BY CRAIG SHELLEY AND ANTHONY COLE
In developing fire protection methods and response guidelines for liquefied petroleum gas (LPG) bulk storage facilities, the chief concern is a massive failure of a vessel containing a full inventory of LPG. The probability of this type of failure occurring can be mitigated or at least controlled to a reasonable and tolerable figure with appropriately designed and operated facilities, coupled with a local fire department/brigade response. Since most LPG fires originate as smaller fires that become increasingly more dangerous, this article will focus on fire protection methods and guidelines in relation to small leaks and fires in LPG spheres. Of greater importance to the fire protection engineer is the more likely event of a leak from a pipe, a valve, or another attached component leading to ignition, a flash fire, a pool fire, and eventually a pressure fire at the source.
LPG PROPERTIES
LPG was first discovered in the 1900s. Its applications and uses range from cooking and refrigeration to transportation, heating, and power generation, making it an all-purpose, portable, and efficient energy source.
LPG consists of light hydrocarbons (propane, butane, propylene, or a mixture) with a vapor pressure of more than 40 psi at 100ºF. At standard temperature and pressure, LPG is a gas. It is liquefied by moderate changes in pressure (i.e., in a process vessel) or by a drop in temperature below its atmospheric boiling point. The unique properties of LPG allow it to be stored or transported in a liquid form and used in a vapor form. LPG vapors are heavier than air and tend to collect on the ground and in low spots. After LPG is released, it readily mixes with air and could form a flammable mixture. This mixture could ignite and cause a vapor cloud explosion (VCE).
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A VCE can occur when a large amount of flammable vaporizing liquid or gas is rapidly released into the surrounding air and is ignited before being diluted in air below the lower flammable limit (LFL). As a release occurs, there will be an area closest to the release that is above the flammable range, an intermediate area that may be in the flammable range, and areas that will be below the flammable range. Mixing via natural currents and diffusion of LPG vapors affect the size and extent of these areas. If these processes continue, eventually the mixture is diluted to below the LFL (Tables 1, 2, 3).
Other characteristics of LPG include
• LPG exerts a cooling effect as a result of vaporization resulting from releases at low pressure (also called autorefrigeration).
• The density of LPG is almost half that of water; therefore, water will settle to the bottom in LPG.
• Very small quantities of liquid will yield large quantities of vapor.
• When vaporized, LPG leaves no residue.
• When LPG evaporates, the autorefrigeration effect condenses the surrounding air, causing ice to form. This is usually a good indication of a leak.
• LPG is odorless; agents such as ethyl mercaptan are added to commercial grades in most countries for better detection.
PRODUCTION AND OPERATIONS
LPG is derived from two main energy sources: natural gas processing and crude oil refining. When natural gas wells are drilled into the earth, the gas released is a mixture of several components. For example, a typical natural gas mixture may be methane or “natural gas” (90%); the remaining percentage of components (10%) is a mixture of propane (5%) and other gases such as butane and ethane (5%). The gas is shipped in tankers or through a pipeline to secondary production facilities for further treatment and stabilization. From these facilities it is sent by bulk carrier or pipeline to various industrial plants, gas-filling operations, and power-generation facilities.
LPG is also collected in the crude oil drilling and refining process. LPG trapped inside the crude oil is called associated gas, which is further divided at primary separation sites or gas-oil separation plants (GOSPs); central processing facilities (CPFs) for offshore installations; or drilling, production, and quarters platforms (DPQs). At these facilities, the fluids and gases produced from the wells are separated into individual streams based on their characteristics and properties and sent on for further treatment.
At refineries, LPG is collected in the first phase of refinement or crude distillation. The crude oil is then run through a distillation column in which a furnace heats it at high temperatures. During this process, vapors will rise to the top and heavier crude oil components will fall to the bottom. As the vapors rise through the tower, cooling and liquefying occur on “bubble trays,” aided by the introduction of naptha. Naptha is straight-run gasoline, and the heavier naptha is generally unsuitable for blending with premium gasolines. Therefore, it is used as a feedstock in various refining processes such as in a reformer. These liberated gases are recovered to manufacture LPG.
In commercial applications, LPG is usually stored in large horizontal vessels called bullets, ranging in volume size from 150 to 50,000 gallons. In industrial applications, LPG is typically stored in large vessels that are spherical or spheroid shaped, the large “golf ball” shaped and oval vessels commonly seen at refineries and similar occupancies. In this article, we will deal primarily with the protection of LPG spheres.
STANDARDS
Various sources of standards and codes exist for dealing with LPG facilities and proper fire protection. Some of these sources include:
• National Fire Protection Association (NFPA) 54, National Fuel Gas Code.
• NFPA 58, Liquefied Petroleum Gas Code.
• NFPA 59, Utility LP-Gas Plant Code.
• American Petroleum Institute (API) 2510, Design and Construction of LPG Installations.
• API 2510A, Fire-Protection Considerations for the Design and Operation of Liquefied Petroleum Gas (LPG) Storage Facilities.
• IP Code of Practice for LPG.
Additional sources of information are available from various organizations such as the British Standards Institute, the World LP Gas Association, the LP Gas Association, and industry producers and suppliers. For the purpose of this article, we will focus on some of the above-mentioned sources that are typically accepted as the industry standard.
FIRE PROTECTION DESIGN CONSIDERATIONS
To reduce the fire risk at LPG facilities, adherence to various design considerations and requirements such as layout, spacing, and distance requirements for vessels, drainage, and containment control will help to limit the extent of fire damage. Additional considerations such as fireproofing, water-draw systems, and relief systems are also important with respect to the integrity of the installation and the risk reduction. These considerations address the various ways to prevent leaks or releases that may lead to a fire.
Equally as important to the prevention of a leak or release is a properly designed, installed, and maintained fire protection system. These systems attempt to minimize or limit the fire damage once a fire occurs. In the situation in which a fire does occur, the levels of required fire protection are affected by several factors such as location and remoteness of the fire and the availability of water.
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To determine if cooling water is required, the anticipated radiant heat flux from an adjacent tank, the maximum tank shell temperatures if the vessel shell is not cooled, and other specific risk management guidelines must be analyzed. API 2510A contains a procedure to identify the point at which cooling water should be applied based on the size of the pool fire and the distance between the vessel and the center of the fire (Figure 1). Additionally, an analysis of the relief valve parameters is necessary to maintain certain internal vessel pressures. Although computer models are available to more accurately anticipate the heat fluxes, this procedure helps to determine if a more detailed study is needed.
Figure 1 considers the radiant heat flux from a pool fire, assuming a 20-mile-per-hour wind. To illustrate this procedure, first locate the diameter of the pool fire along the x-axis. Using an imaginary line from the designated point along the x-axis, locate the corresponding point of intersection on the 7,000 Btu/hr-ft2 line. Next, extend an imaginary horizontal line to the y-axis. The corresponding point of intersection on the y-axis is the distance between the vessel and the pool fire at which cooling water must be applied. For example, if a pool fire is 30 feet in diameter, it is necessary to apply cooling water when the distance between the vessel and the center of the pool fire is approximately 120 feet.
In general, there are three primary methods to apply water for cooling or extinguishing fire on LPG vessels: water deluge, fixed monitors, and water spray. Additionally, portable equipment such as ground and trailer-mounted monitors can be used but should not be considered a primary means of water delivery. This is mainly because of the potentially extended setup times, logistics, and requirement of human intervention that is not necessarily reliable.
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Table 4 describes some of the advantages and disadvantages of the three primary water application methods and the use of portable equipment.
The first method involves the use of a water deluge system and some form of water distributor. This could include high-volume spray heads, perforated pipe, or a distribution weir. An underflow or overflow weir allows water to be evenly distributed over the surface area of a sphere; water flows up the piping network, over the top of the sphere, and out of the weir. This type of water distributor is commonly used but is prone to corrosion from standing water and clogging and requires increased preventative maintenance. Additionally, weirs may not be as effective on bullets and are often greatly affected by wind. The remaining components of this method are similar to other deluge installations. The typical deluge system contains a supply piping network, deluge valve and trim, and branchline-piping network near the top of the sphere. Newer installations are usually activated automatically, whereas older installations are commonly activated manually. The decision of which activation method to use requires evaluation of spacing, available protection, exposures, and other factors.
The principle behind the deluge or weir system for LPG sphere protection is that the geometric shape of the sphere and gravity work together as an advantage. As water is applied to the top of the vessel, the shape of the sphere and the force of gravity facilitate the flow of the water as it covers the surface area of the vessel. This type of protection is very effective to facilitate an even distribution of water over the surface area. Caution should be exercised, however, because paint, corrosion, dust, and other environmental influences can cause changes in the surface of the sphere, resulting in uneven water distribution. Additionally, settling and other conditions inside the weir can also cause uneven water flow over the sphere’s surface.
Fixed monitors, the second method of water application, permit the use of fixed hydrant-mounted monitors or individual monitors connected to the fire main to apply water to the fire area. In this case, water is applied by operators manually opening valves to allow the flow of water to the LPG sphere. This procedure exposes operators to high heat fluxes and places them dangerously close to vessels under fire conditions. It is important to carefully study the plant and vessel layout if this method is elected. Proper placement, location, and quantity of fixed monitors must be reviewed and field tested to ensure that proper application and even distribution of water to all parts of the vessel are accomplished. In some cases, remote activation and operation are suggested when proper spacing of monitors is not a possibility. Additionally, annual testing and preventative maintenance are necessary to ensure parameters have not changed and coverage is still adequate.
The third method of application is the use of water spray systems, comprised of a piping network of spray nozzles that distribute water over the surface area of an LPG sphere. The spray nozzles are positioned to form a grid pattern that facilitates the complete coverage of the sphere’s surface area. Larger orifices and piping should be considered to help reduce blockage because of scale and mussel buildup and other potential problems. It is also important to properly size the strainer to prevent blockage. Inspection of strainers should be part of the preventative maintenance program.
The last method available is the deployment of portable monitors and hoses. This method uses hand-carried portable or trailer-mounted monitors deployed by the fire department. Although it is not one of the three primary methods of water application, preparation and planning for this type of application should not be forgotten. Quantity of monitors, monitor flow calculations, and predetermined hoselays should be reviewed prior to an incident to ensure adequate capabilities are available. This method is considerably more dangerous than the previously mentioned methods because of the exposure of personnel to the hazards and risks associated with LPG firefighting.
When using the four water application techniques discussed previously, a combination of techniques such as the use of a deluge or water spray system and portable monitors provides ample fire protection. A combination of a water deluge/distributor with a fixed water spray system with portable monitor backup from the fire department provides excellent coverage.
A water application rate for these fixed fire protection systems depends on the type of fire situation. When a vessel is exposed to only radiant heat without direct flame contact, a density of 0.1 gpm per square foot of vessel surface area is the minimum. If direct flame contact or impingement occurs, a density larger than 0.1 gpm, up to 0.25 gpm per square foot of vessel surface area is the minimum.
When using fixed or portable monitors, 250 to 500 gpm is the minimum flow that should be considered initially. However, field verification and flow testing are necessary to ensure adequate and proper coverage. Monitor placement must also be field verified against approved plans to ensure acceptable spacing and access.
Vapor, heat, or flame detectors mounted in the vicinity of a vessel can complete automatic activation of these systems. Vapor detection provides early detection and warning, but activation of water application systems must be confirmed through flame detection. Flame detection provides quick activation, but use caution when positioning these detectors to prevent false activation from sunlight. Also consider installing UV/IR combination detectors to reduce the false indication rate. These devices require testing and preventative maintenance programs. An evaluation of the facility is necessary to determine the correct type and location of devices.
RESPONSE
Even with the properly installed fixed fire protection systems, the importance of emergency response to LPG fires cannot be disregarded. LPG fires can escalate quickly, and a lack of manual activities by the fire department can lead to vessel failure. As part of this response, an up-to-date and complete emergency response/preincident plan should include the following:
• Facility name and location.
• Map of facility.
• Emergency phone numbers for key plant personnel.
• Hydrant layouts and capacities.
• Additional water supplies-e.g., ponds, canals. (Are they available in freezing weather conditions?)
• Hoselays and lengths required.
• Multiple response approaches (wind-dependent).
• Vessel inventories.
• Fixed fire protection information.
• Scenarios for both unignited and ignited leaks.
Emergency response/preincident plans should also identify the emergency response structure of the industrial plant as well as the incident/unified command structure that will be used. In some instances, a plant operations person may be acting as the incident commander, with the municipal/town/volunteer department operating in a support role. It is better to have this organized during preincident planning than on the fireground.
The emergency response/preincident plans must be exercised frequently and updated as necessary. Since recent events throughout the world have increased security in industrial plants, obtaining information for preincident planning may be difficult. It is imperative that discussions with plant personnel remain open and take place frequently to maintain a team spirit and facilitate information sharing. Once information is obtained, it is important that fire departments maintain operational security for this information. Emergency response/preincident plans should be secured on the apparatus, and numbered copies should be tracked. These copies should be inventoried on a scheduled basis to maintain operational security.
When responding to an LPG facility, preplans may indicate that responding personnel remain at a staging area, usually near the main gate or other location (depending on wind direction) until directions are received from plant personnel, especially in the case of a leak. We know of instances where vehicles were the source of ignition for a leak. In one case, although not an LPG incident, the fire department apparatus was the source of ignition for a natural gas explosion that destroyed a number of city neighborhood blocks. In another instance, a plant security vehicle investigating a reported leak was the source of ignition, causing a catastrophic loss to the facility.
On arrival, initial assessment of the situation is essential for the safety of personnel. During fire situations, if there has been flame impingement on a vessel, especially on the vapor space, and there is no water cooling of the area or fireproofing on the area of flame impingement, vessel failure could be imminent. If these conditions have been present for 10 minutes or more from the initial impingement (not the fire department arrival), an immediate evacuation is recommended. It is important to remember that the initial time of the notification to the fire department may not be the time the incident started.
If it has been determined that a fire is safe to approach, develop an incident action plan (IAP). This plan does not have to be written initially, but it should contain the following objectives:
• Cooling of exposed storage vessels.
• Water application rates and water supplies available.
• Shutting down fuel supply.
• Monitoring of the surrounding area using combustible gas indicators (CGI).
• Evacuation of nonessential personnel.
• Evacuation routes for responders and plant personnel in case of emergency.
Use unmanned monitors when cooling exposed vessels. As in smaller LPG fire operations, do not extinguish the fire. Instead, shut down the gas supply. Where there is direct flame impingement on an exposed vessel, apply a minimum of 500 gpm of water at every point of flame impingement.
If a liquid pool has not ignited, use water spray to control/dilute the vapors. Water should not contact the spilled material where possible because it will increase the vaporization. Use combustible gas indicators to determine the extent of the vapors. Always remember to position fire apparatus upwind and uphill.
The fire department alone cannot control the incident. Expert advice, cooperation, and remedial actions by plant personnel are needed. Begin to foster this cooperation and identify those plant personnel who can assist the fire department during preincident planning and familiarization visits.
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Since most LPG fires originate as smaller fires that become increasingly more dangerous, quickly using the three primary water application methods can help reduce the risk of vessel failure. Deploying portable monitors and hoses, although not one of the three primary methods of water application, is an important backup to the primary methods. LPG fires can escalate quickly, and a lack of manual suppression by the fire department can lead to vessel failure. You must, however, take control of the fuel source before attempting to suppress the fire. In any case, an emergency response plan, along with proper training and drills, is important to reduce the risk of injuries and promote a quicker and safer response. ■
References and additional reading
American Petroleum Institute (API) 2510, Design and Construction of LPG Installations, American Petroleum Institute, Washington D.C., 2001
API 2510A, Fire-Protection Considerations for the Design and Operation of Liquefied Petroleum Gas (LPG) Storage Facilities, API, Washington D.C., 1996.
FM Global, Guidelines for Evaluating the Effects of Vapor Cloud Explosions Using a TNT Equivalency Method. Factory Mutual Insurance Company, Johnston, Rhode Island, 2001.
Friedman, Raymond. Principles of Fire Protection Chemistry and Physics, 3rd edition. National Fire Protection Association, Quincy, Massachusetts, 1998.
Industrial Fire Hazards Handbook. National Fire Protection Association, Quincy, Massachusetts, 1990.
Understanding Fire Protection for Flammable Liquids. National Fire Protection Association, Quincy, Massachusetts, 2003.
Flammable and Combustible Liquids Code Handbook, 6th edition. National Fire Protection Association, Quincy, Massachusetts, 1997.
Fire Protection Handbook, 19th edition. National Fire Protection Association, Quincy, Massachusetts, 2003.
The SFPE Handbook of Fire Protection Engineering, 3rd edition. National Fire Protection Association, Quincy, Massachusetts, 2002.
Zalosh, Robert G.,. Industrial Fire Protection Engineering. John Wiley and Sons, West Sussex, England, 2003.
■ CRAIG SHELLEY, CFO, EFO, MIFireE, is a fire protection advisor for a major oil company working in the Middle East. He has previously served as the chief of marine operations for the Fire Department of New York and as the chief of the Rutland (VT) Fire Department. He has served as a task force leader for FEMA’s USAR NY TF-1. Shelley has a bachelor’s degree in fire service administration and a master’s degree in executive fire service leadership and has served on two National Fire Protection Association technical committees.
■ ANTHONY COLE, CFPS, CFEI, MIFireE, is a fire protection engineer with a major oil company operating in the Middle East. He previously served with an iron and steel facility in the Middle East as a fire protection engineer and fire chief. Cole has also been a paid firefighter in Ohio and Mississippi and has served on various volunteer fire departments in Ohio, Kentucky, Missouri, and Mississippi. Cole has a bachelor’s degree in fire protection engineering technology from Eastern Kentucky University and is working toward his master’s degree in fire protection engineering from Worcester Polytechnic Institute. He serves on two Society of Fire Protection Engineers committees and three National Fire Protection Association Technical Committees and is a member of the International Association of Fire Chiefs.
