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Integrity of tube-to-header welds in a waste heat boiler

Following  the discovery of leaks from two tube-to-header welds in a Waste Heat Boiler, an Industrial Member requested that TWI identify and evaluate suitable non-destructive testing techniques to establish the integrity of the remaining welds. The aims of the inspection were to detect and sentence surface cracks, sub-surface cracks and original welding flaws.

In response the following tasks were performed:

  • Manufacture of a test block containing six artificial lack of fusion flaws

  • Development of NDT techniques allowing the detection and accurate sizing of all six flaws (with through-wall sizes of 2mm upwards)

  • Establishment of approved procedures for site deployment

The final procedures were based on:

  • A swept beam Phased Array Ultrasonic Testing (PAUT) technique for detection and sizing of flaws embedded/root flaws

  • An encircling coil MPI technique for detection of surface flaws

Figure 1 - Manufactured test block showing three tube-to-header welds
Figure 1 - Manufactured test block showing three tube-to-header welds

The PAUT technique was deployed in a tube scanner, which allows full access even where adjacent tubes are closely spaced. The inspection procedures were approved for site use and were successfully demonstrated to the client at its site in the Middle East.

The equipment is, in general, able to inspect a wide range of pipes, with diameters ranging from 0.84” (21mm) up to 4.5” (114mm), and it can be applied to pipe-to-pipe welds as well as pipe-to-component welds. The PAUT scanner has a height clearance of just 12mm (0.5”) allowing inspections in limited access areas, eg where there are adjacent obstructions such as nearby pipes or other structures.

For more information, please email contactus@twi.co.uk

Figure 2 - Through-wall sizing of 6mm flaw from test block using the PAUT procedure
Figure 2 - Through-wall sizing of 6mm flaw from test block using the PAUT procedure
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