How Do Screw Compressors Handle Boil-Off Gas in LNG Systems?

Liquefied natural gas is stored at extremely low temperatures, but LNG storage is never completely static. Even when a tank or cargo containment system is well insulated, a small amount of heat enters the system. Part of the LNG therefore evaporates and produces what the industry calls boil-off gas, or BOG.

The amount may be relatively small compared with the total LNG inventory, but it cannot simply be ignored. Pressure inside the storage system has to be controlled, and the gas also represents a recoverable energy resource.

This is where BOG compression becomes an important part of LNG infrastructure. Screw compressors are one of the technologies that can be considered when the system requires reliable gas handling across changing operating conditions.

Why Does Boil-Off Gas Need to Be Managed?

BOG is mainly composed of natural gas vapor, with methane normally being its dominant component. It can be generated during LNG storage, loading and unloading operations, transportation, or changes in ambient conditions.

The challenge is not simply removing the gas from a tank.

If BOG is continuously vented, valuable natural gas is lost and emissions increase. Flaring can provide a controlled disposal method in certain situations, but it still means that the energy contained in the gas is not recovered.

A properly designed BOG system instead gives the gas a useful destination.

Depending on the LNG facility, compressed BOG may be sent to a fuel gas system, reinjected into a pipeline, returned to a process stream, or routed through a reliquefaction system. The appropriate arrangement depends on the facility's operating pressure, gas composition, required flow rate, and downstream process.

The compressor therefore sits at an important point between LNG storage and the rest of the gas-handling system.

Why Screw Compressors Can Be Suitable for BOG Service

BOG flow is not necessarily constant.

During normal storage, the generation rate may remain relatively low. During LNG loading, unloading, ship operations, or changes in ambient temperature, the gas load can change significantly. A compressor selected only for one operating point may therefore spend much of its operating life away from its design condition.

Screw compressors have an advantage in applications where capacity needs to be adjusted over a relatively wide range.

Open-type screw compressor designs can use stepless capacity control and adjustable volume ratios to respond to changing gas loads. Instead of treating the compressor as a fixed-flow machine, the operating point can be adjusted according to the actual BOG requirement.

For LNG facilities, this can help reduce unnecessary energy consumption when the gas generation rate is below the maximum design condition.

The mechanical characteristics of a screw compressor are also useful for continuous industrial service. There are no reciprocating pistons repeatedly changing direction, and the rotating elements can be designed for long operating periods when lubrication, sealing, bearings, and cooling are properly engineered.

Capacity Control Is a Major Consideration

One of the more important questions when selecting a BOG compressor is not simply "How much gas can it compress?"

The more useful question is:

How efficiently can it operate when the actual BOG flow is different from the design flow?

An LNG terminal may experience periods of high BOG generation followed by much lighter loads. A compressor with a narrow operating range may require frequent starts and stops, bypass arrangements, or other forms of capacity management.

A screw compressor equipped with stepless capacity control can reduce displacement when the required capacity falls. A range such as 10–100% capacity control can be particularly useful for installations where the BOG load varies significantly.

The volume ratio, or Vi, also deserves attention. The appropriate internal volume ratio depends on the pressure ratio and process conditions. If the compressor's internal compression ratio is poorly matched to the actual system, additional losses can occur.

For engineered BOG packages, capacity control and volume-ratio selection should therefore be considered together rather than treated as separate specifications.

Compressor Materials and Sealing Matter in Gas Service

BOG compression is not the same as conventional refrigeration compression.

The gas being handled is a hydrocarbon mixture, and the equipment operates within an environment where leakage and ignition risks have to be carefully controlled. Mechanical seals, shaft arrangements, bearings, lubrication systems, and pressure-containing components all become part of the safety discussion.

Robust screw compressor designs may use forged steel rotors, high-precision bearings, and specialized mechanical seals suited to demanding industrial service.

Silicon carbide mechanical seals, for example, can be used in applications where durability and leakage control are important. Bearing life also matters because LNG facilities often expect equipment to operate continuously for long periods rather than being treated as intermittent machinery.

The compressor housing and associated piping must also be selected according to the maximum operating and design pressures of the complete process.

In higher-pressure applications, compressor series designed for elevated pressure service can provide additional flexibility, but the final selection must always be based on the actual process conditions.

Explosion Protection Cannot Be an Afterthought

Natural gas is flammable, so BOG compression equipment has to be considered as part of the hazardous-area design rather than as an isolated mechanical package.

An open-drive compressor can provide flexibility because it can be coupled with an appropriate explosion-proof motor or other suitable driver. Variable-frequency-drive operation can also be considered where the project requires variable speed.

However, the motor is only one part of the safety arrangement.

Instrumentation, electrical equipment, seals, ventilation, gas detection, emergency shutdown systems, piping, and pressure protection all have to work together. Applicable hazardous-area classifications and project standards need to be established before equipment is specified.

For projects operating under international specifications, buyers may also encounter requirements related to ATEX, API standards, or other regional and project-specific codes.

The exact compliance route should be confirmed during engineering rather than assumed from a general product description.

Where Are BOG Screw Compressors Used?

BOG compression technology is relevant across several parts of the LNG value chain.

At LNG terminals, compressors can support storage-tank pressure management and send recovered gas toward fuel, pipeline, or reliquefaction systems.

On LNG carriers, BOG handling is closely connected with cargo operations and propulsion or fuel-gas systems. Compressor reliability becomes particularly important because equipment operates within a marine environment where space, maintenance access, vibration, and redundancy can influence the design.

For FSRUs, BOG management has to work alongside regasification equipment and onboard utility systems. The compressor package therefore has to fit within a more complex process arrangement.

Peak-shaving facilities can also use BOG compression as part of their LNG storage and gas supply process.

These applications may look similar from the outside, but the actual compressor requirements can be very different.

Why BOG Systems Are Often Engineered to Order

There is no single compressor configuration that fits every LNG installation.

The required displacement, suction pressure, discharge pressure, gas temperature, methane composition, oil management strategy, driver type, hazardous-area classification, and downstream process can all affect the final package.

For some installations, a single-stage screw compressor may be appropriate. Higher pressure ratios may require compound or two-stage arrangements. Reliquefaction systems can introduce additional requirements for cooling, separation, and gas conditioning.

This is why buyers should be cautious about selecting a compressor solely from a catalog capacity figure.

A compressor capable of handling 10,000 m³/h at one operating point may not necessarily be the best choice for a system requiring stable performance across a much wider BOG envelope.

The complete operating map matters more than one headline number.

What Should Buyers Ask Before Selecting a BOG Compressor?

Before comparing different compressor packages, an LNG project team should establish several basic process parameters:

Normal and maximum BOG flow rate
Minimum and maximum suction pressure
Required discharge pressure
Gas temperature and composition
Required turndown range
Continuous or intermittent operating conditions
Driver and power requirements
Hazardous-area classification
Applicable design and inspection standards
Oil injection and separation requirements
Cooling arrangement
Maintenance and spare-parts requirements

These details allow the compressor supplier to evaluate whether an open screw, compound screw, or another compression technology is appropriate.

For a customized BOG package, the compressor itself is only one component. Separators, coolers, oil systems, controls, valves, instrumentation, and safety devices all contribute to the performance of the finished system.

Turning BOG From a Pressure Problem Into a Gas Resource

BOG is an unavoidable part of LNG storage and transportation, but it does not have to be treated simply as waste gas.

With the right process arrangement, the gas can be recovered, compressed, reused as fuel, reinjected, or reliquefied. The compressor is central to making those options technically practical.

Screw compressor technology becomes particularly interesting where the project requires wide capacity adjustment, continuous operation, compact mechanical arrangements, and flexibility across changing BOG conditions.

SRM TEC SNOWKEY compressor technology, including open-type screw compressor platforms and high-pressure configurations, can be considered for engineered BOG handling packages where the final design is matched to the LNG process.

For project engineers, EPC contractors, and LNG equipment buyers, the key is to evaluate the compressor as part of the complete BOG system rather than as an isolated machine. Flow variation, pressure ratio, gas properties, hazardous-area requirements, capacity control, sealing, cooling, and downstream gas treatment all influence whether a particular compressor package will perform reliably over its operating life.

More information about the SRM TEC SNOWKEY BOG Screw Compressor can be reviewed when evaluating compressor solutions for LNG storage, transportation, and gas recovery applications.

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