A failure had occurred in a petrochemical plant in a weld joint of a de-superheater only six months after being in service. System was commissioned after making necessary repairs using same metallurgy since all the needed resources were long delivery-scheduled items. Leaking of steam through the weld joint revealed the failure location. Analysis of the failed samples by metallurgical techniques, none destructive testing (NDT), scanning electron microscopy (SEM), and Energy Dispersive X-ray (EDX) techniques indicated that the failure was caused by improper design of the injection of the BFW in the de-superheater that producing inner cyclic thermal differential loads leading to thermal fatigue. This paper will discuss the mechanism of the prevailing thermal fatigue failure was explored and explained.
Many equipment and structural failures in petrochemical plants occur in or near weld joints. From the first instance, a failure can mislead the field investigator to quickly blame the weld material. In applications where cyclic loads and high temperature are present, a failure can be recognized as thermal fatigue. By definition, thermal fatigue is a phenomenon that results in sudden fracture of a component after a period of cyclic loading in a region under stress and strain over a period of time. This leads to the development and formation of micro-cracks in the most highly strained locations of the component, which subsequently propagates until failure occurs. Thermal shock can viewed as a single instantaneous load [1]. It can lead to excessive thermal gradients on materials, which lead to excessive stresses [2]. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials. Thermal shock is caused by non-uniform heating or cooling of a uniform material or uniform heating of non-uniform materials while it is constrained so not to expand. When the temperature of the material increases, the increased activity of the molecules causes them to press against the constraining boundaries, thus setting up thermal stresses While in operation, the resultant action of rapid heating and cooling of even thick-walled pipe will cause one part of the wall try to expand or contract while the adjacent section tries to restrain it. Thus, both sections are under stress. When the inside wall cools, it contracts, while the hot metal surface on the outside wall is still expanded. This sets up a thermal stress, placing the cold side in tensile stress and the hot side in compressive stress, which can cause cracks in the cold side of the wall. This paper will present a case history failure due to thermal fatigue.
The failure that had occurred in a petrochemical plant was the second failure at the same weld location in only six months after repairing the first failure. No change either in sprayer system or materials was made after the first failure since needed resources were long delivery scheduled items.
