Simulation of Gas/Liquid Flow in Slug Catchers (includes associated papers 17173 and 17465 )
- A. Boa (Koninklijke/Shell Laboratorium) | J.G. du Chatinier (Koninklijke/Shell Laboratorium)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- August 1987
- Document Type
- Journal Paper
- 178 - 182
- 1987. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 4.6.2 Liquified Natural Gas (LNG), 4.1.5 Processing Equipment, 4.2.5 Offshore Pipelines, 4.2 Pipelines, Flowlines and Risers, 4.1.6 Compressors, Engines and Turbines, 4.1.2 Separation and Treating, 4.2.4 Risers, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.4 Gas Processing, 5.3.2 Multiphase Flow, 4.6 Natural Gas, 4.3.4 Scale, 4.2.3 Materials and Corrosion, 5.2.1 Phase Behavior and PVT Measurements
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Summary. For the Optimization of existing multiple-pipe-type slug catchers and the development of novel configurations, small-scale model tests are used that use a two-liquid system with a density ratio representative of that of the gas/liquid phases in practice. The technique has been validated by field tests with an existing slug catcher.
Trunklines transporting natural gas often operate in the two-phase-flow mode because of liquid injection and/or additional liquid formation by retrograde condensation. Slip in velocity between the gas and liquid phases leads to accumulation of liquid in the pipeline (liquid holdup). The total amount of liquid present at any time in a long-distance two-phase pipeline can be significant. when operating conditions are changed, large volumes of liquid may emerge from the pipeline, maybe as a result of a change in volume flow-i.e., in velocity-or a sphere (or pig) run through the line. The largest slug that can ever occur is that caused by sphering. The occasionally very large volumes of ("live") liquids encountered must be handled and stored onshore as they emerge from the pipeline, preferably without any reduction in velocity, which would be reflected in the gas production. For this reason, a liquid-receiving facility known as a slug catcher is always connected to a two-phase pipeline.
A slug catcher consists essentially of two parts: a separator, which separates the liquid from the mixed stream arriving under normal (steady)flow conditions, and storage, which receives and stores the incoming liquid slug created by upset conditions (such as running a sphere through the pipeline). When a more-or-less continuous slug of liquid arrives, the liquid displaces the gas present in the slug catcher, ensuring an uninterrupted supply of gas to the downstream facilities (compressor, treating plant, and liquefied natural gas plant). Gas lines generally operate at velocities of up to 12 m/s [39 ft/sec], and large slugs will take only a matter of minutes to arrive. Therefore, the holding capacity of the slug catcher must essentially be as great as the volume of the largest slug. Although liquid carry-over must be limited, a slug catcher is not meant to replace a high-efficiency separator.
A great number of slug catchers are in operation throughout the world. They may have a vessel, multiple-pipe, or parking-loop configuration and vary widely in geometry, mainly because of the many degrees of freedom possible in the design. It is very difficult to produce a reliable design on a purely theoretical basis, especially when the geometry is as complex as in the case of a multiple-pipe slug catcher. Then, it is useful to have a suitable laboratory technique available to test a design concept. In this paper, a laboratory-scale modeling technique is presented, together with its application for the improvement of an existing multiple-pipe slug catcher. First, though, a more detailed description of multiple-pipe slug catchers is given.
The Multiple-Pipe Slug Catcher
In slug-catcher design, the multiple-pipe concept finds wide application. A multiple-pipe slug catcher consists of an entrance section, where liquid/gas separation occurs, and an array of parallel downward-sloping bottles (of standard line-pipe size) for liquid storage.
Fig. 1 shows the layout of an existing multiple-pipe slug catcher(in operation in The Netherlands). The names of the various parts are indicated. An incoming liquid slug flows through the splitter into the inlet manifold and then through downcomers into and down the sloping bottles. The downward-flowing slug displaces the gas present in the bottles up through the risers mounted on the bottles; from the risers, the gas flows through the outlet header into the gas-treating plant. The liquid/gas exchange that takes place in the bottles ensures that the gas supply to the downstream facilities remains uninterrupted during liquid slug arrival, provided that excessive liquid carry-over can be avoided.
In this particular slug catcher, only primary bottles are used. The liquid from the inlet header flows directly into the bottles, which have both a separation and a storage function. Fig. 2 shows the layout of another slug catcher (in operation in the U.K.), which has nine primary and four secondary bottles. The secondary bottles have only a storage function and are filled from the bottom through the primary bottles and bottom header.
The decision of how many primary and secondary bottles should be used in a given multiple-pipe slug catcher depends on several factors: the gas flow rate in the pipeline; the liquid storage capacity required; the size of plot available (length available for bottles); and the diameter and slope of the bottles, which determine the maximum slug flow rate that the primary bottles can accommodate without liquid carry-over occurring.
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