14 Gravity Separator Design - Power

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

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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

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14 Gravity Separator Design - Power