In this post I want to share how to size horizontal three phase separator. The steps used in this post were from Surface Production Operations by Ken Arnold & Maurice Steward. You can learn the details and equations used from the book. In the end of this post, I share free spreadsheet on how to size horizontal three phase separator.
The calculation used in the spreadsheet in for preliminary calculation only. As for fabrication, you may need to discuss more with related vendors and EPC constructors.
Required Data for Horizontal Three Phase Separator Sizing
To size horizontal three phase separator, we need the following data:
 Oil rate, water rate, and gas rate
 Operating pressure and operating temperature
 Properties (density, viscosity) of oil and gas, respectively
 Retention time of oil and water, respectively
 Droplet size of liquid and oil to be removed
Retention Time
A certain amount of oil storage is necessary to ensure that the oil reaches equilibrium and flashed gas is liberated. The most common retention time ranges from 3 minutes to 30 minutes, depends on laboratory and field data. If this data is not available, approach in table below can be used for estimation.
Oil Retention Time 

^{o}API Gravity 
Minutes 
Condensate 
25 
Light Crude Oil (3040)  57.5 
Intermediate Crude Oil (20^{o}30^{o})  7.510 
Heavy Crude Oil (less than 20^{o}) 
10+ 
Note: if an emulsion exists in inlet stream, increase above retention times by a factor of 2 to 4 
Similarly, a certain amount of water storage is required to ensure that most of the large droplets of oil entrained in the water have sufficient time to coalesce and rise to the oilwater interface. Retention times for water phase is commonly 3 minutes to 30 minutes, depends on laboratory and field data. If this data is not available, 10 minutes is recommended for preliminary design.
Water Droplet Size in Oil
It is difficult to estimate the size of water droplet that must be settled out of the oil phase. Unless laboratory data is available, good results have been obtained by sizing the oil pad such that water droplets 500 microns and larger are settled out.
Free Spreadsheet of Horizontal Three Phase Separator
Please feel free to download horizontal three phase separator sizing spreadsheet. I tried to make the spreadsheet as simple as possible. Related equation that I found important have been attached in the spreadsheet.
References:
Arnold, Ken, and Maurice Stewart, “Surface Production Operations – Design of Oil Handling and Facilities” Vol 1 3rd Edition”, Elsevier, 2008.