Slug Catchers in Natural Gas Production - NTNU

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Karam – Slug Catchers in Natural Gas Production 3.1 Slug Catcher types CHAPTER 3 SLUG CATCHERS A slug catcher, which is a part of the gas pipeline system, is an essential equipment at the receiving terminal of a multiphase flow processing plant. The specific function of a slug catcher is the separation of the gas and liquid phases as well as the storage of the liquids temporarily. The gas is then sent for further treatment in the gas-treating facilities downstream the pipes. The slug catcher is mainly made up of two different compartments: the first one includes the gas-liquid separator under steady flow conditions while the second consists of the storage where the received liquid is accumulated under operating conditions. The gas will be guaranteed to reach the downstream facilities as the accumulated liquid will displace the existing gas in a relatively continuous pattern. The size of the slug catcher should be determined by the size of the largest slug that is possible to form in the pipeline. The appropriate design of the slug catcher accounts largely to avoid problems at the receiving terminals. In order to prevent the acceleration of the gas/liquid mixture, the inlet diameter of the pipes entering the slug catcher should be the same as that of the pipeline. Normally the slug catcher is made up of a series of pipes that are parallel and inclined in order to give the hold-up volume for the liquid (Shell, 1998). Each one of these pipes in the slug catcher is known as a finger. The upper end of the pipes discharges the gas while the bottom end discharges the liquid. A strong structure and foundation maintain the pipes so as to support the impact of the slug. The slug catchers exist in three different types: the vessel type, the multi-pipe type and the parking loop type. The vessel type can range from a simple to a more complicated knock-out vessel which is mainly used for limited plot sizes such as offshore platforms due to its small size. For large volumes of slugs which implies a volume exceeding 100 m 3 , the multi-pipe or parking loop slug catchers are mainly used. The multi-pipe slug catcher is made up of a liquid and gas separation entry slot and a series of parallel tilted bottles where the liquid is stored. The inflow of liquid gets first through the splitter into the inlet manifold and then down to the bottles moving thus the existing gas up to the gas outlet risers. As a consequence, a continuous gas flow is maintained to the downstream facilities. Therefore, the advantage of this slug catcher category is the ease of operation due to a free flow Page 8 of 56
Karam – Slug Catchers in Natural Gas Production control measure. The gas inlet side and the liquid inlet header are shown in Figures 9 and 10, respectively. The parking loop slug catcher is designed to handle liquid carry-over that can be easily formed in case of counter-current gas/liquid flow. The separation and storage parts are practically separated but the liquid and the gas from the incoming stream are separated in the container. A slug arrival into the separator can be detected by an increase in the liquid volume in the vessel. For precautious measures, the gas is controlled by forcing the liquid to get into the pipe-loop where a pig is present. The latter is responsible for the separation of the liquid and gas. The other side of the loop is now open for the gas to flow in a co-current mode to the downstream facilities. This slug catcher type is mainly used offshore where the separator is located on the platform while the loop is mounted on the seabed. It can also be used onshore to reduce the space used if the pipe-loop is placed parallel to the inlet pipe. Multiphase surges can be classified into three different categories. The latters are hydrodynamic slugs, terrain induced slugs and operationally induced surges. Hydrodynamic slugs, as mentioned previously, form due to an instability in the waves at the gas-liquid interface in stratified flow regimes. On the other hand, the terrain induced slugs form mostly at low flow rates after accumulation and intermittent removal of liquids in dips along the flowline. The operationally induced surges occur usually as the system is forced to change from one steady state to the other such as in pigging operations. In order to say that a pipeline is being operated under slug flow regime, it should be then filled with a number of hydrodynamic slugs. Under such regime, the liquid-gas flow shows a chaotic behavior. 3.2 Vessel slug catcher vs. Multi-pipe slug catcher Multi-pipe slug catcher, also known as finger slug catcher, is preferably used compared to the vessel slug catcher. In case of large volumes of slug handling, which is more frequently experienced in operations, multi-pipe slug catcher is more cost effective. As well, less operational problems are encountered when using the multi-pipe slug catcher. On the other hand, vessel slug catcher is more size effective as it does not require a large space in the processing plant. Several criteria and aspects are to be considered when deciding upon which type of slug catcher is more feasible for the field in question. The performance as handling the incoming slug and the transportation features differentiate the two types of slug catchers. The performance depends chiefly Page 9 of 56
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