The following information was extracted from a publication by Sentin dated 14 June 2021, which provides a provocative viewpoint about possible failures in non-destructive testing (NDT). Procedures and processes combined with effective training, adequate testing and audits can limit failures and catastrophes.
Bridges, nuclear plants and aeroplanes are particularly vulnerable as NDT plays a significant role in ensuring that engineering systems are in good shape and safe to use. This great input is attributed to the test methodology’s capability to detect and analyse welding defects and discontinuities in an engineering system or component without inflicting further damage to the original part or system.
However, if the right procedures are not followed, NDT can lead to various failures and catastrophes. This article focuses on the hiccups and the potential subsequent impacts.
There are three types of failure and catastrophe in NDT:1. Danger: using the wrong NDT method
In some cases, a non-destructive test process may fail to detect some critical flaws in crucial engineering components. In some instances, human factors are to blame for the failures. For instance, lack of the necessary skills/experience could lead to poor inspection, whereby unacceptable defects go unnoticed.
The reliability of an NDT method is the test’s ability to detect critical defects in a system. When a test fails to detect critical defects with significant impact on the integrity of a structure, the undetected problems might escalate to cause structural failure. Consequently, the compromised structure might turn out to pose a safety risk and cause negative economic, environmental and reputational impact.
For instance, in March 2012, an ultrasonic inspection missed five significant axial flaws in the North Anna Power Station steam generator. The test involved a site-specific mock-up for the qualification of the inspection procedure. An encoded phased array technique was used to scan for circumferential defects. However, even with all these approaches in place, some flaws were still missed.
2. Danger: failure due to incorrect interpretation
In some cases, the NDT equipment/system might detect the flaws, but the failure arises when it comes to improper defect sentencing/interpretation. This simply means that the inspector failed to clarify an accurate picture of the significance of the flaw. For instance, an inspector can downplay the seriousness of a defect, terming it as too minute to cause any remarkable structural failure and thus not requiring any intervention. In most cases, the misinterpretation occurs when the assessor lacks the necessary skills and experience. Additionally, external factors such as physical and social environment as well as motivation factors have a remarkable role to play for NDT accuracy.
Non-destructive tests are instrumental in the maintenance of the integrity of various crucial mechanical systems and components. Nevertheless, failures resulting from human factors or inherent errors can occur if particular things are not well taken care of. It is critical that inspectors apply various testing technologies and complementary techniques to ensure that no significant defect in a mechanical system or unit goes unnoticed. Companies employing NDT inspectors should ensure that the experts they engage are sufficiently skilled and experienced. It is crucial to ensure that the testing process is carried out with the utmost care to prevent exposure that can harm personnel and the external environment at large.
3. Danger: wrong procedures or instructions
NDT failures as a result of errors, or omission or misinterpretation of results, can be catastrophic. For instance, if serious defects in a nuclear plant are overlooked in a non-destructive test process, highly radioactive nuclear waste can find its way into the environment, causing pollution, loss of lives and long-term damage.
Radiographic testing (RT) utilises X-rays or gamma rays for internal structure examination. Unfortunately, RT can pose a hazard to human tissue if the person handling the process is overexposed to the film processing chemicals used in the procedure. This danger is mainly pronounced if the process takes place in an area that is not well ventilated. Someone interacting with the process for an extended period in such an environment has a high risk of various health issues as a result of the inhalation of the fumes. Apart from direct health concerns, RT has waste effluent that can cause environmental pollution.
This article piggybacks on a previous article about the necessity for an effective auditing programme.
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