Receiving product from a tanker into storage raises two questions: what will move the product out of the truck, under what suction conditions, and how will the received quantity be verified? A truck unloading system is designed to answer both together. Unlike loading, the critical side is suction: as the compartment empties, air or vapor enters the line, the pump edges toward cavitation, and if the transfer is metered, the meter starts to see gas instead of liquid.
This page covers the driving force for unloading, suction line design, air and vapor elimination, metering of the received quantity and reconciliation with tank levels. Loading and custody transfer governance are covered on their own pages.
What pulls the product out of the tanker?
The unloading arrangement depends on site infrastructure, the truck fleet and the product. The options below are not mutually exclusive, and some sites use more than one.
Arrangement | How it works | What to watch |
Site pump | A fixed unloading pump draws product through a hose or arm connected to the tanker's bottom outlet | Suction line length and losses directly set the pump's suction conditions |
Truck-mounted pump | The vehicle's own pump pushes product into the site receiving line | Flow and pressure vary from truck to truck; the receiving side must tolerate this |
Gravity | Product flows to a lower receiving tank or pump suction on its own | Available elevation is limited and flow falls as the truck empties |
Liquefied gases under pressure are unloaded with different methods, such as vapor balancing and compressor-driven transfer; those applications are covered on the relevant industry page.
Suction side: cavitation and pump inlet conditions
When pressure at the pump inlet approaches the product's vapor pressure at operating temperature, liquid starts to vaporize at the impeller. Collapsing vapor bubbles erode the impeller and cut pump performance, so the available suction head must stay above the pump's requirement with an adequate margin. Unloading puts that margin under particular pressure because:
· The liquid level keeps falling: Static head at the pump inlet drops as the tanker empties.
· Suction lines are often long and include hoses: Friction in hoses, elbows, strainers and valves lowers suction pressure.
· Product temperature varies: A tanker left standing in the sun can warm up, raising vapor pressure.
· Strainers foul over time: Rising differential pressure across a suction strainer quietly increases cavitation risk.
Keeping the suction line short and generously sized, monitoring strainer differential pressure, and reducing pump flow in the final stage are the main design decisions that limit this risk.
Air and vapor at the end of unloading: the main source of metering error
In the last minutes of unloading, the liquid level approaches the outlet, a vortex forms and air is drawn into the line. For metered transfers this is the critical moment: if the meter counts air or vapor as liquid, the received quantity is overstated. Metrological requirements for liquid measuring systems therefore list the gas elimination device among the system's components, and unloading metering skids typically carry an air eliminator and strainer as standard equipment.
· Air/vapor eliminator: Separates and vents air upstream of the meter; it can be arranged to stop flow when it fills with gas or the liquid level drops.
· Low-flow or low-pressure detection: Recognizes the end of the truck and shuts down the pump and metering in a controlled way.
· Keeping the meter full: A layout that keeps the meter and downstream line full lets each unloading start from the same condition.
· Confirming the compartment is empty: The operating procedure should include this check so the metered quantity covers everything in the tanker.
How is the received quantity verified?
Three quantities are compared during unloading: the figure on the tanker's delivery document, the quantity measured by the unloading meter, and the quantity calculated from the level change in the receiving tank. They are not expected to match exactly; what matters is understanding where the differences come from.
· Temperature: If the document, meter and tank readings were taken at different temperatures, volumes are comparable only after conversion to a reference temperature.
· Line fill: Whether the unloading line and tank inlet start empty or full creates a difference between meter and tank.
· Tank gauging resolution: In a large-diameter tank, a small level change may not resolve a single truckload precisely.
· Unmeasured air: Inadequate gas elimination makes the meter read high.
Dynamic metering and static tank gauging complement rather than replace each other, and terminal stock reconciliation uses both. If the unloading measurement is the basis for invoicing, meter proving, sealing and legal metrology requirements need separate assessment.
Selection criteria
Parameter | Why it matters in design | Effect on selection |
Vapor pressure and temperature | Sets the tendency to vaporize and cavitate on the suction side | Pump type, suction line size, air eliminator sizing |
Viscosity | Drives suction losses and unloading time | Pump selection, heating needs, meter technology |
Fleet outlet connections | Defines the site-side interface | Hose or arm, coupling type, breakaway coupling |
Target unloading time | Sets truck waiting time and pump capacity | Flow rate, line size, meter size |
Purpose of measurement | Inventory tracking, supplier checks and invoicing need different rigor | Air eliminator type, proving connections, record keeping |
Receiving tank and gauging | Core data for reconciliation and tank overfill protection | Level instrument, alarms and interlock logic |
Hazardous area classification | Sets the protection method for the pump motor and electrical equipment | Motor, transmitter and panel selection |
Safety and environmental aspects of unloading
The tanker is grounded during unloading and ground continuity is monitored; hoses and couplings are selected for the product. For volatile products, returning displaced vapor from the receiving tank to the tanker is assessed against the project and applicable emissions regulations. Protecting the receiving tank against overfill is also part of the unloading system; it differs from tanker compartment overfill prevention and is built as an interlock tied to the tank's level measurement. Which functions are designed as safety functions is decided by the project risk assessment.
TLY Enerji's role in unloading projects
Starting from your product and site data, TLY Enerji provides engineering support for selecting the pump, flow meter, air eliminator and level measurement. Depending on the project, scope can include equipment supply, integration of the pump and metering system with the PLC or SCADA, site installation, testing, commissioning and maintenance support. The scope of supply is defined together through the technical specification and project data.
What to prepare for an unloading system inquiry
· Products to be unloaded, their vapor pressure or volatility and temperature range
· Fleet type, number of compartments and bottom outlet connection sizes
· Whether the truck-mounted pump or a site pump will be used
· Distance and elevation difference between the unloading point and the receiving tank
· Target unloading time and trucks per day
· How the received quantity will be used: inventory, supplier checks or invoicing
· Existing level measurement and overfill protection on the receiving tank
· Whether vapor return or emissions control is required
Related pages
· Truck loading systems: The loading point chain when delivering from storage into tankers.
· Custody transfer truck loading skids: Proving, sealing and records for sales measurement.
· Tanker truck loading and unloading terminals: Overview of loading and unloading solutions.
· Fuel truck unloading systems: Product verification and inventory reconciliation for fuels.
Questions about truck unloading
Is a meter mandatory on an unloading line?
No. Some sites determine the received quantity only from the receiving tank level change or a weighbridge. However, when several trucks discharge into the same tank in succession, when deliveries are checked per supplier, or when the measurement is the basis for invoicing, a meter on the unloading line improves traceability. The choice depends on the purpose of measurement and the resolution of tank gauging.
Why can the meter read high at the end of unloading?
As the compartment empties, a vortex forms at the outlet and air is drawn into the line. Without an air eliminator, or with an undersized one, that air passes through the meter and can be counted as liquid. A correctly sized air eliminator, low-flow or low-pressure detection of the empty truck, and reduced flow in the final stage keep this error in check.
Truck pump or site pump: how do you choose?
A truck-mounted pump reduces the need for a fixed pump on site, but flow and pressure vary between vehicles, and that variability affects metering and receiving line design. A site pump gives more controlled operation but requires careful suction-side design. Fleet makeup, product and daily truck count usually decide the matter.
When should a difference between meter and tank gauging be investigated?
The trend matters more than any single difference. Once all quantities are converted to the same reference temperature, a difference that keeps growing in one direction, or that concentrates on particular trucks or products, calls for investigation. Check the air eliminator operation, meter proving records, line fill assumptions and the calibration of the tank gauge in turn.
Why does unloading take longer with viscous products?
Higher viscosity increases friction losses in the suction line and makes the pump inlet conditions harder to meet. Viscous products therefore call for larger suction lines, positive displacement pumps and, where needed, heating to keep the product fluid. Any heating must stay below the product's allowable upper temperature limit.