Mechanical Deterioration of tubes and fittings

Mechanical deterioration may materially reduce the service life of heater tubes and fittings. The two most common causes of this are leakage in the tube rolls—the rolled joints between tubes and fittings—and damage during mechanical cleaning. Leakage in the tube rolls may result from faulty roll procedures or workmanship when the tubes were originally installed, or may be caused from thermal upsets during operation. Similarly, damage to a tube during mechanical cleaning may be caused by faulty procedures or workmanship. One of the most common causes is allowing the cleaner to operate in one position for so long that it cuts the tube metal. Machined surfaces of plug-type header fittings can be damaged by contact with cleaning tools. Cleaning by steam-air decoking can cause serious oxidation and other deterioration of tubes unless temperatures are carefully controlled.

Undue force used to close fittings may result in the development of cracks in the fitting body or at the base of fitting ears and may cause excessive wear or distortion of the plugs of U-bend seats, fitting ears, or holding sections and members-dogs or caps and screws. The use of excess force commonly occurs because of improper cleaning of ground surfaces or mismating of plugs to return bends. Training and close supervision of personnel with regard to the proper care, use, and amount of tightening permissible are essential to prevent this damage. Casting or forging defects may also result in cracks in the fitting body or at the base of fitting ears. One common practice to aid in removing plugs and to reduce the chance of damaging the casting is to heat the fittings. Overheating with a torch may cause the fitting to crack. The depth and seriousness of cracks formed by overheating with a torch should be investigated.

Thermal expansion that has not been accommodated can cause deterioration. Tube materials expand when heated. If the expansion can not be accommodated, it can create stresses that are high enough to cause serious weakening and deformation of the tube or fitting. For instance, tube failures have resulted from refractory repair work, which did not allow the tubes to expand, and created high enough local stresses to result in creep rupture.