The fluids flowing through many pipelines are two phase, i.e., a mixture of liquid and gas. Ideally the two phases separate out with the liquid at the bottom section of the pipe. The two phases flow together to a processing facility that separates the two phases. In practice, however, the two phases often travel at different rates which means that they can form slugs whereby the composition of the stream at different sections is almost all liquid or almost all vapor. When slugs of liquid exit the pipeline, they can overwhelm the gas/liquid handling capacity of the process plant they are entering.
Slugs of liquid are can also be created when pigging a line. The pig pushes forward the liquid that has collected in the line, eventually creating a large slug of that liquid. In addition, slugs can be formed in pipelines that cross undulating terrain. Liquid can accumulate at a low point of the pipeline until sufficient pressure builds up behind it. Once the liquid is pushed out of the low point it can form a slug.
Because slugs of liquid can be so damaging to downstream processes, slug catchers are often installed at the exit of the pipeline. They separate the liquid from the mixed phase stream, and then permit the gas and some liquid to move forward. The buffered liquids are then slowly drained into the process. If the slug is created intentionally, for example when the line is being pigged, the slug catcher will probably not be in continuous service but will be used as needed.
Some slug catchers are basically horizontal vessels or knock-out pots in which the two phases separate. Others are “finger or harp type” in which the slug catcher consists of several long pieces of pipe ('fingers'), which together form the buffer volume. This approach makes the handling of high pressure gas simpler. However finger type catchers do have a large footprint.
This YouTube video provides an animation of the operation of a HARP or finger-type slug catcher.