Separator design

Separator Types

Refer to "Separator types"

Separator components

The separator is generally divided into the following functional zones.

• Inlet zone
• Liquid gravity section
• Gas gravity section
• Gas demisting
• Outlet zone

Inlet

The inlet consists of the inlet nozzle, inlet device, and if needed, distribution baffles.  The purpose of the inlet is to provide bulk gas-liquid separation, separate large droplets, distribute the flow, and prevent  foaming and droplet shattering.  Various inlet devices such as a half open pipe, inlet vane device (IVD), and cyclones can be used.  The inlet nozzle and inlet device are sized together.  Distribution baffles, typically perforated plates, attempt to smooth out the flow for the gravity settling sections described below.  Although not part of the separator, the inlet piping plays an important role in separator performance. Piping components such as bends, valves and reducers should be installed as far upstream as possible, allowing for straight section of pipe prior to the inlet.

Liquid gravity section,

This section separates gas bubbles from liquid and, if three-phase separation is needed, oil droplets from water and water droplets from oil.  For horizontal vessels, solids may also be settled out.  Devices such as parallel plate packs may be placed in the liquid phases to coalesce small droplets into larger ones.

Gas gravity section

Similar in purpose to the liquid gravity section, large liquid droplets settle out in the gas gravity section to reduce the amount of liquid reaching the gas demisting section.  High surface internals may be placed in the gas phase to aid in foam breaking.

Gas demisting

At this point in the separator, only small droplets (mist) should be entrained in the gas flow.  Typically, wire mesh, vane packs or cyclones (and combinations) are used as mist eliminators to remove small liquid droplets from the gas stream.

For more details on mist eliminators, refer to “Mist Eliminators”.

Outlet section (nozzles, weirs).

The outlet nozzles remove their respective phases (gas, oil, water, solids) from the vessel while minimizing effects on the flow distribution of the upstream separator sections.  For example, if the gas outlet nozzle is too close to the mist eliminator, higher velocities can occur over the area of the mist eliminator closest to the nozzle resulting in liquid carryover.  In three-phase separators, weirs may be installed to facilitate the removal of the oil and water phases.  In horizontal vessel, nozzles may be installed along its length to remove accumulated solids from the bottom of the vessel.

If the above components are properly designed together, the desired separation requirements should be met.

Separator performance

Refer to discussion of separator requirements in Oil and Gas Separators.  The same separator performance requirements should not be used for all services. Separators in different applications and locations within a process do not necessarily require the same level of performance.  A better approach is to consider the requirements (liquid-in-gas, oil-in-water, and water-in-oil) of the downstream processing equipment.

Sizing Methods

Several methods exist to size separators and vary in complexity and expert knowledge required.  Two  approaches commonly used in the industry are:

o   Design using specific criteria, such as velocity and momentum flux limits.

o   Design through performance calculations, which typically involve sequential modeling of all functional zones as outlined above.

In the criteria based design, for example,  the areas of the gas gravity section and mist eliminator are commonly sized using a K-factor, adapted from Souders and Brown.  The maximum gas velocity, $V_{g}$ (m/s), through the gas flow area or mist eliminator is determined from the K-factor (m/s) in Equation (1):

(1)

Where                                                                                                                                          

ρ_l is the liquid density (kg/$m^{3}$) and

ρ_g is the gas density (kg/$m^{3}$).

The physical properties, droplet size distribution, and liquid loading impact the allowable K-factor.  

The liquid phase size can be sized by a number of methods including velocity limits, degassing time or velocity (bubble removal), droplet (oil and water) removal, or retention time.

In the performance based method, gas space sizing typically involves sequential modeling of all functional zones as outlined above. This approach generally includes predicting the quantity and droplet size of entrained liquid at the separator inlet, followed by calculating drop separation efficiency for the inlet, gas section, and mist elimination device.  In this way, the amount of liquid carryover can be quantified.  The liquid phase sizing can also be similarly performed although there is much less information on liquid-in-liquid and gas-in-liquid entrainment and distributions. These methods are typically used by subject matter experts (especially in troubleshooting liquid carryover issues) as they require expert knowledge of the appropriate entrainment volume and drop size distribution models and mist eliminator separation models.

A generally accepted set of criteria based design guidelines can be found in the document, API Recommended Practice, Ninth Edition, September 2024, Process Design of Oil and Gas Separators and Scrubbers.  The API document provides a stepwise methodology for sizing horizontal two-phase and three-phase separators and vertical two-phase separators.

The API document provides recommendations for:

a. Selection and sizing of inlet devices and inlet nozzle sizing based on inlet momentum flux $\rho$_mV_m² where $\rho$_m is the mixture density and V_m is the mixture velocity.

b. Gas cross section area sizing based on K-factor

c. Liquid cross section area and length sizing based on drop size and velocity

d. Mist eliminator selection and sizing based on K-factor

e. Outlet nozzle sizing based on momentum flux and velocity.

The API document also provides recommendations for inlet pipe configurations and lengths, level control times and spacings for proper operation of the separator, and design considerations for separators on floating facilities.  Other similar sizing guidelines can be found in GPSA Engineering Data Bookand NORSOK Standard P-002.

Separator types