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Thickness Reading

Inspection & Monitoring
mechanical

Definition

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.

Guide

Context

Thickness readings form the time-series data that drives corrosion rate calculations and remaining life projections. In Reliatic, readings are logged per CML with trending charts that show the thickness trajectory over time. Anomalous readings (sudden drops, unexpected increases) are flagged for verification to prevent data quality issues from corrupting corrosion rate calculations.

Examples

  1. CML-HX101-S1-01: Nominal 12.7mm, Previous reading 11.2mm (2024-03-15), Current reading 10.8mm (2025-03-15), Calculated short-term corrosion rate 0.40 mm/year.
  2. A thickness reading of 15.1mm on a component with nominal thickness 12.7mm is flagged as anomalous — likely a measurement error or reading taken on a weld pad rather than the base metal.

Applicable Standards

  • API 510
  • API 570
  • ASME E797

Related Terms

Condition Monitoring Location(CML)

A designated, permanently marked point on equipment where thickness measurements or other condition assessments are taken at regular intervals. CMLs (also called TMLs — Thickness Measurement Locations) are positioned at locations where degradation is expected to be most severe or most representative — areas of high velocity, turbulence, impingement, deadlegs, injection points, and heat-affected zones. Each CML has a unique identifier and documented coordinates.

Corrosion Rate

The rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.

Nominal Thickness

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.

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.

Ultrasonic Testing(UT)

An NDE method that uses high-frequency sound waves to detect internal flaws and measure wall thickness in materials. A transducer sends ultrasonic pulses into the material; reflections from internal surfaces, flaws, or the back wall are analyzed to determine thickness, flaw size, and flaw location. UT techniques include conventional (A-scan), phased array (PA-UT), time-of-flight diffraction (TOFD), guided wave (GWUT), and automated/semi-automated scanning (AUBT).

Where this appears in Reliatic

This concept is used in the Inspections module.