Published on 28 January 2013
Process plant is often operated well past its anticipated end of life due to ever increasing productivity requirements in the face of global competition.
In the furnace and foundry industries, much of this plant operates at high temperature and pressure. If plant systems or individual components are not in serviceable condition, catastrophic and life threatening failures could occur.
Regulatory bodies world-wide are well aware of these risks, so inspection requirements such as regular programmes of Non-Destructive Testing are mandatory for continued operation of elderly plant.
It was under this premise that TWI was recently called in by a leading European process plant operator, to investigate several of its installations.
'We were summonsed to inspect certain furnace stove welds to add a third party independent view' recalls TWI's Steve Kenny. 'The furnace stoves are pressure vessels that transfer hot air to the furnace itself. They are not highly pressurised, but they reach temperatures of 200 - 300o Celsius.'
Separate contractors had previously done some NDT work for TWI's client. But, because the competent authority wanted confirmatory evidence in the form of more volumetric NDT inspection of the welds, a more sophisticated approach was required.
Previously, manual ultrasonic inspection was used. 'But this takes a very long time and it is very subjective' says Kenny. 'So they asked us to look at improving the inspection process. We introduced a technique called time of flight diffraction to inspect the welds in question very quickly, and with greater accuracy and repeatability.'
The welds in question had pre-existing defects which the client already knew about. They were caused by lack of fusion and had been there from day one of construction, 50 years earlier. The client wanted to assure itself that these defects were not going to grow and fail catastrophically in fatigue.
TWI developed the technique on some coupon plates, containing welds with induced defects, to prove that it could be done.
The work was originally due to last about nine months, but the competent authority decided, subject to TWI proving that the technique was viable, that they wanted a lot more welds inspected. The client then asked TWI to continue the programme of work for over 18 months.
'The client and competent authority were encouraged by the results we had given them and were happy with the accuracy of the technique' says Kenny. 'One of the main reasons for adopting TOFD is that it is very repeatable. Data is initially captured on computer and can be stored on CD. You can revisit and inspect the same weld several times and get more or less exactly the same result. You can then monitor if there has been any growth in the defects.'
TOFD is very accurate, particularly at cross section measurement of defects and is being used increasingly in critical engineering applications. The technique uses two probes, in a pitch and catch formation, in other words transmitting and receiving.
Instead of projecting a thin pencil beam signal a large volume of material is effectively 'flooded' with sound. If there is a defect a tip diffraction signal is detected, as opposed to just a reflected beam as in pulse-echo ultrasonic testing.
A circular wave front emanates from the tip of the defect, which is then picked up by the receiving probe. The system is computerised, and is calibrated to triangulate between the two probes, defect tip or tips, and the component back wall. This gives a very accurate position of the tip within the beam.
The claimed achievable tolerance for through-thickness sizing of a defect is ?1mm with TOFD. With more traditional ultrasonic methods it is supposedly ?3mm.
'The criteria for further investigation is that if we find a linear defect 50mm long or greater, that is not associated with the existing lack of fusion, that we already know about, then it is investigated using another ultrasonic testing technique' explains Kenny. 'If you use pulse echo ultrasonics for confirmation you get an even better idea of the shape and characteristics of the defect, than with TOFD.'
Looking back on the project Kenny comments; 'The best thing about the job for me was the way in which TWI established the confidence of the client early on. They quickly seemed to embrace us as being part of their engineering support team, instead of treating us as a sub-contractor'.
For information about TWI's capabilities please contact us.