LPG Truck Unloading Systems

There are two basic ways to unload an LPG tanker: draw the liquid out with a pump and push it into storage, or use a compressor to take vapor from the storage vessel, push it into the truck's vapor space and let the resulting pressure difference drive the liquid across. The right choice depends on the truck's connection layout, the pressure conditions on the suction side, whether the vapor left in the truck at the end should be recovered, and the unloading time you can accept. At most sites the decision rests on all four together rather than on a single criterion.

Product-independent unloading topics, such as suction-side problems, air elimination and reconciling the received quantity with tank levels, are covered on the Truck Unloading Systems page. Here the focus is on how LPG's nature as a pressurized liquid shapes the unloading method.

Unloading with a pump

In the pump method the truck's liquid outlet feeds the suction of a terminal pump, which discharges into the storage vessel; some arrangements use a pump mounted on the truck instead. When the vapor spaces of the two vessels are linked by a balance line, vapor displaced from storage by the incoming liquid flows back to the truck, which limits the pressure drop in the truck as it empties.

The weak point is the suction side. LPG in the truck is saturated, so every bit of friction loss, elevation difference or strainer resistance in the suction line can push the pump inlet below vapor pressure. The liquid then flashes at the inlet, cavitation begins and the last part of the load cannot be drawn out. The risk grows toward the end of unloading as the liquid level falls. Pump types that tolerate small amounts of vapor at the suction, such as sliding-vane pumps, are often considered for this duty, but a short, generously sized, low-resistance suction line remains decisive.

Unloading with a compressor

In the compressor method no liquid passes through a machine. The compressor draws vapor from the storage vessel's vapor space, compresses it and delivers it to the truck's vapor connection. Once truck pressure is slightly higher than storage pressure, liquid flows through a separate liquid line toward the lower-pressure side, into storage. Because the compressor handles only vapor, the NPSH issues that affect pumps do not arise.

When the liquid transfer is complete, the truck still holds pressurized vapor, and this is the second advantage of the method. By changing the valve alignment, the same compressor now draws vapor from the truck and sends it to storage, so a significant part of the remaining product is recovered instead of being vented or flared. Flow reversal is typically handled with an arrangement such as a four-way valve, and a liquid trap at the compressor inlet stops any liquid carried over in the vapor line from reaching the machine.

Pump or compressor?

Some facilities install both: the pump moves the bulk of the liquid, while the compressor handles vapor recovery at the end and situations where the pump struggles. That flexibility is especially useful at terminals receiving a mixed fleet with different connection layouts.

End of unloading: when is a truck empty?

After the liquid is gone, product remains in the truck as vapor under pressure, which is normal. Where vapor recovery stops is set by the operating procedure, taking into account the minimum internal pressure required by the truck owner and transport rules, the product and the time available. That limit should also be built into the compressor control logic so truck pressure is never reduced in an uncontrolled way.

How the received quantity is determined

If a metering skid is installed in the unloading line, keeping the product liquid in the meter is just as important as in loading. Toward the end of a load, vapor starts to arrive with the liquid, and this is the riskiest period for measurement error. In the compressor method, recovered vapor does not pass through the liquid meter, so it should be agreed in advance whether that quantity is captured by weighbridge difference, by separate measurement or by a contractual approach. Skid design is covered on the LPG Metering Skids page and commercial measurement on the LPG custody transfer page.

Safety at the unloading point

·         Shutoff on both lines: Remotely closable valves near the truck connection on the liquid and vapor lines, arranged to close together with the truck's internal valves.

·         Respect for truck design pressure: When pressurizing with a compressor, the truck's allowable pressure must not be exceeded; discharge pressure limits and relief valves are provided.

·         Hose and arm separation: Breakaway couplings, and safe draining of liquid trapped in the hose after unloading.

·         Grounding and permissives: Pump or compressor cannot start until grounding, vehicle position and connection status are confirmed.

The scope of the emergency shutdown chain and the interlock list follow from the risk assessment; details are on the page covering ESD and safety interlock systems for LPG terminals.

TLY Enerji's role in unloading projects

TLY Enerji evaluates the unloading method against your process data and can take on the supply of equipment such as pumps, flow meters, safety valves and control systems, together with the automation concept. Depending on project scope, engineering support also extends to installation, testing and commissioning. Which party supplies the compressor package, the metering skid and site piping is agreed project by project.

What to share when choosing an unloading method

·         Truck and rail car types arriving, and the position of their liquid and vapor connections

·         Daily number of unloadings and acceptable unloading time

·         Storage vessel design pressure, operating fill range and distance to the unloading point

·         Whether vapor recovery at the end of unloading is required

·         Whether the received quantity is set by weighbridge or meter

·         Existing pump or compressor equipment and power supply

Related LPG pages

·         LPG loading and unloading skids: Package scope for pump or compressor unloading equipment.

·         LPG vapor return systems: What the vapor line between truck and storage does.

·         LPG ESD and safety interlocks: Emergency shutoff and interlock logic at the unloading point.

·         Truck unloading systems: The product-independent unloading chain and quantity reconciliation.

Questions about LPG unloading

Is compressor unloading slower than pump unloading?

There is no general rule. With a compressor, transfer rate depends on the pressure difference created between truck and storage, the liquid line size and compressor capacity. With a pump, rate is set by pump capacity, but if suction conditions are poor the pump may not reach its expected flow. Any comparison should use the site's actual line and vessel data.

Why recover the vapor left in the truck?

Once the liquid is gone, product remains in the truck as pressurized vapor. Transferring it to storage with the compressor keeps that product at the site instead of venting or flaring it. How much is recovered is set in the operating procedure, taking into account the minimum pressure the truck must retain and the time available.

Can liquid reach the compressor?

Liquid that condenses or is carried along in the vapor line can reach the compressor and damage it. That is why a liquid trap is installed at the inlet, vapor lines are routed so liquid does not collect, and the pressure difference is not set higher than needed, since over-compressed vapor condenses more readily on cold surfaces.

If trucks have their own pumps, does the terminal need one?

A truck-mounted pump allows unloading without separate terminal equipment. A terminal may still prefer its own pump or compressor if it receives different vehicle types, wants vapor recovery, or wants unloading tied into its own control and interlock system. The decision follows fleet structure and the agreed split of responsibilities.