The net quantity of crude received by truck is found by taking the gross volume metered during unloading, correcting it for temperature and pressure, and then deducting the sediment and water it contains. That result can only be trusted if three conditions hold: the meter sees nothing but liquid, a representative sample is collected over the whole offload, and temperature, density and S&W data for each truck end up on the same record. A crude oil unloading system is laid out to meet those three conditions.
General engineering of unloading pumps, suction lines and air entrainment is covered on the Truck Unloading Systems page; this page focuses on what is specific to receiving crude.
Where trucked crude is received
Crude receipt typically happens where trucks from leases without pipeline access are offloaded: pipeline injection stations, gathering terminals and refinery crude receiving areas. The quantity unloaded there is the basis for settlement between carrier and buyer, and often for payment to the producer as well. Because many trucks of different quality may arrive from different leases on the same day, each offload has to be recorded as a separate batch.
Gas on the suction side and the end of the offload
Crude is agitated and may warm up in transit, so part of the dissolved light ends comes out of solution. The unloading pump lowers pressure on its suction side, which makes gas breakout more pronounced. In the last minutes of an offload, as the trailer nears empty, the pump starts drawing air and vapor. Either way, free gas passing through the meter can inflate the registered liquid quantity.
For that reason an air eliminator ahead of the meter is common on crude unloading lines. Its sizing has to account for viscosity, since bubbles separate more slowly from viscous crude. Low-level or gas detection logic that stops flow automatically at the end of the offload protects the pump from running dry and keeps air out of the measurement.
Solids and strainers
Sand, sludge and wax fragments drawn from the bottom of field tanks settle in the trailer and move into the line near the end of unloading. A strainer ahead of the meter and pump captures them. Basket mesh is chosen by weighing the meter manufacturer's recommendation against the solids load, as an overly fine screen causes high pressure drop with viscous crude. Differential pressure monitoring and quick-clean duplex baskets keep busy receipt points running without interrupting offloads.
Sampling and S&W determination
The most sensitive input to net quantity is the sediment and water fraction. It is determined by analyzing a composite sample drawn in proportion to flow throughout the offload. Methods for crude include Karl Fischer titration, distillation, laboratory centrifuge and field centrifuge; precision depends on both the water level and how representative the sample is. Laboratory results can be used to adjust the declared crude quantity, which is why the sampling system is part of the measurement system. Sampler and mixing options are discussed on the Crude Oil Sampling Systems page.
How net quantity is calculated per truck
The meter ticket issued for each offload is calculated in the following order. Correction factors and proving methods are covered in detail on the Crude Oil Custody Transfer Systems page.
1. Indicated meter volume is multiplied by the meter factor from the most recent proving to obtain gross volume.
2. Using flow-weighted average temperature and pressure and the product density, gross volume is corrected to the reference conditions defined in the contract.
3. Applying the S&W result of the composite sample, sediment and water are deducted from the corrected volume; the remainder is net standard volume.
4. The ticket is recorded with truck ID, source lease, start and end time, meter totalizer readings and sample ID.
For crude, temperature and pressure correction factors are calculated from generalized crude oil curves, and the calculation sequence, rounding rules and intermediate precision are defined by standards so that anyone calculating from the same data reaches the same result. Where a Coriolis meter is used, mass is measured directly and volume is derived from measured density; the contract states which quantity is the basis for invoicing.
Acceptance criteria and rejected loads
Merchantability is assessed before or during unloading. Limits for S&W, density, temperature and, where relevant, vapor pressure or H2S are set in the contract or in the pipeline operator's quality specification. The design should answer these questions up front:
· Pre-check: Will a sample be taken from the trailer for a quick field test before unloading, or will the decision rest on the post-offload lab result?
· Segregation: Can off-spec loads be routed to a tank separate from main inventory?
· Real-time monitoring: Will an online water cut monitor be used to stop the offload or divert flow when water is high?
· Documentation: How are rejected or segregated loads reported to the parties?
Reconciling the haul ticket, meter and tank gauge
Trucked crude usually comes with three quantity figures: the ticket issued at the loading point, the unloading meter ticket and the level change in the receiving tank. Differences can stem from evaporation in transit, water settling, temperature change, heel left in the trailer and measurement uncertainty. Confirming the trailer is fully empty, reporting meter and tank figures on the same reference basis and defining an acceptable tolerance in the contract all help reduce disputes.
Design criteria
Parameter | Why it matters in design | Effect on selection |
Trucks per day and offload time | Sets receipt capacity and queuing | Number of unloading lanes, pump and meter size |
Viscosity and minimum product temperature | Affects pump suction, gas breakout and pressure drop | Pump type, air eliminator size, heating need |
Expected S&W range | Net quantity accuracy and acceptance depend on it | Sampling system, mixing, water cut monitoring |
Solids load | Meter and pump wear, strainer plugging | Strainer type, mesh and DP monitoring |
Invoicing quantity | Volume-based or mass-based settlement | Meter technology and density measurement |
Meter proving approach | Meter factor must stay current | Prover connections or master meter arrangement |
H2S and vapor | Personnel safety and hazardous area | Closed connections, gas detection, ventilation |
TLY Enerji's role at crude receipt points
TLY Enerji supports crude receipt projects with engineering for meter, temperature and pressure instrumentation, water cut monitoring and control valve selection. Depending on scope, supply of that equipment, field installation, integration of flow calculation and records with PLC or SCADA, testing and commissioning can be included. Boundaries with the sampling system, tank gauging and pump station are agreed through the technical specification.
Information to share for an unloading point
· Crude types and source leases to be received, with assays if available
· Trailer capacity, trucks per day and peak-hour arrivals
· Invoicing quantity and the reference conditions to be used
· Contract acceptance limits for S&W, density and H2S
· Existing or planned receiving tank and tank gauging system
· Preferred proving method and frequency
· Expectations for meter tickets, reporting and host system integration
Related pages
· Truck unloading systems: General engineering of unloading pumps, suction side and air entrainment.
· Crude oil sampling systems: Flow-proportional sampling for S&W determination.
· Crude oil custody transfer: Gross-to-net corrections, meter factor and proving.
· Crude oil truck loading systems: Gas, heating and sampling provisions at the loading point.
· Pipeline receipt and delivery metering: Continuous metering where received crude enters the pipeline.
Crude unloading FAQ
Why can the haul ticket and the unloading meter disagree?
Loading and unloading may happen at different temperatures, light ends can evaporate in transit, water can settle in the trailer and some product may remain as a heel. Each measurement also has its own uncertainty. A comparison only makes sense once both figures are corrected to the same reference conditions and reported on the same S&W basis.
How does S&W affect net quantity?
Net standard volume is the corrected volume minus the sediment and water fraction. A small error in the S&W result therefore flows straight into net quantity and the invoice. That is why drawing the sample in proportion to flow over the whole offload, with adequate in-line mixing, is critical.
How is air kept out of the meter at the end of the offload?
An air eliminator ahead of the meter separates free gas before it reaches the meter. In addition, low-level or gas detection logic that senses the trailer running empty stops the pump or closes the valve so that aerated flow is not measured. With viscous crude the air eliminator needs to be sized with extra care.
Can an online water cut monitor replace lab analysis at a receipt point?
Usually not. Online water cut monitoring is valuable for spotting a high-water load immediately and stopping or diverting the offload. The S&W value used for invoicing, however, is in most contracts taken from standard analysis of the composite sample.