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  • Nadirsha Shahabudeen

Slug Catcher in Oil and Gas Industry

Slug Catcher is the name of a unit in the gas refinery or petroleum industry in which slugs at the outlet of pipelines are collected or caught. A slug is a large quantity of gas or liquid that exits in the pipeline.


The Slug Catcher helps protect your valuable production equipment from abrupt large bursts of liquids entering your gas stream. It is typically used downstream of pipeline and pigging operations where large volumes of liquid can collect in the pipeline and are pushed through in a short time. The unit has a liquid level controller and liquid outlet control valve, as well as a crash dump connection. This connection allows you the option of adding a crash level controller and secondary liquid outlet to dispose of sudden large slugs.


The production stream first encounters the inlet diverter, which causes initial separation of gas from liquid. The heavier liquid descends while the gas rises. In the open area of the vessel, the stream slows even further and the larger liquid particles fall from the gas. Next the high efficiency mist extractor captures smaller liquid particles entrained in the gas. If a large liquid slug enters the system, an optional dump valve is triggered to quickly handle the sudden influx. Liquids exit the bottom of the vessel and the natural gas exits the top.


Slug catchers can be used continuously or on-demand. A slug catcher permanently connected to the pipeline will buffer all production, including the slugs, before it is sent to the gas and liquid handling facilities. This is used for difficult to predict slugging behaviour found in terrain slugging, hydrodynamic slugging or riser-based slugging. Alternatively, the slug catcher can be bypassed in normal operation and be brought online when a slug is expected, usually during pigging operations. An advantage of this set-up is that inspection and maintenance on the slug catcher can be done without interrupting the normal operation.


Slugs can be generated by different mechanisms in a pipeline:


Terrain slugging is caused by the elevations in the pipeline, which follows the ground elevation or the sea bed. 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.

Riser-based slugging, also known as severe slugging, is associated with the pipeline risers often found in offshore oil production facilities. Liquids accumulate at the bottom of the riser until sufficient pressure is generated behind it to push the liquids over the top of the riser, overcoming the static head. Behind this slug of liquid follows a slug of gas, until sufficient liquids have accumulated at the bottom to form a new liquid slug.


Hydrodynamic slugging is caused by gas flowing at a fast rate over a slower flowing liquid phase. The gas will form waves on the liquid surface, which may grow to bridge the whole cross-section of the line. This creates a blockage on the gas flow, which travels as a slug through the line.


Pigging slugs are caused by pigging operations in the pipeline. The pig is designed to push all or most of the liquids contents of the pipeline to the outlet. This intentionally creates a liquid slug.


Slugs formed by terrain slugging, hydrodynamic slugging or riser-based slugging are periodical in nature. Whether a slug is able to reach the outlet of the pipeline depends on the rate at which liquids are added to the slug at the front (i.e. in the direction of flow) and the rate at which liquids leave the slug at the back. Some slugs will grow as they travel the pipeline, while others are damped and disappear before reaching the outlet of the pipeline.

Requirements:

  • Level controller and liquid outlet

  • Crash dump level control connection

  • Crash dump liquid outlet connection

  • Thermal pressure relief valve

  • Sight glass/level gauges with gauge cocks

  • High efficiency vane mist extractor

  • Liquid level controller

  • Liquid outlet control valve

  • Pressure indicator with isolating valve

  • Temperature indicator connection

  • Instrument gas supply drip pot with high and low pressure regulators

  • Liquid drain connection

  • Instrument gas header including high and low pressure regulators

  • Engineered lifting lugs for safe handling

  • High quality enamel paint system

  • ASME code stamped and National Board registration

The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.

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