Fuel Truck Unloading Systems

When fuel is received from a tank truck into storage, two errors cost the most: discharging into the wrong tank, and a received quantity that does not match the shipping document. The first can spoil a full tank of product. The second creates a small commercial difference on every delivery that adds up over time. A fuel unloading system is therefore designed not as a simple pump and meter line, but as a control point that ties product verification, measurement and tank data together.

The general engineering of an unloading line, including cavitation and air or vapor entrainment on the suction side and the selection of air eliminators and strainers, is covered on the truck unloading systems page. This page focuses on fuel-specific product verification and quantity reconciliation.

Where fuel is unloaded

·         Depots without a pipeline connection: Regional depots supplied by road from a refinery or a primary terminal.

·         Terminal-to-terminal transfers: Product brought in by truck from another terminal for stock balancing or during maintenance outages.

·         Blend component receipt: Ethanol, biodiesel or additives delivered by truck to a blending terminal.

·         Industrial and power facilities: Generator, boiler or fleet fueling sites that fill their own tanks by truck.

Layers that prevent misdelivery

Discharging the wrong product rarely comes from one big mistake. More often, one of several checks that should back each other up gets skipped. A robust arrangement therefore has several independent layers:

1.       Document match: product, quantity and origin on the shipping document are compared with the expected delivery record at the terminal; without a match, unloading does not start.

2.       Physical segregation: each product has its own unloading connection, labeling and color code, and connections for different product groups can be chosen so they cannot be cross-connected.

3.       On-board product data: on trucks fitted with product recognition devices, the compartment product can be read digitally.

4.       Product check: a sample is taken for density or visual inspection; where the meter also measures density, the measured value can be compared with the expected range at the start of unloading.

5.       Valve interlock: the inlet valve of the receiving tank opens only after the previous steps are confirmed, and only for the tank assigned to that product.

Which layers are automated and which rely on procedures depends on how often the terminal unloads, how many products it handles and the automation already in place. For quality-sensitive products such as jet fuel, a dedicated unloading line and a rule that product does not enter the tank before quality control is complete are common.

Air, vapor and the end of unloading

Toward the end of unloading, air and vapor enter the line as the compartment empties. With gasoline the effect is stronger than with diesel, because the low pressure on the suction side also promotes vaporization. If air or vapor passes through the meter as if it were liquid, the received quantity is overstated. In unloading lines used for custody or reconciliation purposes, the air eliminator is therefore treated as an integral part of the measuring system, and a stop logic is defined to prevent the pump from running dry at the end of the delivery.

Vapor in the storage tank head space is also displaced during unloading. At sites with a vapor balancing line, this vapor can be sent back to the truck; where there is none, tank venting and emission rules need to be assessed separately. In Europe, the tank truck adapters used for bottom unloading fall under the same adapter standard as bottom loading.

Reconciling the received quantity

Every delivery produces three quantity figures: the quantity on the loading terminal's document, the quantity measured by the unloading meter and the quantity calculated from the level change in the storage tank. To compare them, all three must be brought to the same basis, meaning volume corrected to reference temperature, or mass. A truck may be at a different temperature when loaded and when unloaded, so comparing observed volumes directly gives misleading results.

When checking against tank gauging, the readings before and after unloading should take into account the tank temperature and whether connecting lines are full. When the gap between the unloading meter and the tank measurement exceeds the agreed tolerance, a consistent record trail is needed to tell whether the difference comes from the document, the measurement or a physical loss. Stock control and loss monitoring at terminal level are covered on the terminal automation and custody transfer pages.

Overfill protection for the receiving tank

In unloading, the overfill risk lies with the receiving storage tank, not the truck. Standards that define an overfill prevention process for aboveground tanks at petroleum facilities may exclude tanks filled exclusively from wheeled vehicles, so the rule that applies to a truck-fed tank needs to be settled in the project. In practice, available ullage is checked before unloading starts, and the high-level alarm is wired to stop the unloading pump.

Design criteria

What TLY Enerji contributes

For fuel unloading facilities, TLY Enerji provides system design and engineering, supply of pumps, valves, flow meters and instruments, installation, commissioning and maintenance. Automation and control of unloading operations, and transfer of meter data to the site control system, can be included depending on the project. The interface boundary with the tank gauging system and terminal software is defined in the technical specification.

Information to prepare for a review

·         Products and blend components to be received

·         Number of deliveries per day or per hour and average quantity per truck

·         Whether unloading uses the terminal pump or the truck pump

·         Whether the received quantity is used for invoicing or only for reconciliation

·         Number of receiving tanks and existing level and temperature measurement

·         Vapor balancing and tank venting arrangement

·         Current product verification procedures and automation system

Related pages

·         Truck unloading systems: General engineering of the suction side, air eliminator and unloading line.

·         Fuel metering skids: Selecting the receipt meter, temperature correction and proving hardware.

·         Fuel terminal automation systems (TAS): Automating document matching and inventory reconciliation.

·         Fuel custody transfer metering: Making the received quantity commercially acceptable.

Fuel unloading FAQ

Why do the unloading meter and the tank gauge give different results?

They rely on different principles and have different sources of uncertainty. The meter measures product as it flows, while tank gauging calculates volume from level, the tank capacity table and average temperature. Whether connecting lines are full, temperature stratification in the tank and the timing of readings all affect the result. Comparisons should use values converted to the same reference basis, with the acceptable difference defined in advance.

Does unloading with the truck's own pump affect measurement?

It can. With a truck pump, flow rate and pressure vary from vehicle to vehicle, and air ingress at the end of unloading may be less predictable. With a terminal pump, suction conditions are set by the terminal. In both cases, the air eliminator and the end-of-unloading stop logic are the main factors that determine how the measuring system behaves.

What happens if the wrong product goes into a tank?

It depends on which products mixed and in what proportion. A small amount of gasoline in diesel can affect the flash point and safety properties, while in jet fuel even a very small intermix can take the product out of service. Contaminated product is usually downgraded into a lower-grade product based on laboratory analysis or segregated for reprocessing. That cost is what justifies investing in preventive layers.

Does every unloading line need its own meter?

Not necessarily. With few products and infrequent deliveries, a shared metering line can be routed per product, but the volume left in the line at each change and a separate meter factor for each product must then be taken into account. Where intermix is unacceptable or deliveries are frequent, dedicated lines per product are preferred.