Coke drums: Inspection, evaluation and repair

Introduction

The oil refining industry uses delayed coker units to convert heavy crude residues into lighter, higher-value products such as diesel and naphtha. Coke drums, vital components in these units, are subjected to extreme temperature and pressure conditions during their operating cycles, resulting in structural damage over time. The main challenges in managing these drums include the early identification of defects such as cracks, bulges, and material loss, as well as the implementation of effective repairs that extend their service life without compromising operational safety.

This article examines the inspection, evaluation, and repair processes for coke drums, based on the industry practices described in API Technical Report 934-J, which addresses common coke drum problems.

Types of damage to coke drums

Delayed coking is a process that develops in harsh environments to which coke drums are exposed, where a number of damage mechanisms are generated that, if not addressed in time, can lead to failure. The main mechanisms include:

Crevice corrosion: One of the most critical problems is the appearance of cracks in various parts of the drum. Cracks can occur in the circumferential and longitudinal welds, in the drum plate, near the junction between the skirt and the drum, or in areas where the drum has complex geometries. These cracks are caused by thermal cycling, which generates stresses due to rapid temperature fluctuations, especially during the heating and cooling phases of the coking cycle. The most common types of cracks are described below:

• Longitudinal cracks: These occur along the vertical axis of the drum and are usually caused by repetitive thermal expansion and contraction.
• Circumferential cracks: Generally develop around the drum and are often the result of concentrated stresses in welds or in areas where the drum meets other components.
• Cracks in the skirt weld: These cracks occur in the joint between the drum skirt and the drum itself, due to stresses induced by temperature differences and mechanical loads during operation.

Bulging: Bulging is another common problem, caused by plastic deformation of the drum material when subjected to extreme thermal cycling. These can appear on both the inner liner and the drum surface.

Metal loss and coating damage: Corrosion, erosion, and coating delamination are common causes of material loss in coke drums. These losses can be accelerated by aggressive conditions on the inside of the drum, especially in areas of direct contact with coke or corrosive gases. Metal loss reduces the thickness of the drum, weakening its ability to withstand operating pressures.

Drum deformation and tilting: Over time, the drum can develop significant deformations due to thermal cycling and mechanical loads. Drum tilting, bowing, and lifting at the base are problems that can affect equipment stability and alignment. These deformations increase stresses in welds and other critical connections, increasing the risk of structural failure.

Coke drum inspection methods and frequency

These activities depend on several factors, for example, for new drums an initial inspection is required after the first 4 to 6 years of service, but in general, the inspections and frequency will depend on the types of problems, their location, and the method of inspection, as specified in tables 2 and 3 of API Technical Report 934-J.

Some of the most commonly used inspection methods are described below.

Visual inspection (VT): This is one of the first steps in the evaluation process of coke drums. This method allows detecting visible deformations, cracks, and other signs of surface damage and is particularly useful in identifying cracks that have developed characteristic patterns, such as “elephant skin”, a common condition in coke drums that indicates thermal fatigue. High-definition cameras and remote video technology have greatly improved the ability to inspect the inside of the drum without the need for personnel to enter the equipment.

Liquid penetrant inspection (PT): This method is effective for finding surface cracks and determining their extent, and is particularly useful on non-magnetic materials, such as nickel alloy restoration welds. This method is widely used to inspect specific areas of the drum where cracks are known to occur, such as at circumferential welds or near nozzle connections.

Magnetic particle inspection (MT): Magnetic particle inspection is a non-destructive method used to detect surface and sub-surface cracks in ferromagnetic materials. This method is suitable for inspecting weld areas on carbon steel drums, but is not applicable to refurbished welds with nickel alloys or other non-magnetic materials.

Ultrasonic Inspection (UT): Ultrasonic inspection is one of the most advanced and accurate methods for detecting and sizing cracks and other defects in coke drums. There are several ultrasonic techniques, such as Time of Flight Diffraction (TOFD) and Phase Array Ultrasound (PAUT). These techniques allow detection of internal cracks in the drum, including those that originate on the inside surface and propagate outward. The use of TOFD is particularly effective for detecting weld cracks and assessing their size and depth, while PAUT allows detailed imaging of the inside of the drum wall.

Acoustic Emission Testing (AET): This is an advanced technique that allows real-time monitoring of the integrity of drums while they are in operation, using acoustic waves emitted by defects such as cracks or deformations. Acoustic emission sensors, mounted on the drum surface, detect these waves and record their intensity, frequency, and location.

Laser profiling: It is a scanning technique that allows measuring deformations in coke drums, especially bulges in coke drums. This method can detect small changes in drum geometry over time, allowing detailed monitoring of the areas most affected by plastic deformation.

Laser tools are generally mounted on the internal drill stem or drum inlet/outlet nozzles to provide a surface profile.

Coke drum damage assessment

This phase involves a detailed analysis of the results obtained during the inspection to determine the extent of the damage and plan the necessary repairs. The evaluation should consider the following aspects:

• Thermal and mechanical loads: The coke drum is subjected to severe thermal and mechanical cycling, which generates stresses that can cause cracking, deformation, and other damage. It is important to assess how these stresses affect the structural integrity of the drum over time. Data obtained from temperature and strain sensors can help identify the most critical areas where damage is most likely.
• Bulges: Detecting these damages is critical to avoid drum failures. Bulges in coke drums indicate that the material has undergone plastic deformation, which can weaken the drum and increase the likelihood of cracking. This damage can be detected by internal visual inspections or through the use of laser profiling techniques.
• Cracks in the drum skirt: The drum skirt is one of the areas most prone to cracking, due to stresses generated by temperature differences between the drum and its base. Cracks in this area can compromise the stability of the drum.
• Loss of material: Loss of material in coke drums can be due to corrosion, erosion, or delamination of the coating. It is essential to ultrasonically measure the remaining drum thickness to determine if sections of the equipment need to be replaced.

Techniques to extend coke drum life

In addition to repairs, there are techniques to extend the life of the drums, avoiding the need to replace them completely.

• Temporary crack repairs: The application of these types of repairs allows the drum to continue operating until the next scheduled maintenance cycle. These repairs usually involve surface welds to seal the cracks and prevent them from propagating. However, these solutions should only be considered as a temporary measure and a permanent repair should follow.
• Controlled deposit welding and bead welding (CDW/TBW): These are techniques used in the repair of coke drums to minimize thermal stresses and prevent cracking. CDW controls the heat input, preserving the properties of the base material, while TBW applies layers of weld that temper the affected areas, eliminating the need for further heat treatment. These techniques are essential for maintaining the structural integrity of the drum in critical repairs.
• Weld Overlay: Weld Overlay is an advanced technique for reinforcing areas of the drum that have suffered significant wear or damage. This method involves applying layers of weld overlay over the affected surface, restoring its structural strength. Weld overlay can extend the life of the drum, reducing the need for frequent repairs.

Coke drum repair

Generally performed depending on the severity of the damage, this may include welding, section replacement, or structural reinforcement. Some of the most common repair methods are described below.

• Weld repair from the inside: When cracks are detected on the inside surface of the drum and do not penetrate more than 50% of the thickness, repairs can be made by removing the damaged areas and applying restoration welding according to the CDW/TBW process. After welding, the inner drum liner is restored with Weld Overlay to protect against corrosion and erosion.
• Weld repair from the outside: This is practiced when it is necessary to remove the thermal insulation and perform repair welds using a CDW/TBW process, or using a normal weld sequence followed by PWHT. It is essential to follow API Technical Report 934-J recommendations regarding the selection of filler materials and weld parameters.
• Drum section replacement: In cases where damage is extensive, it may be necessary to replace entire drum sections. This requires cutting out the affected areas and welding in new plates. This type of repair is more complex and costly but is essential when damage severely compromises the integrity of the drum.

Conclusions

Inspection, evaluation, and repair of coke drums are critical processes that ensure the safety and efficiency of delayed coker units. The use of advanced inspection techniques, together with a proactive approach to damage assessment and the implementation of appropriate repairs, will extend the life of the drums and reduce unplanned downtime.

By following the recommendations described in API Technical Report 934-J, operators can effectively manage the life cycle of their coke drums, optimizing their performance and minimizing operational risks.

References

1. American Petroleum Institute. (2021). Inspection, assessment, and repair of coke drums and peripheral components in delayed coking units (API Technical Report 934-J). Washington, D.C.: API.