3-Phase Separator Design Basics (With PDF)
whatispiping.com/3-phase-separator-design/
Yeremias K Lusi
November 16, 2020
A Separator is a pressurized vessel used in the exploration and production (E&P)
of an oil and/or gas field to split a multi-phase well stream into a gas stream and
one or more liquid streams. An example of the pressurized vertical and horizontal
separator used in separation & processing include:
a. Scrubber
b. Slug Catcher
c. Free water knock-out
Mechanical Design of separator is done following ASME BPVC Sec. VIII Division 1 or
Division 2 (Class 1 or Class 2).
The selection of Division 1 or Division 2 shall be based on both design and economic
considerations. The use of the Code shall be limited to the following pressure such as:
ASME BPVC Sec. VIII Div. 1 for 200 barg
ASME BPVC Sec. VIII Div. 2 for 689.5 barg
If any of the following conditions apply, the vessels should be constructed in accordance
with Division 2, unless otherwise specified by the Company.
Pressure vessels operating in cyclic service, pressure or temperature induced
Pressure vessels which operate in the creep range of the materials of construction
Pressure vessels where weight savings are a critical factor such as offshore or
marine applications.
If either of the following conditions applies, the vessels shall be constructed in
accordance with Division 2, unless otherwise approved by the Company.
Pressure vessels with a nominal wall thickness greater than 38 mm (1½ in)
Pressure vessels with a design pressure of 10 MPa (1500 psi) or higher.
This article is aim to present an overview of simple explanation of 3-phase separator.
Factors for Separator Designs
Designing an “optimum” set of separators require a balance between the desired size
(volume caused by phases of liquid and gas), operating pressure, separation efficiency,
space limitations (vertical/horizontal), the material of construction, fabrication method and
installation cost.
Let’s briefly look at the common design factors of a separator below.
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1). Size
In general, the sizing of the separator will be carried out by the Process Engineer based
on the target volume to be achieved, flow rate, density, operating temperature and
pressure, and the need to precipitate droplets from gas, oil and water. Size parameters
also include process considerations such as fluid velocity, fluid hold, and storage capacity.
The output size consists of diameter and length/height (seam-seam) depending on the
gas capacity or liquid capacity calculation. The final option of the optimal size is the most
economical.
2). Pressure
The operating pressure is usually fixed by process conditions. Therefore, in some kinds of
literature, separators could be categorized according to their operating pressure.
Low-pressure units:
Medium-pressure units:
High-pressure units:
10 to 180 psi (0.7 to 12.41 barg)
230 to 700 psi (15.85 to 48.26 barg)
975 to 1500 psi (67.22 to 103.42 barg)
This grouping directly affects the selection of the L/D ratio in the early stage of the design
separator. L/D ratio of separator could be selected based on operating pressure and vice
versa.
The optimum L/D ratio may refer to the
above breakdown and will vary following
these parameters:
Allowable stress: affected by
temperature, inside diameter,
TL/TL, and material selection. It
will affect the primary and
secondary stresses of the entire
life of the separator
Table 1. Pressure vs Separator L/D Ratio
Corrosion Allowance: affected
the shell, head, nozzles, and
pressure-part thicknesses calculation output. It will affect the MAWP and MDMT
final results
Joint Efficiency: affected the thickness calculation, welding quality, NDE, and
fabrication requirement.
In mechanical design & analysis, it is recommended to have a suitable margin between
operating and design pressure. Determination of design pressure can follow the guideline
below:
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Table 2. Guidelines for determination Design Pressure
3). Temperature
Operating temperature is determined based on service fluid, ambient site temperature
and considering the maximum and minimum factors, normal and the worst case caused
by start-up, shutdown, operational upset, auto-refrigeration and other sources of cooling.
In mechanical design & analysis, design temperature is used to select the proper material
of construction (MOC), flanges pressure-temperature rating
and for specific case affects the
impact test requirements (e.g per UG20(f) and UCS-66 from ASME VIII
Div.1). Determination of design
temperature can follow the guideline
below:
Table 3. Summary of ASME B16.5 rating for material
group 1.1
Table 4. Guidelines for determination Design
Temperature
4). Orientation: vertical vs horizontal
a). Vertical Separator:
Used for small liquid holdup or in other words; it is mainly applied when there is a
large amount of vapor detected from a small amount of the light and heavy fluid
(less than 10-20% by weight).
Used for easier cleanout because of its smaller bottom area
More easily used to handle liquid surge because of the depth of the space
A smaller area is required especially for offshore platform cases.
A higher climbing ladder and platform for access are required.
b). Horizontal Separator
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Used for large surge volume because of its most efficient large amounts of
dissolved gas are present with the liquid
The horizontal separator has a greater capacity
Can add an additional boot to achieve liquid/liquid or vapor/liquid separation
efficiency
A less static head affects the supports geometry
A bigger area is required but with less climbing ladder & platform.
4). Separation Process
The separation process can be described as either 2-phase or 3-phase vessel.
2-phase separator: It most often used when the fluids contain little water. It will
separate gas from oil in oil fields, or gas from water for gas fields. Click here to
know more about 2-phase separator design basics.
3-phase separators: separate the gas from the liquid phase, and water from oil.
Since free water does not settle out in the time it takes for the oil and gas to
separate, a three-phase separator requires a longer retention time than a two-phase
separator.
5). Retention Time
Retention times are mostly as an input parameter in determined the oil and water
capacity to stay inside the separator. For a given retention time of oil and water, the
procedure for establishing the diameter and length of separator become easier.
Table 5. Residence Time as per API 12J
Configuration of 3-Phase Separator (Horizontal)
3-Phase separator in horizontal orientation consists of a shell, dished ends and mostly
two saddles. In mechanical design perspective, horizontal separators offer less wall
thickness in either shell, head and support compared to the vertical separator in the same
shape and size. The vertical separator will tend to be influenced by wind and seismic load
resulting in greater bending stress, and deflection.
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In process design perspective, the 3-Phase separator is typically similar to the 2-phase
separator, but with additional internals to handle two immiscible liquids (oil and water)
rather than one liquid. In a 3-phase separator, the vessel itself should be designed to
separate the gas that flashes from the liquid, as well as separate the oil and water.
Therefore, in the 3-phase separator, we will find additional control devices for controlling
the liquid level (LLC) and pressure (PCV).
The Simplicity of Separation
steps:
Feed inlet–> hit the Inlet device
–> Gravity settling and
separation of bulk liquid with big
droplet –> Gas exists through
Mist Extractor/Mist
Eliminator/Demister.
The most common configuration of 3phase separator used in the
separation process as shown in the
following sample from Kimray Inc. :
Fig. 1: Liquid level settling rates in 3-phase Separator
1). Horizontal 3-Phase Separator with a Weir Plate
Fig. 2: Horizontal 3-Phase Separator with an Weir
Plate
2). Horizontal 3-Phase Separator with an Oil Bucket & Weir Plate
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Fig. 3: Horizontal 3-Phase Separator with an Oil
Bucket & Weir Plate
Internals of a 3-Phase Separator
Other information that shall be remembered while designing the separator includes
separation efficiency, capacity, and pressure drop. The separation efficiency depends on
the droplet size target (e.g 500-micron water and 200-micron oil) distribution in each
phase of separation, hence to achieve more efficient separation, internals must be
installed inside the separators.
In general, the 3-phase horizontal separator settler consists of three-zone compartments
that automatically determined the internals type selection and installation. (Figure 4).
Inlet compartment
Settling compartment (Liquid-Liquid settling zone)
Outlet compartment (Gas Liquid separation zone)
This device is designed according to
the process requirement, and attach
inside the separator by either welding
or bolting connections and usually will
be finalized by internal Vendor (e.g
from Sulzer) approved by their
Principal.
Internals components included:
Fig. 4: Flow direction and internals installation
a). Inlet Devices
Half-open Pipe
Plate diverter
Vane distributor
Schoepentoeter
b). Sand jetting system and drains
c). Weir Plate (fix/adjustable type)
d). Mist Extractor / Mist Eliminator / Demister
e). Cathodic Protection (mostly Anode type to protect from H2S and CO2)
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f). Vortex breaker
As explained in an early paragraph, it can be concluded that 3-phase horizontal separator
main parts including Head, Shell, Inlet Pipe, Inlet Device, Gas gravity separation section,
Mist Extractor/Mist Eliminator/Demister, Liquid gravity separation section, Manway (for
inspection and maintenance), nozzles and Saddle Supports. Fig. 5 below shows a typical
general arrangement drawing of a 3-phase horizontal separator.
Fig. 6 below shows a typical 3-phase
horizontal separator from Kaji Site –
South Sumatera, Indonesia.
Fig. 5: GA Drawing of a Typical 3-phase Horizontal
Separator
Fig. 6 A typical 3-phase Horizontal Separator
Conclusions
The L/D ratio is one of the factors that influence the optimal design of a separator, without
neglecting other considerations, taking into account the following points:
Process design reason
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Economical reason
Operational reason
Mechanical design reason
In most EPC companies, determination of the type and size of separator must be made
on an individual basis, based on justification and experience in daily-routine work
according to relevant guidance and specification such as GPSA, Clients Specifications,
Separators Handbook, International codes/standard etc.
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