1. What is a LACT?
A Lease Automatic Custody Transfer (LACT) unit is the legally-binding handover point where a producer's crude oil becomes a midstream operator's crude oil. The volume and quality that the LACT measures are the basis of the producer's revenue and the operator's invoice — so the unit is regulated under API MPMS Chapter 6.1 and must achieve a stated total uncertainty (commonly ±0.25% or ±0.35% commercial spec).
A LACT continually pulls oil from a sales tank battery, removes entrained gas, checks BS&W, measures volume, and diverts to a reject tank if quality is off-spec. It must be field-provable by an on-skid Small Volume Prover (SVP) or a portable master meter without taking the lease offline.
2. Component-by-component
| Component | Function | Sizing basis |
|---|---|---|
| Strainer | Protect meter from particulates | 100 mesh (149 µm) std; 200 mesh (74 µm) for Coriolis |
| Air eliminator | Vent gas pulled off the tank with the oil | QAE ≥ Qmax · (1 + GVF) |
| BS&W monitor | Continuous quality measurement; arm diverter | Full pipe at design rate; full-scale 0–3% min |
| Diverter valve | Route off-spec oil to reject tank | Setpoint 1% BS&W typ; 30-s delay; fail-open to reject |
| Custody meter | Volume / mass measurement | Rangeability check + uncertainty target |
| Sampler | Auto-grab oil for composite BS&W + density lab analysis | API MPMS 8.2 — composite ≥ 3 L per batch |
| Prover connection | On-skid SVP or portable hookup | Vprover ≥ pulses / K-factor |
| Block / check valves | Isolation, backflow prevention | ANSI class from MAWP |
3. Meter selection
The first decision is meter type. Each type has fundamentally different physics and different rangeability:
| Type | Principle | Rangeability | Best when |
|---|---|---|---|
| Coriolis | Mass flow via tube oscillation | 100:1 to 1000:1 | API gravity varies > ±2°; want mass not volume |
| Positive Displacement (PD) | Volumetric chambers | 10:1 | Viscous crude, low flow, legacy installations |
| Turbine | Volumetric, rotor RPM | 10:1 | Clean, low-viscosity, high flow |
| Ultrasonic | Transit-time | 30:1 | Large-bore, low ΔP, multiple gravity grades |
For new LACT installs in 2024+, Coriolis dominates: it measures mass directly (no temperature/pressure correction needed for custody volume), accepts the widest flow turndown, and meets ±0.10% meter uncertainty without proving frequency creep. It also tolerates entrained gas and varying API better than any volumetric meter.
4. Prover & proving
API MPMS Ch. 4 requires the prover to accumulate enough meter pulses per pass to give ±0.025% repeatability over 5 consecutive runs:
10,000 pulses is the standard pulse count. A typical 2" Coriolis with K = 10,000 pulses/bbl needs Vprover ≥ 1.0 bbl — so a 1-bbl SVP suffices. A turbine with K = 5,000 pulses/bbl needs ≥ 2.0 bbl. PD meters with low K can push you to a 5-bbl prover.
Standard SVP sizes: 0.5, 1.0, 2.0, 5.0, 10.0 bbl. The SVP is U-tube or piston-style; calibrated by water-draw at the manufacturer and re-calibrated every 5 years per MPMS 4.9.
Proving frequency is a regulatory + commercial decision: monthly is common in 2024; weekly for very high throughput pipelines; quarterly for stable Coriolis installations with documented low drift.
5. Sampler (MPMS 8.2)
Continuous proportional-to-flow grab sampler. A small piston pulls a 0.5–1.5 cc grab on every fixed volume increment of flow (1 grab per 1–10 bbl typical). All grabs accumulate in a composite container; lab analyzes for BS&W (centrifuge), gravity (hydrometer or DMA), and sulfur:
The 3 L minimum gives the lab enough volume to run all required analyses with retain. A 100 BPH lease with 1 grab per 5 bbl pulls about 480 grabs/day → at 1 cc/grab that's 0.48 L/day → batch over a week or month to exceed 3 L.
6. Uncertainty stack-up
Each measurement input contributes independent uncertainty that combines by root-sum-square:
Typical modern Coriolis-based LACT:
| Component | Typical U (%) |
|---|---|
| Coriolis meter (after proving) | ±0.10 |
| Prover (SVP, calibrated) | ±0.025 |
| Temperature (RTD class A) | ±0.05 |
| Pressure (transmitter) | ±0.02 |
| Density (Coriolis built-in) | ±0.05 |
| BS&W monitor | ±0.10 |
| RSS total | ±0.16 |
This meets the commercial ±0.25% target with margin. PD/turbine installations typically stack up to ±0.30–0.40% and need master-meter proving to hit ±0.25%.
7. Worked example — 2,000 BPD lease
Validation case from the calculator: 2,000 BPD = 83 BPH average, sweet 38° API crude, 80 psig 90°F, ±0.25% uncertainty target.
- Qmax = 1.25 × 83 = 104 BPH; Qmin = 0.20 × 83 = 17 BPH → required range = 6:1.
- Coriolis at 100:1 handles this with 16× margin. Pick 2" Coriolis (DN50) per the size table.
- 200-mesh strainer (Coriolis tube protection), 2" basket type, ΔP < 2 psi clean.
- Air eliminator: QAE = 104 · (1 + 0.02) = 106 BPH; chamber ≈5.0 ft³ (30-s liquid retention).
- BS&W monitor: microwave type, 0–3% full scale; auto-divert at 1.0% with 30-s delay.
- Sampler: 1 cc grabs, 1 per 5 bbl → ~400 grabs/day → 0.40 L/day → 3+ L composite over 8 days.
- Prover: K = 10,000 pulses/bbl × 10,000 pulses → V ≥ 1.0 bbl → 1 bbl SVP.
- Uncertainty: RSS of components = ±0.16% < 0.25% ✓.
- ANSI 150 (80 psig ≪ 285); skid 12 ft × 6 ft.
8. References
- API MPMS Ch. 6.1 — Lease Automatic Custody Transfer (LACT) Systems.
- API MPMS Ch. 4 — Proving Systems (4.2 displacement provers, 4.5 master-meter, 4.8 sampling).
- API MPMS Ch. 5 — Metering: 5.2 (PD), 5.3 (Turbine), 5.6 (Coriolis), 5.8 (Ultrasonic).
- API MPMS Ch. 8.2 — Standard Practice for Automatic Sampling of Liquid Petroleum.
- API MPMS Ch. 11.1 / 11.2 — Volumetric correction factors (CTL, CPL).
- API 11N — Operation, Maintenance, and Testing of LACT Equipment.
- NACE MR0175 / ISO 15156 — Sour service materials.
- ASME B16.5 — Pipe Flanges and Flanged Fittings.
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