A common damage mechanism for fired heater tubes in a coking service is Carburization, which if not managed, may lead to loss of containment. Inspection for carburization damage is difficult and destructive examination is the most common method. Fitness for service (FFS) assessment practices are also poorly defined and lack rigorous acceptance criteria. This paper will discuss integrity assessment of carburization damage utilizing in-service ultrasonic inspection, operating conditions, and fitness for service practice based upon API 579-1 FFS methods. In-service inspection is used to define the extent of the carburization damage, from which empirically determined end of life criteria are used to assess the fitness for service and plan for the retirement of the tubes.
Fired heaters in coking service are susceptible to carburization damage, which needs to be predicted and managed to prevent unexpected downtime and expedited replacement costs. Carburization damage occurs when carbonaceous material, i.e., coke, is deposited on a steel surface and exposed to high metal temperatures; such are the internal conditions present in fired heater tubes in coking services. At these high temperatures, the carbon diffuses into the steel microstructure and increases the hardness while reducing ductility. At an advanced state, this reduction in ductility may lead to tube failure if a mechanical or thermal shock is applied. The diffusion of carbon can also cause the formation of deleterious chromium carbides in the steel microstructure, reducing the high temperature corrosion resistance in those areas.
Historically, destructive testing has been used to determine the extent of carburization damage in fired heater tubes and is often only performed after failure occurs. An alternative, proactive assessment can be performed using a smart pig ultrasonic tool that is uniquely designed to detect carburization (e.g., FTIS™ technology†) combined with an empirical algorithm based on measured carburization depths in post-service samples from other heaters. The operating conditions of the heater undergoing assessment can be used to predict carburization rates and therefore plan for retirement at an appropriate end of life date.
