A slug catcher, is a special case of two-phase gas-liquid separator that is designed to handle large gas capacities and liquid slugs on a regular basis, and it is commonly used in gas gathering pipelines. Liquid are usually accumulate in the bottom of gas pipes, especially in offshore pipes, accumulation will continue till the pressure upstream it will be higher enough to push all the liquid as a slug to the gathering station. Such slugs will disturb any production separator in process area, so a slug catcher is necessary to handle the gas and liquid slugs from time to time. Also slug catcher is necessary to handle slugs resulting from pip pigging which is periodically used to sweep the lines of liquids. When the pigs sweep the liquids out of the gathering lines, large volumes of liquids must be handled by the downstream separation equipment.
There are numerous slug catcher designs. Figures 2-14, and 2-15 are both two-phase horizontal slug catcher with liquid “fingers.” Gas and liquid slug from the gathering system enters the horizontal portion of the two-phase vessel, where primary gas-liquid separation is accomplished. Gas exits the top of the separator through the mist extractor while the liquid exits the bottom of the vessel through a series of large-diameter tubes or “fingers.” The tubes provide a large liquid holding volume and routes the liquid to a three-phase free-water knockout (FWKO) for further liquid-liquid separation. The design of an FWKO is discussed in next chapter.