Lease Automatic Custody Transfer (LACT) Units

What Is a LACT Unit?

A Lease Automatic Custody Transfer (LACT) unit is an automated measurement system that transfers crude oil from a producer's lease tanks to a pipeline purchaser without requiring personnel from either party to be present during the transfer. The LACT unit measures the volume, quality, and temperature of the oil, generates an automated custody transfer ticket, and maintains a detailed audit trail for volume reconciliation and royalty payment calculations.

Before LACT units, oil was sold through manual gauging: a gauger from both the producer and purchaser would measure tank levels before and after a truck or pipeline transfer, hand-calculate volumes with temperature corrections, and record the transaction on a paper run ticket. LACT automation eliminates this labor-intensive process, enabling continuous pipeline sales with measurement accuracy superior to manual methods.

LACT Unit Components

Charge Pump and Strainer

The charge pump provides the pressure required to push oil from atmospheric storage tanks through the LACT unit and into the pipeline. Sizing considerations include:

• Pump type: Positive displacement (gear or vane) pumps are preferred for their consistent flow rate regardless of downstream pressure changes. Centrifugal pumps are used for higher flow rates but require flow control valves to maintain stable measurement conditions.
• Suction conditions: Adequate NPSH (Net Positive Suction Head) must be maintained to prevent cavitation. Tank outlet height, pipe friction, and strainer pressure drop all affect available NPSH.
• Strainer: A basket or Y-strainer upstream of the LACT unit protects the meter, sampler, and BS&W analyzer from debris. Differential pressure monitoring across the strainer detects plugging and triggers cleaning alerts.

BS&W Analyzer

The Basic Sediment and Water (BS&W) analyzer measures the water and sediment content of the crude oil. Pipeline specifications typically require BS&W below 0.5% or 1.0%. The LACT controller uses the BS&W reading to:

• Reject off-spec oil: If BS&W exceeds the pipeline maximum, the LACT unit automatically diverts flow back to the lease tanks for further treating. A motorized three-way divert valve routes oil to either the pipeline (on-spec) or back to tanks (off-spec).
• Apply net volume corrections: The BS&W percentage is deducted from gross measured volume to calculate net oil volume for payment. A 1% BS&W error on 1,000 barrels represents a 10-barrel payment discrepancy.
• Analyzer technologies: Capacitance probes (most common for lease LACT), microwave resonance analyzers (higher accuracy for pipeline custody transfer), and centrifuge-based analyzers (laboratory reference method).

Sampler System

An automatic sampler collects a representative composite sample of the oil stream during transfer. This sample is analyzed in a laboratory to determine gravity (API), BS&W, and other quality parameters that affect pricing. Key requirements include:

• Flow-proportional sampling: The sampler takes grabs proportional to volume flow, ensuring the composite represents the entire transfer batch. Typically one grab per 50-200 barrels of flow.
• Sample container: Stainless steel or glass containers maintained at a controlled temperature to prevent wax precipitation or water dropout that would skew lab results.
• Mixing: The sample point must be located downstream of a static mixer to ensure the stream is homogeneous at the sampling location.

Custody Transfer Meter

The meter is the primary measurement device and the most critical component of the LACT unit. Common meter types include:

• Positive displacement (PD) meters: Smith (now part of FMC/TechnipFMC) and Liquid Controls PD meters have been the industry standard for decades. Gear-type meters provide +/-0.02% repeatability and +/-0.25% accuracy before proving.
• Coriolis meters: Micro Motion (Emerson) and Rheonik Coriolis meters directly measure mass flow and density. Mass measurement eliminates temperature and pressure correction uncertainties. Accuracy of +/-0.1% on mass flow.
• Turbine meters: Lower cost alternative with good accuracy (+/-0.25%) but sensitive to viscosity changes and wear. Requires more frequent calibration than PD or Coriolis meters.

Meter Prover

Proving verifies the meter's accuracy by comparing its reading against a traceable reference volume. LACT units typically use one of these proving methods:

• Bidirectional pipe prover: A calibrated pipe section with a displacer sphere. Provides the highest accuracy but requires significant physical space. Common at pipeline interconnects and terminals.
• Compact prover (small volume prover): A piston-based prover (Brooks or Emerson) that provides custody-transfer-quality proving in a fraction of the space. Proving can be completed in 5-10 minutes.
• Master meter proving: A portable or permanently installed reference meter (usually Coriolis) proved against a pipe prover. The master meter then proves the LACT meter. Common for lease LACT units where installing a pipe prover is impractical.

LACT PLC/RTU Programming

The LACT controller manages the entire measurement and transfer sequence:

• Batch control: Starts and stops transfers based on upstream tank levels, downstream pipeline availability, and scheduling. Records batch start and stop volumes, times, and quality data.
• BS&W diversion logic: Continuously monitors BS&W reading and actuates the divert valve when quality exceeds pipeline specifications. Includes configurable delay timers to prevent nuisance diversions from transient spikes.
• Meter factor application: Applies the most recent proving-derived meter factor to gross pulse counts to calculate corrected volume. Stores meter factor history for audit purposes.
• Temperature correction: Applies API Table 6A/6B corrections to convert observed volume at flowing temperature to net standard volume at 60 degrees F. RTD temperature measurement with +/-0.1 degree F accuracy.
• Ticket generation: Automatically generates custody transfer tickets containing batch number, start/stop times, gross and net volumes, API gravity, BS&W percentage, temperature, meter factor, and calculated net standard barrels.

SCADA Integration and Remote Monitoring

LACT data transmitted to SCADA enables remote monitoring of oil sales and volume reconciliation. Operators can view real-time flow rates, cumulative batch volumes, BS&W trends, and alarm conditions from a centralized control room. Historical data supports monthly volume reconciliation between producer and purchaser statements, identifying discrepancies early. Integration with production accounting systems automates revenue booking and royalty calculations, reducing the month-end close process from days to hours.