Liquefied Natural Gas
LNG safety and protecting a facility: Striving to be the safest in industry
NFPA Journal®, January/February 2007
By Keith Olson, Joe Behnke, and Dale Edlbeck
As the demand for natural gas continues to escalate, so too does the business of continually moving ever-greater volumes of Liquefied Natural Gas (LNG) around the world. More exploration, new production facilities, vast shipping fleets, large-scale storage are the new realities — as is the need for effective fire suppression and containment at every point in the process.
With 13 facilities currently approved by the Federal Energy Regulatory Commission and 13 proposed LNG terminals on the table, LNG is beginning to take the forefront in energy conversations across the country.
LNG, which is mainly composed of methane, is used to supplement normal supplies during periods of peak demand. LNG is usually transported as a cryogenic gas in insulated cargo vehicles under U.S. Department of Transportation regulations and is stored in fixed insulated tanks built to ASME and API specifications. Fixed storage tanks can be subject to boiling liquid expanding vapor explosion, but the probability of this event occurring is low.
For those facilities still in the planning phase, having the best equipment in place may well aid in attaining the proper permits to build new facilities and expand transportation opportunities. It is important to note that the fire suppression equipment manufacturer should be consulted before selecting a fire suppression system for the protection of LNG facilities.
Even in those facilities that have already been built, manufacturers, such as ANSUL, can design fully capable retrofit systems that perform and protect as well as systems that have been in place from the beginning.
Effective fire protection for LNG facilities concentrates on three areas: prevention, control, and extinguishment. A wide range of agent and equipment solutions to address each of these objectives is needed. These products are dedicated to preventing LNG fires at every stage of supply, including production fields, liquefaction plants, filling and storage facilities, tankers above and below-deck, and other risk zones.
Dry chemical systems
Large dry chemical systems are used to protect numerous applications within LNG facilities. Areas such as pipeline and distribution facilities, storage tanks, transfer pumps, trenches and sumps, or vessels and docks can all be protected with large dry chemical skid units or systems. These units may incorporate hose reels for manual fire fighting or a high-flow dry chemical turret where longer dry chemical throw is needed, such as to protect loading/unloading arms. In addition to large dry chemical systems, there are twin-agent systems. Twin-agent technology combines the rapid flame knockdown of dry chemical with the securing capabilities of foam. Twin-agent suppression systems are long proven and provide a most effective fire suppression and vapor securing combination. Twin-agent systems are available with dry chemical capacities of 450 to 1,350 pounds (204 to 612 kilograms) and foam capacities of 50 to 200 gallons (189 to 757 liters). All can be fitted with single or dual hoses and nozzles.
Large dry chemical systems may also be fixed pipe/nozzle systems used to protect permanent or fixed hazards such as pumps or vaporizers.An additional application where a fixed pipe/nozzle large dry chemical system may be used in an LNG facility would be to protect the vent stacks on storage tanks. This system would be sized based upon the maximum venting capacity of each vent.
The dry chemical flow rate is based upon the venting capacity of each vent, in cubic feet per second, to provide a minimum of 30-second discharge duration. ANSUL conducted numerous tests in the mid- to late 1960s to determine these requirements. Nozzles protecting vent stacks are located external to the vents, are aimed 12 inches (305 millimeters) beyond the vent opening, and utilize the vent flow to entrain dry chemical to knock down the flame.
Custom large dry chemical fixed pipe systems do not carry listings or approvals but are designed to meet the site-specific requirements for a facility. The system design capability is based upon years of testing and experience.
High-expansion foam systems
While dry chemical and large dry chemical systems are the choice for suppressing LNG fires, High-Expansion Foam Systems (HEF) provide vapor suppression for spills of LNG and reduce heat release and radiant heat feedback on involved LNG pools. Typical hazard areas protected by HEF at an LNG facility include pump areas, secondary containment pits, and the trenches that direct a potential LNG spill into these pits. Experience has shown that HEF is very effective in reducing methane concentrations at ground level during an LNG spill.
Properly designed HEF systems serve a dual purpose. First, the systems provide fire control with substantial reductions in heat release and radiant heat feedback into the LNG pool fire, which, unchecked, would further intensify the fire. The calculations for the foam output of the generators are easily performed, with the best performance being achieved when the foam has a near 500:1 expansion ratio. Also, the minimum desired design application rate should be 6 feet (1.8 meters) per 3 minute per square foot of area as previously determined by fire testing. Simply stated, the foam must be applied at a 6-feet-per-minute rate in order to achieve fire control as defined by a 90 percent reduction in radiant heat feedback within a three-minute period of time.
Second, a HEF blanket over an unignited LNG spill is effective in reducing downwind levels of LNG vapor. An LNG spill will produce a visible vapor cloud. LNG vapors are heavier than air and will drift downwind until the vapors warm sufficiently to where they become lighter than air and disperse into the atmosphere. This presents the potential problem of a downwind ignition source igniting the LNG vapors. A HEF blanket warms the LNG vapors passing through the foam, dispersing the vapors more readily into the atmosphere and thereby reducing LNG downwind concentrations.
High-Expansion Generators are intrinsically safe because they are water powered. They are available in corrosion-resistant 316 stainless steel, designed to meet the requirements of NFPA 11, Low-, Medium-, and High-Expansion Foam, can be fitted with discharge hoods for flow direction, and are listed by Underwriters Laboratories and approved by Factor Mutual. These generators also carry United States Coast Guard approval for marine application.
Protection within a facility
It is important not to overlook what needs safeguarding within the LNG facilities themselves. Throughout facilities where computers, data servers, delicate electronics software, and hardware are used, the extinguisher of choice would be clean agent fire extinguishers. Clean agent fire extinguishers listed by Underwriters Laboratories Incorporated with a 2A rating are able to protect delicate electronics and comply with fire codes’ minimum requirement to protect light “Class A hazards” without any additional fire extinguishers.
For the protection of the LNG facility and all LNG high hazard applications, such as pumps, compressors, flanges, off-loading areas, and loading areas, there is specific and special fire protection equipment available. When fire problems occur at LNG facilities on high hazard applications, there are only two ways to extinguish LNG fires quickly. The first and best method is the removal of the fuel. This is easier said than done. Shutting off product valves usually stops the flow of fuel. Sometimes valves may be located at a substantial distance from the fire, and the fire cannot be allowed to burn out without significant damage to compressors, pumps, equipment, and/or the facility. Therefore, the fire has to be suppressed by specialized fire protection equipment.
When fighting an LNG fire, two problems can be encountered. First, LNG is typically being transferred under pressure due to the operations of pumps and compressors; a second problem is the volume increase associated with the vaporization of LNG. Because of the volume increase vaporizing LNG, special fire extinguishers are needed to deliver a high flow of dry chemical extinguishing agent. The special high-flow fire extinguisher knocks down the fire by inhibiting the chemical reaction.
There are organizations and independent companies in the marketplace that are focusing their efforts on making LNG safe from ship to shore. NFPA 59A, Production, Storage and Handling of Liquefied Natural Gas, takes a broad look at fire protection. It does an excellent job of providing the requirements necessary to safely design, install, and operate LNG facilities. This is evidenced by the safety record of these facilities, which are some of the safest in industry. Safety has been accomplished through requirements for design of the facilities. The standard is very much geared towards prevention of leaks and fires as well as separation distances to control damage in the event of an LNG or natural gas incident.
The NFPA 59A chapters covering design and installation of equipment, including storage tanks, focuses heavily on robust design. Storage tanks are required to be designed to withstand environmental hazards such as earthquakes and floods and are also designed to contain the product in a secondary containment system should the inner tank containing the LNG fail.
The standard includes requirements for piping design to minimize loss of product, with special materials and valves to stop flow in the event of an incident. Separation distances between tanks, between tanks and other equipment, between tanks and buildings, and between tanks and property lines are specified to prevent heat from causing damage and natural gas vapors from reaching ignition sources. Written Emergency Shut-Down (ESD) procedures are required for every facility to contain an incident. The standard defines what is to be covered in the ESD and how it is to be instituted and maintained. A separate chapter defines requirements for transfer operations to assure safe handling of LNG.
Chapter 12 covers Fire Protection, Safety, and Security. The standard takes a performance-based approach to fire protection. Paragraph 12.2 requires “an evaluation based on fire protection engineering principles, analysis of local conditions, hazards within the facility, and exposure to or from other property.” The paragraph goes on to detail what the evaluation is to determine. This requirement covers all aspects of fire protection but does not specify exactly what is to be protected or the method of protection. Limited information is included on detection, water systems, and portable equipment for the suppression of fires.
Chapter 12 does not reference any specific hazards within an LNG facility, nor does it discuss the use of special hazard suppression systems except to reference the various standards covering design of those systems in the Annex. Requirements for special agents and application rates should be included in the standard if they are specific to LNG protection. Detection and alarm requirements could also be strengthened to identify releases and fires in a timely manner. Flame detection should be required by the standard, as it is a very common method of detection within these facilities because of the speed with which it detects a flame.
According to Dennis Kennedy, Vice President and General Manager, Engineered Systems and Foam Products at ANSUL, improvements to NFPA 59A won’t happen overnight and they won’t happen without the incorporation of better requirements for fire protection. The first step to accomplish this is to get more involvement on the technical committee from fire protection representatives working in the LNG industry who are willing to review and improve that part of the standard. In addition, while NFPA is doing its part to oversee the safety of LNG by having a technical committee dedicated to its safety, more needs to be done. To ensure the protection of people and property from potential LNG hazards on a fire suppression level, outside companies and/or manufacturers need to use their expertise to oversee this valuable energy resource.
Dale R. Edlbeck is the manager of technical services for low-pressure C02 systems and commercial suppression products for Ansul Incorporated,
Keith Olson is the manager of technical services, foam products at Ansul and a member of the NFPA 11 Technical Committee.
Joe Behnke is the manager of technical services, engineered systems at Ansul and a member of the NFPA 410 and NFPA 418 technical committees.
In this Section:
|Engineering a Future at NFPA
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|Liquefied Natural Gas
LNG safety and protecting a facility: Striving to be the safest in industry
|Vehicle Fire Problem
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|U.S. Standards Strategy: A Clear and Consistent Message
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|WSC&E Comes To Boston
More than 4,000 fire and life safety, electrical, and security professionals will convene in Boston for the 2007 NFPA World Safety Conference & Exposition®.