Liquid And Gas Separation

Oil and gas processing (Onshore)

Oil & gas separation SEPARATOR:

• Pressure vessel separating well fluids into gaseous/ vapour and liquid components

Main facilities and operations in oil installation 1)Well 1)Well head & choke 2)Flow 2)Flow lines 3)Manifold 3)Manifold facilities 4)Testing 4)Testing of wells 5)Liquid 5)Liquid and gas separation facilities 6)Oil 6)Oil and gas dehydration system 7)Desalting 7)Desalting 8)Storage 8)Storage 9)Shipment 9)Shipment of products

Main facilities and operations in GGS -Oil and gas metering -Instrumentation Instrumentation and control system -Maintenance Maintenance of equipments -Information Information & communication system -Utilities Utilities e.g. power, water and air -Office Office building -Fire Fire fighting facilities -Transportation Transportation -

Oil & gas separation SEPARATORS OF PRODUCING FIELD: Oil & gas separator • Stage separator • Trap • Knock out vessel/drum/trap or •

• • • • •

liquid knock out Flash chamber /vessel/ trap Expansion separator /vessel Gas scrubber (dry/wet ) Gas filter ( dry/wet) Filter /Separator •

Well head and well fluids

ABP ABT

FTHP FTHT

Choke

SBHP FBHP SBHT

Multiphase HC mixture with varying compositions and densities

3.5”/4” 4/5 km. API 5L

ØGas phase ØLiquid phase: 2 immiscible liquids •Oil •Water

Well manifold To HP separator WELL#1 WELL#2

PG

PG

NRV

NRV To MP separator

WELL#3

PG

NRV

To test separator HP

MP Test Headers

Block diagram well fluids processing Well Fluid from header

Gas to GDU/ Consumers

Separator Crude oil to dehydrator Produced water to ETP

Utilization of gas and oil

• Gas is used for value added products -LPG -C2/C3 -Gas to consumers

•Oil is supplied to refineries for distillation

Classification of Installations a)For a)For one or two wells: well head installation (WHI) b)For b)For many oil wells: group gathering station (GGS) c)For c)For gas wells: gas collecting station (GCS) d)For d)For oil storage and shipment : central tank farm (CTF) e)For e)For gas shipment : gas compressor plant (GCP) f)For f)For gas conditioning: Gas dehydration unit (GDU) & gas sweetening unit (GSU) g) For water handling: Effluent treatment plant (ETP) h)Other h)Other plants: LPG recovery, C2-C3 unit, CSU etc.

Well connection configuration Multiphase HC mixture and water flowing through individual lines (preferred) WELL

WELL

WELL

WELL WELL

GGS

WELL WELL

WELL

WELL

Well connection configuration Multiphase HC mixture and water flowing through interconnected lines (not preferred) WELL

WELL

WELL WELL

WELL

GGS

WELL WELL

WELL

WELL

Well connection configuration Multiphase mixture flowing through well pads and group & test lines to GGS

WELL

WELL

WELL

WELL

WELL WELL

Pad

Pad

WELL

GGS WELL

WELL

Well connection configuration Central Tank Form (CTF) Well

Well

Well

Well

Well

Well

Well

Well

GGS

Well

Well

GGS

Well Well

Well

Well Well

Well

Well Well

CTF

GGS

Pump

Typical PFD of GGS Flare header BH

Bath heater

KOD

HP

Flare

MP To consumers Scrubber

BH BH HP MP Test

To flare

Test Oil Stb. To ETP

Heater treater

HT Tank

Pump

Typical PFD of GCS Flare header MeOH or BH KOD

HP

MP

Flare

Consumers Scrubber To flare

Test HP MP Test

Cond. Stb. To ETP

Cond. Stb. Tank

Pump

Oil & gas separation WELL FLUIDS: • Mixture of oil, gas and water / Free gas

•

Impurities / Extraneous material ØNitrogen, CO2, H2S etc., ØWater, Paraffin ØSand, silt etc.,

Oil & gas separation Quality of separated fluids Separated crude oil

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Depending on retention time,

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Free gas content in separated oil = 1.5% to 20%

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Water content of separated Oil

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= 0.05% to 8%

Quality improvement possible by chemical,

equipment, techniques and procedures 

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Oil & gas separation Quality of separated fluids 

Separated water:



Depending on retention time,

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Oil content in eff. Water

= 0.004% to 2.0%

Special methods and separating techniques can improve water quality. 

Difference in sp.gr. of oil and water less than 0.2 results in limited and incomplete separation. 

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Oil & gas separation Quality of separated fluids Separated Gas:

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For a separator with suitable mist extractor,

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Oil content in separated gas = 0.101 to 1.0 gal/mmscf

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In case of gas scrubbers oil content in effluent gas should be less than 0.1 gal/mmscf.

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Oil & gas separation Instruments for measuring quality of separated fluids:

Measurement

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Oil in separated gas

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Gas in separated oil

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Water in separated oil

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Oil in effluent water

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Instrument

Laser liq. Particle spectrometer Nucleonic densitometer BS&W monitor Ultraviolet absorption unit/ Solvent extraction/

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Oil & gas separation Functions of separators Primary functions 1.

Removal of oil from gas

Secondary functions 1. Maintain optimum pressure

2. Removal of gas from oil 2. Maintain liquid seal 3. Removal of oil from water

Oil & gas separation Process of separation:

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Separation of bulk liquid from gas

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Reduction in gas velocity to allow liquids to drop out

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Scrubbing of the gas

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Retaining liquids long enough for free gas separation

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Controlling and maintaining gas oil interface.

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Removal of all products from their respective out

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lets.

Oil & gas separation Classification of separators

Configuration Application

Vertical Horizontal Spherical

Function

Principle of primary sep.

2-phase 3-phase Test sep. Prodn. Sep. L.T.S. Elevated sep. Stage sep. Metering sep. Foam sep.

Operating pressure

Low pr. Sep. (10 - 225psi) Med. Pr. Sep. (225 - 750 psi) High pr. Sep. (750 - 1500 psi Gravity sep. Impingement/ coalescence sep. centrifugal sep.

Oil & gas separation Stage separation of oil and gas:

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Series of separators operating at sequentially reduced pressures.

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Liquid is discharged from high pressure separator into the next lower pressure separator

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Aims at maximum recovery of liquid hydrocarbon and maximum stabilization of separated fluids.

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Economics limits the no. of stages of separation to 3 to 4

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Oil & gas separation Stabilization of crude oil :

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Operation of separator under vacuum / at high temp. ®

Liquid discharged completely stabilized

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Liquid recovery comparable to that of 4-6 stage separation

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Results in increased profit for highly volatile liquids

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Initial cost of stabilizer less than initial cost of multiple stage oil and gas separators.

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Separators Benefits of separation ® Separated gas, crude oil and produced water can be processed easily at low cost ® Removal of water helps in reducing damage due to corrosion ® Less costly MOC can be used if water is removed ® Less energy is required to move single phases 

SEPARATION Depends on following factors: •Pressure •Temperature •Gas Oil Ratio (GOR) •Flow rate •Fluid properties like density, viscosity etc.

Oil & gas separation Principles of separation of “Oil from gas” ® Density difference ® Impingement ® Change of flow direction ® Change of flow velocity ® Centrifugal force ® Coalescence ® Filtering 

Oil & gas separation 

Principles of separation of “Gas from oil

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Settling

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Agitation

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Baffling

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Heat

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Chemicals

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PRINCIPLES OF SEPARATION: Based on one or more of the following: Removal of Oil from Gas Gravity Separation Impingement Change of flow direction Change of flow velocity Filtering Coalescence Centrifugal

Removal of Gas from Oil Settling Agitation Baffling Heat Chemicals

PRINCIPLES OF SEPARATION (OIL FROM GAS) : Gravity Separation Diff. Liquid phases Diff. Densities Diff. momentum

PRINCIPLES OF SEPARATION (OIL FROM GAS) : Gravity Separation: Terminal / Free settling velocity of droplet

Max. allowable gas velocity Vt =  [4 g Dp ( l -  g) / 3  g C’]

PRINCIPLES OF SEPARATION (OIL FROM GAS) : Impingement : If a flowing stream of of gas containing liquid mist is impinged against a surface, the liquid mist may adhere to and coalesce on the surface.

PRINCIPLES OF SEPARATION (OIL OIL FROM FROMGAS GAS): Change of flow direction: §Change in flow direction: Change in inertia §Gas assumes change in direction readily §Liquid gets retarded and flow back

PRINCIPLES OF SEPARATION (OIL OIL FROM FROMGAS GAS): : Change of flow velocity: §Change in flow velocity: Change in inertia §Increase / decrease in velocity: High inertia liquid moves away from gas

PRINCIPLES OF SEPARATION (OIL OIL FROM FROMGAS GAS): : Filtering : §Porous filters are effective to remove liquid mist. §Uses the principles of impingement, change in direction, change in velocity and coalescence.

PRINCIPLES PRINCIPLES OF OF SEPARATION SEPARATION OIL (OIL : FROM FROMGAS GAS): : Coalescing Coalescing of small droplets (mist/fog)

Formation of large droplets

Settling by gravity

PRINCIPLES OF SEPARATION (OIL OIL FROM FROMGAS GAS): : Centrifugal force: §Fluids allowed to flow in circular motion at high velocity. §Centrifugal force throws liquid mist outward against the walls of the vessel.

PRINCIPLES OF SEPARATION (GAS FROM OIL ): Settling : §Non solution gas separate on adequate retention time §Optimum removal of gas – if body of oil is thin

PRINCIPLES OF SEPARATION (GAS FROM OIL ): Agitation/Baffling: §Controlled agitation helps removing non solution gas §Disperses oil in such a manner that gas readily escapes §Allows gas bubbles coalesce and separate

PRINCIPLES OF SEPARATION (GAS FROM OIL ): Heat : §Hydraulically retained gas releases reduction of surface tension or viscosity

on

§Heat reduces surface tension and viscosity of oil. §

PRINCIPLES OF SEPARATION (GAS GAS FROM FROMOIL OIL:): Chemical : §Hydraulically retained gas releases on reduction of surface tension or viscosity §Certain chemicals can reduce surface tension and foaming tendency of oil. §

PRINCIPLES OF SEPARATION (GAS GAS FROM FROMOIL OIL:): Centrifugal force : §Heavier oil is thrown outward against wall of vortex retainer and gas occupies inner portion. §Properly designed vortex allow gas to ascend and liquid to flow downward §

CLASSIFICATION OF SEPARATORS: SEPARATORS 1.Two phase separator ➦Separation of liquid (oil + water) and gas 2.Three phase separator ➦Separation of liquid and gas ➦Separation of water and oil

MAIN SECTIONS/COMPONENTS OF A SEPARATOR HORIZONTAL SEPARATOR

1.Primary separating section 2.Secondary or gravity separating section 3.Coalescing section 4.Sump or liquid section

Two Phase Inlet

A

B

Gas Outlet Mesh Pad C

D Liquid Outlet

Gas Outlet VERTICAL SEPARATOR Mesh Pad Two phase Inlet

A

A – Primary Separation B – Gravity Settling C – Coalescing D – Liquid Collecting

B

D

Liquid Outlet

Oil & gas separation Components of oil and gas separators:

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Primary separation device and /or section

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Secondary gravity settling section

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Mist extractor

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Gas out let

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Liquid settling section

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Oil & gas separation Components of oil and gas separators:

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Oil out let

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Water out let

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Vortex breakers

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Back pressure control valve

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Level control valves

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Pressure relief valves

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Oil & gas separation Essential features of separators:

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Inlet baffle / Impingement baffle / divertor ® Adequate liquid capacity to handle liquid surges ® Adequate vessel diameter and height for vapor disengagement ® Internal baffle / defoaming plates ® Demister pad / mist eliminator ®

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MAIN SECTIONS/COMPONENTS OF A SEPARATOR 1.Primary separating section Inlet Diverter Vane type Mist Extractor

Vapor Outlet

•Separate bulk portion of free liquid from inlet stream

D v Two phase Inlet

Downc omer

Liq. Outlet

••Inlet Inlet diverter diverter // Inlet Inlet baffle baffle // Impingement Impingement baffle baffle

MAIN SECTIONS/COMPONENTS OF A SEPARATOR 2.Secondary or gravity separating section •Settling section •Reduced turbulence •Retention time ••Straightening Straightening vanes/ vanes/ Settling Settling vanes vanes ••Defoaming Defoaming plates plates

MAIN SECTIONS/COMPONENTS OF A SEPARATOR 3.Coalescing section •Coalescer or mist extractor/demister pad •Interwoven mesh •Gets plugged very easily with heavier hydrocarbons or deposits •Requires frequent cleaning Knitted Knitted wire wire mesh mesh type type

MAIN SECTIONS/COMPONENTS OF A SEPARATOR 3.Coalescing section •Coalescer or mist extractor/ demister pad •Does not require frequent cleaning •Better suited to crudes having high wax and deposition tendency Vane Vane type type

MAIN SECTIONS/COMPONENTS OF A SEPARATOR 4.Sump / Liquid Collection Section •Receiver for all liquid separated from well fluid in first 3 sections •Minimum level required for liquid controls •Requirement of surge volume for degassing or slug removal Vortex Vortex breaker breaker

SEPARATOR CONFIGURATION: 1.Vertical separator 2.Horizontal separator 3.Spherical separator 4.Centrifugal separator 5.Compact Separators/Hydro-cyclones •Vertical and Horizontal type separators are the most common in the oil industry

SEPARATOR CONFIGURATION: 1.Vertical Separator (Advantages) •Low to medium GOR streams •When relatively large liquid slugs are expected •Incidence of sand, paraffin, wax •Limited plot space •Ease of level control is desired •Full diameter for gas & liq. flow

Vertical separators ®

Disadvantages

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- Larger diameter for given gas capacity -

More difficult to skid mount and ship More difficult to reach and service top

mounted devices

SEPARATOR CONFIGURATION: 2.Horizontal Separator (Advantages) •High to medium GOR streams •Less difficult to skid mount and ship •Larger volume of gas •Foaming crude •3- separation

Horizontal separators ® ®

Disadvantages

ü

Only part of shell available for gas separation Occupies more area Liquid level control is more critical More difficult to clean produced sand



ü ü ü  

SEPARATOR SELECTION CRITERIA: Vertical Separator Low to medium GOR When large liquid slugs are expected

Horizontal Separator Medium to high GOR For larger volumes of gas

Incidence of sand, paraffin or wax

For foaming crude

Limited plot space For 3- separation Ease of level control is desired

General guidelines for use Vertical Compressor KOD ® Fuel gas KOD ® Degassing boots ® Absorber feed KOD ®

Horizontal Production separator ® 3-Phase separation ® Reflux drum ® Flare KOD ®

SEPARATOR CONFIGURATION: 3.Spherical Separator •Most commonly used for separation of large vol. of gas from extremely small vol. of liquid •High pressure service where compactness is desired •Limited liquid surge capacity

SEPARATOR CONFIGURATION: 4. Centrifugal/compact separators

- Relatively new type of separators - Technology still developing

- Less efficient than other separators

Centrifugal/compact separators • Advantages Less maintenance is involved Less space is required Light in weight Less expensive

Centrifugal/compact separators • Disadvantages •Not suitable for large liquid slugs •Efficiency not as good as other types •Narrow operating flow range for highest efficiency

Comparison of separators

Separator design guidelines ® Sufficient

residence time for both oil and water is provided to enable separation of water from oil and oil from water.

® ® Enough

free space is left at the top for separation of liquid from gas.

SEPARATOR SPECIFICATIONS: Retention time for liquid-liquid separation •Retention time determines the liquid capacity of a separator •API 12J recommendations are available for specifying retention time for 2- phase and 3- separator

SEPARATOR SPECIFICATIONS: Retention time for liquid-liquid separation •API 12J allows equal retention times for both oil and water •If problems such as foaming, wax, or slug flows are encountered, additional retention time may have to be considered

SEPARATOR SPECIFICATIONS : Retention time for liquid-liquid separation API RECOMM. FOR 2- SEPARATION Oil API Gravity

Retention time (min.)

Above 35

1

20 – 30

1 to 2

10 - 20

2 to 4

API RECOMM. FOR 3- SEPARATION Oil API Gravity

Retention time (min.)

Above 35

3 to 5

Below 35, Sep temp > 100 oF

5 to 10

Below 35, Sep temp > 80 10 to 20 oF Below 35, Sep temp > 60 20 to 30 oF

SIZING CRITERIA FOR A SEPARATOR Holdup time:

•Based on the reserve required to maintain good control and safe operation of downstream facilities

h1 h2 h3 h4 h5 h6

HLA HLL

Di

NLL LLL

h7

LLA

h8

T

•Time it takes to reduce the liquid level from NLL to LLL, while maintaining a normal outlet flow without feed make-up

T

T h1 h2 h3 h4 h5

h6

HLA HLL

Di

NLL LLL

h7

LLA

h8

T

SIZING CRITERIA FOR A SEPARATOR Surge time : •Time it takes for the liquid level to rise from NLL to maximum or HLL, while maintaining a normal feed flow without any outlet flow •Based on requirements to accumulate liquid as a result of upstream or downstream variations, e.g. slugs •Normally, surge time is taken as ½ of holdup time •

Design conditions for pressure vessels Pressure Operating Pr (Bar) 0-10 10-35 35-70 > 70 

Design Pr (Bar) MOP + 1 Bar MOP + 10% MOP + 3.5% MOP + 5% 

MOP : Maximum operating pressure

Design conditions for pressure vessels Temperature ®

Max design temperature = Max op. temp + 15 oC

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Min Design temperature = Min op. temp – 5 oC

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Note : Minimum design temperature must take into account of depressurization of the vessel 

Sizing of vertical separators ®

Calculate settling velocity

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vs = k [ (l - v )/ v ]1/2

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l = Liquid density, kg/m3 v = Vapour density, kg/m3 vs = settling velocity, m/s k = correlating factor (Find out from table)

Sizing of vertical separators ® ® ® 

Derate calculated settling velocity by 85% for design margin. Calculate internal diameter Di= [ 4Q/ vs]1/2

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Where  Di = Internal dia, mm  Q = Flow rate, M3/s ® Round the ID to nearest 50 mm. 

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Sizing of vertical separator Height calculation

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h1= Max 15% of dia. or 400mm h2= 150mm for mesh pad h3= max ( 50% of dia. or 600mm) h4= 400mm +d/2; d: inlet nozzle,mm h5= calculate based on 1-2 min residence  time at maximum liquid h6= calculate based on 4-5 min hold up time h7= calculate based on 1-2 min residence time h8= 300mm for bottom connection, 150mm  for side connection H = h1+ h2 + h3 + h4 + h5 + h6 + h7 + h8 t 

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h1 h2 h3 h4 h5

h6

HLA HLL

Di

NLL LLL

h7

LLA

h8

T

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T

Sizing of vertical separators ®

Wall thickness

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t = {PDi / (2SE-1.2P)}+ C

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Di = Internal dia., mm t = wall thickness, mm P = design pressure, barg E = joint efficiency (use 1.0 for seamless shells 0.85 otherwise)

S = Max allowable stress, bar

Sizing of vertical separators ®

Weight calculation

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- Shell weight with ellipsoid head

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Wt= s ( tDiHt+2x1.09xDi2)x10-9

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Where W = Bare vessel weight with ellipsoid head, Kg t  s = CS density = 7865.55 Kg/m3 

Sizing of vertical separators ®

Weight calculation

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- Shell weight with dished head

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Wt = s( tDiHt+2x0.842xDi2)x10-9

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where: W = bare vessel weight with dished head, t Kg 

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s= CS density = 7865.55 Kg/m3

Sizing of vertical separators ®

Weight calculation

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- Weight of the vessel can also be calculated

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from graph based on thickness, length and diameter.    

Nozzle sizing of separators Inlet nozzle ® Size based on normal volumetric flow + 10% ® Limit inlet velocity to 7-13 m/s ® Round nozzle diameter to nearest standard size 

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Gas outlet ® Size based on normal flow arte ® Limit velocity to 15-30 m/s 

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Liquid outlet ® Normal flow rate + 10% ® Limit velocity to 1-3 m/s for HC  2-4 M/S for water 

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SEPARATOR DESIGN : Horizontal 2- Separator

•Calculate settling velocity Vs •Vessel diameter required for droplet separation D1=  Qg/ .Vs.F.(L/D) •Vessel Vessel dia. required for sufficient liquid residence time D2= 3 16Q 16Qltres /3 .(L/D) •Select max. of D1 or D2. •

Qg - Gas flow rate at Pr.& T – m3/s Ql- Tot. liq. Flow rate - m3/s Vs- settling velocity - m/s D - Vessel Dia. -m L/D – Vessel design ratio F - Security factor (0.85) tres – liquid residence time -s

Three phase separation ® Types

of separators

® Vertical

three phase separator

® Horizontal

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3 phase separator

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Inter-phase control with weir

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Inter-phase control with boot

Design guidelines for 3-ph separator ® Settling

time for separation of oil from water and water from oil is calculated based on: ® minimum ® flow

particle size

rates ® density difference ® fluid viscosities. ®

SEPARATOR DESIGN : Horizontal 3- Separator

When heavy liquid volume is not substantial ( 15-20% by wt.) When almost equal vol. of light and heavy liq. are present

DETERMINATION OF SEPARATION STAGES: •Usually Usually carried in more than one stages •Separation Separation of HC mixture into vapor and liquid  in two or more equilibrium phases at successively lower pressures •Storage Storage tank is considered as one stage of separation •In In actual field practice, 2 or 3 stage separation is considered to be optimal

•More stable stock tank liquid •Enhanced liquid recovery

DETERMINATION OF SEPARATION STAGES:

DETERMINATION OF SEPARATION STAGES : •TOTAL GOR : Ratio of total cumulative gas recovered from all stages per bbl of stock tank oil produced

Total GOR, scf/bsto

•Total Total GOR varies with no. of stages for a given crude •Total Total GOR is lower for more no. of stages •When total GOR is lower, more of light fractions remain in oil, thus increasing oil API gravity, thereby yielding higher income

SEPARATOR SPECIFICATIONS: Separator efficiency Liquid carryover in gas •With mesh type ME, efficiency of 98-99% with droplet size smaller than 100

Gas content in oil •Generally < 2 – 5 % by volume

Oil content in water •< < 300 – 500 mg/l due to environmental restrictions

SEPARATOR SPECIFICATIONS: Liquid levels Oil level •Max oil level < 0.65 x ID •Normal Normal oil level = 0.5 x ID, or 1 min. retention time between max. and normal oil level •Low Low oil level = 0.1 x ID, or 12” from bottom, or safe height from normal water level to prevent water carrythrough; whichever is greater

Water level •Height Height corresponding to water retention time (1 to 2 min. normally), or 12”; whichever is greater

PREVENTIVE MAINTENANCE OF SEPARATORS : Regular checks should include: •Daily •Check liquid levels •Check pressures and temperatures

•Yearly •Check pressure relief valves

•Periodically •Lubricate valves •Clean gauge columns •Check drum valves •Check level control valves •Replace broken gauge glasses and pressure gauges whenever reqd. •Check back pressure controls

Oil & gas separation Sizes and Capacities :

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Efficiency of separators(accepted norms): ®

Liquid carry over of 10  droplet size in gas < 0.1 gal/ mmscf.

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Gas carried through oil - 2 to 5% by volume

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Oil content of effluent water < 300 - 500 mg/l

Oil & gas separation Sizes and Capacities :

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Recommended retention time: Oil gravity Retention time oAPI  (min.) 2-Phase > 35 1 Separators 20-30 1-2  10-20 2-4 ®

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3-Phase >35 3-5 separators Below35  100+o F 5-10  80+o F 10-20  60+o F 20-30 

Oil & gas separation Sizes and Capacities : ® Liquid levels 

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Maximum oil level

: < 0.65 I.D.

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Normal oil level

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Low oil level

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: 0.5 I.D. or 1 min. retention time between max. and normal liq. Level : 0.1 I.D. or 12” from bottom

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Water/ inter face level

: Water retention time of 1-2

Oil & gas separation Sizes and Capacities : ®

Design data required: ® Separator

operating temperature and

pressure ® Gas

: Flow rate, Sp. Gr., acid gas content

® Oil

: Flow rate, Sp. Gr., viscosity,

® Water

: Flow rate, Sp. Gr., viscosity,

corrosion and scaling tendencies. 

Oil & gas separation Sizes and Capacities : • Design data required: –Water : Flow rate, Sp. Gr., viscosity, corrosion and scaling tendencies. –Impurities : Quantities and description of deposits and –Vessel

scales

: Type, Pr. Rating, corrosion allowance, connections

and coatings. –Accessories: Codes to be followed, safety devices, and instrumentation desired.

Oil & gas separation Gas capacity

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Based on max. allowable gas velocity

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Max. allowable vap. Velocity (Vs)

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(Vs) = K (  L -  V /  V ) Where K=  4gDp / 3Cd

Oil & gas separation Oil capacity ® Calculation based on retention time of the liquid in the vessel sufficient to obtain equilibrium between liquid and gas. ® Liquid settling volume (W)  W = VL (t x 1440) 

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Where VL = Liquid capacity, bbl/d t

= retention time, min.

Oil & gas separation Design considerations:

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Sized for maximum flow rates

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Should take care of :

ü Heads/ slugs and pumping requirements ü Pumping , gas lift and naturally flowing wells . ® 

Oil & gas separation Capacity of vertical and horizontal separators: ®

Nomographs/Curves available ® To

size the separator

® To

determine the volume of fluid for a

given separator ®

•

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Suitable for preliminary sizing

Oil & gas separation Capacity of vertical and horizontal

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separators: ®

Gas capacity : Does not vary directly with a change in shell length.

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The gas capacity of a horizontal separator is proportional to its length.

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The liquid capacities depend on the liquid retained in the settling section of the separators.

Oil & gas separation 

Selection criteria of separators:

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Vertical separator applications ® Well

fluids having low GOR.

® Well

fluids having sand, etc.,

® Where ® Slug

flow from wells

® Down

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limitation of space exits

stream and upstream equipment.

Oil & gas separation Selection criteria of separators:

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Horizontal separator applications ®

Separation of water from oil

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Foaming crude oils

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Well fluids with high GOR

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Wells with relatively uniform flow

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Skid mounted or trailer mounted

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Stacking of multiple units.

Oil & gas separation Selection criteria of separators:

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Spherical oil and gas separator applications ®

Well fluids having high GOR and constant flow rate

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Installations where both vertical and horizontal space/height limitations exist.

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As gas scrubbers down stream of process units.

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Oil & gas separation Controls of separators:

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Liquid level controllers for gas/ oil and oil/water interfaces.

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Gas back pressure control valves

Oil & gas separation Valves of separators:

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Oil discharge control valve

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Water discharge control valve

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Drain valves

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Block valves

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Pressure relief valves

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Valves of sight glasses

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Oil & gas separation Accessories of separators:

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Pressure guages

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Thermometers

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Pressure reducing regulators

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Level sight glasses

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Safety head with rupture disk

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Piping and tubing

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Oil & gas separation Safety features in separators:

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High and low liquid level controls

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High and low pressure controls

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High and low temperature controls

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Safety relief valves

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Rupture disks

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Oil & gas separation Problems in oil and gas separation:

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Foaming of crude oil ®

Encasing of gas in thin film of oil

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Crudes that are likely to foam ®

crudes with oAPI < 40

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Operating temp < 160 0 F

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Crude viscosity > 50 cP

Oil & gas separation Problems in separation:

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Foaming of crude oil ® Effects ®

of foaming

Reduces the capacities of oil and gas separators

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Carry over of oil in gas

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Control of liquid levels

Oil & gas separation Problems in separation:

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Foam management ® Addition ® Special

crude ® 

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of foam depressants

designs for handling foaming

Oil & gas separation Problems in separation:

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Paraffin

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Reduces efficiency of separators

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Can be removed by steam / solvents

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Can be prevented by coating

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Oil & gas separation Problems in separation:

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Sand / silt / mud etc.,

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Reduces capacities of separators

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Sand removal by periodic draining

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Salt removal by mixing crude with water and draining the water.

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Oil & gas separation Problems in separation:

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Corrosion

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Cause early failure of equipment

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Most corrosive components in gases are CO2 and H2S.

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Oil & gas separation

Operation and maintenance: ® Periodic inspection ® Installation of safety devices ® Safety heads / rupture disks ® Mist extractors ® Hydrate inhibition ® Corrosive inhibition ® paraffin controls 

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Oil & gas separation

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Operation and maintenance in oil and gas

separation: ® High capacity operations ® Pressure shock loads ® Throttling of discharge liquids ® Pressure guages ® Guage cocks and glasses ® Cleaning of vessels ® ®

Oil & gas separation PREVENTIVE MAINTENANCE OF SEPARATORS  Daily Check liquid levels • Check pressure & temperatures • Replace broken guage glasses & pressure guages •

 Periodically Lubricate valves • Clean guage columns • Check level control valves •Check back pressure valves •

€Yearly •Check pressure relief valves

Oil & gas separation TROUBLE SHOOTING LOW LIQUID LEVEL •Fluid dump valve opening too wide or trim cut out •Drain valve opening or leaking •No fluid entering

Oil & gas separation TROUBLESHOOTING HIGH LIQUID LEVEL •Fluid control dump valve closed or plugged •Block valve around dump valve closed •Inlet valve to next vessel closed •Separator overloaded

Oil & gas separation TROUBLE SHOOTING LOW PRESSURE IN SEPARATOR

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Back pressure control valve not working

•Leaking safety relief valve •Inlet valve closed

Oil & gas separation TROUBLE SHOOTING HIGH PRESSURE IN SEPARATOR

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Back pressure control valve not working

• Separator downstream valve closed • Plugged mist extractor

Oil & gas separation TROUBLE SHOOTING ALL THE OIL GOING OUT OF GAS LINE •

Dump valve not open/partially open

• Blocked valve closed in piping to tank • Separator or piping plugged

Oil & gas separation TROUBLE SHOOTING MIST GOING OUT OF GAS LINE

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Vessel too small

• Plugged mist extractor • High liquid level • Foaming problem

Oil & gas separation TROUBLE SHOOTING FREE GAS GOING OUT OF OIL/WATER VALVE •

Too low level in separator

• Dump valve not seating •Block valve around dump valve passing

Oil & gas separation TROUBLE SHOOTING EXCESS GAS GOING TO TANK WITH OIL

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Less retention time

• Foaming oil • Too much pressure drop from separator to tank

Oil & gas separation TROUBLE SHOOTING OIL/CONDENSATE & WATER NOT SEPARATING IN 3 PHASE SEPARATOR

• Paraffin problem hampering water from being free • Not enough retention time • Interface level control not working properly • Leak in adjustable weir • Adjustable weir height to be adjusted

Oil & gas separation TROUBLE SHOOTING DIAPHRAGM OPER. DUMP VALVE NOT OPERATING •

Pilot failure

• Supply gas failure • Out of adjustment • Broken valve stem • Plugged tubing • Ruptured diaphragm • Leak in line from pilot to valve