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8/12/2019 14 Gravity Separator Design - Power
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UpstreamEngineeringCentre
Gravity Separator Design:Theory vs. Practice
Mike Power,Separation Subject Matter Expert,
BP UEC Sunbury
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Agenda
Separation Design Methods
Theory vs. Practice
Upstream vs. Downstream
BP Approach to 3 Phase Separator Sizing
Separator Internals
Inhibitors to Gravity Separation
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Separation Design Approaches
Residence Timee.g. API, GPA Oil Relative Density Retention Time (min.)
< 0.85
> 0.85 > 100oF
80 100oF
60 80oF
3 5
5 10
10 20
20 30
Stokes Law Cut Point
Typical Guidelines:
Good Separation 150 m
Bulk Separation 500 m
Separability
e.g. Bottle Tests
In 1851, George Gabriel
Stokes derived an expression
for drag force in laminar flow& so solving the generallyunsolvable Navier-Stokes
equations.
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Stokes and Residence Time
Settling velocity according toStokes Law:
Example:
How long does it take for a500mm droplet to settle througha distance of 1 meter ?
Water Density: 1000 kg/m3
Oil Density: 760 kg/m3
Oil Viscosity: 4 cP
Settling time = 2 minutes
g
dv owsettle
18
.10002
However:If the oil had a higher density and viscosity,
e.g:
Oil Density: 900 kg/m3
Oil Viscosity: 50 cP
Settling time = 60 minutes
Residence Time Approach takes no account of viscosity
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Stokes Law Approach
Typical Literature Criteria
Good Separation 150 m
Bulk Separation 500 m
Typical Required Specifications
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Downstream Services:
typically constant flows, operating conditions, knowncompositions of liquids and gases
where liquid contain immiscible components, typicallymaximum of three phases
internals kept to a minimum as conditions inseparators well defined and controlled
sizing criteria for separator readily accommodated bywell established design rules
Upstream Services: typically wide range of flows, gas/liquid ratios, fluid
compositions, flow regimes (slug, mist, annular etc.) flowrates and compositions highly dependent on
operation of field & production profiles of wells flow to separators generally highly turbulent separators typically deal with six phases: oil, water,
gas, emulsion, foam and solids good functioning of the separator highly dependent
upon good choice of internals FPSO movement may also need to be accounted for
Upstream versus Downstream
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Current Design Approach
Waterin
Oil,Vol%
Stokes Cut Point, m100 1000
25
2.5
Generic Description
Separator operating performance.
Performance compared against StokesLaw theoretical Cut Point.
Extrapolated to provide Cut Point versusPerformance for new designs.
Internals provided in vessel to produceflow regimes in vessel to bring abouttheoreical Stokes Law type behaviour.
Challenge:Assumed valid for Oils < 10cPApplicability for Oils > 10cP?
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Intent often lost on the journey
What the FEED said What the design
contractor designed
What the fabrication
contractor built
What the construction
contractor installed
What the commissioning
team commissioned
What the Operator
wanted
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Three Phase Separation: Theory vs. ?
What the operator got!
What the Designer Imagined
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Essential Internals
10
Separator Arrangement
Critical Internals for Separation:
1) Inlet Distributor
2) Baffle Plate
3) Demisting Device
4) Vortex Beaker
Other internals as required:sand removal facilities,overflow weir for 3 phaseseparation
Challenge:Are current Design Limitations (V2, inlet/outlet velocitiesetc.) applicable for HP/HT Operation
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Structured Packing & Longitudinal Baffles
Internals greatly simplified: Packing Removed, Longitudinal Baffles Opened, Perforated Baffled, Vane Type Inlet, Cyclonic Demister
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Vessel Inlet Devices
Open Vane Type
Cyclonic Type
Many Inlet Device Designs available
Preferred Inlet Device for typical services:Open Vane Type
Except for Foaming Services:
Cyclonic Type
Both types require baffle plate immediatelydownstream to assist reducing turbulence
Experience of inappropriate selection andinstallation
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Gas Outlet Demisting Devices
Wire Mesh:
Clean Basic Services
Vane Pack:
High Spec < 30bar Cyclonic Device:
High Spec > 30 Bar
3 Common Types
Downcomers requireadequate hydraulic sizing,
well positioned and be
free-flowing
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Vertical Separators
Souders Brown Equation (1934)
U = Maximum superficial gasvelocity (m/s)
k = 0.107 wire mesh demister
= 0.061 no wire mesh
= 0.400 vane pack
l = liquid density (kg/m3)
g= gas density (kg/m3)
HHLL
HLL
D
LLL
FEED NOZZLE
OUTLET
150 mm
MIN
0.85 D
- 150 mm
600 mm MIN
FEED NOZZLE
600 mm
1 MIN
1 MIN
1 MIN
300 mm MIN
600 mm (DRY DRUM)
600 mm MIN
0.6 D
1200 mm MAX
DRY DRUM
Vortex Breaker if Continuous Flowto Pump or Control Valve
5.0
g
glkU
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Vessel Diameter: Impact of k
5.0
g
glkU
Souders- Brown Equation:
Vertical G/L Separator DesignStream Properties
Temperature: 40oC
Pressure: 160 kPaGas Flow: 50,000 kg/hr
Gas Density: 2.9 kg/m3
Liquid Flow: 9800 kg/hrLiq. Density: 690 kg/m3
Demister Type: - Mesh Vane ?K Factor 0.06 0.11 0.4 0.6
Vessel Diam. 2.6m 2.0m 1.1m 0.9m
Challenge: Are the K factors valid for HP/HT
and Viscous Oil Operation
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CFD Modelling of Flow Distribution
For separators in critical services, (slugcatchers, HP /LP separators, Flare
KO drums) CFD analysis performed on the vessel, as well as the first twoinlet piping upstream bends, to identify possible unpredicted fluid flow
behaviour, including FPSO movement.
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Inhibitors to Stokes Law behaviour
Turbulence
Stokes Law not applicable
Inlet to Separators typically highly turbulent
May be increased by vessel internals
Emulsions
Inhibits/prevents oil/water separation
May lead to formation of growing rag layer
Sand
Reduces separation capacity
Plugs internals
Particle stabilised emulsions
Pump & valve wear/damage/blockage
valve/pipeline/equipment erosion/corrosion
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Emulsions: Impact of Water Cut
Emulsion inversion point:-below 30-60% water cut oilis
the continuous phase,-above 30-60% water cut wateris the continuous phase
BP Experience:
Continuous Water generally
better than Continuous Oil
Typically, Separators perform
better at higher watercut
regimes
Gas
Water
Sand
Foam
Oil
Emulsion
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Emulsion Breaking Strategies
Heatreduces viscosity & promotes coalescence
Residence Time with SedimentationPromotes Gravity Settling
Centrifugal Forceenhances settling velocity
Coalescence
utilise plates or vane packs
electrostatics
Chemical Injectiondroplet surface forces destabilised
coalescence promoted
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Sand Removal Systems
Sand fluidisation and removal from separators
Jetting System
Cyclonic Devices
Hybrid System
Jetting nozzles
1. Start of jetting
2. End of jetting
Original sand layer
Fluidised sand particles
Suction point
Nozzle fluidisation flow
Nozzle
Nozzle
Cyclonic Device
Challenge: Are current technologies adequate for increasingly
viscous sand prone reservoirs?
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Foam? Homework?
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Gravity Separator Design: Theory vs. Practice
Thanks for ListeningQuestions?