Ethylene cracking furnace coils

Cracking problems had been experienced during the repair of some cast heat resistant furnace tubes which had been in service for several years. Some of these furnace tubes had been removed during the repair work, and this material was available for post-exposure dimensional and metallographic examination. A preliminary calculation of life was made from the limited operating temperature and pressure data available. This showed that even at the maximum operating temperature, the creep stress rupture life remaining was as great as the service life already experienced and the entire furnace did not require immediate retubing. The metallographic and dimensional data confirmed the level of creep life fraction consumed. It was considered that the life limiting factor in service was likely to be the rate at which oxidation and carburisation were reducing the effective wall thickness. A retubing schedule was developed on the basis of an estimated rate of microstructural change determined from the metallography. No post-exposure creep testing was required.