What’s a slug hunter?

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A slugcatcher is a device that stores and dissipates the energy of liquid or gas slugs in a pipeline, protecting processing equipment. Slugs can form due to various factors, including changes in elevation and hydrodynamic slugging. Slugcatcher designs vary, but generally fall into three categories: ship-type, multi-pipe, and parking lot.

A slugcatcher is the part of a piping system that dissipates the energy of intermittently occurring liquid slugs and temporarily stores the excess material for later processing. Slugs are large volumes of liquid or gas that exit a pipeline as a unit. The slugcatcher is found at the end of a pipeline and acts as a buffer to protect processing equipment. It can be a permanent device designed to buffer all input or only brought online when a slug is expected.

Pipelines that allow for two-phase flow, or the simultaneous transportation of liquid and gas, are prone to slug growth. Oil from a drill site carries pockets of natural gas and water. Pipelines are often subject to hydrocarbon condensation and accumulations of water or processing chemicals. When the flow of liquid and gas in a pipeline is not constant, plugs can form that consist almost entirely of gas or liquid. These accumulations are called slugs.

Variations in the elevation of a pipeline due to the terrain can cause liquid to accumulate at low points and block the flow of gas. The gas is compressed behind the block until the pressure overcomes the weight of the buildup and removes it. A liquid slug is created which is pushed in front of the expanding gas. This process is responsible for both ground slugging and the riser-based slugging common to offshore drilling structures.

Under certain conditions, the flow of gas over a slower moving liquid will cause a wave to form. If the wave height reaches the top of the pipe, slug formation begins. Driven by the gas flow, the slug builds up volume in the direction of flow and loses volume in the opposite direction. The relative rates of accumulation and loss determine whether sustained slug occurs. This formation process is called hydrodynamic slugging.

Pigging involves inserting devices into a pipeline for inspection, maintenance or cleaning. Typically, pipeline flow is not interrupted during this process. As a result, pigging slugs often form when flow is blocked or the contents of the pipe are pushed out of the device. Unlike other cases of slug formation, which are periodic, this type is induced.

Slugcatcher designs vary based on facility needs, however, there are three basic categories. The ship-type slugcatcher is generally a simple storage container that also allows for the separation of the gaseous and liquid phases. A multi-pipe slugcatcher can be implemented from standard piping material and has great flexibility to handle a wide range of flows. In the parking lot slugcatcher, separation and storage are distinct elements. The separator could be a type of vessel while the storage part is a ring of pipelines located at a distance.

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