(128d) Condition Based Recoiling

Radiant coils in ethylene cracking furnaces are operated at high tube wall temperatures. Remaining coil lifetime is determined by metallurgical properties, including carburization and creep resistance, furnace design, and operating conditions.

The cost of a furnace shutdown for recoiling can be significant and may include lost ethylene production. Optimizing furnace recoil intervals, while maintaining furnace availability and reliability, is a key furnace ownership task.

Recoiling a furnace with coils that still have significant remaining lifetime is a suboptimal use of resources. On the other hand, if a recoil is scheduled with coils at end of life, then furnace reliability will suffer and there will be considerable risk of coil failure and unscheduled maintenance work.

Carburization is an important radiant coil damage mechanism. Nowadays there are robotic techniques available to measure coil carburization in the firebox (e.g. REPCO’s Cibot). These measurements are non-destructive and can be done while a furnace is out of operation for other maintenance reasons.

Dow is using carburization data as part of a condition based recoiling program, to optimize furnace recoil intervals along with modeling to assess coils coking tendency and thermal exposure across the entire coil lifetime.

The presentation will cover some of the important coil failure mechanisms, radiant coil inspections with Cibot, focusing on the associated modelling efforts and how inspection data is used and interpreted. Finally, we will explain how this affects our furnace recoiling strategy.