LPG Truck Loading Systems

An LPG tanker is loaded through a closed circuit with a liquid line and a vapor line connected at the same time. Liquefied petroleum gas (LPG) stays liquid at ambient temperature only under pressure, so the product is never open to atmosphere: liquid flows from storage into the truck while vapor compressed in the truck returns to storage through a separate line. Controlled loading also depends on stopping at the permitted fill limit, measuring the quantity by meter or weighbridge, and an emergency shutdown chain ready to act at any moment.

The generic loading chain of pump, strainer, meter, control valve, loading arm and batch controller is covered on the Truck Loading Systems page. This page looks at how that chain changes when the product is pressurized and sits close to its boiling point.

What makes LPG loading different from fuel loading

Propane and butane are gases at atmospheric pressure and liquids at higher pressure, and they vaporize as soon as pressure drops. Three consequences follow. Every arm, hose and coupling is pressure equipment rated for storage and tanker conditions. There is no open top loading; storage vessel and truck remain two connected pressure vessels throughout the transfer. And any local pressure drop, at the pump suction, in the meter or across a valve, can cause flashing of part of the liquid.

Closed loading with liquid and vapor return lines

As the truck fills, the vapor inside it is compressed and its pressure rises. The vapor return line sends this vapor back to the storage vessel and limits the pressure difference between the two, so the loading rate is not throttled by rising back pressure and no vapor is released to atmosphere. Line size, connection arrangement and the commercial treatment of returned vapor are separate questions: sizing is discussed on the LPG Vapor Return Systems page, and the effect on invoiced quantity on the LPG custody transfer page.

Valves, couplings and safety fittings on the truck itself are defined by standards for equipping LPG road tankers. The arms or hoses on the terminal side have to match that equipment in size, coupling type and pressure rating, so the fleet data should be collected and checked at the start of the project.

Fill limit: why a tanker is never filled liquid-full

The volume of liquid LPG changes markedly with temperature. In a pressure vessel filled completely with liquid, a temperature rise leaves no room for expansion and pressure climbs quickly. Transport regulations therefore cap the amount of LPG a tank may hold relative to its water capacity. US transport rules, for example, tabulate the maximum filling density against the product's specific gravity and the tank size, which means the permitted quantity depends on composition. The limit applied on a given project comes from the national dangerous goods regulations in force.

The loading system must enforce that limit reliably. The preset is calculated from the truck's water capacity and product density, and the batch controller closes the flow in steps when it is reached. The truck's fixed maximum level indicator is an independent layer: when it responds, loading stops. Where the propane and butane ratio changes seasonally, a current density value matters for both the fill calculation and the quantity record.

Quantity determination: meter or weighbridge

A weighbridge weighs the truck before and after loading and gives mass directly, at the cost of weighing time and scale capacity for every load. A metering skid in the loading line measures flow during transfer and gives the batch controller its cut-off signal. Direct mass meters are often considered because LPG is commonly traded by mass, and where both methods exist the weighbridge also serves as a comparison. The back-pressure arrangement that keeps LPG liquid inside the meter is explained on the LPG Metering Skids page.

Pump suction and NPSH margin

LPG in storage sits very close to its boiling point. If pump inlet pressure falls below the product's vapor pressure, part of the liquid vaporizes, causing cavitation, unstable operation, lost capacity and an inability to draw the tank down. Net positive suction head (NPSH) available is therefore decisive for the loading pump. Vessel elevation, a short low-loss suction line, strainer and valve losses and minimum-flow protection are evaluated together. LPG is also thin and poorly lubricating, so pump type and seals must suit the service.

Safety equipment at the loading point

·         Remotely operated shutoff valves: Check, excess-flow or remotely closable shutoff valves close to the hose or arm connection on both liquid and vapor lines, to prevent uncontrolled release.

·         Breakaway couplings: Define where the hose or arm separates if a connected truck moves, and seal both ends to limit the release.

·         Grounding and static control: No loading permissive until the truck is connected and ground verification is confirmed.

·         Interlocks and permissives: Arm position, vehicle position and ground status are wired to the batch controller as permissive signals.

·         Attendance: LPG regulations require a qualified attendant at the transfer point from connection until disconnection.

Which functions are tied into the emergency shutdown system, and the reliability they need, is derived from the risk assessment. That topic is covered in detail on the page about ESD and safety interlock systems for LPG terminals.

Design criteria for LPG loading

How TLY Enerji supports LPG loading projects

TLY Enerji provides design, product supply and engineering services for LPG truck loading and unloading facilities. On a loading project this can cover the selection and supply of pumps, flow meters, pressure reducers, safety valves and control systems, together with the automation concept. Depending on project needs, site installation, testing, commissioning and follow-up technical support are also considered. The scope boundary, from the truck connection interface to the control system, is agreed for each project.

Information we need to scope your loading system

·         Product to be loaded and how its blend changes over the year

·         Number, capacity and design pressure of storage vessels, and their elevation relative to the pumps

·         Daily truck count, target loading time and number of bays

·         Tanker types and the size and type of their liquid and vapor connections

·         Whether quantity is set by meter, weighbridge or both

·         Existing emergency shutdown, gas detection and control system infrastructure

·         Plot plan and hazardous area classification

Related pages

·         LPG vapor return systems: The line that carries displaced vapor from the truck back to storage, and how it is sized.

·         LPG loading and unloading skids: Supplying the loading equipment as a pre-assembled package.

·         ESD and safety interlocks for LPG terminals: Emergency shutoff valves and interlock logic in detail.

·         LPG metering skids: How the meter in the loading line keeps the product in the liquid phase.

·         Truck loading systems: The product-independent loading chain and staged flow profile.

LPG loading FAQ

Is a vapor return line mandatory for LPG loading?

In closed loading, the vapor line limits the pressure rise in the truck so loading finishes in a reasonable time, and it keeps vapor out of the atmosphere. Whether it is used depends on tanker and vessel design, the applicable regulations and the custody transfer arrangement. If a vapor line is used, the contract should also state how the returned vapor is treated in the commercial quantity.

Who decides how full an LPG tanker can be?

The maximum fill is set by dangerous goods transport regulations together with the tank manufacturer's data. Regulations express the permitted LPG quantity relative to the tank's water capacity, usually as a function of product density. The loading system applies that value as a preset, while the truck's maximum level indicator remains an independent check.

If we have a weighbridge, do we still need a meter?

Not necessarily. A weighbridge gives mass on its own and serves as the primary measurement at many sites. A meter, however, allows automatic cut-off at a set quantity, simultaneous loading at several bays and real-time records. The choice depends on truck count, scale capacity, custody transfer requirements and the automation target.

Why do LPG loading pumps wear out quickly?

The most common cause is poor suction conditions. When inlet pressure drops below the product's vapor pressure, the liquid vaporizes, cavitation sets in and seals and bearings deteriorate fast. Elevation between vessel and pump, suction line losses and minimum-flow protection should be checked together at the design stage.

Does a breakaway coupling replace an emergency shutoff valve?

No. The two devices do different jobs. A breakaway coupling ensures that the connection parts in a controlled way and both ends close if the truck moves. Emergency shutoff valves stop flow on events such as gas detection, fire or an operator command. At LPG loading points both are normally evaluated together.