Choosing the Right NDT Method for Pressure Equipment

Separate the questions before choosing a method

Wall thickness, surface-breaking indications, internal discontinuities and material identity are different objectives. Ultrasonic thickness testing measures remaining wall at examined locations; it is not automatically a comprehensive weld examination. Positive material identification (PMI) concerns material identification and does not establish that a component is free of cracks or other flaws.

For pressure equipment, the credible damage mechanism, component geometry and material should drive the technique and coverage. Ask the NDT provider to state both the objective and the limits of the proposed work.

Scope non-destructive testing around the examination objective before selecting a technique or booking field work.

Understand the practical differences

Magnetic particle testing (MT) is used for surface and near-surface discontinuities in suitable ferromagnetic materials. Penetrant testing (PT) targets surface-breaking discontinuities on suitable non-porous surfaces. Neither should be specified without considering material suitability and surface condition.

Ultrasonic examination (UT) and radiography can provide information about features within a component, but their sensitivity and interpretation depend on the technique, geometry and discontinuity. Eddy current testing (ET) has applications in suitable conductive materials and component arrangements. These are not interchangeable menu choices; competent personnel need to select and qualify the approach for the actual scope.

Treat access and preparation as technical inputs

Coatings, temperature, roughness, insulation and access to one or both sides can influence whether the proposed technique is effective. Explain these constraints before mobilisation. Radiography also requires coordinated site controls; its feasibility cannot be decided from the defect question alone.

Define examination locations and coverage explicitly. “NDT completed” is not enough to show which weld, surface or volume was examined. A report should tie results to asset identity and a location reference that another examiner can reproduce.

• What damage or uncertainty is being investigated?
• Is the material known and suitable for the method?
• What preparation and access does the technique require?
• Who sets the acceptance basis and reviews indications?
• How will unexamined areas and limitations be reported?

Decide what happens after an indication

An indication needs interpretation against the agreed basis; it is not always a defect requiring repair. Equally, a report with no relevant indications only applies to the examined coverage and sensitivity. Agree an escalation route for additional examination or engineering assessment before field work begins.