Different Types OF Slug Catchers

First of All, that’s better to have a review on two phase flow regimes

Pipelines that transport both gas and liquids together, known as two-phase flow, can operate in a flow regime known as slugging flow or slug flow. Under the influence of gravity liquids will tend to settle on the bottom of the pipeline, while the gases occupy the top section of the pipeline. Under certain operating conditions gas and liquid are not evenly distributed throughout the pipeline, but travel as large plugs with mostly liquids or mostly gases through the pipeline. These large plugs are called slugs.

Slugs exiting the pipeline can overload the gas/liquid handling capacity of the plant at the pipeline outlet, as they are often produced at a much larger rate than the equipment is designed for.

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.

·       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.

·       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.

·       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.

Different Type of Slug Catcher

1-   Harp/Finger Type Slug catcher

A finger type slug catcher consists of several long pieces of pipe ('fingers'), which together form the buffer volume. The advantage of this type of slug catcher is that pipe segments are simpler to design for high pressures, which are often encountered in pipeline systems, than a large vessel. A disadvantage is that its footprint can become excessively large.

Features

• Economical way to catch large slugs (thousands of barrels)
• Gives predictable particle separation in the 50micron and up sizes
• Predictable separations up to tens of thousands of barrels
• Can be shipped in pieces for oil field assembly with line pipes

2-   Horizontal Vessel Type Slug Catcher

• Can give small particle separation (10 microns) where there is more liquid and lower gas flow.
• Useful as three phase separator.
• Becomes expensive and heavy when large sizes are required.
• Good separation up to 5 - 700 bbls. slug size.

3-   Vertical Separator Vessel

• Useful where small particle separation (10 microns) is required and gas flow is large in relation to liquid slug size. 
• Equipment is expensive and heavy when large sizes are required.
• Good separation useful up to 5 - 700 bbls. slug size.

4-Pipe Fitting Type Slug Catcher

This type of separation equipment typically has an impingement plate to knock out bulk liquids and a vertical column to form a gravity type separator, but it usually has insufficient area to effectively remove small particles. Normally, it is just used to catch the slugs of liquid and hold them. For economic reasons, these slug catchers are usually designed as pipe and fittings, rather than as pressure vessels. The pipe fitting type slug catcher provides good slug separation and slug storage volume at a reasonable cost. Small particle separation is poor, but it improves at low flow rates. A slug catcher of this type can be used to protect a centrifugal type separator and the combination will give separation and slug storage capacity.

Features

• Very economical where small liquid slugs are to be caught.
• Particle separation is poor and relatively unpredictable.
• Catches slugs up to 150 - 200 bbls.

Refrences

Samir Dalvi, 2015 , “Fundamentals of Oil & Gas Industry for Beginners”;

Taylor Forge Company, Engineered Solutions to Separation Problems

Wikipedia Online Encyclopedia