PREDICTING THE REMAINING LIFE OF CRACKING COILS: AN APPROACH BASED ON MATERIAL DEGRADATION

This paper presents a case study of a predicting the remaining life of cracking coils of an ethylene cracking furnace based upon material degradation. ENGEMASA has developed a new approach for predicting the remaining life of a cracking coil furnace, based on the perspective of the tube material degradation due to the damage accumulated, as illustrated in Equation 1.

Equation (1)

where,

REL: Remaining Equipment Life

L_new: Design Life of New Equipment

D_acc: Accumulated Damage

In this approach, the remaining coil tube life is determined by the material accumulated damage taking into account three main thermoactivated degradation mechanisms acting synergistically, which are, Carburization, Creep, and Oxidation, as indicated in Equation 2.

Equation (2)

where,

D_{carburization}: Accumulated Damage Due to Carburization

D_creep: Accumulated Damage Due to Creep

D_oxidation: Accumulated Damage Due to Oxidation

A sound wall reduction due to damage accumulation approach is introduced to assess the equipment life reduction. Due to the operational temperature and coke deposition during the cracking process, on the inside surface of the tube, the carburization takes place leading to the degradation of the material from the inside diameter inwards the bulk of the material. On the other hand, from the outside surface inwards the volume of the material the oxidation is taken into account, leading to material degradation due to its microstructural depletion at high temperatures and oxidative environment. On the bulk of the material, the creep mechanism is considered through the API 530 formula, where the remaining wall thickness (after disconsidering the wall thickness lost due carburization and oxidation) is evaluated according to operational conditions as pressure and temperature, as well the Larson-Miller curve for the material.

All three mechanisms are time and temperature-dependent, meaning that their influence is determined by the operational conditions.

Real operational condition records were used to evaluate the methodology showing a good correlation. Samples removed from equipment at the end of life were used to validate the results. The obtained results allowed to establish a relationship among the operating conditions, the theoretical remaining life, and the degradation state of the samples.

The methodology presents as a useful supporting tool for cracking furnace monitoring, reliability, and decision-making. The uncertainties in the approach are directly related to the quality and frequency of the information acquired from the field operational controls and could be reduced by combining with NDT methods and material sampling removal guidelines.