Crude Oil Terminals

Unlike a refined product that leaves a refinery to a specification, crude oil is a mixture whose properties change with its source, its field and over time. Density ranges from light to heavy and sulfur content from sweet to sour, and the oil may carry varying amounts of water, sediment and dissolved gas. That variability shapes both measurement and transfer: the gross volume a meter registers is not the net quantity of oil being sold. The gap is closed by temperature and pressure corrections, by determining sediment and water (S&W), and by making sure the sample behind that determination truly represents the batch.

At terminals that receive, store or forward crude arriving by truck or pipeline from production sites, the metering system therefore has to answer three questions at once: how much liquid passed, how much of it was oil, and how can both parties verify the result?

Properties that affect metering and transfer

·         Variable density and composition: Crudes from different fields, or production that changes over time, alter density and the volume correction calculation. A fixed-density assumption produces errors when quality shifts, which is why online density measurement is often considered.

·         Sediment and water (S&W): Water and solids in the crude are deducted from the quantity sold. The accuracy of the water determination depends on the arrangement and characteristics of every element of the sampling system, not on a single instrument.

·         Viscosity and temperature: Heavy, viscous crudes influence meter technology selection, pressure drop and pumpability. Some crudes lose much of their fluidity in cold weather; whether heating is needed is assessed from product analysis.

·         Dissolved gas and vapor: When pressure drops, crude can release light hydrocarbon gases. Free gas passing through a meter causes measurement error, so gas elimination and a back-pressure arrangement that keeps the liquid in a single phase are considered together.

·         Hydrogen sulfide (H2S): Naturally present in some oil and gas deposits, it is taken into account in gas detection for personnel working at sampling points and tank openings, in material selection and in closed-system design.

From gross volume to net standard volume

In crude oil trading, the invoiced quantity is usually the net standard volume. The meter measures gross volume at operating conditions; that figure is corrected for temperature and pressure effects to reference conditions, and the S&W percentage determined in the laboratory from the sample is then deducted. Industry standards define the calculation sequence, rounding rules and correction factors so that the same result is obtained regardless of who performs the calculation. The meter itself is verified at regular intervals against a prover or master meter, and its meter factor is updated.

Main measurement points at a crude terminal

Safety and working environment

Occupational safety agencies have highlighted that high concentrations of hydrocarbon gases and vapors can escape from hatches opened for manual gauging and sampling on crude oil tanks, creating both fire and explosion risk and oxygen-deficient atmospheres. Gauging and sampling methods that do not require opening hatches, closed sample containers and electrical equipment matched to the hazardous area classification are therefore considered from the start of the design.

TLY Enerji's role

For crude oil metering applications, TLY Enerji can provide engineering support covering selection of measurement technology to suit process conditions, supply of Coriolis, ultrasonic and other flow meters together with pressure, temperature and level instruments, system integration, installation, testing and commissioning, and PLC- and SCADA-based monitoring. The scope of supply and division of responsibilities are defined for each project from the technical specification and fluid analyses.

Crude oil solutions

·         Crude oil truck loading systems: Extra measures for gas, vapor and viscosity during loading.

·         Crude oil truck unloading systems: Determining the net quantity of crude received by truck.

·         Crude oil metering skids: Selecting meter, density, water cut and sampling components.

·         Crude oil custody transfer systems: Corrections, provers and meter factors behind the commercial quantity.

·         LACT units: Automatic custody transfer from the production lease to the pipeline.

·         Crude oil sampling systems: Representative samples and reliable water determination.

·         Pipeline receipt and delivery metering systems: Continuous high-flow metering and batch tracking.

·         Tanker truck loading and unloading terminals: Fluid-independent map of loading and unloading solutions.

Frequently asked questions

Why does the crude oil quantity on the invoice differ from the meter reading?

The meter registers everything passing through it at operating temperature and pressure, including water and sediment along with the oil. For the invoiced net quantity, that gross volume is first corrected to reference conditions, then the S&W percentage determined from the sample is deducted. The meter factor obtained from proving is also applied in the calculation.

Which meter type is best for crude oil?

There is no single answer. The choice depends on viscosity and density range, flow rate, gas content, allowable pressure drop and the proving method. Where light and heavy crudes share the same line, the meter's behavior across different viscosities and how readily it can be proved deserve particular attention.

Why is sampling treated as part of the metering system?

Because the net quantity is calculated from the S&W content measured on the sample. If the sample does not represent the whole batch, the net result will be wrong no matter how good the meter is. The sampling point, stream mixing and flow-proportional collection are therefore designed with the same care as the meter itself.