Creep Damage Analysis of Delayed Coker Unit (DCU) Heater Tubes of a Petroleum Refinery

Introduction

Process heater tubes in the petroleum industry that are made up of 5Cr-0.5 Mo steels are typically designed for a service period of 100,000 hours, considering creep damage. The finite life of heater tubes is governed by the tube material, process environment, and tube operating conditions, primarily the operating temperature and pressure. However, depending upon actual operating conditions, the service life of such tubes may vary greatly [1,2]. Whereas better control of operating parameters and proper maintenance may lead to a service life much greater than normal design life, shortened life, and premature or sudden failures are, however, observed from time to time.  Factors accounting for shorter tube life include routine operation above the design temperature, flame impingement, coke buildup, or short-term high rates of corrosion.

Components exposed to elevated temperatures are more susceptible to creep damage. Creep is a time-dependent process where materials exposed to elevated temperature and stress over a long period of time will gradually deform and eventually fail [3]. The safety of the plant and its operating budget can be greatly impacted by how accurately we can estimate creep life for further operation of the components [4].

It is reported that replacing heater tubes prematurely can cost a medium-sized refinery up to one million dollars per year or more in unnecessary capital costs [5]. Therefore, constant emphasis should be placed on the health assessment of tubes and extending their reliable operation beyond the design life. Among the available methodologies to assess the creep life of a component, a fitness-for-service (FFS) Level III assessment utilizing accelerated stress rupture tests has some advantages over other methodologies. The major advantage is derived from an increased accuracy by obtaining actual test results from a heater tube exposed to the operating environment in question.  Additional benefits include a significant reduction in test duration and minimal requirements for data extrapolation for creep life assessment [4,6]. 

In this article, we review a study performed to predict the remaining life of a delayed coker unit (DCU) heater’s 5Cr-0.5 Mo steel tubes after a service life of just 5.5 years at nominal temperatures 627°C (1160.6°F) and 32 MPa (4.64 ksi). Delayed coking is a thermal cracking process used in petroleum refineries to upgrade and convert petroleum residuum into liquid and gas product streams, leaving behind a solid, concentrated carbon material known as petroleum coke. The feed to the coker is usually the residue of a vacuum distillation unit, which has a high content of asphaltenes, resins, aromatics, sulfur, and metals. The schematic diagram of the process unit is shown in Figure 1.