Two-Phase Separator Design Basics

A Separator is a type of pressure vessel that is used to separate the gas and liquid from a two-phase mixture. The two-phase separator separates out the liquid and gas phases from the mixture.

Pressure vessels are widely used in the process plant industry. Pressure vessels serve various functions such as

  • short term hold-up, i.e. day tanks, surge vessels,
  • pressurized storage storages, i.e. bullets, Horton spheres,
  • 2 phase (V/L) / 3 phase (V/L/L) separators,
  • Special purpose vessels such as reactors, columns, jacketed vessels.

Most common shape of pressure vessels is cylindrical shell with dished ends. Other types of end closures such as conical, hemispherical are also used when appropriate. For large pressurized storages, spherical shape may be chosen. The Standard Engineering codes used for design of 2 phase separators are GPSA guidelines, API 12J.  

Orientation of Pressure Vessels

Pressure vessels can be installed in vertical or horizontal orientation.

Vertical Pressure Vessels

Vertical orientation is preferred to horizontal orientation owing to the following advantages:

  • Lower plot space required: In most cases length (or height) of pressure vessels is more than the diameter. Therefore, the layout space required is lower when a vessel is placed vertically.
  • Pumps are commonly used for the transfer of liquids from pressure vessels. The vertical vessel provides a higher NPSH available for the pump, as the operating level is at a higher elevation. This is advantageous for the design of the pumps.
  • The vertical installation provides better utilization of vessel volume as the working volume between the high and low operating levels. This is illustrated in Fig. 1 below:
Vertical vs Horizontal Pressure Vessel
Fig. 1: Vertical vs Horizontal Pressure Vessel

Horizontal Pressure Vessels

A vessel may be oriented horizontally when higher mechanical strength is needed to support the weight. This is especially important in the case of high-pressure vessels and very long vessels (high L/D ratio). Horizontal vessels can be provided with two or more saddles.

Two-Phase (V/L) Separator Design

Depending on fluid phases the separators can be classified into two groups.

  • Two-Phase Separator and
  • Three-Phase separator

Two phase separators handles two-phase fluids. One is gaseous phase and the other is liquid phase. While a three-phase separator can separate out three phases; normally gas, oil and water (two liquid phase and one gas phase). In the following paragraphs, we will briefly explore the design basics of two-phase separators.

Selection of separator: Horizontal or Vertical

As a rule, a vertical drum should be chosen when the ratio of vapor to liquid volume is large (750 or more). The vertical drum is often preferred since the separation efficiency does not vary with the liquid level in the drum. Also, the plot space required is lower for the vertical drum.

The figure given below (Fig. 2) is used as guidance for selection for the orientation of separators.

Separator Selection Guide Chart
Fig. 2: Separator Selection Guide Chart

Choice of Separator Internals

Separator Internals are provided to increase the efficiency of the separator and reduce the entrainment. The Internals available commercially are Demister Pads, Vane packs, Multicyclones or swirl decks. The size of droplets present in the two-phase flow entering the drum decides the type of internals to be used. Droplet size depends on the flow regime of the inlet pipe. Diameter of the inlet pipe should be selected to avoid dispersed, annular or mist flow. The approximate size of droplets present in the vapour phase is given by:

Droplet Size in a Separator
dDrop diameterm
Dpipeline internal diameterm
gacceleration due to gravitym/s2
kSouder’s Brown proportionality constantk/s
σsurface tensionN/m
Wmass flowratekg/s
νkinematic viscositym2/s
μdynamic viscosityNs/m2
vgas / vapour phase 
lliquid phase 

Determining Separator Diameter

The design methods are based on Souder’s-Brown equation

Maximum Allowable velocity inside a separator

The maximum allowable velocity of the vapour phase is given by the value of vmax calculated by Souder’s-Brown equation. The diameter of a vertical separator is calculated based on the value of vmax.

In case of horizontal vessel, the full cross section area for flow of vapour is calculated based on the value of vmax. This is in tern used to calculate vessel diameter.

Typical Values of Proportionality Constant, k

Different values of the proportionality constant k are applied for the internals and orientation of the separator.

 k value, m/s
Vertical knock-out drums 
Empty knock-out drum0.08
Empty Compressor suction drums0.04
Flare knock out drum0.07
Horizontal knock-out drums 
Empty, for bulk separation0.1
Empty Compressor suction drums0.05
Vertical demister mat KO drum0.105
Horizontal demister mat KO drum0.15
Table showing values of proportionality constant, k for knock-out drums

Deciding Height of Separator

Determining Height of Separator
Fig. 3: Determining Height of Separator

Height of the separator or drum is calculated considering the following requirements:

  • Between high liquid level and the inlet pipe allow the larger of 0.3 vessel dia or 300 mm.
  • From the top of the inlet nozzle to the tan line allow the larger of 0.9 vessel dia or 900 mm.

Fig. 4 shows a typical separator used in the oil and gas industry.

Separator used in an oil and gas industry
Fig. 4: Typical Horizontal Separator used in an oil and gas industry

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Nisarg Sonawane

A passionate & performance driven Chemical Engineer with multifaceted personality and intellectual curiosity in process Engineering. With my refined skill sets, I bring along a Deep Conceptual understanding of Engineering Fundamentals coupled with cognitive business knowledge. Ability to effectively communicate and work in Vibrant high performing Teams to enhance delivery of strategic goals meeting the bottom-line objectives of business through high quality Engineering Deliverables and solutions.

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