Whether top loading is acceptable at a fuel terminal comes down to three questions: which product is being loaded, which vapor control rules apply to it, and whether the truck fleet is equipped for bottom loading. For volatile products such as gasoline, top loading without vapor collection is restricted by regulation in some regions, the EU being one example. Top loading can still make sense for less volatile products such as diesel, at sites serving trucks without bottom-loading adaptors, or at existing terminals planning a phased conversion. In those cases the goal is to make the system safer and cleaner with respect to vapor.
The general engineering of top loading arms, drop tubes and platforms is covered on the top loading systems page. This page deals with fuel-specific issues: gasoline vapor, splash loading, static electricity in low-conductivity fuels and the decision to convert to bottom loading.
Where top loading of fuel is still found
· Trucks without bottom-loading equipment: Tankers whose compartments lack API adaptors, vapor collection adaptors and overfill sensors can only be filled through the hatch.
· Low-volatility products: Gasoline vapor rules usually target gasoline; bays loading diesel or heating oil may be subject to different vapor requirements, which should be confirmed against local regulations.
· Existing sites in transition: Where conversion is phased, some bays continue top loading for a period.
· Rail tank cars: Top loading arms are used for rail cars as well as road tankers.
Gasoline vapor: from open hatch to closed circuit
As a compartment fills, the air and fuel vapor inside are pushed out. With an open hatch, that mixture goes straight to atmosphere, at the height where the operator is working. For gasoline this means both volatile organic compound emissions and a flammable atmosphere.
As a regional example, EU Directive 94/63/EC on petrol storage and distribution requires vapors displaced during loading to be returned through a vapor-tight connection to a vapor recovery unit at the terminal, and requires terminals with road tanker loading to provide gantries meeting its bottom-loading specification. In the US, federal performance standards for bulk gasoline terminals require the terminal's and the truck's vapor collection systems to be connected during every loading. Whatever the project location, the local emissions regulation is what counts.
Vapor can be collected during top loading: arms with vapor collection heads that seal the hatch opening are available, including variants with vapor return lines for gasoline. How tight that seal is depends on the hatch type and on the operator seating the head correctly. What then happens to the collected vapor, whether a vapor recovery unit (VRU) or a vapor combustion unit (VCU), is a separate decision covered on the vapor recovery systems page.
Avoiding splash loading
Letting product fall freely into the compartment (splash loading) agitates the liquid surface and creates droplets, which increases vapor generation and electrostatic charging. Directive 94/63/EC requires that, where top loading is permitted, the arm outlet be kept near the bottom of the compartment. Fuel terminals therefore use drop tubes sized to compartment depth, and starting at reduced flow until the tube outlet is submerged is common practice.
Static electricity and switch loading
Low-conductivity fuels such as diesel and jet fuel can pick up electrostatic charge during flow and release it slowly. Grounding the truck and arm, withholding the loading permissive until ground is verified, and limiting flow at the start of loading are therefore basic precautions. Loading diesel into a compartment that last carried gasoline (switch loading) needs particular care, because residual gasoline vapor combined with charge carried by the low-conductivity product can raise the ignition risk. Flow limits and procedures are set from industry guidance on static and ignition prevention and the terminal's own safety assessment.
Overfill prevention and operator safety
In bottom loading the overfill sensors sit on the truck itself. In top loading the same job usually relies on a level probe mounted on the arm or hatch head together with the preset quantity. Wiring that probe into the batch controller or bay permissive system reduces reliance on the operator watching the level.
Top loading puts the operator at height on top of the tanker, so the platform, folding gangway, fall protection and monitoring of the arm's parked position all form part of the safety chain. Fixed-reach arms need accurate truck spotting, so bays serving vehicles of different sizes need arms with suitable reach. Because bottom loading is done at ground level, it removes much of this risk.
Upgrade options for an existing top loading bay
· Drop-tube or telescopic arms to eliminate splash loading
· Vapor collection heads on gasoline bays, tied into the terminal vapor line
· A level probe on the arm or head, wired into the loading permissive
· Permissive logic that blocks loading until truck grounding is verified
· A staged low, high and low flow profile in the batch controller
· Review of platform, gangway and fall protection arrangements
· Arm parked position and gangway position added to the permissive chain
Deciding whether to convert to bottom loading
Decision factor | Why it matters | Effect on conversion |
Product and regulation | Gasoline vapor rules may make conversion mandatory | Gasoline bays first; low-volatility bays assessed separately |
Fleet equipment | Bottom loading needs adaptors, vapor adaptor and overfill sensors on the truck | Fleet conversion must be planned alongside the terminal |
Vapor handling | Collected vapor needs a processing unit or balancing line to go to | Timing of VRU or VCU investment |
Bay capacity | Bottom loading can fill several compartments at once | Arm count and pump capacity are recalculated |
Existing structure | Canopy, platforms and piping may be reused or may be in the way | Phased conversion or bay-by-bay renewal |
Continuity of operations | Deliveries must continue during conversion | Bay-by-bay transition plan and temporary mixed operation |
During a mixed period the same terminal may run both top and bottom loading bays. Having the terminal automation system check product-to-bay and truck-to-bay compatibility reduces the risk of a truck being sent to the wrong bay.
Where TLY Enerji fits in
TLY Enerji provides system design, product supply, engineering, installation, commissioning and maintenance for fuel tanker loading and unloading facilities. At top loading bays that can mean assessing the existing system, improving loading control and permissive logic, supplying and commissioning flow measurement and control equipment, or engineering support for conversion to bottom loading. Whether items such as loading arms, platforms or a vapor processing unit are included is agreed project by project.
What we need to assess your bays
· Products loaded at each bay, and which bays handle gasoline
· Hatch types, compartment counts and bottom-loading readiness of the fleet
· Whether the terminal has a vapor collection header and a vapor processing unit
· Existing loading arms, platform and gangway arrangement
· Current batch controller, grounding and overfill permissive setup
· Target schedule and continuity requirements if conversion is planned
Related pages
· Top loading systems: Fluid-independent engineering of arms, drop tubes and platforms.
· Bottom loading for fuel terminals: Adaptor, vapor return and overfill interfaces after conversion.
· Vapor recovery and vapor return systems: Processing collected vapor in recovery or combustion units.
· Grounding and overfill prevention systems: Tying ground verification and overfill signals into the permissive chain.
· Fuel truck loading systems: Product segregation, additive injection and loading rate limits.
Questions about top loading of fuel
Can gasoline still be top loaded?
That depends on the emissions regulation where the terminal operates. In the EU example, vapors displaced during gasoline loading must be collected and sent to a vapor recovery unit, and gantries meeting bottom-loading requirements are expected; where top loading is still permitted, the arm outlet must be kept near the compartment bottom. Confirm what local rules require for gasoline before making an investment decision.
Does a diesel bay need vapor collection too?
Diesel has a much lower vapor pressure than gasoline, so rules aimed at gasoline vapor do not always cover diesel bays. However, if the same compartment carried gasoline on its previous trip, gasoline vapor can still be present. Fleets that switch products should therefore assess vapor and static risks together rather than by product alone.
Does the type of top loading arm affect measurement accuracy?
Measurement is done by the meter and batch controller upstream of the arm, so the arm type does not set accuracy directly. Vapor and foam from splash loading do make visual checks of the compartment harder, and how product left in the arm is drained at the end of loading can affect the quantity that physically reaches the truck. Custody transfer requirements are assessed separately.
Must all bays be converted to bottom loading at once?
No. In a phased approach, gasoline bays and lanes used mostly by bottom-loading trucks can be converted first while other bays continue top loading for a time. A phased transition needs the capacity of the vapor processing unit, the fleet conversion schedule and the product mix per bay to be planned together.