Understanding slug catchers: Operation, types and maintenance in gas plants

Antonio Zavarce Franciosi.

December 24, 2023

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Introduction

In the oil and gas industry, the efficiency and safety of production processes are crucial. Among the essential components for this type of environment are “slug catchers”, devices designed to handle specific phenomena in oil and gas flow lines. This article explores in depth the damage mechanisms that can affect slug catchers in gas plants, a topic of vital importance for professionals in the sector.

What is a slug catcher?

A slug catcher is equipment used in natural gas processing plants to separate and store condensate and other liquids that may be transported along with natural gas in pipelines. These “slugs” or liquid bumps can be caused by changes in flow velocity, pressure variations, or irregularities in the topography of the terrain through which the pipes pass.

The slug catcher acts as a buffer, collecting these slugs and allowing a more uniform flow of gas to the processing facility. This not only improves process efficiency, but also protects downstream equipment, such as compressors and dehydrators, from potential damage.

Types of slug catchers

Vessel type slug catcher: The vessel type slug catcher is a simple two-phase separation vessel. Although cup separation efficiency is not critical for a slug catcher, cup volume is. The cup needs to be large enough to accommodate large slugs of liquid produced by a pipe, especially during pipe cleaning with a pig. Since an oil and gas pipeline generally operates at very high pressure, the large vessel must be designed to withstand a high design pressure as well.
Finger type slug catchers: The finger type slug catcher offers a solution to the economic problem of having to design a large buffer vessel at high design pressure. Finger-type slug catchers use large diameter pieces of tubing instead of a conventional cup to provide cushioning volume. Since it is easier to design a pipe to withstand high pressures compared to a vessel, this design is advantageous in that regard. However, a large number of pipes are required to provide sufficient volume and this results in a large footprint for the slug catcher.
Parking loop slug catchers: The parking loop slug catcher combines features of cup and finger type slug catchers. A vessel is used for basic gas-liquid separation, while liquid buffer volume is provided by parking loop-shaped fingers. From these fingers, liquid slowly drains to downstream processing equipment.

How is a slug catcher made up?

The composition of a slug catcher can vary depending on the specific design and application requirements, but generally includes the following main components:

Separation vessel or main body

Main structure: It is the largest component of the slug catcher, designed to withstand the pressure and volume of the slugs. It can be a large horizontal container or a series of parallel tubes (finger-type).
Separation internals: Include baffles, impingement plates and other internal devices designed to improve the separation of gas and liquid phases.

Entry and distribution

Inlet pipe: Conducts the flow of gas and liquid to the slug catcher.
Distributors: They help to evenly distribute the incoming flow within the slug catcher.

Drainage and liquid collection systems

Drains: Allow the controlled elimination of accumulated liquids.
Liquid Storage Containers: Temporarily store separated liquids before processing or disposal.

gas outlet

Gas outlet pipe: Conducts the separated gas to the next process or installation.
Control valves: Regulate the flow of outgoing gas and maintain the desired operating pressure.

Control and monitoring systems

Instrumentation: Includes pressure, temperature and level sensors to monitor conditions inside the slug catcher.
Control systems: They allow automated operation and adjustment of process parameters.

Security systems

Pressure relief valves: Prevent overpressure in the system.
Gas and liquid detectors: Alert about leaks or abnormal levels.

Accessories and supports

Structural supports: Provide stability and support to the equipment.
Insulation: Maintains temperature and protects against corrosion under insulation.

Connections and auxiliary valves

Isolation and maintenance valves: Facilitate maintenance and repair.
Connections for instrumentation and sampling: Allow the taking of samples and the connection of additional instruments.

Damage mechanisms in slug catchers

Corrosion: Corrosion is one of the main enemies of slug catchers. This can be accelerated by the presence of CO2, H2S, and water in natural gas. Internal corrosion can weaken the walls of the slug catcher, leading to structural failure. Corrosion under insulation (CUI) is also a concern, where moisture builds up under the equipment’s insulation, causing damage not visible externally.
Erosion: Erosion is caused by solid particles present in the gas and liquid flow, which impact against the internal surfaces of the slug catcher. Over time, this can lead to thinning of the walls and eventually failure. Erosion is particularly problematic in areas where flow changes direction or speed.
Vibration Fatigue: Slug catchers are subject to vibrations caused by the pulsating flow of gas and liquids. These vibrations can induce fatigue in materials, especially at connection points and welds. Vibration fatigue can lead to crack formation and eventually catastrophic failure.
Slug Impact Damage: Slugs that enter the slug catcher can have significant mass and velocity, which can cause impact damage. This is especially true if the slug catcher design has not adequately considered the dynamics of the expected slugs.
Design and manufacturing problems: Inadequate design can be a significant source of problems. This includes not only the size and shape of the slug catcher, but also the selection of materials and the design of internal components. Additionally, manufacturing errors, such as faulty welds, can be critical points for failure.

Inspection and maintenance strategies for slug catchers in gas plants

Inspections and maintenance of a slug catcher are essential to ensure its efficient and safe operation in a gas processing plant. These activities focus on preventing, identifying and correcting potential problems that could affect the operation and integrity of the equipment. Below are the types of inspections and maintenance commonly applied to slug catchers:

Routine inspections

External visual inspection: Review of structural integrity, looking for signs of corrosion, mechanical damage, leaks, or deformations , and verification of the state of insulation, paint and coatings.
Inspection of instrumentation and control systems: Verification of the correct operation of sensors, transmitters, control valves and alarm systems.
Inspection of connections and accessories: Check valves, flanges, gaskets and other components to detect leaks or wear.

Detailed periodic inspections

Internal inspection: Requires plant shutdown for internal access, focusing on the detection of corrosion, erosion, impact damage, and the integrity of welds and internals.
Non-destructive testing (NDT): Includes ultrasound, radiography, magnetic particle inspection and liquid penetrants to detect cracks, corrosion and defects in welds.
Thickness measurement: To evaluate the reduction in wall thickness due to corrosion or erosion.

Preventive Maintenance

Cleaning and dredging: Removal of sediment, waste and liquid accumulations that can cause corrosion or blockages.
Replacement or repair of worn components: Includes the replacement of valves, gaskets, and other elements susceptible to wear.
Updating corrosion protection systems: Application of new coatings, paints and cathodic protection systems.
Instrument calibration: Ensure the accuracy of pressure, temperature and level gauges.

Predictive Maintenance

Historical data and trend analysis: Using monitoring data to predict potential failures and plan interventions.
Modeling and simulation: To evaluate the behavior of the slug catcher under different operating conditions and predict problems.

Emergency response

Safety system testing: Regular verification of pressure relief valves and leak detection systems.
Emergency training and drills: Preparation of personnel to respond efficiently to emergency situations.

Documentation and registration

Inspection and maintenance log: Maintain a detailed history of all inspections, findings and corrective actions.
Updating manuals and procedures: Ensure maintenance practices are up to date with the latest regulations and technologies.

Maintenance and inspections of slug catchers must be performed by qualified personnel and in accordance with industry regulations and standards, such as those of the API (American Petroleum Institute) and ASME ( American Society of Mechanical Engineers ). The frequency and extent of these activities depend on several factors, including the type of slug catcher, operating conditions, environment, and equipment maintenance history.

Mitigation and maintenance strategies

Corrosion Monitoring and Analysis: Implementing real-time corrosion monitoring systems and performing periodic analyzes can help identify and mitigate corrosion before it causes significant damage.
Erosion control : Using erosion-resistant liners and optimizing the design to minimize areas of high flow velocity can significantly reduce the risk of erosion.
Anti-vibration design : Incorporating design elements that reduce vibration, such as mounts and shock absorbers, and performing fatigue analysis can extend the life of the equipment.
Design optimization : The design of the slug catcher must be optimized considering the specific characteristics of the gas and liquid flow. This includes simulating fluid dynamics to predict and mitigate the impact of slugs.
Inspections and preventive maintenance : Performing regular inspections and preventative maintenance is key to identifying and repairing problems before they become critical failures.

Conclusion

In the vast and complex world of the oil and gas industry, slug catchers emerge as crucial components, whose efficiency and safety are pillars in the management of production processes. This article has navigated through the depths of its operation, varied designs and, above all, the challenges inherent in its maintenance and operability.

From cup slug catchers to innovative finger designs to parking loop hybrids, each configuration addresses specific needs and addresses unique challenges. A detailed understanding of these systems not only reveals the intricate engineering behind them, but also the critical importance of their maintenance and inspection strategies to ensure their optimal functioning.

The second part of this analysis dives into the damage mechanisms that can afflict this equipment, highlighting corrosion, erosion, vibration fatigue, slug impact damage, and design and manufacturing issues. These factors, if not properly managed, can lead to catastrophic failures, underscoring the importance of rigorous preventive and predictive maintenance. Mitigation strategies, from corrosion monitoring to design optimization, are not just precautionary measures, but essential steps to extend the life of this equipment and ensure safe operations.

Ultimately, this article not only serves as a technical guide for professionals in the sector, but also as a reminder that excellence in slug catcher management is a reflection of the commitment to efficiency, safety and sustainability in the oil and gas industry.