This multiphase flow is received in a large vessel in the crude oil processing and storage facilities where the oil, water and gases are crudely separated to remove the slug, hence the name.
A multiphase flow in a pipe or pipeline sometimes results in a slug flow where the fluid gets segregated into plugs of liquids and gases flowing after one another in the same pipeline. These plugs of liquids and gases are called slugs. Slug flow can be formed in a pipeline due to a variety of reasons such as terrain ups and downs, higher gas velocity compared to liquid velocity, pigging etc.
When liquid slugs arrive in an onshore oil processing facility, the liquid flowrate is significantly higher than the normally anticipated liquid flow for which the processing equipments are designed. Hence a buffer volume is needed to convert the slugs of oil and water into steady liquid flow going to the oil processing facilities. This buffer volume is provided by the slug catcher.
Types of slug catcher
- Vessel type slug catcher is a simple two phase separation vessel. While the separation efficiency of the vessel is not critical for a slug catcher, the vessel volume is. The vessel needs to be large enough to accommodate large liquid slugs produced by a pipeline, especially during pipeline pigging. Since a oil and gas pipeline usually sees a very high pressure the large vessel has to be designed to withstand a high design pressure as well.
- Finger type slug catcher provides an answer to the economic problem of having to design a large buffer vessel at high design pressure. Finger type slug catchers use pieces of large diameter pipes instead of a conventional vessel to provide a buffer volume. Since pipe is easier to be designed to withstand high pressure compared to a vessel, this design is advantageous in that respect. However, large number of pipes is required to provide sufficient volume and this results in a large footprint for the slug catcher.
- Parking Loop slug catcher combines features of the vessel and finger type slug catchers. A vessel is used for basic gas liquid separation, while the liquid buffer volume is provided by parking loop shaped fingers. From these fingers the liquid is slowly drained to the downstream processing equipments.
A slug catcher provides a buffer volume to accommodate the intermittent high liquid flow in the form of a slug. Slug performs a very crude gas liquid separation and the liquids in the vessel can then be slowly drained to the downstream processing facilities at their design flow.