In a fuel metering skid, the meter, the temperature and density correction and the proving components are chosen according to the viscosity of the products, how many products share the line, what the measurement is for and how the meter will be verified on site. Two skids that look alike on the frame can end up with quite different equipment lists when one serves in-terminal transfers and the other measures pipeline receipts. The skid-mounted approach lets these components be engineered together before they reach site.
Where metering skids sit in a fuel terminal
· Pipeline receipt: Measuring batches from a refinery or trunk line as they enter the terminal; usually a custody point.
· In-terminal transfer: Tank-to-tank transfers, blend preparation or segregating contaminated product; normally for process and stock control.
· Loading lines: Meter groups at the head of each bay, designed as part of the truck loading system.
· Blend component metering: Adding ethanol, biodiesel or paraffinic components to the base product in ratio.
Meter technology and product properties
Dynamic measurement of liquid hydrocarbons uses positive displacement, turbine, Coriolis and ultrasonic meters. For fuels, the product properties that drive the choice are:
Product property | Effect on measurement | What is considered in selection |
Viscosity and viscosity range | Viscosity changes from gasoline to diesel and additized grades; the calibration of some meter types is viscosity-sensitive | Viscosity range of products sharing a meter and the need for a meter factor per product |
Frequency of product changes | Each change means the meter runs on a different fluid | Dedicated meter per product or shared meter with product-specific factors |
Volatility | Vaporization at low pressure can let vapor pass through the meter | Air eliminator, back pressure and meter position in the line |
Density variation | Affects conversion from volume to mass or to reference volume | Fixed density value or on-line density measurement |
Cleanliness and particulates | Wear and blockage risk in meters with moving parts | Strainer mesh and maintenance access |
Coriolis meters measure mass, density and temperature at the same time and calculate volume from mass and density, which can help on lines with frequent product changes and on blend components of variable density. Positive displacement and turbine meters have long been used in fuel loading and transfer and fit volume-based measurement practice directly. Ultrasonic meters are considered for large-diameter pipeline receipts. No single technology suits every fuel application; the choice is made together with flow range, line size, product slate and proving method.
The hardware side of temperature and density correction
Fuel sales and stock quantities are usually stated as volume corrected to reference temperature. A published calculation method provides temperature and pressure volume correction factors for refined products. To apply it, the skid needs a temperature measurement close to the meter, pressure measurement where relevant and product density data. Placing the temperature sensor so that it sees the same flow conditions as the meter, and providing a second test thermowell for verification, are points skid designers commonly watch.
Where density data comes from depends on the product slate. On a single-product line, a fixed laboratory value may be enough. On lines where blends and bio-component ratios change, on-line density measurement or a density-capable meter is preferred. The flow computer or preset controller doing the calculation should record which correction method was used for which product. The commercial side of calculation rules and reference conditions is covered on the custody transfer page.
Proving components
Because fuel meter performance can shift over time and with product changes, how the meter will be proved is planned into the skid from the start. The main methods are:
· Displacement prover (fixed or portable): Compares a known volume with the meter under dynamic conditions; the skid carries the valve arrangement and connection points.
· Master meter: A verified meter connected in series for comparison, either mounted on the skid or brought in as a portable unit.
· Volumetric test measure: Common at loading bays: the meter delivers into a vessel of known volume and the result is checked.
The method depends on meter type, flow rate and whether the measurement is commercial. A dedicated code of practice exists for proving meters on loading gantries, and it notes that local regulations and contracts may take precedence. On shared meters with a factor for each product, proving has to be repeated for each product.
Typical skid components
Component | Function | Standard / option |
Strainer | Protects meter and control valve from particles | Standard |
Air eliminator | Keeps air and vapor out of the meter | Standard on receipt and unloading lines |
Meter | Volume or mass measurement | Standard; technology per product |
Temperature and pressure measurement | Volume correction and back pressure control | Standard |
Density measurement | Mass or reference volume calculation | Option depending on product slate |
Control or back pressure valve | Flow control and keeping the liquid phase in the meter | Per application |
Flow computer or preset controller | Correction calculation, records and data transfer | Standard; type per application |
Proving connections | Prover or master meter tie-in | Common in custody applications |
Sampling point | Confirming product quality and density | Per project |
Process skid versus custody skid
A process transfer skid and a custody receipt skid may use the same meter type. The difference lies in whether temperature and density measurements can be verified, the proving connections, sealing of parameters, event logs and assessment of the measuring system as a whole. Legal metrology sets separate maximum permissible errors for the complete measuring system and for the meter, so the meter's own performance does not represent the whole system. Governance, records and sealing for custody measurement are explained on the fuel custody transfer page.
Hazardous areas and assembly integrity
Fuel metering skids are usually installed where an explosive atmosphere may occur. Area classification determines the protection method and wiring of electrical equipment on the skid. Under European practice, when items that each carry their own conformity documentation are combined into a functional assembly, any additional ignition hazards arising from the combination must be assessed. In short, the meter's or transmitter's own certificate does not by itself demonstrate conformity of the whole skid.
What TLY Enerji contributes
TLY Enerji helps identify suitable metering technology for fuel applications based on product and process data. Depending on the project, supply of Coriolis, ultrasonic and other flow meters, temperature and pressure transmitters and control valves, together with system integration, installation, commissioning and post-commissioning support, can be provided. The fabrication and test scope of the skid and the scope of supply are defined against the technical specification.
Data needed to select a metering skid
· Location and purpose: receipt, transfer, loading or blend component metering
· Products on the line with viscosity and density ranges
· Minimum, normal and maximum flow with operating pressure and temperature
· Whether the result is used for invoicing and the required output: reference volume or mass
· Preferred proving method and any prover or master meter already on site
· Hazardous area classification and layout constraints
· Control system or terminal automation the flow computer will connect to
Related pages
· Fuel custody transfer metering: Commercial basis of volume correction, legal metrology and records.
· Custody transfer truck loading skids: General structure and proving of custody loading skids.
· Fuel truck loading systems: Product-specific requirements for meters on loading lines.
· Fuel truck unloading systems: Reconciling the receipt meter with tank gauging.
Fuel metering skid FAQ
Can one meter measure several fuel products?
Yes, but each product needs its own meter factor and proving has to be done product by product. Passing products with very different viscosities through the same meter changes the measurement behavior of some meter types. The volume left in the line at each product change also has to be considered. For quality-sensitive products, a dedicated meter is preferred.
Why leave prover connections on the skid?
Meter performance can shift with product, flow rate and time. Without prover or master meter connections, proving means removing the meter or re-piping the line, which stops operations and tests the meter under conditions different from its real service. Providing the connections from the start is far easier than adding them later.
Is density measurement needed on every skid?
No. On single-product lines with stable density, a laboratory value may be enough. On lines where blend ratios change, product comes from several sources or reporting is by mass, on-line density measurement is considered. With a Coriolis meter, density can be taken from the meter, but whether that value can be used in commercial calculations must be confirmed separately.
Can a process skid be upgraded to custody service later?
It may be technically possible, but it usually takes more than changing the meter. Verifiability of temperature measurement, proving connections, the flow computer's record and sealing features and assessment of the measuring system as a whole all need to be revisited. If the purpose of measurement might change in future, it should be considered in the original design.