LNG Safety Concerns: Hazards, Storage, and Tank Purging

Liquefied natural gas (LNG) is one of the most useful forms of natural gas in use today for both domestic and industrial use. Cryogenically chilled and liquefied natural gas is typically composed of methane and smaller proportions of other hydrocarbon compounds.

This liquid mixture has the advantage of saving valuable storage space due to its reduced volume when compared to natural gas in its gaseous state. While specialized equipment allows this fuel to be stored, transported, and handled safely, there are still misconceptions about its potential hazards.

In this article, we will consider the main LNG safety concerns, including flammable vapor, cryogenic exposure, oxygen displacement, and pressure buildup. We will also discuss storage safeguards and the role of nitrogen in LNG tank purging.

What Is LNG?

Liquefied natural gas (LNG) is natural gas that has been subjected to sub-zero temperatures at which point it condenses into a liquid state. An LNG fraction can be produced by super-cooling natural gas to very low temperatures at which the newly formed liquid occupies barely 1/600th of its volume in a gaseous state.

LNG requires specially designed cryogenic tanks to maintain its low temperature. Heat entering the tank causes some of the liquid to vaporize, producing boil-off gas that must be managed to control pressure. Although LNG is generally considered non-toxic, its extremely low temperature and the properties of its vapor present hazards that require appropriate storage and handling controls.

At What Temperature Does Natural Gas Liquefy?

According to the U.S. Department of Energy, natural gas is cooled to approximately -260°F (-162°C) to produce LNG for shipping and storage. At approximately atmospheric pressure, the liquid occupies about 1/600th of the volume of natural gas in its gaseous state.

How Is LNG Transported?

LNG is usually stored and transported in specially designed tanks. For maritime transportation, manufacturers employ large LNG container ships to move the product across vast distances.

These sea-going vessels use double-hull construction and specialized cargo containment systems to help protect and insulate the LNG. Overland transport also requires specially designed cryogenic tanks with insulation and pressure relief equipment. Depending on the vessel and its systems, marine nitrogen generation systems may support inerting, line purging, and maintenance activities. These applications must follow the vessel’s approved operating procedures.

LNG Risks and Safety Concerns

For decades, the LNG manufacturing industry has been safely transporting the fuel in large quantities. However, as with all industrial processes that involve the use of potentially combustible materials, there have been some notable accidents in the past.

For example, during WWII in Cleveland, Ohio, a poorly constructed containment tank failed and leaked into nearby sewers in a residential area. The released vapor subsequently ignited killing over 100 persons and injuring about 200. Although incidences like the one mentioned above have become a rarity due to strict enforcement of storage, and transport standards, the potential LNG hazards remain the same.

Potential Hazards Associated with LNG Spills

The most common LNG spillage risks include:

• Ignition of flammable vapor, potentially resulting in flash fires, pool fires, or jet fires
• Damaging overpressure if a flammable vapor cloud ignites in confined or congested conditions
• Rapid phase transition when LNG contacts water, potentially producing a physical explosion without combustion
• Cryogenic burns from contact with the liquid or extremely cold equipment
• Thermal stress and brittle fracture in materials not designed for cryogenic temperatures
• Oxygen displacement and asphyxiation in areas where vapor accumulates
• Pressure-related vessel failure, including boiling liquid expanding vapor explosions (BLEVEs) involving pressurized vessels under certain failure conditions

Is LNG Flammable?

The major component of LNG is methane. LNG does not burn in its liquid state, but the vapor released as it warms can form a flammable mixture with air. Methane is flammable at concentrations of approximately 5% to 15% by volume in air. If this mixture encounters a suitable ignition source, a fire can occur.

Is LNG Toxic?

LNG is generally considered non-toxic, but this does not mean exposure is harmless. High concentrations of natural gas vapor can displace oxygen, creating an asphyxiation hazard, particularly in enclosed or poorly ventilated areas.

Direct contact with LNG can also cause severe cryogenic burns. Appropriate protective equipment, gas monitoring, and handling procedures are therefore essential wherever exposure may occur.

LNG Spills in Water

When LNG is accidentally spilled into water, heat transfer causes the liquid to vaporize rapidly. Under certain conditions, this can produce a rapid phase transition, resulting in a physical explosion without combustion. Although LNG does not leave a persistent oil-like slick, the release can still present hazards from extreme cold, flammable vapor, and localized overpressure. The absence of a lasting liquid residue should not be taken to mean that the spill is harmless.

LNG Spills on Ground

When LNG is spilled on a ground surface, it behaves similarly as with water. The fuel will quickly form vapor and dissipate harmlessly and completely as long as an ignition source is not brought into contact with its vapor which is flammable.

LNG Storage Safety Measures

Safe LNG storage depends on several layers of protection. These measures are designed to prevent releases, identify abnormal conditions, and limit the consequences of an incident.

Common LNG storage safeguards include:

• Cryogenic tanks, piping, and materials suited to the operating temperature
• Boil-off gas management and pressure relief systems
• Gas detection, fire detection, and emergency shutdown equipment
• Spill containment designed to limit the spread of released LNG
• Regular equipment inspections, maintenance, and personnel training

The Pipeline and Hazardous Materials Safety Administration outlines these and other LNG facility safety measures. The appropriate safeguards depend on the facility, storage system, and applicable requirements.

LNG Tank Purging with Nitrogen

LNG tanks may require purging during commissioning or preparation for maintenance. Unlike physical cleaning, nitrogen purging changes the atmosphere within the tank to help control combustion hazards.

Nitrogen gas is used due to its inert, non-combustible properties. Before introducing natural gas, it can displace air to help prevent a flammable mixture from forming. During maintenance preparation, it may be used to displace residual hydrocarbon vapor as part of an approved procedure.

Effective nitrogen purging requires a procedure suited to the tank’s condition and intended use, with gas testing to verify completion. An inerted tank is not safe for personnel entry, as nitrogen creates an oxygen-deficient atmosphere. Entry requires separate preparation and authorization under applicable requirements.

An on-site nitrogen gas generator can supply nitrogen for the operation. Equipment selection should account for the required flow, pressure, purity, dryness, and project duration.

NiGen Has Nitrogen Purging Solutions for LNG Tanks

At NiGen, we offer a broad range of turnkey industrial air solutions to clients in the energy sector. Our offerings include nitrogen generators, pipeline maintenance, and pipeline cleaning services to support industrial operations. For temporary project requirements, we also offer nitrogen generator rentals. Our team can help evaluate equipment options based on your required nitrogen flow, pressure, purity, and site conditions.

For more information about our trusted nitrogen generation solutions, please contact us today!