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PETROLEUM ENGINEERING HANDBOOK
Fig. 12.2—Horizontal skid-mounted three-phase well tester on offshore drilling platform off coast of Brazil.
knockout vessel installed upstream or downstream of the separators. If the water is emulsified, it may be necessary to use an emulsion treater to remove it. Figs. 12.2 through 12.5 are illustrations of three-phase separators.
Secondary Functions of Oil and Gas Separators Maintain Optimum Pressure on Separator
For an oil and gas separator to accomplish its primary functions, pressure must be maintained in the separator so that the liquid and gas can be discharged into their respective processing or gathering systems. Pressure is
maintained on the separator by use of a gas backpressure valve on each separator or with one master backpressure valve that controls the pressure on a battery of two or more separators. Fig. 12.6 shows a typical low-pressure gas backpressure backpressure valve, and Fig. 12.7 shows a typical high pressure pressure gas backpressure backpressure valve used to maintain the desired pressure in separators. The optimum pressure to maintain on a separator is the pressure that will result in the highest economic yield from the sale of the liquid and gaseous hydrocarbons. This optimum pressure can be calculated theoretically or determined by field tests. GAS
FLOAT NOZZLES
MIST EXTRACT
INLET FLUID E L E M E N T NONWEIGHTED F L O A T . MOTOR VALVE (DMV)
WEIGHTED FLOAT WEIGHTED F L OAT SECTION
A-A
SECTION B-B
Fig. 12.3—Schematic of typical horizontal three-phase oil/gas/water separator.
Fig. 12.4—Schematic of a typical vertical three-phase oil/gas/water separator.
OIL AND GAS SEPARATORS
12-5
GAS OUTLET
CENTRIFUGAL-TYPE INLET SEPARATING
OIL LEVEL FLUID - - - - -
NONWEIGHTED
Fig. 12.5—Schematic of a typical spherical three-phase oil/gas/water separator.
Fig. 12.6—Low-pressure-gas backpressure valve.
Maintain Liquid Seal in Separator
To maintain pressure on a separator, a liquid seal must be effected in the lower portion of the vessel. This liquid seal prevents loss of gas with the oil and requires the use of a liquid-level controller and a valve similar to those shown in Figs. 12.8 and 12.9. A lever-operated valve similar to the one shown in Fig. 12.10 can be used to
Fig. 12.7—High-pressure-gas backpressure valve.
maintain the liquid seal in a separator when the valve is operated by a float that is actuated by the oil level in the separator. The oil discharge control valve shown in Fig. 12.9 can be actuated by a float-operated pilot (not illustrated), by a floatless liquid-level controller similar to the one shown in Fig. 12.11, or by a torque tube-type (dis placement) liquid-level controller similar to the one shown in Fig. 12.8.
12-6
Fig. 12.8—Torque-tube (displacement)-type liquid-level controller.
Special Problems in Oil and Gas Separation Separating Foaming Crude Oil
When pressure is reduced on certain types of crude oil, tiny spheres (bubbles) of gas are encased in a thin film of oil when the gas comes out of solution. This may result in foam, or froth, being dispersed in the oil and creates what is known as “foaming” oil. In other types of crude oil, the viscosity and surface tension of the oil may
Fig. 12.10—Lever-type valve for controlling oil discharge from oil and gas separators. Valve is float operated.
PETROLEUM ENGINEERING HANDBOOK
Fig. 12.9—Diaphragm-motor-type oil-discharge control valve.
mechanically lock gas in the oil and can cause an effect similar to foam. Oil foam will not be stable or long-lasting unless a foaming agent is present in the oil. Crude oil is more likely to foam when (1) the API gravity is less than 40° API, (2) the operating temperature is less than 160°F, and (3) the crude oil is viscous, having a viscosity greater than 5,000 SSU (about 53 cp). Foaming greatly reduces the capacity of oil and gas separators because a much longer retention time is required to separate adequately a given quantity of foaming crude oil. Foaming crude oil cannot be measured accurately with positive-displacement meters or with conventional volumetric metering vessels. These problems, combined with the potential loss of oil and gas because of improper separation, emphasize the need for special equipment and procedures in handling foaming crude oil. There are many special designs of separators for handling foaming crude oil. The special horizontal separator for handling foaming oil shown in Fig. 12.12 is one of the simpler, more effective units available for this service. The special degassing element used on the inlet of this separator shown in Section CC of Fig. 12.12 gently agitates the well fluid and assists in removing gas from the oil and in breaking foam bubbles as they flow through the inlet element. The defoaming plates, which extend from near the inlet end to near the outlet end of the separator, are spaced 4 in. apart and are shaped with an apex at the vertical center of the separator. The plates that are immersed in oil assist in removing nonsolution gas from the oil and in breaking foam in the oil. The plates that are above the oil/gas interface in the gas section of the separator remove
OIL AND GAS SEPARATORS
12-7
oil mist from the gas and assist in breaking foam that may exist in the gas section of the vessel. The 6-in.-thick knitted-wire-mesh mist extractor (located below the gas outlet) removes the remainder of the liquid mist from the gas and breaks or removes the remaining foam bubbles from the gas. The vertical separator shown in Fig. 12.13 can be used to handle foaming crude oil. As the oil cascades down the plates in this unit, the foam bubbles will be distorted and broken. This design can increase the capacity of the separator to handle foaming oil by 10 to 50%. The main factors that assist in “breaking” foaming oil are settling, agitation (baffling), heat, chemicals, and centrifugal force, These factors or methods of “reducing” or “breaking” foaming oil are also used to remove entrained gas from oil. They are discussed on Pages 12-13 through 12-15. Many different designs of separators for handling foaming crude oil have evolved. They are available from various manufacturers-some as standard foamhandling units and some designed especially for a specific application. Paraffin
Paraffin deposition in oil and gas separators reduces their efficiency and may render them inoperable by partially filling the vessel and/or blocking the mist extractor and fluid passages. Paraffin can be effectively removed from separators by use of steam or solvents. However, the best
LEGEND
Fig. 12.11—Floatless liquid-level controller and diaphragm-motor oil-control valve on high-pressure oil and gas separator.
1
Fig. 12.12—Horizontal oil and gas separator with special internals for separating foaming crude oil.
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PETROLEUM ENGINEERING HANDBOOK
TABLE 12.4-MEASUREMENTS OF EFFLUENT FLUIDS QUALITY Measurement
Instrument
Oil in effluent gas Gas in effluent-oil Water in effluent oil Oil in effluent water Oil in effluent water
Laser liquid particle spectrometer Nucleonic Densitometer BS&W monitor (capacitance measurement unit) Ultraviolet absorption unit Solvent extraction/infrared absorbance
the results must be expertly analyzed and interpreted to obtain reliable and reproducible results.
Classification of Oil and Gas Separators EXTRACTOR FLU IN
NONWEIGHTED FLOAT
Fig. 12.23-Schematic of typical vertical two-phase oil and gas separator.
Classification by Configuration
Oil and gas separators can have three general configurations: vertical, horizontal, and spherical. Vertical separators can vary in size from 10 or 12 in. in diameter and 4 to 5 ft seam to seam (S to S) up to 10 or 12 ft in diameter and 15 to 25 ft S to S. Vertical separators are shown in Figs. 12.4, 12.13, 12.14, and 12.20 through 12.24. Horizontal oil and gas separators are manufactured with monotube and dual-tube shells. Monotube units have one cylindrical shell, and dual-tube units have two cylindrical parallel shells with one above the other. Both types of units can be used for two-phase and three-phase service. A horizontal oil and gas separator is usually preferred over a dual-tube unit. The unit has a greater area for gas flow as well as a greater oil/gas interface area than is usually available in a dual-tube separator of comparable price. The separator will usually afford a longer retention time because the larger single-tube vessel retains a larger volume of oil than the dual-tube separator. It is also easier to clean than the dualtube unit. In cold climates, freezing will likely cause less trouble in the unit because the liquid is usually in close contact with the warm stream of gas flowing through the separator. The design normally has a lower silhouette than the dual-tube unit, and it is easier to stack them for multiple-stage separation on offshore platforms where space is limited. Horizontal separators may vary in size from 10 or 12 in. in diameter and 4 to 5 ft S to S up to 15 to 16 ft in diameter and 60 to 70 ft S to S. Horizontal separators are shown in Figs. 12.2, 12.3, 12.12, 12.15, and 12.25 through 12.27. Spherical separators are usually available in 24 or 30 in. up to 66 to 72 in. in diameter. Spherical separators are shown in Figs. 12.5 and 12.28. Classification by Function
The three configurations of separators are available for two- and three-phase operation. In the two-phase units, gas is separated from the liquid with the gas and liquid being discharged separately. In three-phase separators, well fluid is separated into gas, oil, and water with the three fluids being discharged separately. Classification by Operating Pressure
Fig. 12.24-Typical field installation of a vertical two-phase oil and gas separator.
Oil and gas separators can operate at pressures ranging from a high vacuum to 4,000 to 5,000 psi. Most oil and gas separators operate in the pressure range of 20 to 1,500 psi.
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OIL AND GAS SEPARATORS
Separators may be referred to as low pressure, medium pressure, or high pressure. Low-pressure separators usually operate at pressures ranging from 10 to 20 up to 180 to 225 psi. Medium-pressure separators usually operate at pressures ranging from 230 to 250 up to 600 to 700 psi. High-pressure separators generally operate in the wide pressure range from 750 to 1,500 psi.
GAS OUT FLOAT NOZZLE
TYPE MIST EXTRACTOR
Classification by Application
Oil and gas separators may be classified according to ap plication as test separator, production separator, lowtemperature separator, metering separator, elevated separator, and stage separators (first stage, second stage, etc.). Test Separator. A test separator is used to separate and to meter the well fluids. The test separator can be referred to as a well tester or well checker. Test separators can be vertical, horizontal, or spherical. They can be two phase or three-phase. They can be permanently installed or portable (skid or trailer mounted). Test separators can be equipped with various types of meters for measuring the oil, gas, and/or water for potential tests, periodic production tests, marginal well tests, etc. Production Separator. A production separator is used to separate the produced well fluid from a well, group of wells, or a lease on a daily or continuous basis. Production separators can be vertical, horizontal, or spherical. They can be two phase or three phase. Production separators range in size from 12 in. to 15 ft in diameter, with most units ranging from 30 in. to 10 ft in diameter. They range in length from 6 to 70 ft, with most from 10 to 40 ft long. Low-Temperature Separator. A low-temperature separator is a special one in which high-pressure well fluid is jetted into the vessel through a choke or pressure-
NONWEIGHTED
FLOAT
SECTION’A-A’
Fig. 12.25--Schematic of typical horizontal two-phase oil and gas separator.
reducing valve so that the separator temperature is reduced appreciably below the well-fluid temperature. The tem perature reduction is obtained by the Joule-Thompson effect of expanding well fluid as it flows through the pressure-reducing choke or valve into the separator. The lower operating temperature in the separator causes condensation of vapors that otherwise would exit the separator in the vapor state. Liquids thus recovered require stabilization to prevent excessive evaporation in the storage tanks. Metering Separator. The function of separating well fluids into oil, gas, and water and metering the liquids can be accomplished in one vessel. These vessels are commonly referred to as metering separators and are availa ble for two- and three-phase operation. These units are available in special models that make them suitable for accurately metering foaming and heavy viscous oil.
Fig. 12.26--Horizontal monotube two-phase oil and gas separator. Unit is skid mounted with accessories. Operator is checking liquid level in separator.
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PETROLEUM ENGINEERING HANDBOOK
GAS OUTLET 2’ SAFETY POP
MIST EXTRACTOR
GAUGE COCK
OIL OUTLET
Fig. 12.27—Typical horizontal dual-tube two-phase oil and gas separator.
Fig. 12.28—Schematic of a typical spherical two-phase oil and gas separator with float-operated lever-type oilcontrol valve.
A two-phase metering separator separates well fluids into liquid and gas and measures the liquid in the lower portion of the vessel. A typical two-phase metering separator is shown in Fig. 12.29. A three-phase metering separator separates the oil, water, and gas and measures only the oil or both the oil and water. Metering of the liquid is normally accomplished by accumulation, isolation, and discharge of given volumes in a metering compartment in the lower portion of the vessel. Fig. 12.30 illustrates a three-phase metering separator in which the free water is measured with a positivedisplacement meter. The metering separator shown in Fig. 12.31 is designed especially for separating large volumes of foaming and/or viscous oil. This unit uses hydrostatichead liquid-level controllers to measure the oil accurately on a weight basis rather than by volume. It uses pressure flow into and out of the dual compartments and does not rely on gravitational flow. The unit shown in Fig. 12.31 is a two-phase vessel with two combination separating and metering compartments operating in parallel on an alternate basis. It is equipped with controls and valves that are arranged to permit constant flow of well fluid into the vessel. With pressure flow into and out of each of the two compartments, this separator can handle much larger volumes than separators with two compartments that rely on gravity flow from upper to lower compartments. These units are furnished with hydrostatic-head liquidlevel controls for metering foaming oil or float-operated controls for nonfoaming oil. Foam Separator. Oil and gas separators that handle foaming crude oil are generally referred to as foam separators. For a discussion of the design and application of separators for handling foaming oil refer to Pages 12-6 and 12-7. Fig. 12.29—Schematic of a vertical two-phase metering separator. Liquid is metered in integral metering compartment in lower portion of vessel.
Elevated Separators. Separators can be installed on platforms at or near tank batteries or on offshore platforms so that the liquid can flow from the separator to storage
