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Nominal Thickness

Inspection & Monitoring
mechanical

Definition

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.

Guide

Context

Nominal thickness is the reference value in Reliatic's CML data. The long-term corrosion rate is calculated from nominal thickness to current thickness. When original thickness records are unavailable, nominal thickness may be estimated from pipe schedule tables or measured on non-corroded sections of the equipment.

Formula

Nominal Thickness = Minimum Required Thickness + Corrosion Allowance + Manufacturing Tolerance

Examples

  1. A 12-inch Schedule 40 carbon steel pipe has a nominal thickness of 10.31mm (per ASME B36.10). The minimum required thickness is 5.2mm, leaving 5.1mm of corrosion allowance plus tolerance.
  2. Nominal thickness of a vessel shell is 25mm (15mm minimum required + 6mm CA + 4mm manufacturing over-tolerance), as documented on the original design data sheet.

Applicable Standards

  • ASME B36.10
  • ASME BPVC Section VIII
  • ASME B31.3

Related Terms

Minimum Required Thickness(t_min)

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.

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.

Retirement Thickness

The 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.

Thickness Reading

A measured wall thickness value obtained at a CML using non-destructive examination techniques, typically ultrasonic thickness gauging. Thickness readings are the primary quantitative data for calculating corrosion rates and remaining life of static equipment. Each reading is recorded with the CML identifier, measurement date, measured value, measurement technique, and the technician.

Where this appears in Reliatic

This concept is used in the Inspections module.