Minimum Required Thickness(t_min)
Definition
The minimum wall thickness required by the applicable design code to safely contain the operating pressure at the design temperature, considering the allowable stress of the material, weld joint efficiency, and applicable code margins. This is the absolute minimum thickness for pressure containment — below this value, the equipment exceeds its design basis and must be retired, repaired, or de-rated.
Guide
Context
Minimum required thickness is calculated per the original design code (ASME VIII for vessels, B31.3 for piping) and is a fixed reference value for each component. It determines the retirement threshold and the corrosion allowance consumed. In Reliatic, t_min is stored per CML and used in automated remaining life calculations.
Formula
t_min = PR/(SE - 0.6P) for cylindrical shells (ASME VIII Div.1), where P=design pressure, R=inside radius, S=allowable stress, E=joint efficiency
Examples
- For a vessel shell: P=35 bar, R=500mm, S=138 MPa, E=0.85: t_min = (3.5 x 500)/(138 x 0.85 - 0.6 x 3.5) = 15.0mm.
- For a pipe: the minimum required thickness per B31.3 is 4.8mm. The measured thickness is 6.2mm, leaving 1.4mm above t_min as remaining margin.
Applicable Standards
- ASME BPVC Section VIII
- ASME B31.3
- API 510
- API 570
Related Terms
The wall thickness specified at the time of design and fabrication, before any in-service degradation. Nominal thickness is the starting point for remaining life calculations and is the thickness that should be measured at a CML immediately after construction. It includes the minimum required thickness plus the corrosion allowance plus any manufacturing tolerance.
Retirement ThicknessThe minimum acceptable wall thickness below which equipment must be retired from service, repaired, or re-rated. Retirement thickness equals the minimum required thickness for pressure containment plus any additional thickness required by the applicable code (such as structural minimum for external loads, corrosion allowance for future service, or minimum for NDE reliability). When measured thickness reaches retirement thickness, the equipment has exhausted its design margin.
Corrosion Allowance(CA)Additional wall thickness added to the minimum required thickness during the design phase to compensate for expected material loss due to corrosion, erosion, and other degradation mechanisms during the equipment's design life. Corrosion allowance is the buffer between the as-built thickness and the retirement thickness — once consumed, the equipment has reached its design end of life.
Design ConditionsThe maximum temperature, pressure, and other parameters for which equipment is designed and rated. Design conditions define the mechanical design basis and determine the minimum required wall thickness, material selection, and construction code requirements. Actual operating conditions should remain within the design envelope — operation outside design conditions requires engineering assessment and potentially re-rating or modification.
Fitness-for-Service(FFS)An engineering assessment methodology used to determine whether equipment with identified flaws, damage, or degradation can continue to operate safely and reliably for a defined future period. FFS evaluations quantify the remaining structural capacity of degraded equipment using the procedures in API 579-1/ASME FFS-1, considering the specific type and extent of damage relative to the design margins.
Where this appears in Reliatic
This concept is used in the Inspections module.