle loads are considered (WRC checks)
Designers often calculate nozzle local loads stresses between 80–99% of allowable. But key questions remain: Are these nozzle loads actually present at MDMT? Is the WRC pressure input the same as the operating pressure at MDMT, or closer to design pressure? Should these stresses be considered tensile?
When external pressure applies
Vessels may still operate at MDMT under external pressure, but the Code gives no guidance on coincident ratios in this case.
For components like tubesheets
Establishing MAWP is particularly difficult in heat exchangers when both shell-side and tube-side pressures are involved. A tubesheet calculated MAP (and therefore coincident ratio) at MDMT may not correctly represent all pressure scenarios, such as when one side is under vacuum.
Lessons from European Codes
The European standard EN 13445-2, Annex B uses a similar concept to ASME but with important clarifications. For instance, stresses must include internal and external pressure as well as dead weight, and for heat exchangers, tube-end restraints must be considered.
Similarly, PD 5500, Annex D makes comparable observations, reinforcing the need for engineering judgment rather than blind reliance on stress ratios. Particularly Note 2 states that the calculated tensile membrane stress may vary with the minimum design temperature, in cases such as auto-refrigeration during depressurization, and the coincident ratio should be evaluated allowing where appropriate for the possibility of re-pressurization while the vessel is still code (e.g hydraulic overfill).
Conclusion
A low coincident stress ratio method can really reduce the vessel MDMT, but in many real-world cases – global loads, local stresses, external pressure, or complex components – it may not provide a safe answer. Designers must carefully interpret the Code, apply judgment, and when uncertain, choose conservative assumptions.