Inspectioneering Journal

Reliability of Coke Drums

Part 1

By Eivind Johnsen at DNV Inc., Lynne Kaley, Vice President of R&D at Pinnacle, and Andy Tallin at DNV Inc. This article appears in the November/December 1996 issue of Inspectioneering Journal.

Petroleum coke production is an important source of revenue for many refineries. While cooking units were initially constructed to deal with a waste product, these units are now of significant economic value.

Worldwide, an estimated 3.8 million barrels of petroleum coke are produced per day. Figure 1 shows the distribution of coke production; nearly 50% of the production is concentrated in the US> Delayed coking is the most common coking process, accounting for as much 3.3 MMbpd.

Figure 1- Coking capacity worldwide
Figure 1- Coking capacity worldwide

Coke drums are an essential component in the delayed coking process. These drums are used to separate petroleum coke from lighter hydrocarbons. Many drums have reliability problems. Because the drums are central to the coking process, poor drum reliability can lead to frequent shutdowns and low productivity.

Coke drums are constructed of a vertical cylindrical shell with an elliptical top and a conical bottom. They range from 60' to 80' in height and 15' to 30' in diameter. Skirts, attached near the shell to cone weld area, are used to support the drums. Most coke drums are made of C-1/2 Mo or Cr-Mo steel and internally clad with stainless steel.


In delayed coking, coke drums are operated in pairs where only one drum is filled at a time. Coking cycles, from fill start to fill start, last between 24 and 48 hours.

Figure 2 shows the process inlet temperature in a drum. Prior to filling, the drum is pre-heated with overhead vapor gas from its twin drum. Filling is the period of stable inlet temperatures. After filling, the coke is cooled by a steam strip and a water quench and then it is removed from the drum. During the cycle, the process temperature varies from 1500 F to 9000 F.

Figure 2- A process cycle for a coke drum
Figure 2- A process cycle for a coke drum

Damage Mechanisms

Coke drums have been studied by a number of refining and engineering companies. These studies have identified low cycle fatigue (LCF) caused by cyclic thermal stresses as the main damage mechanism. LCF is the fatigue mechanism that controls failure occurring before 10,000 cycles - the equivalent of about 40 years of operation. LCF cracking can occur in both the shell and the skirt of the drum.

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