Clean Air Program
from the Federal Transit Administration
Summary Assesment of the Safety, Health, Environmental and System Risks of Alternative Fuel
(for the full report click here)

3.3.4 Liquefied Natural Gas General Description

Liquefied natural gas (LNG) is produced by cooling natural gas and purifying it to a desired methane content. The typical methane content is approximately 95% for the conventional LNG produced at a peak shaving plant. Peak shaving involves the liquefaction of natural gas by utility companies during periods of low gas demand (summer) with subsequent regasification during peak demand (winter). It is relatively easy to remove the non-methane constituents of natural gas during liquefaction. Therefore, it has been possible for LNG suppliers to provide a highly purified form of LNG known as Refrigerated Liquid Methane (RLM) which is approximately 99% methane.

The primary advantage of LNG compared to CNG is that it can be stored at a relatively low pressure (20 to 150 psi) at about one-third the volume and one-third the weight of an equivalent CNG storage tank system. The big disadvantage is the need to deal with the storage and handling of a cryogenic (-160C, -260F) fluid through the entire process of bulk transport and transfer to fleet storage. Safety Issues

(a) General Properties Affecting Fire Hazards

Even though the end product of the use of CNG and LNG for vehicular applications is essentially the same, the general properties affecting safety are quite different. On one hand, LNG is a more refined and consistent product with none of the problems associated with corrosive effects on tank storage associated with water vapor and other contaminants. On the other, the cryogenic temperature makes it extremely difficult or impossible to add an odorant. Therefore, with no natural odor of its own, there is no way for personnel to detect leaks unless the leak is sufficiently large to create a visible condensation cloud or localized frost formation. It is essential that methane gas detectors be placed in any area where LNG is being transferred or stored.

The cryogenic temperature associated with LNG systems creates a number of generalized safety considerations for bulk transfer and storage. Most importantly, LNG is a fuel that requires intensive monitoring and control because of the constant heating of the fuel which takes place due to the extreme temperature differential between ambient and LNG fuel temperatures. Even with highly insulated tanks, there will always be a continuous build up of internal pressure and a need to eventually use the fuel vapor or safely vent it to the atmosphere. When transferring LNG, considerable care has to be taken to cool down the transfer lines in order to avoid excessive amounts of vapor from being formed.

The constant vaporization of the fuel also has an interesting effect on the properties of the fuel, unless it is a highly purified form of LNG, i.e., RLM. The methane in the fuel will boil off before some of the other hydrocarbon components such as propane and butane. Therefore, if LNG is stored over an extensive period of time without withdrawal and replenishment the methane content will continuously decrease and the actual physical characteristics of the fuel will change to some extent. This is known as "weathering" of the fuel.7

Another consideration is that under low temperatures, many materials undergo changes in their strength characteristics making them potentially unsafe for their intended use. For example, materials such as carbon steel lose ductility at low temperature, and materials such as rubber and some plastics have a drastically reduced ductility and impact strength such that they will shatter when dropped.

As before, many of these potential issues have been identified and addressed in the various codes that have been developed by the NFPA and under the Uniform Fire Code. For example, the NFPA has the following national standards and codes applicable to LNG:

NFPA 59A -- Standard for Production, Storage, and Handling of Liquefied Natural Gas

NFPA 57 (draft) -- Standard for Liquefied Natural Gas Vehicular Fuel Systems (final code expected to be published in 1995)

(b) Fire Hazards During Transport

LNG may either be liquefied on-site or it can be delivered to fleet storage using a standard 10,000 gallon LNG tanker truck. In general, only the largest fleet operators would find on-site liquefaction to be advantageous. Typical LNG storage vessels, including those used on the tanker truck, have the following basic components:

Inner pressure vessel made from nickel steel or aluminum alloys exhibiting high strength characteristics under cryogenic temperatures

Several inches of insulation in a vacuum environment between the outer jacket and the inner pressure vessel. Stationary tanks often use finely ground perlite powder, while portable tanks often use aluminized mylar super-insulation.

outer vessel made of carbon steel and not normally exposed to cryogenic temperatures

control equipment consisting of loading and unloading equipment (piping, valves, gages, pump, etc.) and safety equipment (pressure relief valve, burst disk, gas detectors, safety shut off valves, etc.)

The double walled construction of the LNG tanker truck is inherently more robust than the equivalent tanker truck design for transport of other liquid AMFs. Therefore, the transport of LNG is safer from the perspective of fuel spills resulting from a tank rupture during an accident. A rupture of the outer vessel would cause the loss of insulation and result in an increased venting of LNG vapor. While this is of concern, it is relatively minor compared to the prospect of an LNG spill.

An explosion of an LNG container is a highly unlikely event that is possible only if the pressure relief equipment or system fails completely or if there is some combination of an unusually high vaporization rate (due to loss of insulation) and some obstruction of the venting and pressure relief system preventing adequate vapor flow from the inner pressure vessel with a resultant pressure build up. If the pressure builds up to the point where the vessel bursts, the resulting explosion is known as a BLEVE (boiling liquid expanding vapor explosion) with the container pieces propelled outward at a very high velocity.7 This is a highly unlikely event due to the extensive requirements for pressure relief including pressure relief valves and burst discs that are built into the design codes. (There have been no reports in the literature reviewed of any BLEVE occurring with LNG.)

In the event that the LNG vessel is ruptured in a transport accident and the LNG is spilled, there will be a high probability of a fire because a flammable natural gas vapor/air mixture will be formed immediately in the vicinity of the LNG pool. In an accident situation, there is a high likelihood of ignition sources due to either electrical sparking, hot surface, or possibly a fuel fire created from the tanker truck engine fuel or other vehicles involved in the accident. The vapor cloud from an LNG pool will be denser than the ambient air; therefore, it will tend to flow along the ground surface, dispersed by any prevailing winds.

When spilled along the ground or any other warm surface, LNG boils quickly and vaporizes. A high volume spill will cause a pool of LNG to accumulate and the boiling rate will decrease from an initial high value to a low value as the ground under the pool cools. The heat release rate from an LNG pool fire will be approximately 60% greater than that of a gasoline pool fire of equivalent size.

(c) Fire Hazards During Transfer to Fleet Storage

The transfer of LNG from a tanker truck to fleet storage is a complex process that involves the active participation of both the tanker truck driver and a representative of the fleet operator. A partial listing of some of the steps involved provides some indication of the safety precautions that are necessary.7

After the truck is chocked and the engine is shut off, a grounding cable is attached to the truck to ground any electrostatic discharge.

A flexible liquid transfer hose is attached to the tanker and purged with LNG to remove all air.

A fleet operator representative will open the storage vessel liquid fill line and the driver will open the trailer's main liquid valve.

The driver will control the pressure in the trailer tank via a pressure building line where LNG is vaporized and returned to the tank to maintain a pressure differential of at least 15 psi between the tanker and the storage vessel.

The driver will use a mechanical means to maintain a tight connection at the hose coupler to compensate for differential expansion.

The safety features that are typical of truck storage transfer of LNG include equipment design such as trailer liquid valves that are interlocked with the truck brake system to prevent fuel transfer before the truck is properly secured; remote-controlled, redundant liquid valves; storage vessel alarms to prevent overfill; and long drain lines for safety-directing vented LNG vapor.

The complexity of the fuel transfer arrangement creates the potential for leaks and spills through human error and equipment failure. One of the particular concerns is that the fuel transfer equipment goes through a continuous cycle of cool down to cryogenic temperatures and warm up to ambient temperature. This type of thermal cooling can create additional stresses on equipment and sealing devices which could result in decreased reliability over time.

(d) Fire Hazards During Fleet Storage

LNG storage facility requirements for a total on-site storage capacity of 70,000 gallons or less are defined in the draft NFPA 57 -- Standard for Liquefied Natural Gas (LNG) Vehicular Fuel Systems. NFPA 59A -- Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG) is applicable to storage volumes above 70,000 gallons. Both of these standards address similar issues including siting of the storage tank, provision for spill and leak control, and the basic design of the storage container and LNG transfer equipment.

One of the major provisions at any LNG storage facility is the requirement to provide an impounding area surrounding the container to minimize the possibility of accidental discharge of LNG from endangering adjoining property on important process equipment and structure, or reaching waterways. This requirement ensures that any size spill at a fleet storage facility will be fully contained and the risk of any fire damage will be minimized.

(e) Other Hazards

LNG has a unique safety hazard among the AMFs because of the potential exposure of personnel to cryogenic temperatures. Workers can receive cryogenic burns from direct body contact with cryogenic liquids, metals, and cold gas. Exposure to LNG or direct contact with metal at cryogenic temperatures can damage skin tissue more rapidly than when exposed to vapor. It is also possible for personnel to move away from the cold gas before injury.

The risk of cryogenic burns through accidental exposure can be reduced by the use of appropriate protective clothing. Depending upon the risk of exposure, this protection can range from loose fitting fire resistant gloves and full face shields to special extra protection multi-layer clothing.

Another unusual hazard associated with aged LNG will arise in the unlikely event that there is a large spill of LNG onto a body of water. This could occur in an accident situation involving an LNG transport vehicle container rupture and spill into an adjacent water body. The hazard is known as a rapid-phase transition (RPT) -- in this case a rapid transformation from the liquid phase to vapor. If significant vaporization occurs in a short time period, the process can, and usually does, resemble an explosion.8

The RPT "explosion" phenomenon for LNG on water has been observed in a number of situations and has been studied extensively in both laboratory and large scale tests. The temperature of the water and the actual composition of the LNG are important factors in determining whether an RPT will take place. It should also be noted that RPTs have been obtained for pure liquefied propane with water temperature in the range of 55C (130F). Health Issues

The principal constituents of natural gas, methane, ethane, and propane, are not considered to be toxic. The American Conference of Governmental Industrial Hygienists (ACGIH) considers those gases as simple asphyxiants, which are a health risk simply because they can displace oxygen in a closed environment. The Occupational Safety and Health Administration (OSHA) has set a time-weighted average (TWA) personal exposure limit (PEL) of 1,000 ppm for propane. A number of the minor constituents of natural gas have ACGIH listed threshold limit values (TLVs), including butane - 800 ppm, pentane - 600 ppm, hexane - 50 ppm, and heptane - 400 ppm. The effective TLV for an average natural gas composition, considering all of these limits, is about 10,500 ppm.3

Unlike CNG, LNG cannot be odorized; therefore, there is some concern about the ability of personnel to detect TLV concentrations. This is another reason to ensure that methane detectors are in place wherever personnel may be exposed. Environmental Issues

There are no significant environmental hazards associated with the accidental discharge of LNG.