In LPG loading, the vapor return line balances pressure between two vessels by carrying the gas compressed in the tanker vapor space back to the vapor space of the storage vessel. Without it, liquid entering the tanker compresses the vapor above it: tanker pressure rises, the pump works against higher back pressure, and loading slows. The vapor return line is the basic means of keeping transfer closed, with nothing vented to atmosphere, and of keeping loading time reasonable. Sizing comes down to one question: can the line carry that vapor flow within an acceptable pressure difference?
How vapor balancing works
In both the storage vessel and the tanker, LPG exists as liquid with saturated vapor above it. During loading, liquid is drawn from storage and pumped into the tanker. The storage liquid level falls and its vapor space grows, while the tanker vapor space shrinks. When a vapor return line connects the two vapor spaces, excess vapor in the tanker flows to the storage vessel and fills the volume freed there. Pressures in the two vessels stay close, and the transfer is driven by the differential pressure the pump provides.
During unloading the direction reverses. As liquid leaves the tanker, its vapor space grows and pressure falls; the vapor line sends vapor from storage to the tanker to offset that drop. With compressor unloading, the vapor line becomes more than a balance line: the compressor draws vapor from storage and pushes it into the tanker to displace the liquid. The choice of method is covered on the LPG Truck Unloading Systems page.
What vapor return changes
· Loading time: Because tanker pressure does not build up, pump flow stays steadier throughout loading. With an undersized vapor line, tanker pressure climbs as loading progresses and the flow rate drops.
· Emissions and safety: Excess vapor in the tanker does not have to be released to atmosphere; the transfer stays closed.
· Storage vessel pressure: As liquid is drawn off, pressure in the storage vapor space tends to fall. Returned vapor limits that fall and helps preserve the pressure margin at the pump suction.
· Quantity calculation: Returned vapor carries mass. For custody transfer, how that mass is treated must be defined separately.
Vapor return, vapor recovery and BOG are different things
These three terms are often confused. Vapor return in LPG service is vapor exchanged between two pressurized vessels; the vapor is not processed, it simply goes back where it came from. Vapor recovery refers to loading fuels or solvents from atmospheric tanks, where collected vapor is treated in a vapor recovery unit (VRU) as an emissions control function. In LNG service, vapor return operates at cryogenic temperature and is tied directly to the handling of boil-off gas (BOG) continuously generated in the facility. Because of these differences, an LPG vapor line should not be sized with the design approach of the other two applications.
How to size a vapor return line
As a first approximation, the volumetric flow the vapor line must carry is close to the volumetric flow of liquid entering the tanker: each unit volume of liquid displaces about the same volume of vapor. Two effects add to that. The first is the temperature difference between tanker and storage; if product or shell in the tanker is warmer, extra vaporization occurs during loading. The second is partial flashing of liquid as it enters the tanker because of the pressure drop in the loading line. Sizing weighs these effects together with the target loading rate.
Line diameter, length, number of bends and valves, the vapor side of the hose or arm and the couplings are chosen together so that the pressure difference stays within the allowable margin. That margin is set by the head the pump can deliver and by the design pressures of the tanker and storage vessel. The line should be checked for the longest routing on site and the lowest operating temperature, since in cold weather part of the vapor can condense inside the line.
Sizing input | Why it matters | Effect on design |
Target loading rate | Directly sets the displaced vapor volume | Main input for line size and the vapor side of hose or arm |
Temperature difference between tanker and storage | Creates extra vaporization or condensation | May require a margin on design flow or condensate drainage |
Product composition | Propane-rich product has a different vapor pressure and vapor density | Pressure drop calculation and pressure rating are product-specific |
Route length and elevation changes | Determine pressure loss and where liquid can collect | Line slope, drain points and diameter |
Tanker and storage design pressure | Caps the allowable pressure difference | Limits the pressure loss accepted in the vapor line |
Metering approach | Decides whether returned vapor enters the quantity calculation | May call for separate metering on the vapor line |
Condensation and liquid holdup
LPG vapor can condense back to liquid in cold sections of the line. Liquid collecting at low points restricts vapor flow and causes pressure fluctuations. The vapor line is therefore sloped so that liquid can drain back to a vessel wherever possible, and unavoidable low points get a drain arrangement. The vapor side of the hose or arm should also be arranged so it can be emptied at the end of transfer.
Valves and flow direction
Vapor flows toward storage during loading and toward the tanker during unloading. Where one line serves both modes, one-way check valves can defeat its purpose, so the type and location of protective devices are chosen with both flow directions in mind. Like the liquid line, the vapor line is part of the pressurized system: an emergency shutoff valve near the tanker connection, a breakaway coupling and its link to the shutdown logic are assessed together with the liquid side.
How returned vapor affects metering
While liquid entering the tanker is metered, vapor flows back from the tanker to storage at the same time, and that vapor carries some product mass. Legal metrology rules for liquefied gases may require that, where a gas-phase connection exists between supplying and receiving vessels, the returned gas is measured and subtracted from the delivered quantity. Some custody transfer skids therefore add a second meter on the vapor line and calculate net delivery as liquid in minus vapor returned. How returned vapor is reflected in the invoice depends on national regulations and the contract; details are on the LPG Custody Transfer Metering Systems page.
How TLY Enerji contributes
TLY Enerji provides design, procurement and engineering services for LPG truck loading and unloading stations. Depending on project scope, support can include selection and supply of valves, pressure instruments and, where required, a flow meter for vapor measurement on the vapor line, as well as integration of vapor and liquid line signals into the control system. Storage-side vapor connections and the tanker-side interface are clarified together against the project's technical specification.
Data needed to review a vapor line
· Target loading and unloading rates and the number of bays operating simultaneously
· Location of the storage vessel vapor nozzle and routing to the skid
· Vapor connection sizes and design pressures of the tanker types served
· Products handled and expected temperature range, especially summer and winter extremes
· For an existing line, its size and length and any loading-time problems observed
· Contract terms on whether returned vapor enters the quantity calculation
Related pages
· LPG truck loading systems: The liquid side of closed loading and fill limits.
· LPG truck unloading systems: The active role of the vapor line in compressor unloading.
· LNG vapor return systems: Vapor return at cryogenic temperature and its link to BOG.
· Solvent vapor recovery and vapor return systems: Vapor handling for emissions control at atmospheric tanks.
· LPG custody transfer metering systems: Where returned vapor fits in the quantity calculation.
Questions about LPG vapor return
Can LPG be loaded without a vapor return line?
Technically, loading can proceed while tanker pressure is allowed to rise, but as pressure builds the pump faces more back pressure, flow drops and loading takes longer. Venting excess vapor to atmosphere is not an acceptable method for safety and emissions reasons. That is why vapor return lines are widely used in bulk LPG loading; whether one is used should be assessed against regulations, tanker equipment and operating conditions.
Why can the vapor line be smaller than the liquid line?
The vapor line carries vapor displaced from the tanker. Its volumetric flow is close to the liquid flow, but its density is far lower, and allowable velocity and pressure loss are judged differently from the liquid line. As a result, the vapor side is smaller than the liquid side in most applications. An undersized vapor line, however, lets tanker pressure rise as loading progresses, so the size should be confirmed by calculation on every project.
Does returned vapor affect invoicing?
It can. Vapor corresponding to part of the liquid loaded into the tanker flows back to storage and carries product mass. Some systems under legal metrology control require returned gas to be measured and subtracted, while some contracts apply a different reconciliation method. Which approach applies should be settled with the regulations and contract at the start of the project.
Why does liquid collect in the vapor line?
LPG vapor can condense where ambient temperature falls or where the line runs in shade. The liquid formed collects at low points, narrows the vapor path and causes pressure fluctuations. Sloping the line, avoiding low points or providing drains keeps the problem under control.