Sep-2020

Managing the life cycle of coke drums

Coke drums have finite lives. Understanding where they are in their life cycle is key to managing spending on repair or replacement

ABBY KING, N CHITHRANJAN and MITCH MOLONEY
Becht

Article Summary

Coke drums are fatigue machines. Fatigue damage accumulates with every cycle and cannot be reversed. Fatigue damage is not easily measured like corrosion, and will manifest in cracking over time, occurring first at the highest stress areas. The location and rate at which this fatigue damage occurs is different for every coker and is dependent upon numerous factors such as the design and operation of the drums (see Figure 1). Understanding how much fatigue damage has accumulated along with how quickly it will continue to accumulate and when to take action is critical to managing the life cycle of coke drums.

As fatigue accumulates in coke drums, more frequent inspection and repairs are needed. There is no true ‘end of life’ for coke drums, instead there is a turning point when outages for inspection and repair or unplanned downtime with reactive repairs outweigh the cost of replacing the drums. Predicting the point when these costs will increase allows time to weigh the economic benefits of the repair and/or the replacement options. Without this forward looking assessment, many sites have found themselves having to react by making very costly repairs, not necessarily because they are the best option but to buy time to plan for drum replacement.

For these reasons, it is critical to know where in the life cycle coke drums are, although this step is easier said than done. Coke drums fall outside of programmes that are used for other pressure vessels, tanks, and piping, such as risk based inspection. Recognising this gap, Becht developed a coke drum reliability assessment tool, BechtCokers, and associated work process to save clients the time and expense of other methods of analysis to understand the current phase of life of their drums (see Figure 2). As part of this assessment, a life cycle view is created for the coke drum from the day that it is placed in service. That allows proactive optimisation of the economic trade-off between coke drum fatigue life and operating costs and the margin value realised.

Since this approach directly links design, condition, and operation of the drums, Becht’s team includes process, mechanical, materials, inspection, and reliability subject matter experts. The work process also facilitates multi-disciplinary discussions between site personnel. Reliable operation of coke drums requires good communication across disciplines to understand the impact of operational changes to the life cycle of the drums and the resulting changes that need to be made to the inspection and maintenance plans.

Becht’s approach uses a ranking of critical factors to benchmark drums at a single site or across several sites’ drums versus the historical performance of other drums in industry. This benchmarking allows us to establish the phase of life and the estimated life fraction consumed to date as a percentage. This approach also provides a predictive model for estimated remaining life in number of cycles and years, depending upon current and future operations.

Equipped with this knowledge, an owner can plan inspection and maintenance activities that will help ensure reliable operation of the drums by reducing the likelihood of through wall cracks, and structural skirt damage. As the end of life approaches, economic analysis can be applied to the factors that affect the life of the drum, such as operational changes, skirt replacements, structural weld overlay, and drum replacement to make the best financial decisions. In some cases, owners have found they can defer significant coke drum replacement capital investment with the right strategies.

Life cycle analysis
When projecting the life of coke drums, we evaluate individually three parts of the drum: shell girth seams, cone, and skirt. This helps identify the limiting component and customise the mitigation plans for different life profiles. The evaluation includes classifying the drums into one of the following three phases of life:
- Phase 1: Minor problems
       Proactive minor maintenance
       Baseline and routine inspection,
       as justified
- Phase 2: Predictable crack growth
       Increased inspection
       Planned repairs
       Optimised repairs during planned shutdowns
- Phase 3: Maintenance intensive
      More frequent shutdowns
      Higher risk of unplanned outages
      Repair vs replace economic evaluation

For the purposes of this article, we will focus on the critical factors used to evaluate the shell girth seams. The two major categories of factors that affect the life of girth seams are the design and operation of the drums. The design is a fixed condition, so let us first examine the critical factors of design that have the greatest impact on drum life.

Material of construction
The most common materials for the shell and cone of coke drums are 1 Cr — ½ Mo and 1-1/4 Cr — ½ Mo, with almost all drums being constructed in low chrome alloys. The vast majority of drums in industry have a 410 SS lining. The BechtCokers tool is calibrated for those materials as well as carbon steel, C — ½ Mo and 2.25 Cr, although they are less common in industry. For materials outside this envelope, materials engineers are consulted for additional calibration as necessary to determine the effects on the fatigue life of the drums.