Biofuel and Ethanol Custody Transfer Metering Systems

In biofuel sales, the commercial quantity is the metered volume converted to reference temperature (or to mass) with a product-specific method, and for blends it also includes a separate record of each component. The logic of fuel custody transfer still applies, but two differences change the design: ethanol and biodiesel expand with temperature differently from fossil fuels, and the bio-component quantity inside a sold blend is often a figure that must be reported on its own.

The general layout of a custody transfer skid, proving arrangements and sealing are covered on the custody transfer truck loading skids page, and the basics of correcting fuel volumes to reference temperature on the fuel custody transfer page. This page focuses on what is specific to bio-components.

Why volume correction is product-specific

Liquid fuels expand as they warm, so the same mass shows a larger volume on a hot day. To make the commercial quantity independent of temperature, the measured volume is converted to reference temperature with a correction factor that depends on product density and temperature. The correlations behind that factor differ by product family. For ethanol this is explicit: API publishes a dedicated measurement standard for the density and volume correction factors of denatured fuel ethanol, and a separate standard for blends of ethanol with gasoline components.

For biodiesel (FAME) and HVO, the correction method to apply has to be settled by product, market and contract. In practice that means configuring a separate calculation method and parameter set for each product code in the flow computer or preset controller. A correction made with the wrong table creates a systematic error in the invoiced quantity, however accurate the meter is.

Which quantity is invoiced for a blend?

A product blended during loading can generate three quantity figures: the fossil component meter, the bio-component meter and, where fitted, a meter on the blended stream. Each component is corrected to reference volume at its own temperature with its own method. The sum of the components and the blended-stream measurement may not match exactly, so the contract should state in advance whether invoicing is based on the total blend or the component totals, and how any difference is handled.

Component-based reporting

In many markets the bio-component quantity is reported separately to track blending mandates, tax treatment or incentive schemes. What must be reported, on what basis and how often depends on national legislation and should be settled before the system is designed. From the metering side, what matters is that each load record keeps the following together in a tamper-evident, traceable form:

·         Product code and blend recipe

·         Observed volume, metering temperature, density used and reference volume for each component

·         Correction method and meter factors applied

·         Achieved blend ratio and deviation alarms

·         Parameter changes and event logs

Under the US biodiesel quality program, participating marketers are expected to monitor the accuracy of blend ratios, an example of component records serving quality assurance as well as tax purposes.

Legal metrology and accuracy class

Continuous, dynamic measuring systems for liquids other than water fall under OIML R 117 and, in the European Union, the relevant annex of the Measuring Instruments Directive (MID). In these documents, measuring systems for loading and unloading road tankers sit in accuracy class 0.5, and the permissible error for the complete measuring system is wider than the error defined for the meter alone. The conversion device that calculates reference-condition quantities is treated as part of the measuring system. Whether a given point of sale is subject to legal metrology control, and which approvals apply, is decided by national legislation; the approval of a single meter does not demonstrate conformity of the whole system.

Proving: a separate meter factor for each product

Ethanol, FAME, HVO and fossil fuel differ in viscosity and density, so a meter may not run with the same factor on every product. On custody lanes the meter factor is established per product, with the actual product and close to operating flow. Proving can use a fixed or portable prover, a master meter or a calibrated vessel; method and interval are set by contract and regulation. Because Coriolis meters measure mass, density and temperature at the same time, they can feed the correction calculation directly, which is useful on bio-component lines where density varies.

Design criteria

How TLY Enerji contributes

Depending on project scope, TLY Enerji can provide engineering support for selecting the right meter technology for each product, supplying meters and temperature and density instruments, transferring metering data to the PLC and SCADA layer, installation, testing and commissioning. Legal metrology approvals and national reporting obligations are addressed with the operator at project start, and the supply boundary is set through the technical specification.

Points to settle before requesting a quotation

·         Products to be sold, blend levels and the sales basis for each

·         Volume correction methods and reference temperature to be applied

·         Component reporting requirements and the receiving system

·         Legal metrology scope and approval status of existing equipment

·         Preferred proving method and interval

·         Loading rate, number of arms and existing loading control infrastructure

Related pages

·         Ethanol metering skids: Technology selection and temperature effects on ethanol lines.

·         Biofuel metering skids: Viscosity and temperature effects on FAME and HVO metering.

·         Biofuel blending systems: The blend point where component quantities originate, and ratio control.

·         Fuel custody transfer metering systems: Reference temperature correction and sales practice for fossil fuels.

·         Custody transfer truck loading skids: Custody skid components and proving arrangements.

Biofuel custody metering questions

Can standard petroleum correction tables be used for ethanol?

Ethanol changes volume with temperature differently from fossil fuels, and API publishes a separate measurement standard giving density and volume correction factors for denatured fuel ethanol, plus another for blends of ethanol with gasoline components. The method to apply should be fixed by contract and national legislation, and the flow computer configured with the correct parameter set for each product.

How is the bio-component quantity in a blend documented?

In ratio blending each component is measured on its own meter and corrected to reference volume using its own temperature and correction method. The load record stores component quantities, metering temperatures, the method used and the achieved ratio together. Keeping these records tamper-evident and traceable is the basis for whatever reporting the regulations require.

Can one meter handle custody measurement of both biodiesel and diesel?

It can, but because the products differ in viscosity and density, the meter factor must be established separately for each product and proved with the actual product. The controller should select the correct factor and correction method automatically from the product code. The effect of the previous product left in the line after a product change should also be addressed, along with how the contract treats it.

Is legal metrology mandatory for biofuel sales?

That depends on the legislation of the country where the product is sold. In the European Union, dynamic measuring systems for liquids other than water are regulated under the Measuring Instruments Directive, with road tanker loading generally in accuracy class 0.5, and OIML R 117 provides the international framework. The approval of one meter does not by itself make the whole measuring system compliant; the system is assessed as a whole.