Guidelines for vessel selection

What is Pressure vessel?

Pressure vessel is a container that holds fluid at a pressure above ambient pressure. the function of a vessel in a process unit is to either provide holdup time or to make a separation between the various phases of a mixed process stream.


They can be divided into two categories:

  • Vapor / liquid separators, which are used to separate the vapour and liquid part of a mixed stream.

  • Liquid / liquid separators, which are used to separate the two liquid phases of a mixed stream.

Selection Criteria for Vapor-Liquid Separators:

Vapour-liquid separator vessels can be selected based on following criteria

  • Orientation

  • Type of feed inlet

  • Type of internals

  • Type of heads


There are vertical and horizontal vessels, both have their advantages. Depending on the application one has to decide on the best choice between the alternatives.

The main advantage of a vertical vessel is that it needed only a smaller area, mostly used in offshore facilities. Horizontal vessels is easier to accommodate large liquid slugs.

Type of feed inlet:

Inlet Nozzle:

The size of the feed nozzle and the type of feed inlet device (if any) have an effect that can be accomplished on the vapor / liquid separation. Normally, the feed nozzle is sized to restrict the feed momentum.

Inlet device: Various inlet devices are available to improve the vapour/ liquid separation. Among others the following inlet devices may be installed:

  • Deflector baffle

  • Slotted tee distributor

  • half-open pipe

  • 90 ° elbow

  • Tangential inlet with annular ring

  • Schoepentoeter

Type of internals:

After passing through the feed inlet, the vapor stream will still contain liquid in the form of droplets. A separation device can reduce this entrainment significantly. They are used for two reasons:

  • To minimize entrainment: Of the drum services having such a requirement, suction drums for reciprocating compressors are the most notable examples.

  • To reduce the size of a vessel

Vessel Head:

Flat heads: A flanged top cap is often used in tiny vertical vessels, which also provides access to the ship.

Hemispherical heads: For an exceptionally big high-pressure vessel, a hemispheric head should be regarded

Selection criteria for Liquid / Liquid Separators vessel:

Vessel should provide vapour space.

Initial vessel dimensions can be selected based on an overall liquid residence time,

Vessels also providing vapor space, with overflow and underflow baffles, which are separating the liquid / liquid separation compartment from compartments for liquid drawoff compartments .

The sizing of these horizontal vessel involves the sizing based on the separation of vapor / liquid as mentioned above and the sizing of the separation of liquid / liquid.