LNG Vapor Return Systems

In LNG loading, the vapor return line takes the vapor displaced by incoming liquid, plus vapor generated as the tanker cools, back to the storage side through a separate connection. This keeps tanker pressure under control and avoids venting gas from the tanker. The facility pays for it: returned vapor raises storage tank pressure and loads the boil-off gas (BOG) system. Because that gas carries energy, it may also have to be reflected in the commercial accounts, depending on how the delivered quantity is determined.

Loading as a whole is covered on the LNG truck loading page and continuous BOG metering on the BOG measurement page. Here the focus is the return line itself, pressure balance and the knock-on effects of returned vapor.

Vapor return, vapor recovery and BOG management: three different things

The three terms are often confused, yet each solves a different problem:

At a fuel terminal, collected vapor goes to a processing unit for emissions control and plays no part in quantity accounting. With LNG, returned vapor is saleable natural gas: once it reaches the facility it joins the BOG system and directly consumes its capacity. That is why LNG vapor return is engineered as part of the process and metering system rather than as emissions equipment.

Where the returned vapor comes from

·         Volumetric displacement: Liquid entering the tanker pushes out an equal volume of vapor.

·         Cooling the tanker shell: If the inner vessel is warmer than LNG, part of the incoming liquid flashes on contact with the wall.

·         Line and arm cool-down: Vapor formed in warm connections at the start of loading also heads for the return line.

Returned vapor is therefore not simply proportional to the volume loaded: a tanker that arrives warm after a long trip produces a much larger vapor peak early in loading, and the return line and BOG system have to cope with it.

Pressure balance and loading rate

If vapor cannot escape, tanker pressure rises and loading has to slow down or stop. Pressure loss in the return line sets the pressure difference between tanker and storage tank, so it also caps the achievable loading rate; line size, hose or arm size and in-line valves are chosen to keep that difference acceptable at the target rate. On the storage side, check that the BOG compressor or other BOG routes can absorb the extra load during loading hours.

In some small-scale applications a tanker can be filled without a return line by top-spray filling, where the incoming liquid condenses the vapor space. That depends on the tanker's equipment, starting temperature and procedure, so the return line should not be dropped before the tanker types to be served are known.

Design points for a cold vapor line

·         Materials and insulation: Returned vapor can be close to boiling temperature; piping and seals are selected for it, and insulation limits heat ingress and external icing.

·         Sizing: Peak vapor flow during cool-down and the target loading rate are considered together.

·         Isolation and safety: Like the liquid line, the vapor line is protected by an emergency shutdown (ESD) valve and, on the tanker side, a breakaway coupling; details are on the cryogenic ESD page.

·         Pressure monitoring: Pressure on the tanker and storage sides is used to control loading rate and permissives.

·         Purging and inerting: Nitrogen purging of the hose or arm before and after connection, with drain points routed to a safe header.

·         Tie-in point: Whether the line returns to the tank vapor space or to the BOG header is decided together with tank pressure control.

LNG vapor at boiling temperature is initially heavier than air and rises only as it warms, so the orientation of drains and relief outlets on the vapor line deserves specific attention in the layout.

Returned vapor in custody accounting

Returned vapor carries energy from the tanker back to storage. OIML R 117-1:2019, the legal metrology recommendation, prohibits a connection between the vapor spaces of the supplying and receiving tanks in LNG measuring systems unless the returned vapor is measured and deducted from the delivered quantity, and it requires that vapor measurement to be better than 20 percent. Put simply, in a meter-based legal-for-trade loading system the return line is either metered or not installed. An industry handbook on LNG custody transfer likewise notes that the energy balance of a transfer accounts for gas returned to the other party.

Where quantity is determined by weighbridge, the mass of returned vapor shows up in the weight difference; for energy-based invoicing, the contract defines how the returned gas share is calculated. These points are developed on the LNG custody transfer metering page.

How it differs from LPG vapor return

LPG is kept liquid by pressure at ambient temperature, so its vapor return links two pressure vessels and carries vapor at ambient temperature. LNG sits near atmospheric pressure at cryogenic temperature; its returned vapor is cold, needs suitable materials and insulation, and ties straight into the facility's BOG system. Wherever a vapor connection exists, the custody treatment of returned vapor still has to be defined.

Beyond road tankers

EN 1473, covering onshore LNG installations, lists road tanker and rail car connections with vapor return, and ship manifold connections, among plant boundary limits. Shore-side marine transfer arms are now covered by EN ISO 16904, which replaced the withdrawn EN 1474-1, and ship cargo measurement accounts for vapor volume alongside liquid volume. This page stays with road tankers.

How TLY Enerji contributes

TLY Enerji provides system design, instrument and valve supply, engineering, installation, commissioning and maintenance for LNG road tanker loading and unloading facilities. For a vapor return line this can cover selecting pressure and temperature instruments, ESD valves and, where needed, returned-vapor metering, then integrating and commissioning them. The supply boundary is set per project in the technical specification.

Data needed to assess a vapor return line

·         Tanker types to be served and whether they have a vapor connection

·         Target loading rate and number of simultaneous loadings

·         Typical arrival condition of tankers: cold or partly warmed up

·         Storage tank operating pressure range and current BOG system capacity

·         Tie-in point and routing of the return line

·         Quantity determination method and how returned vapor is treated in the contract

·         Nitrogen supply, vent headers and ESD interface

Related pages

·         BOG measurement systems: Metering the BOG system that returned vapor joins.

·         LNG truck loading systems: The loading process vapor return belongs to.

·         LPG vapor return systems: Vapor balancing for a pressurized gas at ambient temperature.

·         Solvent vapor recovery and vapor return: How emissions-driven vapor recovery differs.

·         LNG custody transfer metering: Where returned gas fits into energy-based custody transfer.

LNG vapor return FAQ

What is the difference between a vapor return line and a vapor recovery unit (VRU)?

A vapor return line only balances pressure: it carries displaced vapor back to the supplying side through a closed connection without treating it. A vapor recovery unit processes vapor collected during gasoline or solvent loading, for example by adsorption or condensation, to cut emissions. At an LNG terminal the returned vapor is natural gas that joins the BOG system, so the relevant concept is BOG management rather than a VRU.

Does returned vapor have to be metered?

It depends on the purpose of measurement. In meter-based legal-for-trade LNG loading systems with a vapor connection, returned vapor must be measured and deducted from the delivered quantity. When quantity is set by weighbridge, returned vapor mass is reflected in the weight difference, but its treatment in the energy calculation is contractual. From a process standpoint, knowing the returned vapor flow also helps manage the BOG load.

Should the return line go to the storage tank or to the BOG header?

Both are used. Returning to the tank vapor space gives a simple pressure balance but adds all returned vapor to tank pressure. Returning to the BOG header routes vapor straight to compressors or other BOG routes, but the header and compressor must handle the extra load during loading. The decision weighs tank pressure control, BOG capacity and the number of simultaneous loadings.

How does a warm tanker affect loading?

LNG entering a warmed-up inner vessel partly flashes on contact with the wall. Early in loading this sends far more vapor through the return line and drives tanker pressure up quickly, so loading rate is usually kept low in this phase. A facility sized only for cold tankers may struggle to absorb the extra BOG load.

Are LNG and LPG vapor return lines designed the same way?

The aim is the same: balancing pressure between two vessels during loading. The conditions are not. LPG travels under pressure at ambient temperature, LNG near atmospheric pressure at cryogenic temperature. An LNG return line therefore carries cold vapor, needs cryogenic materials and insulation, and feeds the facility's BOG system.