When deterioration cannot be effectively identified or monitored in service, obtaining tube samples for destructive examination may be appropriate. Metallographic examination can be performed for deterioration mechanisms, such as decarburization, carburization, hydrogen attack, and stress cracking. Physical testing of creep life can be appropriate for severe services and for affirming any calculated remaining life. Oftentimes, calculated remaining tube life includes several assumptions of tube operating history that can lead to inaccurate results. The density of scale samples can be measured and may provide information on the tube operating history. Furthermore, physical properties of the tubes can be measured that can assist in damage assessments.
Prior to removal of a tube section, consideration should be given to the ability to make the repair weld between the new tube section and the existing tube sections.
In many heaters, tubes are not accessible for an internal visual inspection. Some companies make a practice of thoroughly inspecting all tubes that are condemned and removed from a heater, regardless of the reason for the tubes’ removal. This inspection is made by cutting a tube into short sections of 2 ft – 3 ft (60 cm – 90 cm) so that the inside surface can be examined. Measurements for metal-wall thicknesses are made at the ends of each section. In some cases, the sections are split longitudinally, thus exposing the entire inside surface for examination. The ends of the tube rolled into the fitting should be removed for examination. They may then be inspected to determine the general condition and effectiveness of the rolled joint.
When external deterioration, including that due to oxidation, scaling, cracking, and external corrosion is suspected, especially in the case of convection tubes, the use of ultrasonic (UT) base intelligent pigging should be considered to carry out full inspection of the serpentine coil. Intelligent pigging will provide complete condition assessment of regions of the coil (convection, cross-over, radiant, etc.) which it passes through. As a less comprehensive alternative, representative tubes may be removed from the heater and then cleaned and examined thoroughly. The selection of the tubes to be removed may be guided by the tube locations in the heater, the length of time the tubes have been in service, and the general appearance of the tubes in the area. If the tubes chosen for inspection are found to be defective or unfit for further service, other tubes in the same area and of the same or similar age and general appearance should also be inspected.