Time-Dependent Risk Control of Crucial Tube Furnace With SIL Requirement Considered

Hydrogenator feed tube furnace is of great safety concern due to its rigorous operation condition (up to 550°C, 17MPa) and flammable/explosive materials (H2, crude oil) involved in refining installations. Unlike instruments where the failure may not necessarily cause direct hazards, the failure of tubes in high temperature furnace always cause severe damage and downtime-caused economic loss. Normally the main cause of tube failure is time-dependent material degradation mechanism (such as creep, fatigue and oxidation). The occurrence of two tube rupture accidents gives birth to the consideration whether it is necessary to add a new isolation/mitigation layer in tube furnace in order to control time dependent risks. Taking advantage of the progress of life and probability prediction techniques (such as API581), the time-dependent risk of high pressure tube furnace was studied by taking into account of contributions provided by control system and safety instrumented functions (SIF). The frequency of severe initial events (leakage or rupture) of furnace tube was studied and fault tree analysis method was used to analyze the combined failure probability of time-dependent tube failure rate, basic process control system (BPCS) as well as safety instrumented system (SIS). The necessity of adding a mitigation function to reduce consequence caused by tube failure (leakage or rupture) was studied in order to control time dependent risks. A mitigation SIF at tube’s late service stage to reduce risks was proposed, which is of great significance for furnace safety.