Slug Catcher: Definition, Types, Selection & Design Steps

What is a Slug Catcher?

A slug catcher is a piece of static equipment in the form of a vessel or piping network. It contains sufficient buffer volume to handle the largest expected slug from the oil & gas pipeline systems or flowlines. Flowlines carrying multiphase fluids (Crude Oil, Gas, Water, mixtures) often form damaging slugs. Slug catchers protect the equipment and support from abrupt failure due to forces generated because of slug flow. They are normally located (Fig. 1) before transferring the fluid into the processing equipment. Slug catchers are widely used in multiphase flow lines to serve the following functions:

• To protect the downstream system and equipment from damaging effects of slug flow.
• To efficiently manage high volumes of liquids (slugs) generated.
• To separate part of the liquid from gases in multiphase gas processing plants (Product Separation). The preliminary separation of liquid and gas phases occur due to density difference inside the slug catcher.
• To reduce the probability of further slug formation in the downstream processes.
• To allow the liquid to follow into downstream facilities and equipment at a lower rate which can be properly handled.
• Slug Catchers can also work as a temporary storage device.

Depending on type and frequency of slug generation, slug catchers can be used permanently or intermittently. When slugging behavior is difficult to predict, they are used continuously. However, for purposely generated slugs like during pipeline pigging operation, slug catchers are used during requirement only.

Types of Slug Catchers

Based on their design construction, four types of slug catchers are found in industrial use. They are:

1. Vessel Type Slug Catcher
2. Finger Type Slug Catcher
3. Stored loop type/ Parking Loop type Slug catcher and
4. Hybrid Slug Catcher

Vessel Type Slug Catcher:

As the name suggests, vessel type slug catchers are basically a two-phase separation vessel. The main design consideration is the vessel volume to contain the largest expected slug from the pipelines. However, the vessel type slug catchers must be strong enough to sustain the high pipeline pressures. The vessel can be horizontal or vertical. Vertical vessels have more efficiency as compared to the horizontal vessel slug catchers. They are simple for design and maintenance but with an increase in volume requirement and pressure, this equipment becomes too costly.

Finger Slug Catcher or Harp Slug Catcher:

A number of large-diameter pipes are used to construct Finger type or Harp slug catcher. As piping design to withstand high pressure is easier, finger type slug catchers are widely used due to its economic reason. Fig. 2 below shows a typical example of a Harp Slug catcher.

Finger type slug catchers consist of

• Fingers having three distinct sections
  • Gas/Liquid Separation Section
  • Intermediate Section, and
  • Storage Section
• Gas Risers that are connected to each finger at the transition zone between separation and intermediate sections.
• Gas equalization lines located on each finger within the slug storage section.
• Liquid header configured perpendicular to the fingers.

Stored loop type/ Parking Loop type Slug catcher:

The features of both the vessel and finger types are combined into a Parking Loop slug catcher. The Separation of gas and liquid phase occurs in the Vessel, while the parking loop-shaped fingers provide the buffer volume for storing the liquid.

Hybrid Slug Catcher:

The high efficiency of a vessel separator and the large storage volume of a harp slug catcher combines in a hybrid slug catcher.

Slug Catcher Type Selection

The basic selection between the “Vessel Type” and “Finger Type” is done considering

• Economical aspects (Slug Catcher Design and Installation costs)
• Equipment characteristics
• Volume Requirement (Thumb rule is to use Vessel type for volumes less than 100 m³. For high steady-state gas flow: vertical type and for low steady-state gas flow: horizontal type)
• Site Condition like Space Requirements. Finger type slug catchers occupy huge plot space.
• Design Pressure (Low Design Pressure-Vessel Type; High Design Pressure-Finger Type)
• Transportation feasibility, etc.

Slug Catcher Design Steps

As Slug catchers are used in both oil/gas multiphase production systems and in gas/condensate systems to handle the dangerous effects of slugs, they must be properly sized to dampen the slugs to a level that the downstream processing equipment can easily process. Information on liquid handling rate, pigging frequency, slugging possibility, slug duration, and ramp-up rates, etc are required to estimate the required size of a slug catcher. To determine the slug arrival duration, volume, and other relevant information, a transient simulation of the arrival scenarios like pigging, flow ramp-up, etc. is normally performed. The main slug catcher design steps consist of the following:

• Determination of Slug Catcher function and location.
• Based on Design criteria and data, estimating slug catcher volume (Dimensions)
• Selecting Slug catcher types or configuration considering the design and economic consideration
• Feasibility study

Working of a Slug Catcher

The basic working of a slug catcher is explained nicely in the following animated video:

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