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The two most common methods of measuring the volume of petroleum liquids are tank gauging and liquid metering. The problems associated with tank gauging are (1) itrequires that an operator make an accurate liquid level determination by climbing to thetop of the tank to be gauged, (2) that an operator make an accurate average liquidtemperature determination, (3) that an operator make an accurate sediment and water 

content analysis and (4) that the tank be static, which means that no liquid can enter or leave the tank during gauging. Once the contents of the tank are removed, it is necessaryto regauge the tank. Since crude oil/condensate is sold on the basis of temperature, APIGravity and the amount of Basic Sediment and Waste (BS&W), it is very important tomake accurate measurements. The greatest effect on volume is temperature. Crude oilwill expand and contract at the rate of 2% per 40 F temperature change. Theaccumulation of lease production and on $50.00 USD per barrel oil, would be thefollowing:

The simplest and most effective way to transfer the ownership of liquid hydrocarbons

 between a buyer and a seller is through the use of an accurate liquid meter. With the aidof additional components, the liquid meter is capable of unattended measurement withmaintained accuracy's of 0.25% or better. This measurement system is commonlyreferred to as Lease Automatic Custody Transfer (LACT) Unit when ownership istransferred at a production lease. When ownership is transferred away from a productionlease, such as a transfer between Pipeline Companies, a measurement system may bereferred to as an Automatic Custody Transfer (ACT) Unit.

The basic components and function of a LACT unit are:

1. Charge Pump and Motor- Largely overlooked and undersized, special care should be

taken into consideration during sizing to ensure correct NPSH is available to preventcavitation, and discharge pressure is enough to overcome pressure drop through theLACT to allow the required flow and pressure to the pipeline pump inlet.

2. Strainer/Air Eliminator- Strains solids larger than the perforations in the removable basket with liner. They should have differential pressure gauge indicators to show pressure drop caused by debris accumulation and be cleaned periodically. Essential to prevent premature meter wear or breakage. The air eliminator is located on the top of the

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strainer at the highest part of the system to allow air to be discharged and not metered.This should be piped with a soft-seated check valve to prevent air from being introducedinto the system during shutdown.

3. Sample System- Installed with an upstream static mixer usually flow proportional,

isokinetic, and tubed to a vapor tight storage vessel sized to allow 25 to 30 days storage.The vessel is provided with a recirculation pump, the samples are mixed and then drawnoff to be checked for composite API Gravity and BS&W during the delivery period.

4. BS&W Monitor and Probe- Installed downstream of the sampler and upstream of the3-way divert valve, this unit consists of a flanged probe that monitors the flowing streamfor basic sediment and water and communicates with the monitor unit that is normallyinstalled in the control panel. The monitor is usually set at 1.0%, and is wired to thesolenoid valve controlling the 3-way valves on the bad oil divert line. These will send oilto be treated if high. BS&W signal is received for a given time. When a good oil signal isreceived for a set time, than the valve will return to normal flow position.

5. Meter- Installed downstream of the 3-way valve and downstream of a properly sizedthermal relief valve. The meter measures the product stream and allows totalization either through a local totalizer or electronic pulses to a flow computer. This meter should be(PD) positive displacement or( Coriolis) Mass Flow. The meter will provide signals to theflow computer or PLC to allow: Sample pacing, Totalization, Meter Proving, Meter Failure.

6. Meter Instrumentation- Downstream of the meter a spool consisting of: Temperaturetransmitter with platinum RTD installed in a SS thermo-well. Pressure transmitter with pressure gauge mounted with a 3-way gauge valve. Test SS thermo-well used to calibratethe temperature transmitter. The temperature and pressure transmitter are used to send alive reading to the flow computer for compensation.

7. Check Valve- Downstream of the meter to prevent backflow to the meter in case thedownstream block & Bleed valve is left open and the opposite meter train is running.

8. Block and Bleed Valves- Located downstream of the check valve at the end of the runand is the main line divert valve separating them to and from lines to the prover 4-waydivert valve. This is to ensure all fluid is being diverted to the prover during proving, or afalse meter factor could be obtained during proving of the meter.

9. Back Pressure Valve- On the skid outlet to maintain pressure above the vapor  pressure of the fluid being metered and provide a constant pressure and flow on the meter during proving.

10. LACT Control Panel- This can be located on the skid with PLC controls and manual proving connections would then be required, for a prover counter, detector switch plug in, power for the counter, and a portable pulse generator for PD meter. An electronictemperature averager could then be used in lieu of temperature transmitter; however, due

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to their flexibility and relative cost, flow computers are rapidly replacing them. If locatedin the MCC room, the panel could then be equipped with a PLC, flow computer, and printer to allow for automatic proving batch reports by pushing a button, providing the prover's 4-way valve is equipped with a remote actuator, and pressure and temperaturetransmitters were installed. The control panel could have the following functions:

• Start and Stop, off High and Low Switches.• Hand-Off-Automatic Switch.• BS&W Divert Controls.• Meter failure. Monitor Failure.• Internal Battery Back-up

11. Calibrated Bi-Directional Meter Prover- Because of the versatility of a bi-directional prover in tight offshore spaces and it's cost associated with normal offshoreflow rates; the bi-directional prover has become the prover of choice versus the uni-directional small volume prover.

Design and Consideration

Customer's SpecificationFirst and foremost, If your LACT Unit does not meet your buyer's or shipper's standard specification, youmay find yourself with a tank full of oil, having to shut down production because of your LACT skid not being approved, or they just notified that you want to come online with a skid they have never seen. Gethold of the specifications and get them involved early in the fabrication of the unit.

High Viscosity / Sandy Oil Can Cause Several Problems:

Does your PD meter require high viscosity clearances, or is your product sandy, should you be looking atCoriolis type meters. If your meter factor, or maintenance is up, you might look at a different type of meter.

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