Quick answer: When a tank truck or drum is filled, incoming liquid displaces vapor that must go somewhere. Vapour recovery at loading captures that displaced vapor through a vapor collection line and either balances it back to the source tank (vapor balancing) or routes it to a recovery unit. Bottom-loading racks with dry-break vapor couplings, overfill protection, and vapor collection are the standard for tank trucks; drum and IBC stations use local capture hoods or lances. Systems are sized for the loading rate, and safety devices prevent flammable mixtures and overfill.
Key figures at a glance
- Application: truck/tank-truck loading racks and drum filling
- Stage 1 principle: displaced vapour returned during loading (vapour balancing)
- Recovery: adsorption/absorption units at the loading rack
- Benefit: recovers product and meets loading-emission limits
- Design point: sized to peak loading vapour flow
Loading operations are one of the most visible VOC sources on a terminal because they are frequent, fast, and open to the atmosphere unless controlled. This guide covers where the vapors escape, the two main control strategies, and what a compliant loading-vapor system looks like.
Where vapors escape during truck loading and drum filling
The loss is displacement-driven: every litre of liquid loaded pushes out roughly a litre of vapor-laden gas from the receiving vessel. With volatile products that displaced gas is VOC-rich. Splash (top) loading makes it worse by generating extra vapor through agitation, which is why bottom loading is preferred. Drum and IBC filling releases vapor from the open bung or fill point unless it is captured locally.
Vapor balancing vs vapor recovery
There are two distinct strategies, and they are often confused:
- Vapor balancing returns the displaced vapor from the vessel being filled straight back into the vapor space of the tank supplying the liquid. No recovery unit is needed — the vapor simply moves from one vessel to the other as liquid moves the opposite way. It is simple and effective for closed transfers.
- Vapor recovery collects the displaced vapor and sends it to a recovery unit (condensation, absorption, adsorption, or compression) that turns it back into product. This is used where balancing is not possible or where emissions limits require actual recovery rather than transfer.
Many terminals use balancing during tanker delivery to storage (Stage 1-type transfer) and recovery at the truck loading rack.
Loading rack vapor collection systems
A modern truck loading rack uses bottom loading with dry-break liquid couplers and a matching vapor collection coupler. As product flows in, vapor is drawn out through the vapor hose into a collection header and on to balancing or recovery. Overfill protection (optic or thermistor sensors in the truck) and grounding/bonding are interlocked with the loading permissive so product cannot flow unless vapor collection and safety systems are connected and healthy.
Drum and IBC filling: local vapor capture
Drum and IBC stations cannot use a sealed vapor coupler the way a truck can, so they rely on local capture: a fill lance with an integral vapor extraction port, or a capture hood over the fill point drawing vapor to a small recovery or abatement unit. The goal is the same — collect the vapor at the point of release before it reaches the operator’s breathing zone or the atmosphere.
Sizing and safety considerations
Loading-vapor systems must be sized for the peak loading rate, because that sets the peak vapor flow — not the daily throughput. Undersized collection lines create back-pressure that pushes vapor out around seals. Safety design covers flammability (keeping the vapor stream managed and, where needed, arrestors between rack and recovery), static control through bonding and grounding, and overfill prevention. All of it is interlocked so a fault stops loading.
Emission regulations for loading operations
Loading emissions are regulated across the EU and elsewhere: terminals loading petrol and volatile products are generally required to control loading-rack vapors to defined limits, with recovery or destruction and periodic testing to prove performance. This is why vapour recovery at the rack is usually a permit condition rather than an optional efficiency measure.
Loading-rack and drum-filling design factors
Truck-loading and drum-filling vapour recovery lives or dies on how well the collection side is engineered. Vapour has to be captured at the point of displacement — the loading arm, the drum bung, or the fill nozzle — with tight vapour-tight connections, because any vapour that escapes collection can never be recovered downstream. Bottom loading with vapour recovery couplings is the norm for road tankers; drum and IBC filling needs dedicated extraction hoods or lances.
The recovery unit is then sized to the rack’s peak simultaneous loading, allowing for how many bays load at once. Under-sizing shows up as vapour breakthrough during busy periods; over-sizing wastes energy and can pull air into the system, raising the oxygen level and the explosion risk. Stage 1 vapour balancing — returning displaced vapour to the supplying tank during transfer — is often used alongside active recovery to reduce the duty.
For volatile products, the captured vapour stream is close to saturation, so knockout of entrained liquid ahead of the recovery stage protects the equipment and keeps recovery efficiency high across the loading cycle.
Keeping loading-rack recovery efficient across the duty cycle
Loading racks work in bursts, and a vapour recovery system that performs at peak loading can still lose efficiency during the quiet periods between trucks — so turndown is as important as peak capacity. A unit sized only for maximum simultaneous loading may cycle or run inefficiently when a single bay is active, while one that cannot handle the peak lets vapour break through during busy periods. Matching the recovery unit to the rack’s full duty cycle, not just its maximum, is what keeps capture high throughout the day.
The collection side has to hold up across that cycle too: vapour-tight loading couplings, well-maintained hoses and seals, and effective liquid knockout ahead of the recovery stage keep efficiency stable whether one bay or several are loading. Stage 1 vapour balancing, returning displaced vapour to the supplying tank, reduces the active recovery duty and helps the unit stay in its efficient range.
The practical result of designing for the whole duty cycle is a rack that meets its emission limit continuously and recovers product on every transfer, rather than one that passes a peak test but leaks vapour during the intermittent real-world operation that defines a loading facility.
Frequently asked questions
What is a tank truck vapor recovery system?
It is the vapor collection and recovery equipment at a loading rack that captures the vapor displaced when a tank truck is filled and either balances it back to the source tank or sends it to a recovery unit, instead of venting it.
What is the difference between vapor balancing and vapor recovery?
Vapor balancing returns displaced vapor directly to the supplying tank’s vapor space; vapor recovery collects the vapor and processes it back into product. Balancing transfers vapor between vessels, recovery actually recovers it.
Why is bottom loading better for vapour recovery?
Bottom loading fills from beneath the liquid surface, generating far less vapor than splash (top) loading and allowing a sealed vapor coupler to collect the displaced vapor — which top loading through an open hatch cannot do cleanly.
Related Rodanco resources: See Rodanco’s vapour recovery unit overview and industrial vapour recovery systems page for the recovery technology behind loading-rack systems.
Where Rodanco fits: Rodanco advises on industrial and loading vapour recovery — matching collection and recovery to the loading rate and the applicable emission limits. See our Industrial Vapor Recovery Systems and Vapour Recovery Unit pages, or contact the Rodanco team through the contact page.