In this post we discusses the nature, function, importance, and common failure mechanisms of coke drums.
What is a Coke Drum?
A coke drum is a type of equipment used in oil refineries during the coking process. The coking process is a thermal cracking process used to convert heavy oil residues and other heavy hydrocarbons into lighter, more valuable products such as gasoline, diesel fuel, and petroleum coke, a carbonaceous solid that is often used in various industries like metallurgy.
Coke drums are vertical pressure vessels made of thick steel walls to withstand high temperature and pressure. These drums typically operate in pairs or sets, with one drum being filled and heated while the other is being emptied and cooled.
Figure 1: KNPC – Kuwait – Coke Drum (630 Megtons, 9.8m diameter, 43m length)
Coke Drum Operation Procedure
Here’s a simplified step-by-step of how a coke drum is typically used:
Figure 2: Coke Drum Procedure – Flow Diagram
1. Filling: The drum is filled with a heavy oil feedstock, which can include residuals from other refining processes.
2. Coking Phase: The filled drum is heated, typically by introducing hot vapors at the bottom of the drum. This causes the feedstock to crack, forming lighter hydrocarbons that rise as vapors and a solid residue known as petroleum coke.
3. Quenching: Once the coking phase is complete, the drum is cooled, typically by introducing water. This quenching process solidifies the remaining coke and cools the drum in preparation for the next step.
4. Decoking: The cooled, solid coke is cut out of the drum, often using high-pressure water jets. The cut coke falls to the bottom of the drum, where it is collected and removed.
5. Switching: Once the decoking process is complete, the cycle starts again. If multiple drums are used, the operation switches to the next drum, allowing for continuous operation of the coking unit.
Coke drums experience extreme thermal and mechanical stresses due to the cyclic nature of their operation, and can be susceptible to issues such as bulging, cracking, and the “banana effect” (thermal deformation). They require regular maintenance and monitoring to ensure safe and efficient operation.
What are Coke Drums Used for?
Coke drums are used in the oil refining process known as coking. The primary function of a coke drum is to convert heavy oil residues into lighter, more valuable hydrocarbon fractions and petroleum coke, a carbon-rich solid material.
Why are Coke Drums Important?
Here’s why coke drums are essential in the oil and gas industry:
1. Enhancing the Value of Crude Oil: Not all fractions of crude oil are equally valuable. Lighter fractions like gasoline and diesel fuel are generally more desirable because they can be used directly as fuels. Heavier fractions are less valuable because they are more challenging to use and often require further processing to be useful. Coking is a method to break down these heavy fractions into lighter, more valuable products, thus enhancing the overall value of the crude oil feedstock.
2. Petroleum Coke Production: The coking process produces petroleum coke, a carbon-rich solid that is often used in various industries such as metallurgy, power generation, and cement production. Petroleum coke can also be used as a source of carbon in chemical processes or as a fuel in power plants and other industrial processes.
3. Managing Residuals: Heavy residuals from the crude oil distillation process can pose a disposal problem. By transforming these residuals into valuable products, coking helps solve this problem.
4. Maintaining Continuous Refinery Operations: Coke drums typically operate in pairs or sets, which allows the coking unit to operate continuously. While one drum is being filled and heated, the other can be emptied and cooled, thereby ensuring a steady flow of products.
Coke drums, therefore, play a critical role in the oil refining process. They allow refineries to maximize the value of their feedstocks and produce a wider range of products. However, the high temperatures and pressures they operate under make them susceptible to various operational and mechanical issues, and they require regular inspection and maintenance to ensure their safe and efficient operation.
Problems with Coke Drums? (Common Failure Mechanisms)
1. The Banana Effect
The “banana effect” in coke drums refers to the phenomena where the drum warps or deforms into a curved (or banana-like) shape due to uneven thermal stresses experienced during the heating and cooling cycles.
In a typical coking operation, the coke drum is subjected to rapid heating during the coking phase, and then rapid cooling during the quenching phase. These rapid temperature changes cause thermal expansion and contraction of the steel material of the coke drum, which should ideally be uniform throughout the structure.
However, due to a variety of factors such as non-uniform heating and cooling, different rates of coke deposition within the drum, and the inherent design and construction of the drum itself, certain areas of the drum can expand or contract more rapidly than others. This can lead to uneven thermal stresses in the material, which over time, can cause the drum to deform.
This deformation, if severe, can affect the efficiency and safety of the coking operation. It’s called the “banana effect” because severe deformation can cause the drum to curve or warp in a manner similar to the shape of a banana.
To mitigate the banana effect, refinery engineers employ several strategies, including using high-strength materials, designing the drum for optimal heat distribution, monitoring the drum’s temperature and pressure during operation, and carrying out regular inspections and maintenance to detect and correct any deformation early on. Additionally, new designs and technology to make the process more uniform and to reduce the temperature variations are continually being developed.
2. Drum Bulging and Cracking
Drum bulging and cracking are issues related to the physical integrity of coke drums in oil and gas refineries. These problems, similar to the banana effect, stem primarily from the intense thermal cycling that the drums undergo during operation.
Bulging of a coke drum is a deformation that occurs when certain sections of the drum wall expand more than others due to uneven thermal stresses. This uneven expansion can be caused by various factors, including non-uniform heating or cooling, variable coke deposition rates, and differences in the mechanical properties of the drum material. Bulging can cause operational issues such as difficulty in cutting the coke, and in severe cases, it can lead to mechanical failure of the drum.
Cracking in coke drums, on the other hand, is the formation of fractures or cracks in the drum material. The primary cause of cracking is also thermal cycling, which results in repeated expansion and contraction of the drum material. Over time, this cyclic stress can lead to the formation of cracks, particularly at weak points in the drum structure such as welds or areas of high stress concentration.
Both bulging and cracking can be exacerbated by factors such as rapid heating or cooling rates, high operating temperatures and pressures, poor drum design or construction, and lack of maintenance. The repeated thermal cycling that coke drums undergo makes them particularly susceptible to these types of deformation and damage.
To manage these issues, refineries use a variety of strategies, including regular inspections and maintenance, careful control of operating conditions, use of high-strength materials, and optimized drum design. Advanced techniques such as acoustic monitoring and infrared thermography can also be used to detect and monitor bulges and cracks before they become critical. Additionally, regular shutdowns for maintenance and inspection can help catch these issues early before they lead to catastrophic failure.
3. Skirt Cracking Repairs
Skirt cracking is a common issue that can affect coke drums in oil and gas refineries. The skirt is the portion of the drum that supports it and connects it to the structure or foundation. Cracking in the skirt is often due to the high thermal and mechanical stresses that the drum undergoes during operation.
Skirt cracking can have serious consequences, as it can lead to a loss of support for the drum, which in turn can cause additional stresses and potential failure of the drum. Therefore, it’s crucial to detect and repair skirt cracks as early as possible.
Repair methods depend on the severity and location of the cracks, and could include:
- Welding: For minor cracks, repair might involve grinding out the crack and then filling it with a weld. This method is known as weld repair or weld overlay.
- Bolting: If the crack is in a location that’s difficult to weld, or if welding isn’t a viable option due to the material properties or the size of the crack, bolting could be used to repair the crack. Bolting involves drilling holes on either side of the crack and then installing a bolt to hold the two sides together.
- Plate Replacement or Addition: For severe or complex cracks, it might be necessary to cut out the cracked portion of the skirt and replace it with a new plate. Alternatively, a reinforcing plate could be added to the outside of the skirt to provide additional support.
- Full Skirt Replacement: In the most severe cases, it might be necessary to replace the entire skirt. This is a major operation that would likely require a shutdown of the drum and is typically used as a last resort.
It’s also important to address the underlying causes of the skirt cracking, which could include issues such as poor material selection, inadequate design, aggressive operating conditions, or lack of regular maintenance and inspection. By addressing these root causes, future cracking can be minimized or prevented.
*Note: As with all maintenance and repair operations in a refinery environment, safety is a primary concern. All work should be carried out by trained and experienced personnel, and appropriate safety measures should be in place.
4. Corrosion under Insulation (CUI)
Corrosion Under Insulation (CUI) is a significant concern in the oil and gas industry, including refineries. It’s a type of damage that occurs to pipes, vessels, and equipment that have been insulated for heat conservation or personnel protection purposes.
CUI is caused by the ingress of water (due to rain, wash water, condensation, or any source of moisture) into the insulation. This water can then come into contact with the underlying metal surface, creating an environment that promotes corrosion. This type of corrosion can be especially problematic because it is hidden under the insulation and may not be detected until it has caused significant damage.
CUI can lead to the degradation of the equipment, resulting in leaks or even catastrophic failures if not detected and mitigated in time. It’s a significant issue due to the difficulty in detection and the potentially serious consequences of the corrosion.
Materials typically affected by CUI include carbon steel, low alloy steels, and austenitic stainless steel, with the temperature range for the most severe CUI typically being between -4 °C (25 °F) and 175 °C (350 °F).
Mitigation & Prevention Techniques:
- Material Selection: Using materials that are more resistant to corrosion, such as high-alloy metals, can help prevent CUI.
- Insulation Selection: Some types of insulation are more resistant to water absorption, or are designed to shed water more effectively. Selecting these types can help prevent CUI.
- Protective Coatings: Applying a protective coating to the metal surface before installing the insulation can provide a barrier against corrosion.
- Inspection and Maintenance: Regular inspection and maintenance is key to detecting and addressing CUI before it causes significant damage. This might involve removing the insulation to inspect the metal surface, or using non-destructive testing methods that can detect corrosion without removing the insulation.
- Design: Equipment and piping can be designed in a way to minimize the risk of water ingress, such as by avoiding horizontal surfaces where water can pool, and by providing proper drainage.
CUI is a complex problem that requires a comprehensive strategy involving material selection, protective measures, and regular inspection and maintenance to effectively manage.
5. Blowdown Piping Fatigue Failure
Blowdown piping in refineries is designed to quickly depressurize a process unit when necessary, such as during an emergency or a shutdown. This process is called “blowdown,” and the piping that carries out this task must handle high pressure, high temperature, and often corrosive fluids, making it particularly susceptible to various forms of stress.
Fatigue failure is one of the potential problems that can occur with blowdown piping, and it refers to the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. In the context of blowdown piping, the cyclic loading comes from the repeated pressurization and depressurization that the pipes experience during operation.
Here are a few key points related to blowdown piping fatigue failure:
- Nature of Fatigue Failure: Fatigue failure typically begins with the formation of small cracks, which can then propagate with each load cycle. These cracks may not be visible or detectable until they reach a critical size, at which point the pipe can suddenly fail.
- Causes: Factors contributing to fatigue failure include the frequency and magnitude of the pressure cycles, the type of material the pipe is made from, the temperature, the presence of corrosives, and the design and fabrication of the pipe (including welds, bends, and other features that may concentrate stress).
- Prevention and Mitigation: Strategies for preventing and mitigating fatigue failure include selecting appropriate materials and pipe designs, maintaining operating conditions within the design limits, applying protective coatings to resist corrosion, and performing regular inspections and non-destructive testing to detect early signs of fatigue cracks.
- Inspection and Monitoring: Advanced techniques such as acoustic emission testing, vibration analysis, and digital radiography can help detect early signs of fatigue. In addition, continuous monitoring of pressure cycles, temperature, and other operational parameters can provide valuable data for assessing the fatigue life of the piping.
- Corrective Actions: If fatigue cracks are detected, actions could include repairing or replacing the affected section of pipe, changing operating conditions to reduce stress, or redesigning the system to eliminate features that concentrate stress.
Blowdown piping fatigue failure can have serious consequences, including leaks or catastrophic ruptures that can lead to safety and environmental incidents. Therefore, it’s critical to manage the risk of fatigue through a combination of good design, material selection, operational control, and regular inspection and maintenance.
6. Severe Cone Damage (Cone Replacement Welding)
Cone replacement is a critical repair strategy often utilized in pressure vessels or coke drums in oil and gas refineries. The cone refers to the conical-shaped bottom or top section of these vessels. Due to their shape and the mechanical and thermal stresses they endure, these sections can experience issues such as deformation, cracking, corrosion, or erosion.
When the damage to a cone section is severe, simply repairing the affected area might not restore the structural integrity of the vessel. In these cases, it may be necessary to replace the entire cone section. This involves cutting out the damaged cone and welding a new one in place.
This operation must be performed by highly skilled welders and must comply with strict safety and quality standards. The welding process must ensure that the new cone is properly aligned and attached securely to the remaining vessel, that the weld is free of defects, and that the finished vessel can withstand the operational stresses it will be subjected to.
Cone Replacement Procedure:
- Preparation: The old cone is cut away and the edges of the remaining vessel are prepared for welding. This usually involves grinding to create a beveled edge that will provide a good fit with the new cone and ensure a high-quality weld.
- Fit-up: The new cone is aligned with the remaining vessel and temporarily held in place, often with the aid of alignment clamps. The fit-up must be checked carefully to ensure proper alignment and spacing for the weld.
- Welding: The cone is welded to the vessel, usually in multiple passes to build up a weld of the required thickness. The type of welding used will depend on the specific application and material, but might include techniques such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), or submerged arc welding (SAW).
- Inspection: After welding, the weld and the adjacent areas are inspected for defects. This may involve non-destructive testing methods such as ultrasonic testing or radiographic testing.
- Post-weld Heat Treatment (PWHT): Depending on the material and the application, it may be necessary to perform post-weld heat treatment to relieve stresses in the weld and improve the mechanical properties of the weld and heat-affected zone.
Cone replacement welding is a major operation that typically requires a shutdown of the vessel. However, it can be an effective way to restore the structural integrity and extend the life of a vessel that has suffered severe damage.
Coke drums are critical pieces of equipment in the oil refining industry, primarily utilized in the delayed coking process. This process is vital for converting heavy, low-value residues into higher value products, making coke drums a cornerstone in maintaining the efficiency and profitability of refineries.
In this post, we highlight the potential failure mechanisms of coke drums, including the “Banana Effect,” which involves the warping of the drum’s shape due to uneven heating or cooling. As well, other failure mechanisms such as drum bulging and cracking, skirt cracking repairs, corrosion under insulation (CUI), blowdown piping fatigue failure, and severe cone damage. Each of these issues can seriously impair a drum’s function and efficiency, requiring preventative maintenance and regular inspections to prevent costly damage and unplanned shutdowns.
We also highlight details on these failure modes and offers potential solutions or ways to mitigate these issues. The focus is on ensuring the longevity of the coke drums, understanding their critical role in refinery operations, and the financial implications associated with their potential failure.