emulsions and oil treating equipment selection, sizing and troubleshooting

It ispreferable to heat the inlet so that more gas is liberated in the boot,although this means that fuel will be used in heating any free water inthe inlet.The height of the external wa

Treating Equipment

Linacre House, Jordan Hill, Oxford OX2 8DP, UK

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08 09 10 10 9 8 7 6 5 4 3 2 1

A Note from the Authors vii

4.5 Choosing the Proper Filter 227

Appendix A: Definition of Key Water Treating Terms 267

C.2 Determination of Dissolved Oil and Grease 278

C.5 Analysis of Variance of Analytical Results 279

Gulf Equipment Guides series serves as a quick reference for the design,selection, specification, installation, operation, testing, and trouble-shooting of surface production equipment The Gulf Equipment Guidesseries consists of multiple volumes, each of which covers a specific area

in surface production equipment These guides cover essentially thesame topics included in the “Surface Production Operations” seriesbut omit the proofs of equations, example problems and solutions whichbelong more properly in a handbook This book contains fewer pagesand is therefore more focused The reader is referred to the correspondingvolume of the “Surface Production Operations” series for further detailsand additional information such as derivations of some of the equations,example problems and solutions and suggested test questions

Emulsions and Oil Treating Equipment: Selection, Sizing and bleshooting is the second volume in the Gulf Equipment Guidesseries Each guide serves as a quick reference resource The series isintended to provide the most comprehensive coverage you’ll findtoday dealing with surface production operation in its various stages,from initial entry into the flowline through gas–liquid and liquid–liquid separation; emulsions, oil and water treating; water injection;hydrate prediction and prevention; gas dehydration; and gas condition-ing and processing equipment to the exiting pipeline.

Trou-Featured in this volume are such important topics as emulsions,oil treating, desalting, water treating, water injection systems, andany other related topics This volume as well as all volumes in theGulf Equipment Guides series, serve the practicing engineer andsenior field personnel by providing organized design procedures;details on suitable equipment for application selection; and charts,tables, and nomographs in readily useable form Facility engineers,process engineers, designers, operations engineers, and senior produc-tion operators will develop a “feel” for the important parameters indesigning, selecting, specifying, and trouble-shooting surface produc-tion facilities Readers will understand the uncertainties and assump-tions inherent in designing and operating the equipment in thesesystems and the limitations, advantages, and disadvantages associatedwith their use

Crude Oil Treating Systems

1.1 Introduction

Conditioning of oil-field crude oils for pipeline quality is complicated

by water produced with the oil Separating water out of produced oil isperformed by various schemes with various degrees of success Theproblem of removing emulsified water has grown more widespreadand oftentimes more difficult as production schemes lift more waterwith oil from water-drive formations, water-flooded zones, and wellsstimulated by thermal and chemical recovery techniques This chap-ter describes oil-field emulsions and their characteristics, treatingoil-field emulsions so as to obtain pipeline quality oil, and equipmentused in conditioning oil-field emulsions

1.2 Equipment Description

1.2.1 Free-Water Knockouts

Most well streams contain water droplets of varying size If they collecttogether and settle to the bottom of a sample within 3–10 min, they arecalled “free water.” This is an arbitrary definition, but it is generallyused in designing equipment to remove water that will settle outrapidly A free-water knockout (FWKO) is a pressure vessel used toremove free water from crude oil streams (Figure 1.1) They are located

in the production flow path where turbulence has been minimized.Restrictions such as orifices, chokes, throttling globe valves, andfittings create turbulence in the liquids that aggravate emulsions Freewater, at wellhead conditions, frequently will settle out readily to thebottom of an expansion chamber

Sizing and pressure ratings for these vessels are discussed in the

“Gas–Liquid and Liquid–Liquid Separation” volume, this series tors affecting design include retention time, flow rate (throughput),temperature, oil gravity (as it influences viscosity), water drop size

Fac-distribution, and emulsion characteristics Abnormal volumes of gas

in the inlet stream may require proportionately larger vessels as thesegas volumes affect the throughput rate A simple “field check” todetermine retention time is to observe a fresh sample of the wellheadcrude and the time required for free water to segregate

In installations where gas volumes vary, a two-phase separator isusually installed upstream of the FWKO The two-phase separatorremoves most of the gas and reduces turbulence in the FWKO vessel.The FWKO usually operates at 50 psig (345 kPa) or less due to the ves-sel’s location in the process flow stream Internals should be coated orprotected from corrosion since they will be in constant contact withsalt water

1.2.2 Gunbarrel Tanks with Internal and External Gas Boots

The gunbarrel tank, sometimes called a wash tank, is the oldestequipment used for multi-well onshore oil treating in a conventionalgathering station or tank battery Gunbarrel tanks are very common

in heavy crude applications such as in Sumatra and East Kalimantan,Indonesia, and in Bakersfield, California

The gunbarrel tank is a vertical flow treater in an atmospherictank Figure 1.2 shows a “gunbarrel” tank with an internal gas boot.Typically, gunbarrels have an internal gas separating chamber or

“gas boot” extending 6–12 ft (2–4 m) above the top of the tank, wheregas is separated and vented, and a down-comer extending 2–5 ft (0.6–1.5 m) from the bottom of the tank A variation of the above gunbarrelconfiguration is a wash tank with an “external” gas boot This configu-ration is preferred on larger tanks, generally in the 60,000-barrel range,where attaching an internal gas boot is structurally difficult In eithercase, the gunbarrel tank is nothing more than a large atmosphericsettling tank that is higher than downstream oil shipping and waterclarifier tanks The elevation difference allows gravity flow into thedownstream vessels

FIGURE 1.1 Cutaway of a free-water knockout

Because gunbarrels tend to be of larger diameter than verticalheater-treaters, many have elaborate spreader systems that attempt

to create uniform (i.e., plug) upward flow of the emulsion to take imum advantage of the entire cross section Spreader design is impor-tant to minimize the amount of short-circuiting in larger tanks.The emulsion, flowing from an upstream separator and possibly

max-a hemax-ater, enters the top of the gmax-as sepmax-armax-ation section of the gmax-as boot.The gravity separation section removes flash gas and gas liberated as

a result of heating the emulsion The emulsion flows down thedown-comer to a spreader, which is positioned below the oil–waterinterface Exiting at the bottom of the down-comer, the emulsion rises

to the top of the surrounding layer of water The water level is trolled by a water leg or automatic level control The emulsion passagethrough the water helps collect the entrained water and converts

con-Gas Oil

Water Emulsion

Gas Outlet

Gas Separating Chamber

Water Wash Section

Oil Settling Section

Oil Outlet

the emulsion into distinct oil and water layers Oil accumulates at thetop and flows out through the spillover line into the oil settling tank.Water flows from the bottom of the tank, up through the water leg,and into a surge or clarifier tank The height of the water leg regulatesthe amount of water retained in the vessel The settling time in thevessel for the total fluid stream is usually 12–24 h Most gunbarrelsare unheated, though it is possible to provide heat by heating the incom-ing stream external to the tank, installing heating coils in the tank, orcirculating the water to an external or ‘jug’ heater in a closed loop It ispreferable to heat the inlet so that more gas is liberated in the boot,although this means that fuel will be used in heating any free water inthe inlet.

The height of the external water leg controls the oil–water interfaceinside the vessel and automatically allows clean oil and produced water

to exit the vessel Example 1.1 illustrates this design consideration

1.2.3 Determination of External Water Leg Height

Given:

Oil gravity at 60
F 36
API

Water specific gravity 1.05

Height of oil outlet 23 ft

Height of interface level 10 ft (for this example)Height of water outlet 1 ft

Solution:

Determine the oil specific gravity

Oil specific gravity ¼ 141:5

131:5 þ
API¼ 141:5

131:5 þ 36¼ 0:845

1 Determine the oil gradient

Since the charge in the pressure with depth for fresh water is0.433 psi/ft of depth, the change in pressure with depth of fluid whosespecific gravity is SG would be 0.433 (SG); thus, the oil gradient is

Oil gradient ¼ ð0:433Þð0:845Þ ¼ 0:366 psi=ft:

2 Determine the water gradient

Water gradient ¼ ð0:433Þð1:05Þ ¼ 0:455 psi=ft:

3 Calculate the height of the oil and the height of the water inthe tank.

Ho¼ height of oil outlet  height of interface level ¼ 23  10 ¼ 13 ft

Hw¼ height of interface level  height of water outlet ¼ 10  1 ¼ 9 ft:

4 Perform a pressure balance

Hydrostatic pressureinside tank

Water Emulsion

Gas Outlet

Chamber

Oil Outlet

The design details for the spreader, water leg, and gas separation tion vary for different manufacturers These details do not signifi-cantly affect the sizing of the tank, provided the spreader minimizesshort-circuiting No matter how careful the design of the spreaders,large wash tanks are very susceptible to short-circuiting This is due

sec-to temperature and density differences between the inlet emulsionand the fluid in the tank, solids deposition, and corrosion of thespreaders

Standard tank dimensions are listed in API Specification 12F(Shop Welded Tanks), API Specification 12D (Field Welded Tanks),and API Specification 12B (Bolted Tanks) These dimensions areshown in Tables 1.1, 1.2, and 1.3, respectively

Gunbarrels are simple to operate and, despite their large size, arerelatively inexpensive However, they have a large footprint, which iswhy they are not used on offshore platforms Gunbarrels hold a large

TABLE 1.1

Shop welded tanks (API specification 12 F)

(a) Field UnitsNominal

Capacity

(bbl)

Approximate

WorkingCapacity (bbl)

OutsideDiameter(ft–in.) Height(ft–in.) Height of OverflowConnection (ft–in.)

Capacity

(bbl)

Approximate

WorkingCapacity (m3)

OutsideDiameter(m) Height(m)

Height ofOverflowConnection (m)

volume of fluids, which is a disadvantage should a problem develop.When the treating problem is detected in the oil outlet, a large volume

of bad oil has already collected in the tank This oil may have to betreated again, which may require large slop tanks, recycle pumps,etc It may be beneficial to reprocess this bad oil in a separate treatingfacility so as to avoid further contamination of the primary treatingfacility

Gunbarrels are most often used in older, low-flow-rate, onshorefacilities In recent times, vertical heater-treaters have become soinexpensive that they have replaced gunbarrels in single-well

TABLE 1.2

Field welded tanks (API specification 12D)

(a) Field Units

Nominal Outside Diameter (ft–in.)

Nominal Height (ft–in.)

Height of Overflow Line Connection (ft–in.) Pressure Vacuum

Nominal Outside Diameter (m)

Nominal Height (m)

Height of Overflow Line Connection (m) Pressure Vacuum

TABLE 1.3

Bolted tanks (API specification 12B)

(a) Field UnitsNominal

Capacity

(42-gal bbl) Numberof Rings

InsideDiametera(ft–in.)

Height ofShellb(ft–in.)

CalculatedCapacityc(42-gal bbl)

Capacity

(42-gal bbl) Numberof Rings

InsideDiameterd(m)

Height ofShellb(m)

CalculatedCapacityc(m3)

The calculated capacity is based on the inside diameter and height of shell.

d The inside diameter is an approximate dimension The values shown are less than the bottom bolt-circle diameters.

installations On larger installations onshore in warm weather areas,gunbarrels are still commonly used In areas that have a winter seasonthey tend to become too expensive to keep the large volume of oil at ahigh enough temperature to combat potential pour-point problems.

1.2.4 Horizontal Flow Treaters

Horizontal flow treaters are not common Figure 1.4 illustrates onedesign, which consists of a cylindrical treating tank incorporatinginternal baffles The internal baffles establish a horizontal flow pat-tern in the cylindrical tank, which is more efficient for gravity separa-tion than vertical flow and is less subject to short-circuiting

The oil, emulsion, and water enter the vessel and must followthe long flow path between the baffles Separation takes place in thestraight flow areas between the baffles Turbulence coupled with highflow velocities prevents separation at the corners, where the flowreverses direction Tracer studies indicate that approximately twothirds of the plan area of the tank is effective in oil–water separation

In addition to gravity separation, the emulsion must be collectedand held in the treater for a certain retention time so that the emul-sion will break In horizontal flow treaters, the emulsion collects

Oil Water hw/z

between the oil and water; however, the horizontal flow pattern tends

to sweep the emulsion toward the outlets The emulsion layer maygrow much thicker at the outlet end of the treater than at the inletend Accordingly, it is much easier for the emulsion to be carriedout of the vessel with the oil

1.2.5 Heaters

Heaters are vessels used to raise the temperature of the liquid before itenters a gunbarrel, wash tank, or horizontal flow treater They areused to treat crude oil emulsions The two types of heaters commonlyused in upstream operations are indirect fired heaters and direct firedheaters Both types have a shell and a fire tube Indirect heaters have athird element, which is the process flow coil Heaters have standardaccessories such as burners, regulators, relief valves, thermometers,temperature controllers, etc

Indirect Fired Heaters

Figure 1.5 shows a typical indirect fired heater Oil flows throughtubes that are immersed in water, which in turn is heated by a firetube The heat may be supplied by a heating fluid medium, steam,

or electric immersed heaters Indirect heaters maintain a constanttemperature over a long period of time and are safer than the directheater Hot spots are not as likely to occur if the calcium content ofthe heating water is controlled The primary disadvantage is thatthese heaters require several hours to reach the desired temperatureafter they have been out of service

Emulsion Inlet Emulsion Outlet

Direct Fired Heaters

Figure 1.6 shows a typical direct fired heater Oil flows through aninlet distributor and is heated directly by a fire box The heat may

be supplied by a heating fluid medium, steam, or an electric immersedheater Direct heaters are quick to reach the desired temperature, areefficient (75–90%), and offer a reasonable initial cost Direct fired hea-ters are typically used where fuel gas is available and high volume oiltreating is required On the other hand, they are hazardous and requirespecial safety equipment Scale may form on the oil side of the firetube, which prevents the transfer of heat from the fire box to the oilemulsion Heat collects in the steel walls under the scale, whichcauses the metal to soften and buckle The metal eventually rupturesand allows oil to flow into the fire box, which results in a fire Theresultant blaze, if not extinguished, will be fed by the incoming oilstream

1.2.6 Waste Heat Recovery

A waste heat recovery heater captures waste heat from the exhauststacks of compressors, turbines, generators, and large engines Heatexchangers are used to transfer this heat to a heating fluid medium,which in turn is used to heat the crude oil emulsion

1.2.7 Heater-Treaters

Heater-treaters are an improvement over the gunbarrel and heater tem Many designs are offered to handle various conditions such as

sys-Oil Outlet

Crude Oil Inlet

Crude Oil Emulsion Heat or Fire

FIGURE 1.6 Cutaway of a horizontal direct fired heater

viscosity, oil gravity, high and low flow rates, corrosion, and coldweather When compared to gunbarrels, heater-treaters are less expen-sive initially, offer lower installation costs, provide greater heat effi-ciency, provide greater flexibility, and experience greater overallefficiency On the other hand, they are more complicated, provide lessstorage space for basic sediment, and are more sensitive to chemicals.Since heater-treaters are smaller than other treating vessels, theirretention times are minimal (10–30 min) when compared to gunbar-rels and horizontal flow treaters.

Internal corrosion of the down-comer pipe is a common problem.Build-up of sediment on the walls or bottom of the treater can causethe interface levels to rise and liquid to carry over and/or oil to exitthe treater with salt water Bi-annual inspections should be performed

to include internal inspection for corrosion, sediment build-up, andscale build-up

1.2.8 Vertical Heater-Treaters

The most commonly used single-well treater is the vertical treater, which is shown in Figure 1.7 The vertical heater-treater con-sists of four major sections: gas separation, FWKO, heating and water-wash, and coalescing-settling sections Incoming fluid enters the top

heater-of the treater into a gas separation section, where gas separates fromthe liquid and leaves through the gas line Care must be exercised tosize this section so that it has adequate dimensions to separate thegas from the inlet flow If the treater is located downstream of a sepa-rator, the gas separation section can be very small The gas separationsection should have an inlet diverter and a mist extractor

The liquids flow through a down-comer to the base of the treater,which serves as a FWKO section If the treater is located downstream

of a FWKO or a three-phase separator, the bottom section can be verysmall If the total wellstream is to be treated, this section should besized for 3–5 min retention time to allow the free water to settleout This will minimize the amount of fuel gas needed to heat the liq-uid stream rising through the heating section The end of the down-comer should be slightly below the oil–water interface so as to

‘water-wash’ the oil being treated This will assist in the coalescence

of water droplets in the oil

The oil and emulsion rise through the heating and water-washsection, where the fluid is heated (Figure 1.8) As shown in Figure 1.9,

a fire tube is commonly used to heat the emulsion in the heating and

Fire Tube

Oil/Water Interface

Gas Equalizer Mist Extractor

d

h

FIGURE 1.7 Simplified schematic of a vertical heater-treater

water-wash section After the oil and emulsion are heated, the heatedoil and emulsion enter the coalescing section, where sufficient reten-tion time is provided to allow the small water droplets in the oil con-tinuous phase to coalesce and settle to the bottom As shown inFigure 1.10, baffles are sometimes installed in the coalescing section

to treat difficult emulsions The baffles cause the oil and emulsion

Fluid

Out Drain

Gas Out

Free-Water Knockout Section

Heating and Water- Wash Section

Oil Settling Section

Gas Separation Section

FIGURE 1.8 Three-dimensional view illustrating oil and emulsion risingthrough the heating and water-wash

to follow a back-and-forth path up through the treater Heating causesmore gas to separate from the oil than is captured in the condensinghead Treated oil flows out the oil outlet, at the top of the coalescingsection, and through the oil leg heat exchanger, where a valve controlsthe flow Heated clean oil preheats incoming cooler emulsion in theoil leg heat exchanger (Figure 1.11) Separated water flows out throughthe water leg, where a control valve controls the flow to the watertreating system (Figure 1.12).

As shown in Figure 1.13, any gas flashed from the oil due to ing, is captured on the condensing head Any gas that did not con-dense flows through an equalizing line to the gas separation section

heat-Hot Air Fire Tube

Emulsion Stack

Thermometer

Fuel Gas Inlet

Thermostat

Safety Fuel Gas Scrubber

FIGURE 1.9 Cutaway showing a typical fire-tube that heats the emulsion inthe heating and water-wash section

As shown in Figure 1.14, a vane-type mist extractor removes the uid mist before the gas leaves the treater The gas liberated whencrude oil is heated may create a problem in the treater if it is not ade-quately designed In vertical heater-treaters the gas rises through thecoalescing section If a great deal of gas is liberated, it can createenough turbulence and disturbance to inhibit coalescence Equallyimportant is the fact that small gas bubbles have an attraction for sur-face-active material and hence water droplets Thus, they tend to keep

liq-FIGURE 1.10 Baffles, installed in the coalescing section, cause the emulsion

to follow a back-and-forth path up through the oil settling section

the water droplets from settling out and may even cause them to carryover to the oil outlet.

The oil level is maintained by pneumatic or lever-operated dumpvalves The oil–water interface is controlled by an interface level con-troller or an adjustable external water leg

Standard vertical heater-treaters are available in 20- and 27-ft(6.1 and 8.2 m) heights These heights provide sufficient static liquidhead so as to prevent vaporization of the oil The detailed design

of the treater, including the design of internals (many features ofwhich are patented), should be the responsibility of the equipmentsupplier

Clean Oil In

Well Fluids Out

Clean

Oil Out

Incoming Well Fluids

FIGURE 1.11 Heated clean oil preheats incoming cooler emulsion in the oilleg heat exchanger

1.2.9 Coalescing Media

It is possible to use coalescing media to promote coalescence of thewater droplets These media provide large surface areas upon whichwater droplets can collect In the past the most commonly used coa-lescing media was wood shavings or ‘excelsior,’ which is also referred

to as a ‘hay section.’ The wood excelsior was tightly packed to create

an obstruction to the flow of the small water droplets and promoterandom collision of these droplets for coalescence When the dropletswere large enough, they fell out of the flow stream by gravity.Figure 1.15 shows a vertical heater-treater utilizing an excelsior

Oil Dump Valve

Heat Exchanger

Oil Outlet

FIGURE 1.12 Cutaway illustrating oil and water legs

The use of an “excelsior” allowed lower treating temperatures.However, these media had a tendency to clog with time and weredifficult to remove Therefore, they are no longer used.

1.2.10 Horizontal Heater-Treaters

For most multi-well flow streams, horizontal heater-treaters are mally required Figure 1.16 shows a simplified schematic of a typicalhorizontal heater-treater Design details vary from manufacturer tomanufacturer, but the principles are the same The horizontalheater-treater consists of three major sections: front (heating andwater-wash), oil surge chamber, and coalescing sections

nor-Gas Equalizing Line

Incoming fluids enter the front (heating and water-wash) sectionthrough the fluid inlet and down over the deflector hood (Figure 1.17)where gas is flashed and removed Heavier materials (water and solids)flow to the bottom while lighter materials (gas and oil) flow to the top.Free gas breaks out and passes through the gas equalizer loop to the gasoutlet As shown in Figure 1.18, the oil, emulsion, and free water passaround the deflector hood to the spreader located slightly below theoil–water interface, where the liquid is “water-washed” and the freewater is separated For low gas–oil-ratio crudes, blanket gas may berequired to maintain gas pressure The oil and emulsion are heated asthey rise past the fire tubes and are skimmed into the oil surge chamber.

As free water separates from the incoming fluids in the front tion, the water level rises If the water is not removed, it will continue

sec-to rise until it displaces all emulsion and begins sec-to spill over the weirinto the surge section On the other hand, if the water level becomestoo low, the front section will not be able to water-wash the incomingoil and emulsion, which would reduce the efficiency of the treater.Therefore, it is important to accurately control the oil–water interface

Shell

Vanes Gas

Inlet

Liquid Outlet FIGURE 1.14 Vane-type mist extractor removes the liquid mist before thegas leaves the treater

in the front section The oil–water interface is controlled by either aninterface level controller, which operates a dump valve for the freewater (Figure 1.19), or a resistance probe If the water outlet valve sticksopen, all the water and oil run out, exposing the fire tube or heat source.

As shown in Figure 1.20, a level safety low shutdown sensor isrequired in the upper portion of the front (heating and water-wash)section This sensor assures liquid is always above the fire tube

If the water dump valve malfunctions or fails open, the liquidsurrounding the fire tube will drop, thus not absorbing the heat gener-ated from the fire tube and possibly damaging the fire tube by over-heating Thus, if the level above the fire tube drops, the level safetylow shutdown sensor sends a signal that closes the fuel valve feedingthe fire tube It is also important to control the temperature of thefluid in the front (heating and water-wash) section Therefore, a tem-perature controller, controlling the fuel to the burner or heat source,

is required in the upper part of the heating–water-wash section(Figure 1.21)

Excelsior

FIGURE 1.15 Vertical heater-treater fitted with excelsior, between the fles, which aids in coalescence of water droplets

baf-Drain Free Water Out

Clean Oil Out

Treated Water Out

Emulsion Clean Oil

Coalescing-Settling Section

Surge Section

Oil and Emulsion

Fluid In

FIGURE 1.16 Simplified schematic of a horizontal heater-treater

Free Water Outlet

Clean Oil Outlet

Treated Water Outlet

Fluid Inlet

Gas Outlet

Surge Section

Drain

Drain

Heating Section

Coalescing Settling Section

FIGURE 1.17 Three-dimensional view of a horizontal heater-treater flow pattern

Oil & Emulsion

Gas Out

Oil Oil Out

LeffWater Out

Oil Surge Chamber

Deflector Around FiretubeFree WaterOut Spreader

Emulsion Inlet

Gas Equalizer

FIGURE 1.18 Schematic of horizontal heater-treater showing the oil, sion, and free water passing around the deflector hood to the spreader locatedslightly below the oil–water interface where the liquid is “water-washed” andthe free water separated

emul-Float Interface Level Control

Water Outlet Valve Closed Water Level in Heating − Water-Wash Section

FIGURE 1.19 Oil–water interface in the heating and water-wash section iscontrolled by an interface level controller

A level controller, in the oil surge section (Figure 1.22), ates the dump valve on the clean oil outlet line This dump valveregulates the flow of oil out the top of the vessel, which maintains

oper-a liquid poper-acked condition When the cleoper-an oil outlet voper-alve is open,the pressure of the gas in the surge chamber forces the emulsion toflow through the spreader and push the clean oil outlet throughthe clean oil collector (Figure 1.23) When the clean oil outlet valvecloses, the flow of emulsion to the coalescing-settling section stops,and gas is prevented from entering the coalescing-settling section(Figure 1.24)

The oil and emulsion flow through a spreader into the back orcoalescing section of the vessel, which is fluid packed The spreaderdistributes the flow evenly throughout the length of this section.Because it is lighter than the emulsion and water, treated oil rises tothe clean oil collector, where it is collected and passes the treaterthrough the clean oil outlet The collector is sized to maintainuniform vertical flow of the oil Coalescing water droplets fall coun-tercurrent to the rising oil continuous phase

Level Safety Low Fuel Shutdown Sensor

FIGURE 1.20 Level safety low sensor, located at the top of the wash section, shuts off the fuel to the heat source (fire-tube) on low liquidlevel

heating–water-The front (heating and water-wash) section must be sized to dle settling of the free water and heating of the oil The coalescing sec-tion must be sized to provide adequate retention time for coalescing tooccur and to allow the coalescing water droplets to settle downwardcountercurrent to the upward flow of the oil.

han-Most horizontal heater-treaters built today do not use fire-tubes.Heat is added to the emulsion in a heat exchanger before the emulsionenters the treater In these cases the inlet section of the treater can befairly short because its main purpose is to degas the emulsion before itflows to the coalescing section

Some heater-treaters are designed with only the coalescing tion In these cases the inlet is pumped through a heat exchanger to

sec-a tresec-ater thsec-at opersec-ates sec-at sec-a high enough pressure to keep the oil sec-aboveits bubble-point Thus, the gas will not evolve in the coalescingsection of the treater

1.2.11 Electrostatic Heater-Treaters

Some horizontal heater-treaters add an electrostatic grid in the lescing section Figure 1.25 illustrates a simplified schematic of a typ-ical horizontal electrostatic treater The flow path in an electrostaticheater-treater is basically the same as in a horizontal heater-treater,except that an electrostatic grid is included in the coalescing-settlingsection, which helps to promote coalescence of the water droplets

heat-The electrostatic section contains two or more electrodes, onegrounded to the vessel and the other suspended by insulators An elec-trical system supplies an electric potential to the suspended electrode.The usual applied voltage ranges from 10,000 to 35,000 VAC, and thepower consumption is from 0.05 to 0.10 kVA/ft2 (0.54–1.08 kVA/m2)

of grid The intensity of the electrostatic field is controlled by theapplied voltage and spacing of electrodes In some installationsthe location of the ground electrode can be adjusted externally toincrease or decrease its spacing to the “hot” electrode Optimum field

Emulsion Level

Oil Level Controller Float

Weir

Clean Oil Outlet Valve

FIGURE 1.22 Level controller in the oil surge section operates the clean oildump valve

intensities vary with applications but generally fall within the range

of 1000–4000 V/in (39–157 V/mm) of separation The use of an tric field is most effective whenever the fluid viscosity is less than

elec-50 cp at separating temperature, the specific gravity differencebetween the oil and water is greater than 0.001, and the electricalconductivity of the oil phase does not exceed 106mho/cm

The electrical control system that supplies energy to the trodes consists of a system of step-up transformers (either single- orthree-phase) in which the primary side is connected to a low-voltagepower source (208, 220, or 440 V) and secondary windings are designed

elec-so that the induced voltage will be of the desired magnitude (Figure 1.26)

As shown in Figure 1.27, oil and small water droplets enter the lescing section and travel up into the electrostatic grid section, wherethe water droplets become “electrified” or “ionized” and are forced to col-lide The electrodes have electrical charges that reverse many times a

coa-Coalescing Section

Surge Section

Gas Pressure

Outlet Valve

Clean Oil Collector

FIGURE 1.23 Pressure of the gas in the surge section forces the emulsion toflow through the spread

Emulsion Level

Oil Level Controller

Clean Oil Outlet Valve

FIGURE 1.24 When the clean oil dump valve closes, the flow of emulsion tothe coalescing settling section stops and the gas is prevented from entering

Fire Tube or Heat Source Water

Emulsion Spreader

Grids Electrical Coalescing Section

FIGURE 1.25 Simplified schematic of a horizontal electrostatic heater-treater

second; thus, the water droplets are placed in a rapid back-and-forthmotion The greater the motion of the droplets, the more likely the waterdroplets are to collide with each other, rupture the skin of the emulsify-ing agent, coalesce, and settle out of the emulsion Because of the forcedcollisions, electrostatic heater-treaters typically operate at lower tem-peratures and use less fuel than standard heater-treaters The time in

Circuit Breaker Ammeter

Low Voltage

Electricity From Power Source

High Voltage

Charged

Grid

Signal Light

Transformer

FIGURE 1.26 Electrical control system of an electrostatic heater-treater