Drum separators exist in almost all of the oil and gas refinery or petrochemical plants. They are the number one choice for separating mixtures of multiple phases. Among all types of separators, two-phase vertical drums have been received the most attentions due to their high applications and well known design procedures. Therefore, it is essential for engineers to know the fundamentals to design a vertical separator.

Main applications of separators are:

  • Knock-out drum (usually prior to compressor)
  • Process condensate recovery drum
  • Utility steam- condensate drum
  • Blow down drum
  • Gas line slug catcher
  • Reflux drum (usually horizontal)

In the following section, we will review some important criteria to design a two-phase vertical separators. Note that for the purpose of this article, we do not provide detailed mathematical equations. Interested readers may refer to technical instructions for detailed information.

Techniques for designing a two-phase vertical separator

First step is to specify whether the separator is vertical or horizontal. As a rule of thumb, select a vertical type if the gas to liquid ratio (V/L) is high.

Overly, there are two major parameters, which have to be determined:

  1. Separator’s diameters
  2. Separator’s height. From bottom to top tangent line.

Diameter calculation (D)

Separator’s diameter is determined based on the terminal velocity (Ut) of the hypothetical droplet. Terminal velocity is the velocity at which the sum of upward forces acting on the liquid droplet becomes equal to sum of downward forces. After specifying Ut,, consider a somewhat lower velocity for the total gas superficial velocity in the drum (e.g. Ug=0.8 Ut). Now, having gas velocity (Ug), one can calculate separator’s diameter by the use of total inlet gas volumetric flow rate: A=\frac { \pi { D }^{ 2 } }{ 4 } =\frac { { Q }_{ g } }{ { U }_{ g } }

Terminal velocity can be calculated as:  { U }_{ t }=K\sqrt { \frac { \left( { \rho }_{ L }-{ \rho }_{ V } \right) }{ { \rho }_{ V } } }  Where K is a function of droplets size, gas and liquid density, viscosity and operating pressure. ({ \rho }_{ L }=Liquid density, { \rho }_{ V }= Gas density)

As a good rule of thumb, one can take K=0.11  (SI unit).

If demister mesh is used, add 15 centimeters to calculated diameter.

Height calculation (HT)

Figure 1

Separator’s height is primarily determined based on hold-up and surge residence times. Hold-up time (TH) is the time that takes the bottom liquid level to reach from low level (LLL) to normal operating liquid level (NLL) when outlet liquid line is blocked. Similarly, surge time (TS) is the time that takes the bottom liquid level to reach from normal liquid level (NLL) to high liquid level (HLL) when outlet liquid line is blocked.

TH and TS are case-dependent. Therefore, they are different for every unique application. As a rule of thumb, TS= TH/2. Consider at least 5 minutes residence time from LLL to HLL or 30 centimeters height (whichever is greater). Here, height calculation is done based on inlet liquid flow rate to separator alongside with the pre-determined separator’s diameter. Distance between LLL to NLL = (HN). Distance between NLL to HLL = (Hs)

For the other section’s height, the following rough figures are provided as a rule of thumb:

  • Consider at least 25 centimeters from bottom tangent line to LLL = (HLL)
  • Consider at least 40+d centimeters from HLL to inlet nozzle centerline (where d is nozzle diameter) = (HI)
  • Consider at least 60 centimeters from inlet nozzle’s centerline to demister mesh or D/2 (Whichever is greater) = (HD)
  • Consider at least 15 centimeters for demister mesh thickness. (HM)
  • Consider at least 30 centimeters from demister to top tangent line. (HU)

Finally, according to figure 1, the overall separator’s height can be calculated as HT= HLL + HN + Hs + HI + HD+ HM+ HU

demister - design a vertical separator

Typical demister pad

Inlet nozzle configuration

Using an inlet diverter is recommended for a better separation. The working principle of these diverters is by changing the flow direction instantaneously, causing the liquid and vapor separate by means of their momentum difference. There are various configurations available which can be used according to their applications: Schoepentoeter- Tangential (cyclonic)- horizontal half pipe- impingement plate- vertical slotted pipe

separator inlet nozzle - design a vertical separator


separator inlet nozzle - design a vertical separator

Impingement baffle

Vessel head type

In order to design a vertical separator, vessel head type shall be specified. Vessel head is a function of both pressure and diameter. As a rule of thumb, if vessel diameter is more than 5 meters, hemispherical head is recommended. Otherwise, for higher pressure (>10 bar) use Elliptical type and for low pressure it is allowable to use Torispherical (dished).

Elliptical heads are more economical than hemispherical and has a radius of D/4.

Some other design tips of a vertical separator

  • If vessel inside diameter is less than 1 meter, flange heads are acceptable.
  • For vessel inside diameter more than 3 meters, use vent and drain connections with a minimum size of 4 inches.
  • Normally, it is a good practice to consider a minimum elevation of 3 meters for bottom tangent line, if a pump downstream is available. It can be done by use of skirts.
  • There are several criteria for sizing the inlet and outlet nozzles (for nozzle’s flange types click here). As a rule of thumb, one can simply consider them the same size as the corresponding lines (inlet and outlet pipes)
  • For all hard switches such as level switch low low (LSLL) and level switch high high (LSHH), use an independent nozzle, not from the stand pipe.

Last stage is to determine vessel thickness and other mechanical considerations. This is usually done according to ASME BOILER & PRESSURE VESSEL CODE- VIII (Rules for construction pressure vessels).


Generally, the optimal separator’s design is done via trial and error. It means the calculation shall be done for several L/D ratio (height to diameter). Then after shell thickness calculation, the finished price (including material and construction) for each L/D ratio shall be estimated. Finally, the optimal design of a vertical separator is the one with lowest price and minimum installation space requirements.

Note: Information presented in this article is for general knowledge only and shall be used under supervision of separator’s experts.