A LACT unit (lease automatic custody transfer unit) is a packaged system that takes treated crude oil from lease stock tanks, delivers it into a gathering pipeline or other transportation without an operator present, and measures that delivery on a custody transfer basis. In one automatic sequence it monitors sediment and water, diverts off-spec oil, samples, meters and records every delivery. A LACT is therefore not just a meter skid but a functional package combining acceptance, measurement and documentation where crude changes hands.
Hardware choices shared with any crude oil meter skid, such as meter type, density and water cut measurement, are covered on the Crude Oil Metering Skids page. This page focuses on what sets a LACT apart: lease-site conditions, the divert logic that acts as a quality gate, and unattended operation.
Where LACT units are used
A LACT sits at the lease outlet, where oil passes from the producer to a carrier or purchaser. In a typical arrangement, well fluids go through separation and treating, sales-quality crude collects in a tank battery, and the LACT starts when the tank reaches a set level, pumping oil into the gathering line. Because leases are usually unmanned and the measurement is audited by more than one party, the unit must make its own decisions and log every step.
Crude oil custody transfer is governed by contracts and regional rules. In the American Petroleum Institute Manual of Petroleum Measurement Standards (API MPMS), LACT metering systems are now addressed in Chapter 6.4A, published at the end of 2023, which supersedes the former LACT chapter. The standard frames design and selection decisions; which requirements apply on a given project is set by the contract and the pipeline operator's tariff or connection terms.
A typical LACT flow sequence
LACT units come in several configurations. The sequence below reflects a common arrangement; whether each element is needed depends on site pressures, crude properties and the sales agreement.
1. Charge pump and strainer: Draws crude from an atmospheric or low-pressure tank and provides enough pressure for the meter and sampling system; the strainer removes solids. Avoiding gas breakout on the suction side is a key point in pump and tank-connection design.
2. Sediment and water (BS&W) probe: Continuously monitors the water and sediment content of the stream and signals the control system when the reading exceeds the contractual acceptance limit.
3. Divert valve: Routes off-spec oil away from the sales line and back to treating or to a tank, so that oil unfit for sale is not measured as delivered.
4. Automatic sampler with mixing: Collects a flow-proportional composite sample representing the whole batch. Water, sediment and density results from the laboratory feed the net quantity calculation.
5. Meter with temperature and pressure measurement: Measures the gross quantity; temperature and pressure are used to correct it to base conditions. Meter technology is chosen according to crude viscosity and flow range.
6. Prover connections: Valved connections that allow periodic proving with a permanent or portable prover, or against a master meter.
7. Back pressure valve: Keeps the line full and single-phase at the meter and sample point, and helps prevent dissolved light ends from flashing to vapor in the meter.
8. Flow computer and control panel: Manages start and stop permissives, divert decisions, sampler pacing and the delivery ticket, and keeps event and alarm logs.
The quality gate: divert logic
The feature that most clearly separates a LACT from a plain meter run is that the fitness-for-sale decision is made before measurement. Sediment and water content is compared continuously with the acceptance limit set by the pipeline operator or buyer. When the limit is exceeded, the divert valve changes position, and oil flowing during that period either bypasses the sales meter or is recorded separately. Design attention goes to placing the probe where the stream is well mixed and free water has not separated, feeding valve position feedback into the audit log, and defining in the operating procedure how diverted oil is re-treated and returned to sale.
With emulsion-forming or waxy crude, or crude whose temperature swings with the seasons, the probe reading can differ from the laboratory result. For that reason on-line water monitoring is treated as a decision and alarm tool, while the sediment and water value used in the commercial calculation is generally taken from the sample. How to draw that sample representatively is covered on the Crude Oil Sampling Systems page.
Unattended operation and audit trail
The unit usually starts automatically on a tank high-level signal and stops at low level. Before starting, permissives are checked for the pump, valve positions, sampler and instruments. During a run, the flow computer logs gross and corrected quantities, average temperature and pressure, divert time and sampler status. When the batch closes, the delivery ticket is combined with laboratory results to calculate net quantity. At remote leases these data can be sent to a central office via SCADA; configuration changes and access to sealed parameters should be logged in a way both parties can audit.
Site-specific design criteria
Parameter | Why it matters in design | Effect on selection |
Daily lease production and tank capacity | Determines how often and for how long the unit runs | Pump and meter size, flow range, single or dual meter |
Crude viscosity and temperature swing | Affects meter performance and pump suction | Meter technology, heat tracing, strainer selection |
Dissolved gas and vapor pressure | Gas breakout at suction or in the meter corrupts measurement | Back pressure setting, tank connection layout, need for a gas eliminator |
Sediment and water level, emulsion tendency | Drives divert frequency and probe reliability | Probe type and position, mixing, return line to treating |
Paraffin and solids content | Can build up in probes, meters and sample lines | Cleanable design, heat tracing, maintenance access |
Pipeline inlet pressure | Sets the head the pump must overcome | Pump type and back pressure valve range |
Proving method | The contract may require periodic meter proving | Permanent prover, portable prover connections or master meter |
Power, climate and hazardous area | Infrastructure at remote leases is limited | Panel protection, heating, cabling and communications |
How TLY Enerji can support a LACT project
TLY Enerji can provide engineering support for evaluating process data and selecting measurement technology suited to the crude being handled. Depending on project scope, this may include supply of meters, density, pressure and temperature instruments, control valves and pumps; integration of the flow computer with PLC or SCADA; and site installation, testing, commissioning and follow-up technical support. The scope of supply for samplers and water monitors, and who assembles the package, is defined together on each project from the technical specification.
Site information for a LACT review
· Lease production profile, number and size of tanks, planned delivery frequency
· Crude density, viscosity, pour point and temperature range
· Typical and maximum sediment and water content, and the contractual acceptance limit
· Dissolved gas and vapor pressure data, tank operating pressure
· Inlet pressure of the receiving pipeline and the operator's measurement requirements
· Required proving method and frequency
· Available power, communications and remote monitoring at the site
· Hazardous area classification and climate conditions
Related pages
· Crude oil metering skids: Details of meter, density and water cut hardware selection.
· Crude oil sampling systems: What the LACT sampler needs in order to collect a representative sample.
· Crude oil custody transfer systems: From gross quantity to net standard volume, and how it is verified.
· Pipeline receipt and delivery metering: Measurement where gathering lines fed by LACT units enter or leave trunk pipelines.
LACT unit questions
How does a LACT unit differ from a crude oil metering skid?
A metering skid is the hardware that determines the quantity and properties of flowing crude. A LACT adds the fitness-for-sale decision, the divert valve, a charge pump and automatic start and stop tied to tank level. It is therefore treated as a functional package designed for unattended delivery from the lease into a pipeline.
What happens to measurement when the divert valve operates?
When sediment and water content exceeds the acceptance limit, oil is routed away from the sales line and back to treating or a tank, and it is not counted as delivered. Logging divert time and valve position lets both parties audit the delivery ticket. How diverted oil is later returned to sale is defined in the operating procedure.
How is the LACT meter proved?
The unit is designed with valved connections for a permanent prover, a portable prover brought to site, or a master meter. The meter factor obtained from proving is applied by the flow computer to subsequent deliveries. The method and frequency depend on the contract and the pipeline operator's requirements.
Is API MPMS Chapter 6.1 still the reference for LACT?
According to the API 2025 publications catalog, LACT metering systems are now covered by Chapter 6.4A, which supersedes the former Chapters 6.1 and 6.7. If an existing specification still refers to the older chapter, it is worth agreeing with all parties at project start which edition will govern.
How does gassy crude affect a LACT?
Dissolved gas can break out where pressure drops, at the pump suction or around the meter, inflating the measured volume and compromising the sample. Tank-to-pump piping, suction conditions and back pressure downstream of the meter are therefore evaluated together. Crude should also be adequately stabilized before it reaches the unit.