01 slug catchers

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SLUG CATCHERS


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SLUG CATCHER Definition

Slugcatcher 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 orliquid that exists in the pipeline.


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SLUGS

• 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 mostlygases through the pipeline.

• These large plugs are called slugs.


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Slugs existing in the pipeline can overload the gas/liquid handling capacityof 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, whichfollows the ground elevation or 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 plug.


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•Hydrodynamic slugging is caused by gas flowing at a fast rate over a

slower flowing liquid phase. The gas will form waves on the liquidsurface, 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 withthe 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 plug.


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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 (ie. in the direction of the 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 dampened and disappear before reaching the outlet of

the pipeline.


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PURPOSE OF THE SLUG CATCHER

• A slug catcher is a vessel with a significant buffer volume to store the

largest slugs expected from the upstream system.

• The slug catcher is located between the outlet of the pipeline and the

processing equipment.

• The buffered liquids can be drained to the processing equipment at a

much slower rate to prevent overloading the system. As slugs are a

periodical phenomenon, the slug catcher should be emptied before the

next slug arrives.

•

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


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• This is used for difficult to predict slugging behaviour found in terrain

slugging, hydrodynamic slugging or riser based slugging.

• The slug catcher can be by-passed 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.


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TYPES OF SLUG CATCHER


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Types of Slug catcher

Slug catchers are used at the terminus of offshorepipelines:

to catch large slugs of liquid in pipelines, to hold theseslugs temporarily, and then to allow them to follow into

downstream equipment and facilities at a rate at which

the liquid can be handled properly.

Slug catchers may be either a vessel or constructed ofpipes.


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TYPES OF SLUG CATCHER

Three following categories:

1. vessel type;

2. multiple-pipe type;3. parking-loop type.



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Vessel-type slugcatcher

The geometry of the

vessel-type slug catcher

could range from asimple knock-out vessel

to a more sophisticated

lay-out.

This type is preferred inthe case of limited plot

sizes (e.g. offshore

platforms).



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Multiple-pipe slug

catcherMulti-pipe slug catchers are

made of pipeline materials.

This type allows for wideranges of flow situations and

configurations with different

shapes of the available ground

area.

The advantage of a multiple-pipe slug catcher is that it is

easy to operate since no flow

controls are required.



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Multiple-pipe slug

catcher (continue1) The configuration of apipe-type slug catcher

consists of the

following parts:

Fingers for gas/liquidseparation and liquid

storage capacity

Gas equalization lines

located on each finger.

Liquid header collectingliquid from each finger.



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Slug catcher design is dependent on several factors, ofwhich the most important are:

Pigging operation and changes in flow rates.

Slug catcher size should be chosen on the basis ofbalancing the cost of frequent pigging operations and the

capital reduction of smaller slug catchers.

Multiple-pipe slug catcher (continue3)



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Separating and storageparts are virtually

disconnected.

It consists basically of alarge separator with the

liquid outlet connected toa long single pipe-loop.

The incoming gas/liquidstream is separated in the

vessel.

This type of slug catcher is particularly suited to offshore applications where theseparator can be placed on the platform and the parking loop on the sea-bed.


Parking loop slug catcher


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Slug catcher design

Slug catcher volume (VSC) must be able to:1. intercept the maximum possible slug size emerging

from the two-phase pipeline at any moment (VINT).

2. It should also contain a buffer volume of

condensate (VBUFFER) in order to guarantee liquid

supply to treating facilities downstream of the slug

catcher.

(VSC) = (VINT) + (VBUFFER)

The intercepting capacity of the slug catcher is determined bythe nature and operating philosophy of the system.

If the pipeline is to be pigged regularly, it can be sized accordingto the amount of liquid expected to accumulate between pigging

intervals.


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Slug catcher design

For long pipelines, sphere-generated volumes(SGV) based on equilibrium hold-up conditions can bevery large and a special operating and pigging

philosophy may have to be devised to limit the size

of a slug catcher.

The size of (VBUFFER) is largely determined by thecharacteristics of the liquid-treatment facilities and

required plant flexibility.

01 slug catchers

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