5 Effective NDT Methods for Pressure Vessels

As pressure vessels often contain fluids and gases that are hazardous or toxic, any vessel leak or failure risks becoming a serious health and safety incident. Fortunately, Non-Destructive Testing (NDT) methods are available to identify any damage or flaws in the vessels that could cause such an event. The development and implementation of an NDT testing regime, by a qualified operator, will minimise this risk.

Detection of surface defects

1) Visual Testing
Visual testing is a common NDT method. It picks up defects like corrosion and physical damage. This method requires easy access to the surface, which presents challenges in some sites. It also relies on the eyesight of the inspector and good light conditions. Visual NDT methods can be enhanced by the use of magnification equipment. A visual inspection can also be done remotely using computerised video systems.

2) Magnetic particle testing
This method is only suitable for ferromagnetic materials like steel. An advantage is that it’s a portable and inexpensive NDT method that detects cracks and defects that break the surface of the material. It’s a two-stage process. A magnetic current is run through the component. Any surface defects will interrupt the lines of magnetic flux, so when the operator sprays magnetic particles onto the surface they are attracted to collect in any defects. The use of fluorescent magnetic inks makes the particles more visible to the NDT operator.

3) Eddy current testing
An electrical current is passed through a coil close to the surface of the pressure vessel, which induces eddy currents. A defect in the surface will restrict the eddy current flow. Where the vessel is manufactured from non-magnetic materials, this method of NDT testing provides a measurement that is indicative of the depth of the defect.

Detection of internal defects

4) Ultrasonic testing
Ultrasonic testing needs a good surface finish and utilises high-frequency sound waves to reflect defects such as cracks. It can also provide wall thickness data. It works through the transfer of sound waves from an ultrasonic probe to the component being tested and needs a coupling medium, usually a water or gel. Manual application of ultrasonic testing is slow over a large surface area, which can be a limitation of the method. Ultrasonic testing is also available as an automated technique, with hard copy results.

5) Radiographic testing
Radiography works by detecting material loss. Radiation is applied and any variation passing through the material is measured and captured on film. Radiography is popular as an NDT method as the radiographic film provides the NDT operator with a hard copy of the results. Digital radiography is an even quicker method, with the results available on a computer screen in seconds, rather than captured on film.
Find out more
A robust NDT and inspection regime will combine the most appropriate methods to detect surface defects, with those that offer the detection of internal flaws. Investment in an NDT programme will ensure safe operational service and avoid asset failure. Get in touch to find out more.

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