Component:

• Serpentine coils in a cylindrical viscosity-breaker furnace, fabricated from 9CrMo steel

Situation:

• Residue oils are admitted to the viscosity breaker for thermal cracking
• This process leads to a gradient of temperature, carburisation and fireside corrosion along the coil, with consequent variation in failure probability

Objective:

• To determine the variation of remaining life across the coil
• To develop the optimum plan for tube replacements

Approach:

• Probabilistic life assessment

Results:

• Remaining life of individual tubes, at various probability levels, determined as a function of position in the coil.
• Tubes 15-19 predicted to have reached a critical probability level.

Benefits:

• Accurate prediction of remaining life, as a function of position in the coil, verifiable against progressive inspection data
• Optimum, risk-based, tube replacement plan