Lease Automatic Custody Transfer (LACT) Principles

Lease Automatic Custody Transfer (LACT) unit principles, accurate measurement of oil assures correct revenues for our company.

Accurate measurement of oil assures correct revenues for our company. Measurement can be done either by gauging and testing a lease tank or by using a Lease Automatic Custody Transfer (LACT) unit.

A LACT unit is for the automated transferring and measuring of oil, condensate and natural gas liquids. The unit measures quantity and quality of the product transferred from seller to buyer. The buyer may receive the product by pipeline, tanker truck, barge or ship.

Buyers will pay only for product with small quantities of impurities. The impurities can be water and basic sediments such as sand, clay, etc. The common term for these impurities has been Basic Sediment and Water (BS&W). Today, A.P.I. standards refer to BS&W as Sediment and Water (S&W). Because BS&W is the more familiar term, it will be used in this module.

Oil Revenue

Revenue is based on oil volume and oil gravity. Volume is calculated using the 42-gallon barrel as the standard unit. However, because oil expands and contracts with changes in temperature, all volumes are converted to 60°F. Net oil volume is the total volume transferred minus the volume of  BS&W.

The other factor in determining revenue is gravity. Gravity, referred to as A.P.I. gravity, is the weight or density of the oil. A.P.I. gravity is expressed in degrees with a 40° A.P.I. oil being lighter than a 20° A.P.I. oil. Because oil gravity is affected by temperature, all gravity calculations are converted to 60°F measurements. Gravity measurement usually affects revenues because lighter oils (higher A.P.I. gravity) mean higher prices.

LACT Measurement

At a custody transfer facility or tank battery, oil measurement can be done by manual gauging and testing or by installing a LACT unit. Gauging and testing is discussed in another module. A LACT unit moves treated oil from the storage tank, tests directly for excess BS&W, samples for BS&W and gravity, measures temperature-adjusted volume and transfers oil, all automatically.

Installation of  LACT units includes these advantages:
- Improved measurement by eliminating errors in gauging and testing;
- Increased revenue because of less time in the storage tank resulting in increased volume and higher A.P.I. gravity;
- Decreased costs because of fewer stock tanks;
- Reduced paperwork for pumper and office staff;
- Reduced chances of oil spills;
- Increased automation with operation 24 hours per day.

Field Facility

A field facility, sometimes called field separation and storage facility or tank battery, contains equipment for separating oil, gas and water.

Although field facilities vary, the following may be included :

Manifold or Header - Valves and piping which allow the operator to direct flow from each well.

Chemical Injector - A device which can add chemicals to the well stream when needed.

Separators - Vessels which separate the well stream into gas, oil and free water. Some facilities may have both a production separator and a test separator.

Gas Meters - Device with measure the produced gas.

Heater Treater - A vessel which uses heat and chemicals to complete the separation of emulsified oil, gas and water.

Storage Tank - One or more tanks which store the treated oil until it is transferred by a LACT unit. Storage tanks are called lease tanks, surge tanks, sales tanks or LACT tanks.

LACT Unit - A combination of devices which moves, tests, samples and measures oil produced at the facility.

LACT Unit

LACT units are typically skid mounted and installed as a unit at a field facility. The complexity and arrangement of devices may vary but the typical unit includes :

Pump - This device moves oil from the storage tank through the LACT unit into the pipeline.  

Strainer - A device which removes solids from the oil.

Deaerator (Air and Gas Eliminator) - This device removes any gas or air from the oil. On some LACT units the strainer and air eliminator are combined into one device.

BS&W Probe - An inline probe which senses the BS&W content of the oil.

BS&W Monitor - A control device which interprets the signal from the BS&W probe and signals the diverter valve.

Sample Probe - A probe that collects samples of the oil which are later used to determine BS&W content and gravity.

Sample Container - A device for collecting and storing samples taken by the probe. The buyer and seller take a sample from this container to determine BS&W content and gravity.

Diverter Valve - A three-way valve which either sends flow through the treating process or to the meter.

Meter - This device measures the volume of oil being transferred.

Prover Loop Connections - Valves and fittings which provide quick connections for checking meter accuracy with a portable prover.

Back-Pressure Valve - A valve downstream of the meter which maintain a constant pressure within the LACT unit.

Controi Panel - This is the nerve center of the LACT unit. It starts, monitors and stops the unit.

Other equipment which may be on some LACT units includes a sample container agitator pump, a four-way valve on the sampler, a lease allowable counter, an additional check meter or a booster pump on the discharge line.

LACT UNIT OPERATION

1. Oil Flow

A LACT unit provides automatic transfer of oil from the storage tank to the buyer. The oil is tested for BS&W content. When the BS&W content is below the monitor setting, the oil is sampled, measured and transferred to the buyer. The pump, as directed by the control panel, moves oil from the storage tank into the LACT unit.

A strainer removes solid debris. Next the deaerator removes free air and gas and vents them into a safe area. The BS&W probe senses the BS&W content and transmits data to the monitor. The BS&W monitor controls the direction of oil flow by setting the diverter valve. Oil with a BS&W content below the monitor set-ting is directed through the meter. If the BS&W content is higher than the monitor setting the oil is recycled through the treating system.

When acceptable BS&W content is detected, in proportion to volume, the sampler takes samples from the oil stream. Samples are taken based on volume or spaced-time interval. These samples are combined in the sample container. Later a single sample is withdrawn for analysis and the contents of the sample container emptied back into the oil flow. Oil with BS&W content below the monitor setting is measured by the meter. From the meter the oil flows through the prover connections and back pressure valve. The oil then flows to the buyer’s pipeline or tanker.

2. Control Panel

The nerve center of the LACT unit is the control panel. It starts the flow of oil after receiving signals from sensors on the storage tank. The control panel also stops delivery when : the storage tank reaches a preset low level, a meter failure occurs, the allowable has been reached, or a failure occurs in the LACT unit.

On some LACT units the control panel has additional capabilities. These include: 24-hour programming of delivery, automatic reset of the unit after meter failure, restart after a power failure, automatic recirculation of the tank bottoms, shut down after predetermined volume or setting off a beacon or alarm when there are failures in the unit or there is a high BS&W content.

3. Storage Tanks

Treated oil is stored in storage tanks. A field facility will have one or more storage tanks. The number and size depend on the production and whether sales are to a pipeline or tanker. The storage tank is equipped with oil level controllers. Some tanks may have two level controllers and other tanks may have three. Both combinations of controllers have a high level switch.

The high-level switch prevents the storage tank from overflowing by shutting-in the lease. When two controllers are used, the other controller serves as a LACT switch and a low level switch. This controller then starts and stops the LACT unit. When there are three controllers, one of the remaining switches starts the LACT unit and the other, the low-level switch, stops the LACT unit. These switches are either float controlled or tank head pressure controlled.

4. Pump

Usually the head pressure in the storage tank is not sufficient to move oil at a constant pressure through the LACT unit constant pressure is required for correct sampling and accurate volume measurement. A pump provides the constant flow. Pumps can be either centrifugal or positive displacement.

Centrifugal pumps are the most commonly used. They provide less vibration and more uniform flow for meter proving. Positive displacement pumps are the rotary type, e.g., gear, lobe, etc. The pump’s operation is directed by the control panel. Signals from the storage tank switches start or stop the pump. However, some pumps can be automatically started at the same time each day, and stopped after so much volume or time; other pumps can be manually operated.

5. Strainer and Deaerator

Solid debris and air or gas in the oil can cause inaccurate sampling and volume measurement. The strainer removes solid debris such as pipe scale, welding splatter, sand, pebbles, etc. For centrifugal pumps the strainer is usually downstream of the pump. However, if a positive displacement pump is used the strainer is upstream of the pump.

The deaerator removes gas or air from the oil. Free gas or air can cause pump cavitation, the meter to spin or the sampler to collect a non-representative sample. Oil collects in the deaerator which allows gas and air to break out of the oil. Usually a float attached to a valve allows air or gas to escape when too much air or gas has accumulated in the deaerator.

On some LACT units the strainer and deaerator are combined into a single component. However, each functions separately and the combined component is always installed upstream of the metering device.

6. BS&W Probe and Monitor

The function of the BS&W probe and monitor is to sense the BS&W content exceeds of the oil. If the BS&W content the monitor setting, the BS&W monitor shuts down delivery and auto-matically directs oil flow through the treating facility.

The probe is an inline sensor mounted on a vertical section of the flow line downstream of the strainer and deaerator. It is on a vertical section of pipe to prevent fluid stratification and the build up of solids on the probe. Both can cause incorrect readings.

The BS&W monitor is an electronic sensing in-strument. Based on data from the probe, the monitor sets the diverter valve. In addition, the monitor has features for calibration, an indicator for clean oil, an indicator for excess BS&W, a limit setting for acceptable BS&W and a setting for the control functions. 

7. Diverter Valve 

The diverter valve directs the flow of fluid based on signals from the BS&W monitor. This valve either directs flow back through the treating system, to a collection tank for reprocessing or to the metering device.

Whenever the BS&W content exceeds the monitor setting, the oil is diverted to the treating system. Usually this action is on a 30-second delay so that small amounts of BS&W do not activate the valve.

Whenever the BS&W content is below the monitor setting, the valve automatically switches flow back to the metering device for sales. Diverter valves can be either electrically operated or pneumatically (air) operated. For either type of operation the valve is a fail safe valve that diverts flow to the treating system whenever instrument air or electrical power is lost.

8. Sampler

The sampler provides liquid from which the following are determined :
- Type of crude or condensate
- A.P.I. gravity
- BS&W content

The sampler operates only when fluid is being pumped through the metering device. The sampler consists of a probe, a sample control device and a composite sample container with an agitator. Sample control valves are either solenoid- operated or motor-driven. The soienoid-operated valve causes a sample to be taken once for each unit measured by the metering device. Motor-driven valves sample based on time, usually one or two samples per minute.

The sample probe is a tube inserted into a vertical run of piping, usually downstream of the BS&W probe. A typical probe has a tapered end which extends into the center of flow. Liquid flow is directed into the taper. Other sample probes may withdraw samples using a pressure drop or small pump.

As samples are taken, they flow into the composite sample container. This is a pressure vessel with a sloping bottom. A sample is withdrawn periodically based on contract requirements, typically twice a month. Before the sample is taken, the composite sample container is agitated using a small pump.

Mixing with the pump assures that the drawn sample is representative. The sample is taken for determining BS&W content gravity and of the product. After the sample has been drawn, the container is emptied into the upstream portion of the LACT unit. After emptying the container, the container is cleaned regularly to remove BS&W residue.

9. Metering Device

The metering device is the most important single component of the LACT unit. A positive displacement (PD) meter is commonly used for measuring oil. However, some LACT units use turbine meters. In either case, the meter measures the volume of liquid delivered.

Most PD meters are equipped with either an automatic temperature compensator (ATC) or an automatic temperature gravity (ATG) selector unit. These temperature compensators correct the gross volume measured into net volumes at 60°F. The ATG also has an adjustment for gravity. The typical PD meter is a sliding vane meter which isolates a measured volume, increments the counter for one measured volume and then releases the volume back into the stream. The meter may have an impulse transmitter for the sampler, an allowable counter, a calibrator, a resettable counter and sometimes a ticket printer.

10. Prover Loop Connections

To maintain the accuracy of the metering device, the meter of the LACT unit is proved regularly. The prover loop connections consist of prover connections, a block and bleed valve and two block valves. These connections provide quick hook-ups for meter proving.

11. Back-Pressure Valve

The last component through which oil flows on the LACT unit is a back pressure valve. This valve maintains a constans pressure for the meter and the piping. Except when positive displacement pumps are used, this valve controls the flow rate through the unit.

MAINTAINING ACCURACY

1. Meter Proving

Meter Proving is a physical test of meter accuracy. By measuring the flow through the LACT meter and a calibrated prover of known volume, a correction factor or meter factor can be determined. Usually the proving of a LACT meter is done by a contractor. However, to ensure that company interests are protected, an company representative is present to witness the proving.

The common proving devices are : truck mounted pipe prover, permanently installed pipe prover or master meter. The meter proof is performed by placing the LACT meter in series with the prover. For the test run all volume measured by the prover is also measured by the LACT meter. Also, only product measured by the LACT meter is measured by the prover. At the end of each measured volume, the LACT meter measurement is compared to the prover measurement. For this comparison to be valid, both volumes are converted to a common base at standard conditions of temperature and pressure. The source of these conversions is the A.P.I. Petroleum Measurement Tables.

The typical prover is the bidirectional pipe prover mounted on either a truck or trailer. A bidirectional prover consists of:

- A loop of pipe for the calibrated volume;

- A sphere displacer which travels through the loop to isolate the volume;

- A four-way valve for changing the direction of flow through the prover;

- Vent valves on either end of the pipe loop to release trapped gas or air;

- Two detectors on either end of the pipe loop which signal passage of the displacer;

- A prover counter/controller which counts pulses from the LACT meter and receives signals from the two detectors;

- Shielded cables which connect the prover counter/controller to the detectors and LACT meter.

To prove the LACT meter, the prover is connected to the LACT unit. Several trial runs are made to bring the prover and meter into stable temperature conditions and purge the prover of gas and air. On each run the displacer moves from one detector to the other. The flow is reversed and the displacer returns to its starting position. This is known as a round trip.

Following the trial runs, the meter accuracy test is performed. A test requires five out of six consecutive round trips to give volumes on the meter within 0.05% of each other. When this condition has been met, the readings of the LACT meter and the prover are combined to calculate the new meter factor.

Several requirement for accurate proving include :

- Proving is done under the same conditions as the LACT meter normally operates;

- The prover has the capacity to provide long enough proving runs;

- Sufficient runs can be made to estabilish validity;

- Calibration of the prover is traceable to the National Bureau of Standards test measures;

- A certificate of calibration is available and present during proving.

2. Control Panel Operation

The control panel assures that no liquid delivery is made if the LACT unit malfunctions. It controls starting and stopping flow as well as indicating the possible cause for a malfunction. Most LACT units have two lamps mounted above the skid. A green light indicates the unit is working correctly. A red light indicates that the unit has been shut down. Indicator lights on the face of the control panel provide additional information for determining cause of shut down. The indicator lights can show the following:

• Power supply on
• Clean oil
• Bad oil
• Low level
• Meter failure
• Allowable complete
• Meter failure reset

3. Routine Checks

Deaerator Valve - The exterior vent valve of the deaerator must be open at all times. The deaerator cannot function when this valve is closed. Some contracts require that this valve be sealed open.

Vibration - A well-designed LACT unit has little vibration with constant flow rate and pressure. Vibration is evidence that a LACT component is about to malfunction. Causes of vibration might include: cycling back pressure valve, cycling diverter valve or a pump problem.

Storage Tank Level - Periodically the storage tank level should be checked. A low level switch failure could be loading the LACT unit with air or tank bottoms. A dirty strainer may be reducing the unit’s flow rate, causing the unit to run longer than necessary.

Normal Flow Rate - On most LACT units the control panel has a control circuit which monitors flow rate. The control circuit shuts down the unit when the flow rate deviates by ± 10% of the rate used during proving. Normal flow should be within the meter’s flow range found on the identification plate. If the normal rate or the proving rate is not within the range, measurement error occurs.

Gravity Measurement - The setting on the automatic temperature gravity selector should be checked by authorized personnel. The amount of compensation applied as temperature changes, can be obtained from Table 6A of the A.RI. Petroleum Measurement Tables. The gravity readings should be compared to the ATG setting for accuracy.

Sample Container Level - The sample con-tainer level should be checked using the sight glass or piston stem clearance. Knowing the delivery amount of liquid for the LACT unit, the sample level should increase about the same each day. Radical level changes in the container can signal a malfunction of the automatic sampler.

BS&W Monitor - The setting for acceptable BS&W content on the monitor is established by contract. If the LACT unit is diverting liquid because of high BS&W content the setting on the monitor should not be raised. Overriding the acceptable BS&W content causes bad samples to be taken. This action results in more volume transferred, but can result in lease shut down, inaccurate volume measurement by the meter or contract termination. Remember BS&W volume is subtracted from gross liquid volume for payment, so higher BS&W sales results in lower profits.

Inspection - For continued accurate automatic delivery, a LACT unit needs regular inspection. Inspection is checking for leaks, listening for unusual noises, observing and testing the lights, observing the whole unit, checking the above items and following the LACT unit manufacturer’s maintenance instructions.

4.Records

For accuracy each lease must have records of its production. Run tickets from LACT units, begin the revenue accounting process for sales. Run tickets are either printed by the imprinter on the LACT meter or completed on a regular basis by a representative for the buyer and a witness from ARCO. Typical data on a run ticket includes: lease name, lease number, time, date, meter number, volume of product, amount of BS&W, and net barrels transferred.

Detail learning in video visual:

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• [accordion]
• 1. How LACT Units Work in Oil and Gas Production
• 
• 2. ASTM D4007 Water and Sediment in Crude Oil by the Centrifuge Method
• 3. ASME B 31.3-2020 ASME Code for Pressure Piping
• 4. PLP E-10-2003 Lact Units 2nd Ed by Rosen