The document discusses different methods for sizing three-phase separators, including retention time theory, droplet settling theory, and fluid carryover specification combined with estimated droplet size distributions. The droplet settling method calculates the separator dimensions based on the cut-off droplet diameter to be separated, while the fluid carryover method is more complex and utilizes inlet and outlet fluid quality specifications along with estimated droplet size distributions. Maintaining larger dispersed phase droplets upstream of the separator can improve separation efficiency by reducing retention time and fluid carryover.

1. Low shear basics
Importance of the droplet size parameter in
three-phase separator sizing methodology

2. Low shear basics
Gravity separator working principle
• Separators are used to segregate gas from liquid and/or one liquid from
another, such as water from oil.
• Two factors are necessary to induce segregation:
1) process fluids must be insoluble in each other
2) process fluids must be different in density.
• Gravity separation uses the force balance of gravity, buoyancy and drag
acting on the dispersed droplets, which yields in terminal settling velocity
of the droplet in:
Liquid phase: 𝑣𝑡 =
𝑔∙𝑑2∙(𝜌 𝑐−𝜌 𝑑)
18𝜇 𝑐
Gas phase: 𝑣𝑡 =
4∙𝑔∙𝑑∙(𝜌 𝑙−𝜌 𝑔)
3∙𝐶 𝑑∙𝜌 𝑔
0,5

3. Low shear basics
Separator types
• Based on the geometry:
– Horizontal separators
– Vertical separators
– Spherical separators (seldom used)
• Based on phase separation:
– 2 phase separators (gas-liquid / oil-water)
– 3 phase separators (gas, oil, and water)
• Based on operating pressure:
– High pressure separators ( ̴1500psig/100barg)
– Intermediate pressure separators ( ̴750psi/50barg)
– Low pressure separators ( ̴250psi/17barg)

4. Low shear basics
Separator internals
Separator performance mainly depends on the type of internals that can be found
in main separator parts:
- Inlet zone: various inlet diverter internals change momentum of the fluid, thus
causing initial separation of liquid from the gas.
- Gravity/Coalescing zone: mesh/plate/vane/matrix packs promote the gravity
separation of the dispersed droplets from the continuous phase as the fluids
flowing from the inlet section to the outlet nozzles.
- Gas outlet zone: various mist type
extractors catch small-sized liquid
droplets not separated from the gas
stream. Droplets are allowed to
coalesce and fall down by gravity to
the liquid phase.

5. Low shear basics
Separator sizing constraints
Following constraints determine the vessel’s diameter-length ratio combination for the
optimal performance:
• Gas capacity constraint (allow the liquid droplets to fall from the gas phase to the
liquid volume). Gas has to meet certain dryness specification.
• Separation of water from oil (allow the water droplets to settle from the oil phase
into the water phase). Oil has to meet certain specification for content of basic
sediments and water (BS&W).
• Separation of oil from water (allow the oil droplets to rise from the water phase
into the oil phase). Water has to meet certain purity specification.
The minimum droplet diameter (liquid droplet in gas phase/oil droplet in water
phase/water droplet in oil phase) to be removed determines largely the separator
design. Depending on the relative ratio of the different phases aforementioned
constraints will govern the separator design.

6. Low shear basics
Separator sizing methods
Various sizing techniques are employed by engineers in order to design
proper separation equipment. Some sizing methods give simple and robust
estimations, while others are more complicated, but provide information for
design conditions of downstream equipment. Following sizing methods are
available in the literature:
• Retention time theory.
• Droplet settling (cut-off diameter) theory.
• Fluid carry over specification combined with the estimated droplet size
distribution and associated dispersed phase volumes.

7. Low shear basics
Retention time method
• For a horizontal separator relationship of vessel diameter 𝐷 and effective
length 𝐿 𝑒𝑓𝑓 can be found as:
𝐷2
∙ 𝐿 𝑒𝑓𝑓 =
𝑄 𝑤∙𝑡 𝑟𝑤+𝑄 𝑜∙𝑡 𝑟𝑜
𝐶∙𝐹 𝑙
• Input data required:
– Oil and water flow rates, 𝑄 𝑜, 𝑄 𝑤
– Selected retention times for oil and water, 𝑡 𝑟𝑜, 𝑡 𝑟𝑤
For water and oil retention time determination, batch test is required. If not
possible, guidance for the recommended retention times for various operating
conditions can be found in standard API 12J.

8. Low shear basics
Retention time method - pitfalls
- Results of batch testing for retention time has no guarantee for the
separation performance in a real plant.
- API recommendations for retention time are too vague and are not
customized based on actual feed characteristics.
- Retention time method cannot provide any estimation of carry over
fraction to the outlet streams.
- Effect of separator internals is not taken into account.

9. Low shear basics
Droplet settling method
• For gas-liquid separation relationship of vessel diameter 𝐷 and effective length 𝐿 𝑒𝑓𝑓 can be
found as:
𝐿 𝑒𝑓𝑓 ∙ 𝐷2 ∙ 𝐹𝑔
ℎ 𝑔
= 𝐶 ∙
𝑇 ∙ 𝑍 ∙ 𝑄 𝑔
𝑃
𝜌 𝑔
𝜌𝑙 − 𝜌 𝑔
∙
𝐶 𝐷
𝑑 𝑑
1/2
(Subscripts «𝑔» and «𝑙» are for gas and liquid phases respectively)
• For liquid-liquid separation relationship of vessel diameter 𝐷 and effective length 𝐿 𝑒𝑓𝑓 can be
found as:
𝐿 𝑒𝑓𝑓 ∙ 𝐷2 ∙ 𝐹𝑐
ℎ 𝑐
= 𝐶 ∙
𝑇 ∙ 𝑍 ∙ 𝑄 𝑐
𝑃
𝜌 𝑐
𝜌 𝑑 − 𝜌 𝑐
∙
𝐶 𝐷
𝑑 𝑑
1/2
(Subscripts «𝑐» and «𝑑» are for continuous and dispersed phases respectively)
• Input data required:
– Oil, water, gas flow rates, 𝑄 𝑜, 𝑄 𝑤, 𝑄 𝑔
– Selected cut-off droplet diameter 𝑑 𝑑 to be separated.

10. Low shear basics
Droplet settling method - pitfalls
- Cut-off droplet diameter to be separated assumes that all the dispersed
droplets above cut-off diameter will be separated.
- In order to estimate a carry over fraction, droplet size distribution curve
and estimation of volume percent of the dispersed phase are required.
- Quantification of these data requires a number of assumptions to be
made. Additionally, these data depend on numerous variables which
change with time, such as level of turbulence caused by upstream
equipment, gas-liquid ratio and water cut, as well as fluid temperature.
- Effect of separator internals is not taken into account.

11. Low shear basics
Fluid carry over method
• This methodology utilizes the desired outlet fluid quality specifications combined
with estimated inlet droplet size distributions and associated estimated dispersed
phase volumes.
• These calculations are more complex and details can be found in following papers:
– Song, J.H., Jeong, B.E., Kim, H.J. et.al. «Three-Phases Separator Sizing Using
Drop Size Distribution», OTC20558. https://doi.org/10.4043/20558-MS
– Bothamley, M. «Gas/Liquid Separators: Quantifying Separation performance»,
part 1-3. Oil & Gas Facilities.
https://doi.org/10.2118/0813-0021-OGF
– Bothamley, M. «Quantifying Oil/Water Separation Performance in Three-
Phase Separators» Part 1. https://spe.org/en/print-article/?art=2830

12. Low shear basics
Fluid carry over process methodology
Gas-liquid separation Liquid-liquid separation
1) With an assumption of the annular gas-liquid
flow regime in the pipe upstream the separator,
liquid entrainment fraction can be calculated by
entrainment correlation.
1) Estimate the amount of dispersed water in oil phase and the amount
of oil dispersed in water phase. Flow pattern map can be employed to
estimate flow regime and to make predictions on amount of dispersed
phase. Another approach mentioned in the literature is to split total
entrained volume for the dispersed water and oil into the same ratio as
the original oil-water ratio. Both methods provide rather qualitative
estimation.
2) Estimate droplet size distribution based on
shearing and coalescence of the choke and the
pipe segment between the choke and the
separator.
2) Calculate droplet size distribution based on flow conditions and
turbulent energy dissipation rate. An upper-limit log normal distribution
or Rosin-Rammler distribution is recommended in the literature for this
purpose. With the absence of experimental data some suggestions for
values of the fitting parameters can be found in the literature.
3) Calculate terminal velocity of liquid drops in gas
phase using Souders-Brown type of equation.
3) Calculate terminal velocity of dispersed phase droplets using Stoke’s
Law.
4) With input gas flow rate, liquid carry over
specification and the gas-liquid interface level
calculate retention time needed for entrained
liquid drops to separate from the gas phase down
to desired level.
4) With input oil and water flow rates, phase carry over specification, and
the gas-liquid interface level calculate retention time needed for
dispersed droplets to rise/settle to the phase interface.
5) Using input flow rate for three phases, required retention times for each phase to separate down to a desired carry over
level, and adjusting the gas-liquid interface level it is possible to calculate the optimal dimension for the three-phase separator.

13. Low shear basics
Fluid carry over method –
Hysys simulation
– Hysys process simulation software allows to model real separator performance with a
carry over setup.
– With a correlation based model the simulator can calculate the expected carry over
fractions based on the configuration of the vessel, feed conditions, and the operating
conditions.
– As an input data Hysys requires the specification of the dispersed phase volumes and
the dispersed droplet size distribution.
– The ProSeparator correlation allows to calculate liquid carry over into the gas phase
based only on the inlet gas flow rate, the gas-liquid physical properties, and the inlet
pipe size.
– Hysys can not simulate performance of the separator internal devices (coalescer, mesh
pad, etc). The only input geometry parameters are the nozzles design, the weir and the
boot geometry specifications.

14. Low shear basics
Importance of fluid specification
• In recent years more and more attention has been paid to the specification of the
droplet size distribution and the amounts of dispersed fractions in oil, water and
gas phases.
• Separator performance is greatly dependent on these parameters.
• Attention should be paid to the sources of shear that generate small droplets and
increase the fractions of the dispersed phases.
• When designing a process facility, a holistic approach is beneficial regarding the
choice of the correct equipment.
• The low shear type equipment must be considered where droplet shearing is
expected.
• Analysis of the droplet size distribution and the levels of the carry over fractions in
the outlet streams can be used as a debottlenecking technique for optimizing the
separator performance.

15. Low shear basics
Effect of equipment on droplet size
The smallest droplets present the greatest challenge to the separator performance.
Thus, it is important to maintain the droplets of the dispersed phase as large as
possible.
When production fluids flow from the reservoir, they naturally encounter turbulent
zones, such as casing perforations, bends, choke and control valves.
Shearing of the droplets
occurs there. However,
when fluids flow in the
straight pipe, coalescence
is enabled. The effect of
different types of control
valves on average droplet
size of the dispersed oil in
water is shown:
(Graph courtesy of Typhonix AS)

16. Low shear basics
Effect of equipment on droplet size
Due to higher average droplet sizes in the
fluid flow the separator efficiency can be
improved. The benefits gained by
maintaining higher droplet sizes with a low
shear control valve upstream a separator
allow following:
- Lower retention time needed for the
specified carry over fraction.
- Reduced carry over fraction with given
retention time.
- Chemicals reduction is possible if they
are used in the process.
(Qualitative estimate)

17. Low shear basics
Effect of equipment on droplet size
The correct choice of pump can be critical for the droplet shearing. The effect of
different types of pump on the average oil droplet size in produced water can be seen
from the graph:
(Graph courtesy of Typhonix AS)

18. Low shear basics
Additional sources
To read more in detail about retention time and droplet settling separator sizing
methods:
Arnold, K.E., Koszela, P.J.: Droplet-Settling vs. Retention-Time Theories for Sizing
Oil/Water Separator. 1990. SPE Production Engineering. V05 Issue 01.
https://doi.org/10.2118/16640-PA
Boukadi, F., Singh, V., Trabelsi, R. et.al.: Appropriate Separator Sizing: A Modified
Stewart and Arnold Method. 2012. Modelling and Simulation in Engineering. Volume
2012. https://doi.org/10.1155/2012/721814
http://petrowiki.org/PEH:Oil_and_Gas_Separators#Separator_Sizing