Bottom loading fills each tank truck compartment through standardized adaptors at the underside of the vehicle, with the operator making every connection from ground level. Whether a bottom loading bay works smoothly depends less on the pump and meter than on three truck-to-terminal interfaces matching each other: the liquid adaptor and loading coupler, the vapor collection connection, and the overfill sensor socket. If any one of them is mismatched, either no loading permissive is issued or vapor displaced from the compartments escapes uncontrolled.
The pump-to-arm loading chain is covered on the truck loading systems page and the full comparison with top loading on the top loading page; this page concentrates on the truck-to-terminal interface and vapor return.
How a bottom loading sequence runs
Safety permissives are established before any product flows. A typical order is shown below; the detailed procedure is set by the terminal itself:
1. The truck is spotted in the bay, secured, and the ground connection is made.
2. The truck's overfill sensor cable is plugged into the rack monitor, which checks the sensors and the ground circuit.
3. The vapor return hose or arm is connected to the truck's vapor collection adaptor.
4. A bottom loading arm is coupled to the liquid adaptor of each compartment to be filled, and the compartment-to-product assignment is checked against the order.
5. Once all permissives are present, the batch controller delivers the preset quantity; loss of any permissive stops the flow.
6. At the end of loading the couplers are closed, arms are returned to their parked position, and the vapor and sensor connections are removed.
Because product enters from below, the rising liquid pushes the vapor space toward the top of the compartment and into the collection line. With no open manhole as in top loading, the vapor connection is the only exit path, which is why the vapor line is an integral part of a bottom loading system rather than an accessory.
Three interfaces between truck and terminal
Liquid adaptor and loading coupler
API RP 1004 sets out uniform vehicle features around a standard 4-inch adaptor so that trucks from different carriers can be loaded with the same equipment at terminals shared by several suppliers. In Europe, EN 13083 defines performance requirements and critical dimensions for bottom loading and unloading adaptors on the tank, and the EU petrol vapor directive 94/63/EC specifies a terminal-side liquid coupler that mates with a 4-inch API male adaptor. The practical consequence is that coupler selection is dictated by the fleet that will use the bay, not by terminal preference alone.
Vapor collection connection and vapor return line
EN 13081 defines the vapor collection adaptor on the tank and the mating coupler on the fixed installation, together forming a vapor-tight path. Directive 94/63/EC also caps the back pressure allowed at the vapor collection adaptor. The vapor line diameter and routing are therefore sized together with the total simultaneous loading rate per bay; an undersized line can end up limiting the loading rate from the vapor side.
Two concepts need to be kept apart. US EPA rules define a vapor collection system as the equipment that carries vapors displaced during loading, and a vapor processing system as the equipment that recovers or oxidizes them, and they require the terminal and tank truck vapor collection systems to be connected during each loading. A vapor return line is a transport path; whether the collected vapor goes to a recovery unit or a combustion unit is a separate investment and regulatory decision.
Overfill sensor and loading permissive
The operator cannot see inside the compartment, so overfill prevention relies on a level sensor in each compartment and a rack monitor at the terminal. Directive 94/63/EC lists 2-wire thermistor, 2-wire optic and 5-wire optic sensors, or compatible fail-safe equivalents, on the vehicle side, while EN 13922 sets technical requirements for overfill prevention systems on tank vehicles. The rack monitor must recognize the sensor types present in the fleet and must refuse to start loading if it detects a fault in its own circuit. In many installations the same unit also verifies truck grounding and can add a vehicle identification permissive.
Compartment loading and arm arrangement
Bottom loading allows several compartments to be filled at the same time, which is the main lever for shortening time in the bay. The number of arms per bay is set by the typical compartment count, whether one truck receives more than one product, and which product header each arm is tied to. When different products go into the same truck, verifying the compartment-to-product match against order data becomes important. EN 14116 defines a digital interface for product recognition devices between tank and terminal; whether such an interface is used depends on how the fleet is equipped.
Operator safety and bay layout
The operator works at grade, so climbing onto the truck, gangways and fall protection largely disappear from the design. The emphasis moves to the ground: bay slope, spill collection and drainage, reach to connection points, and accurate vehicle positioning. Verifying truck grounding against static electricity and selecting equipment for the bay's hazardous area classification are natural parts of the interface design.
Design criteria
Parameter | Why it matters in design | Effect on selection |
Adaptor and sensor equipment of the truck fleet | If terminal couplers and rack monitor do not match the fleet, the truck cannot be loaded | Coupler type, supported sensor types and transition arrangements |
Product group and vapor pressure | Adaptor standards cover defined product classes; vapor load varies by product | Need for vapor return, seal and coupler materials |
Compartments filled simultaneously | Sets the number of arms per bay and the time in the bay | Arm count, product header connections, controller channels |
Flow rate per arm and per bay | Drives vapor line back pressure and surge risk | Arm size, vapor line size, control valve characteristic |
Vapor processing method | Where the collected vapor goes is a separate investment decision | Vapor line routing and interface to the processing unit |
Applicable regulations | Petrol vapor and tank vehicle rules differ by region | Mandatory interfaces, testing and record requirements |
Hazardous area classification | Determines the protection method for electrical equipment around the bay | Rack monitor, arm position switch and cabling selection |
How TLY Enerji supports bottom loading projects
Depending on project scope, TLY Enerji provides engineering support for defining interface equipment from fleet and product data, supplying loading arms, valves, flow meters and instruments, integrating permissive signals into the control system, and site installation, testing and commissioning. Which side supplies the arms, couplers, rack monitor and vapor line is agreed on the basis of the technical specification.
Information we need to prepare a proposal
· Products to be loaded and whether vapor return is required for each
· Adaptor standard, compartment count and overfill sensor types of the incoming fleet, and the mix if the fleet is not uniform
· Number of compartments and arms to be loaded simultaneously per bay
· Target flow rate per arm and expected trucks per bay per day
· Existing or planned vapor processing facility and tie-in point
· System that will receive the permissives: batch controller, PLC or terminal automation system
· Bay layout and hazardous area classification
Related pages
· Top loading systems: How the two methods compare on safety, vapor and ergonomics.
· Truck loading systems: Engineering of the loading chain from pump to loading arm.
· Multi-bay truck loading terminals: Planning bays and product arms at terminal scale.
· Bottom loading at fuel terminals: Gasoline vapor regulations and fuel fleet interface issues.
· Vapor recovery and vapor return systems: Choosing how collected vapor is processed.
Frequently asked questions
Are vapor return and vapor recovery the same thing in bottom loading?
No. Vapor return is the transport of vapor displaced from the compartment through the truck's vapor collection adaptor and the terminal line. Vapor recovery is the processing of that vapor in a unit that recovers it; combustion is another processing option. A terminal can have a vapor return line and handle the vapor by a method other than recovery. Which method is required depends on the product and the applicable regulations.
What happens if the truck's overfill sensors are not compatible with the rack monitor?
If the monitor cannot recognize the sensors, or detects a fault in the circuit, no permissive is issued and the bay will not deliver product to that truck. That is why the sensor types in the fleet are listed at the design stage and checked against what the rack monitor supports. Where the fleet is mixed, monitors that recognize more than one sensor type are considered.
Is bottom loading also used for LPG tank trucks?
The adaptor, vapor coupler and overfill sensor interfaces described here are defined for liquid petroleum products carried close to atmospheric pressure. LPG is transported as a liquid under pressure, so the tank equipment, connections and vapor balancing logic are governed by different standards. Metering and vapor return in LPG loading need their own assessment, which is covered on the LPG terminal pages.
Why is ground verification handled together with overfill prevention?
Both are permissives that must be in place before loading starts, and both are hooked up during the same connection routine at the bay. Some rack monitors combine overfill prevention, static ground verification and optional vehicle identification in one self-checking unit that blocks loading on a fault, so the permissive logic sits at one point and reaches the batch controller or PLC as a single signal.