Slug Catcher Design
Slug catchers are used in both oil/gas multiphase production systems and in gas/condensate systems to mitigate the effects of slugs, which are formed due to terrain, pipeline operation in the slug-flow regime, or pigging. A slug catcher is generally not needed for single-phase liquid lines such as treated oil or produced water because slug flow is not encountered in single-phase operation; however, the need for slug catchers should be evaluated if pigging is expected.
Slug Catcher Design Process
The goal of slug catcher design is to properly size the slug catcher for the appropriate conditions. The process consists of the following steps:
- Determine slug catcher functions.
- Determine slug catcher location.
- Select preliminary slug catcher configuration.
- Compile design data.
- Establish design criteria.
- Estimate slug catcher size and dimensions.
- Review for feasibility; repeat as necessary.
Slug Catcher Functions
The slug catcher functions may be summarized as follows:
- Process stabilization;
- Phase separation;
Each slug catcher can serve one or more functions, and each function is detailed in the following sections.
Process stabilization is the primary purpose of the slug catcher. In a typical steady-state operation, multiphase production fluids from the flowline enter the production facilities at constant temperature, pressure, velocity, and flow rate. Process control devices such as pressure control valves and level control valves are used to maintain steady operating conditions throughout the process facilities. During non-steady-state conditions, such as start-up, shutdown, turndown, and pigging, or when slugging during normal operation is expected, the process controllers alone may not be able to sufficiently compensate for the wide variations in fluid flow rates, vessel liquid levels, fluid velocities, and system pressures caused by the slugs.
A slug catcher provides sufficient space to dampen the effects of flow rate surges in order to minimize mechanical damage and deliver an even supply of gas and liquid to the rest of the production facilities, minimizing process and operation upsets.
The second main function of the slug catcher is to provide a means to separate multiphase production fluids into separate gas and liquid streams in order to reduce liquid carryover in the gas stream and gas re-entrainment in the liquid stream. Gas/liquid separation also occurs, but the efficiency of separation is usually not sufficient to meet oil and gas product specifications. The gas stream may need additional treating to remove entrained liquids prior to treating, compression, or flaring. The liquid may need additional treating for gas/oil/water separation and crude stabilization.
Slugs that result from pigging can often be significantly larger than terraininduced slugs or slugs formed while operating in the slug-flow regime, particularly for gas/condensate systems with long flowlines or pipelines. In these situations, the condensate processing and handling systems may not be sized to quickly process the large slug volume that results from pigging. The slug catcher then acts as a storage vessel to hold the condensate until it can gradually be metered into the process or transported to another location.
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