Coriolis or positive displacement meter for truck loading? Deciding with product and operating data

Short answer: both technologies are used for custody transfer truck loading, and the right choice depends more on the product and the operation than on the meter itself. If the product is sold by mass, if its density varies from batch to batch, or if products of different density share the same line, a Coriolis meter shortens the calculation chain because it measures mass and density directly. If the product is sold by volume at a reference temperature, is clean and has a stable viscosity, and the site already has volumetric proving infrastructure, a positive displacement (PD) meter is often a strong candidate.

This assessment applies to single-phase liquids; the additional constraints of LPG, LNG, products containing solids and products loaded hot are covered separately below. Skid components, sealing and the delivery ticket are described on the custody transfer truck loading skid page and are not repeated here; this article focuses only on the meter technology decision.

Start by fixing the billing unit

Before discussing technology, the contract should state which quantity is being sold: mass, volume at a defined reference temperature, or volume at metering (operating) conditions. Each technology delivers one of these directly and the others through conversion.

·         Sales by mass: A Coriolis meter measures mass directly. A PD meter measures volume; reaching mass requires density from an in-line density meter or from batch analysis. Inferred mass systems that combine a volume meter with an in-line density meter are addressed separately in an addendum to API MPMS Chapter 21.2.

·         Sales by volume at reference temperature: The operating volume measured by a PD meter is converted to reference volume by temperature and, where required, pressure correction. With a Coriolis meter, reference volume is obtained by dividing the measured mass by the density at reference temperature; the density measured in the line is not the reference density, so this conversion also needs a correction method.

·         Sales by operating volume: If the contract defines the quantity as volume at metering conditions, this is the native output of a PD meter; a Coriolis meter calculates the same value from mass and line density.

For crude oil, refined products and lubricating oils, the temperature and pressure correction method is given in API MPMS Chapter 11.1; that document does not cover LPG or NGL. For these products the conversion method must be defined separately, whichever technology is selected.

Worked example: mistaking line density for reference density

Example scenario (assumed values): 25 t of diesel is loaded into a compartment through a Coriolis meter and is sold by volume at 15 °C. The product is at 30 °C; density is taken as 830 kg/m³ at this temperature and 840 kg/m³ at 15 °C. Dividing the mass by the reference density gives a reference volume of about 29.762 m³. Using the line density gives about 30.120 m³; the difference is 0.359 m³, roughly 1.2%.

This difference is more than twice the 0.5% maximum permissible error that OIML R 117-1:2019 sets for a measuring system in accuracy class 0.5. Which density and which method will be used for conversion should be defined before the meter is selected; the example is not a real project result or a design value.

Product data that change the decision

Viscosity and its change with temperature

In a PD meter, measurement is affected by slippage, the liquid passing through the narrow clearances between the moving elements and the housing. As viscosity rises, slippage falls, but the pressure drop across the meter increases; a high pressure drop can cause premature wear of bearings and seals. Operating at a viscosity clearly different from the calibration viscosity introduces measurement error. For products whose viscosity changes strongly with temperature, the viscosity range over which the PD meter will be proved should therefore be defined at the outset.

Coriolis mass measurement is in principle independent of viscosity; viscosity mainly affects pressure drop and therefore sensor size. For high-viscosity products, particularly with large sensors, suitability and pressure drop should be confirmed through the manufacturer's sizing study.

Viscosity and temperature also matter for legal metrology. OIML R 117-1:2019 Table 2 assigns accuracy class 1.0 to liquids with a dynamic viscosity above 1000 mPa·s, and to liquids normally in class 0.3 or 0.5 that are measured below −10 °C or above +50 °C. National legislation and the competent body decide which class applies; the table also notes that a better accuracy may be specified.

Density and product variability

Because a Coriolis meter also measures density, the need for a separate density meter in mass-volume conversion is reduced; a sudden shift in density may signal a wrong product or water ingress, but it does not replace laboratory analysis. With a PD meter, density comes from an in-line density meter or batch analysis; if density varies between batches, how this value enters the delivery record is part of the design.

Solids, abrasiveness and corrosion

A PD meter has close-tolerance moving parts; solid particles wear these clearances and shift the meter factor over time. A strainer is indispensable, but it does not by itself make an abrasive product suitable. The fact that OIML R 117-1 requires an endurance test only for meters whose moving parts are in direct contact with the measured liquid, exempting Coriolis, ultrasonic and electromagnetic meters, also shows that wear is a design issue for PD meters.

A Coriolis meter has no moving parts, but its thin-walled vibrating tubes are sensitive to erosion and localized corrosion. Pitting or stress corrosion can initiate fatigue cracking in a vibrating tube even in a material that looks acceptable for the piping. For corrosive or abrasive products, the tube material should therefore be selected for the specific product with the manufacturer's assessment.

Gas, vapor and phase stability

Both technologies measure correctly only single-phase liquid; API MPMS Chapter 5.2 likewise does not apply to two-phase fluids. OIML R 117-1 requires a gas elimination device where air or undissolved gas may enter; a gas separator is needed if the pressure at the pump inlet can fall below atmospheric pressure or the saturated vapor pressure of the product. For liquids above 20 mPa·s at 20 °C a gas elimination device is not mandatory; instead, measures that prevent gas entry are required, such as keeping the pump inlet pressure above atmospheric or automatically stopping the flow when it drops.

A PD meter counts gas passing through it as volume. In a Coriolis meter, entrained gas disturbs the tube vibration and can cause errors in both mass and density readings; suitable sensor selection, orientation and multiphase alarms help limit this effect but do not replace system measures such as gas elimination or back pressure. In viscous products loaded hot, dissolved gas coming out of solution can create a similar risk of measurement differences.

LPG, NGL and LNG need separate treatment

·         LPG: A liquefied gas under pressure. OIML R 117-1 section 5.4 requires the product to remain liquid in the meter during measurement, which puts back-pressure control at the center of the design regardless of technology. A connection between the vapor phases of the tanks is allowed only if the returned gas is measured and subtracted from the delivered quantity. Liquefied gases under pressure measured at or above −10 °C are in class 1.0 in Table 2.

·         NGL: Light hydrocarbon liquids whose composition depends on the source; they are not the same product as LPG, and variable density and vapor pressure may favor mass measurement.

·         LNG: A cryogenic product; under OIML R 117-1 section 5.11, LNG delivery systems indicate the quantity in mass, and they are in class 1.5 in Table 2. LNG loading falls outside this comparison and should be handled with its own constraints.

Operating data: loading profile, pressure drop, number of products and maintenance

·         Loading profile: Loading starts at low flow, continues at high flow and slows again at the end. OIML R 117-1 requires the operating flow rate to stay between the minimum and maximum flow rates of the system except at the start, at the end and during interruptions. In a Coriolis meter, zero stability governs accuracy at low flow; in a PD meter, the effect of slippage grows at low flow with low-viscosity products. In small compartments the slow steps make up a larger share of the total, so the meter's behavior during these steps should also be verified.

·         Minimum measured quantity: The minimum measured quantity of the measuring system cannot be smaller than the largest minimum measured quantity of its constituents, such as the meter and gas elimination device. The smallest compartment or delivery volume should be compared with this value.

·         Pressure drop and pump: Both meter types can create a significant pressure drop with viscous products. Whether the existing pump covers the total loss of strainer, gas eliminator, meter, control valve and loading arm at the target flow rate should be calculated before the technology decision.

·         Multi-product line: OIML R 117-1 accepts that a meter may measure different liquids with a separate adjustment for each liquid or with a single adjustment; which applies is stated in the type approval certificate. Because the PD meter factor depends on viscosity, a separate factor and proving per product is a common requirement. Coriolis mass measurement depends less on the product; the need for per-product verification is still set by legislation and contract.

·         Cleaning and maintenance: Wear parts in a PD meter such as rotors, bearings and seals need periodic maintenance, and the meter is re-proved after maintenance. A Coriolis meter has no wear parts; maintenance consists mainly of zero checks, monitoring of deposits in the tubes and evaluation of diagnostic results. For products that thicken as they cool, heating against solidification during shutdowns may be needed with either technology.

The verification method influences the technology choice

OIML R 117-1 requires the measuring system to be designed so that it can be verified under conditions of use; for tests with the pumps running, it cites standards suitable for continuous operation such as a pipe prover, a volume standard with a flow diverting mechanism or a weighing instrument. When the reference measures the same quantity as the meter's native output, the verification chain becomes shorter.

·         Volumetric prover: Compared directly with the volume output of a PD meter. If a Coriolis meter is proved against a volumetric prover, its volume is calculated from measured mass and density, so the density measurement also enters the verification chain.

·         Master meter: API MPMS Chapter 4.5 covers the use of displacement, turbine, Coriolis and ultrasonic meters as master meters for single-phase liquid hydrocarbons. Where there is no prover at the bay, series comparison with a verified master meter can be a practical route; traceability of the master meter must be established separately.

·         Mass reference: If the product is sold by mass, a weighing-based reference can be compared directly with the mass output of a Coriolis meter. Which data are used to reconcile differences between a weighbridge and a meter is the subject of a separate article.

Existing prover infrastructure and historical meter factor records are a real cost item in the technology decision. Inspection, calibration and approval steps within legal metrology are carried out by authorized bodies; the verification method that will be accepted should be agreed with them at the start of the project.

Comparison table: which question, which data

Decision criteria for Coriolis and PD meters

Unsuitable conditions and limits

·         A Coriolis meter is a weak candidate where the existing pump cannot cover the additional pressure drop with a viscous product, where gas entry cannot be prevented by system design, or where the manufacturer has not confirmed the compatibility of the tube material with the product.

·         A PD meter is a weak candidate for products containing abrasive solids, for products whose viscosity varies widely without per-product factor management, or where the product is sold by mass and no density measurement is planned.

·         An air pocket or gas slug arriving as a tank runs down creates a risk of overspeed and mechanical damage in a PD meter and of measurement error in a Coriolis meter; in both cases the remedy is system design, not the meter.

·         Type approval of a meter does not by itself show that the loading system is suitable for custody transfer; the measuring system is assessed as a whole.

·         This article does not cover LNG loading design, hygienic design or hazardous-area equipment selection.

Data to share for a quotation

Data form for a Coriolis and PD meter assessment

Frequently asked questions

Is a Coriolis meter not more accurate for every product?

The accuracy on a data sheet is a value for the meter under reference conditions. What counts in custody transfer is the result of the measuring system together with the product, installation, gas elimination, conversion and verification. For a clean product of stable viscosity sold by volume, a well-proved PD meter can meet the required class; product and operating data, not the data sheet, drive the decision.

Can both technologies be used at the same loading bay?

Yes. Different meters can be chosen for different product lines, or one meter type can serve as a master meter to verify the other; API MPMS Chapter 4.5 covers the use of displacement and Coriolis meters as master meters. The batch controller and recording system must handle both output types correctly.

Which should be chosen for products loaded hot, such as bitumen or heavy oil?

There is no general answer. High viscosity reduces slippage in a PD meter but increases pressure drop for both technologies; OIML R 117-1:2019 Table 2 associates liquids measured above +50 °C and viscosities above 1000 mPa·s with class 1.0. Heating, solidification during shutdowns, dissolved gas release and pressure drop must be evaluated with the product data.

Related pages

·         custody transfer truck loading skids: The measuring system around the meter, proving connections and sealing.

·         Coriolis mass flow meters: Operating principle, sizing parameters and approval options.

·         truck loading systems: Loading profile, batch controller and loading permissives.

·         truck loading skid RFQ scope: How skid, pump, arm and automation boundaries are defined in a quotation.

·         reconciling weighbridge and flow meter differences: Which data to place side by side when comparing against a mass reference.