This paper describes non-destructive testing systems that were constructed to inspect tube-to-tubesheet welds of a heat exchanger operating in a chemical plant such as urea products.
A phased array ultrasonic testing system inserts a testing device into the tube, rotates it and scans the tube-to-tubesheet welds, thereby detecting any weld defects that are not only parallel to a tube axis but also in any direction, and displaying the size and location of the defects on a computer screen using system software. These welds have conventionally been inspected by liquid penetrant testing and visual testing as surface inspections during fabrication at shop and periodic inspection at plant site. This new method, however, provides a practical way to the inspection of weld inside.
In addition, in a photographic imaging system, an imaging device is placed on the tube end to take photos of the inner surface of tube and the outer surface of tube including the tube-to-tubesheet weld. These photos are analyzed any suspected damage area, then displaying the location of the area on a computer screen using system software. In the periodic inspections at site, the presence or absence of a local corrosion dent or other damage are determined by visual testing and finger touch, but it is concerned that hundreds of welds will not be covered by these methods sufficiently. Consequently, this new method enables us to eliminate lacks of inspection and the secular change of each weld will be easily grasped with the suitable records.
The above testing systems perform in a short amount of time and are highly reliable methods with inspection records. For this reason, in particular, the application of the new method to a periodic inspection at site is expected to reduce the period until the plant resumes operations, and also to remove any concerns about emergency shutdown of an operating plant due to damages in the tube-to-tubesheet welds.