Pipes that allow two-phase flow, or the simultaneous transport of liquid and gas, are subject to plugging.
A slugcatcher is the part of a piping system that dissipates the energy of intermittently produced liquid slugs and temporarily stores any excess material for later processing. Slugs are large volumes of liquid or gas that come out of a pipe as a unit. The slugcatcher is located at the end of a pipe and acts as a buffer to protect processing equipment. It can be a permanent fixture designed to store all inputs or connect only when a slug is expected.
Pipes that allow two-phase flow, or the simultaneous transport of liquid and gas, are subject to plugging. Oil from a drilling site carries pockets of natural gas and water. Pipelines often experience hydrocarbon condensation and buildup of water or process chemicals. When the flow of liquid and gas in a pipe is not constant, plugs can form that consist almost entirely of gas or liquid. These accumulations are called slugs.
Variations in elevation of a pipeline due to terrain can cause liquid to collect at low points and block gas flow. The gas is compressed behind the blockage until the pressure overcomes the weight of the buildup and displaces it. A liquid mass is created and pushed against the expanding gas. This process is responsible for both shore pounding and standpipe pounding, common in offshore drilling installations.
Under certain conditions, gas flowing over a slower moving liquid will cause a wave to form. If the wave height reaches the top of the tube, packet formation begins. Pushed by the flow of gas, the slug accumulates volume in the direction of flow and loses volume in the opposite direction. The relative rates of accumulation and loss determine whether the result is a sustained slug. This formation process is called hydrodynamic slugging.
Pigging involves inserting devices into a pipeline for inspection, maintenance, or cleaning. Pipe flow is normally not interrupted during this process. Consequently, pigging plugs often form when the flow is blocked or the contents of the tube are pushed out of the device. Unlike other cases of slug formation, which are periodic, this type is induced.
Slugcatcher designs vary depending on the needs of the facility, however there are three basic categories. The container-type slugcatcher is generally a simple storage container that also allows separation of gas and liquid phases. A multi-tube slugcatcher can be implemented from standard pipe material and has great flexibility to handle a wide range of flows. In the parking loop slugcatcher, separation and storage are different elements. The separator can be a kind of container, while the storage part is a pipe circuit located at a distance.