Designing for Plant Fire Protection
By Dominique Dieken, P.E., CFPS, HSB Professional Loss Control
Because of the Increasingly Competitive Nature of the Electric Power Generation Market, Reduced plant staffing and stricter OSHA (Occupational Safety and Health Administration) requirements for employee fire fighting capabilities, builders of new generating stations need to place increased emphasis on fixed fire protection systems. Owners, investors, operators and insurers want to minimize the impacts and consequences of fires, which can damage critical equipment and reduce availability and reliability of power supplies.
Drafting specifications for a new station requires knowledge of the related codes and standards governing the design and installation of water supplies, suppression systems and life safety equipment. The National Fire Protection Association (NFPA) is the most recognized authority, publishing codes, standards and recommended practices in various fire protection areas (see table). NFPA 850, Electric Generating Stations and High Voltage Direct Current Converter Stations, contains the general protection criteria for the plant and for the individual equipment. Specific design and installation requirements are contained in other NFPA standards.
Prompt detection of fire is critical to employee evacuation and for notification of the fire department and plant emergency organization. All fire alarms should be announced on a central panel, plainly visible in the control room. The alarm system, in accordance with NFPA standards, should include the announcement of fire alarms, supervisory signals and trouble signals. Alarms require immediate action. Supervisory signals indicate an abnormal condition that should be investigated and corrected. Trouble signals indicate an adverse component or hardware condition such as an interrupted circuit, ground fault or power supply problem and should be repaired by qualified personnel. The current state-of-the-art alarm system is a microprocessor-based addressable system. Each detector or component has an identification, or address, and is connected to the control unit by a common conductor. The control unit and initiating devices (i.e., detectors) communicate on a regular frequency to verify that everything is operating normally.
Protection of the power block requires both passive (building features) and active (suppression) protection. Two-hour fire-rated construction, separation distances of at least 30 feet, or suppression systems such as water curtains should be provided to all exposed operations, which include offices, storage areas/warehouses, maintenance shops, control rooms, electrical rooms, cable spreading areas, battery rooms, fuel handling and transfer areas, turbine generators, boilers, fire pumps and diesel generators.
Boilers. Boiler or steam generator burners should be equipped with the recommended combustion safeguards in accordance with NFPA standards. Where fuel oil is used, the firing aisles should be protected by automatic sprinklers, including 30 feet beyond the burners and over nearby cable trays. Firing aisle floors should be of solid construction with containment or drainage to prevent burning liquid from exposing other burner levels.
Combustion turbines. Most combustion turbine packages are installed with pre-engineered gaseous suppression systems, typically CO2. Protection is usually provided for the accessory, turbine, load and generator bearing compartments. As with any gaseous fire suppression, CO2 needs a minimum concentration (34 percent) to extinguish fire, which needs to be increased for specific hazards. Openings in the enclosure, faulty dampers or hardware can compromise this concentration. A common fallacy is that vendors are reluctant to prove the integrity of the fire suppression systems by performing a discharge/concentration test, due to the cost. The unsuccessful operation of a fire suppression system can result in damage equivalent to 45 percent of a unit`s installed cost. Therefore, every gaseous fire suppression system should undergo a complete acceptance test, including a discharge/concentration test during which the actual agent concentration is measured with an analyzer at various points within the enclosure.1
Lube oil systems. Most turbine-generator fires involve oil. Oil escaping from the turbine/generator lube system can ignite on nearby hot turbine surfaces and involve the area below the turbine operating floor. An uncontrolled fire at the bearings, usually from a ruptured seal, can result in damages of up to 25 percent of the turbine generator unit`s cost. The preferred protection for steam turbine-generators is automatic sprinklers under the entire turbine operating floor, including any mezzanines between the equipment floor and the turbine level, and under the condenser. Spill containment in the form of drains, curbs, pits and/or trenches should be incorporated into the building design to contain accidental lube oil releases, from both a fire protection and environmental standpoint. Note that the intent is to contain and control a fire; therefore, sprinkler protection needs to be provided over all areas that are subject to oil flow, accumulation or spray. Where containment is impractical, a foam-water sprinkler system should be considered. The foam blanket will smother the burning liquid and help prevent fire from spreading. Automatic fire suppression also needs to be provided over the turbine and generator bearings and any lube oil lines, including under the turbine lagging. The intent is not to protect the bearings but to provide protection from an oil fire.
A 1985 EPRI report indicates that fire damage outweighs the effects of applying water to a steam turbine from a properly designed suppression system. Preaction waterspray systems are best suited for turbine fire protection because of their high assurance against inadvertent discharge (see Power Engineering, April 1998). The favored preaction waterspray configuration requires two nozzles at the 10 o`clock and 2 o`clock positions over each bearing. The hydraulic demands of the bearing and turbine deck systems should be added since both systems can be expected to operate simultaneously.
Cable trays. Cable trays present a unique hazard in that fire damage to even a small area of cable can cause extensive disruption to operations. Cable trays should be routed away from major fire exposure hazards, sources of ignition, boiler fronts and flammable or combustible liquids. The accumulation of coal dust in a cable tray can result in spontaneous combustion and subsequent fire. If coal dust or an oil spill potential is present, the cable trays should be covered and solid bottom trays should not be used. If exposures cannot be avoided, fire retardant coatings or specifically designed blankets can be applied to the cables and the tray.
Cooling towers. Where practical, non-combustible towers are preferred. As incredible as it seems, cooling towers have burned down while in operation. Combustible towers should be provided with automatic sprinkler protection, including protection over the electric fan motors. Wet pipe (climate permitting), dry pipe and deluge systems are suitable for a counterflow tower, although deluge systems provide a higher degree of protection. Only deluge systems are adequate for cross-flow towers due to the differences in placement of heat detectors and sprinklers inside the tower. Vibration monitoring with a fan interlock to trip the fan motor as well as a waterflow interlock and trip are necessary; otherwise the high draft will not allow adequate water penetration to the seat of the fire. It is accepted practice to install a switch to reactivate the fan in case of an inadvertent trip.
Control Rooms, Computer Rooms and Electrical Areas. One of the most common concerns from fire protection laymen is the application of water to electrical/electronic equipment. Many studies conducted by insurance carriers and government agencies have shown that the water damage caused by sprinklers is insignificant when compared to the damage caused by fire. The reason for this is that most fires are controlled by only a few sprinklers which would affect a relatively small area. Electrical and electronic equipment can be cleaned, rinsed with deionized water and dried with minimal damage.
Designers should give special attention to the design of the control room, since the evacuation of control room personnel may be delayed for emergency shutdown procedures. Since a view of the turbine deck is usually desired, the glass between the control room and the turbine deck should be fire rated or protected by window sprinklers or fire-rated shutters. In accordance with life safety regulations, at least two exits must be available from any room. In the case of a turbine fire, the exit to the turbine deck may be inaccessible, and another exit with a 2-hour fire rated path is necessary. The need for proper exits was demonstrated in an unfortunate incident at a cogeneration facility which took the lives of three operators in 1992.2 Control rooms and computer rooms should also be provided with preaction sprinkler protection (similar to turbine-generator bearings) or a halon-alternative gaseous fire suppression system, other than carbon dioxide. While halon-alternative gaseous fire suppression systems provide a good level of protection, a preaction sprinkler system can be less expensive to install and maintain while providing a satisfactory level of protection.
Cable tunnels and vaults present an extreme challenge to manual fire fighting due to their limited accessibility and minimal visibility in a fire. In addition, burning PVC insulation liberates hydrogen chloride (HCl) gas. Burn tests conducted and witnessed by HSB Professional Loss Control have shown that even `fire retardant` cable insulation will burn quite readily once ignited. Also, these areas are typically a vital component of the plant and fire damage would cause an extended shutdown.
Cable penetrations through fire-rated walls or floors should be designed to be readily sealed with listed/approved firestopping materials such as ceramic and mineral fibers, silicone, foams and intumescent materials. Their fire ratings should match those of the wall or floor. Note that urethane foam, even if "fire retardant," is not an equivalent firestopping material.
Transformers. Outdoor oil-filled transformers containing over 500 gallons of oil should be protected by separation distance, rated fire walls or automatic waterspray systems. Studies by Factory Mutual have indicated that approximately one out of ten transformer failures will be followed by fire. If adequate exposure protection is not provided, a fire involving one transformer can easily damage adjacent transformers or important structures. Loss experience reveals that repairs to fire-damaged transformers usually are not justified. A risk-based study by Hartford Steam Boiler concluded that waterspray deluge systems are usually not economically justified for transformers under 300 MVA if adequate separation distance or properly designed fire walls are provided.
Once the hazards and their protection criteria have been determined, an adequate and reliable water supply needs to be provided.
Deluge sprinkler systems for cooling towers can have demands exceeding 1,500 gpm. With the required 500 gpm for hose streams, this can add up to over 2,000 gpm. Per NFPA 850, the minimum required duration of a water supply is 2 hours, which would result in a required volume of 240,000 gallons in the case above. Note that many insurers require a longer duration depending on the location and remoteness of the plant. To provide redundancy, two fire pumps, one diesel and one electric, are usually installed in parallel, with either pump meeting the full fire flow requirements. Although economically attractive, a cooling tower basin is generally not a reliable source since it will be emptied at some times for repairs or cleaning, leaving the entire plant without protection. The exception to this would be a split basin, with pump suction capability from either basin, or a fire pump taking suction from a second cooling tower, provided that the required volume can be supplied.
A system of underground water mains needs to be provided. This should consist of a minimum 6-inch loop around the power block, with individual legs to the equipment/buildings provided with hydrants and sprinkler systems. The location of fire main isolation valves should be arranged such that any section of the loop can be isolated without impairing fire protection systems. Valves should be of the post indicating type. Hydrants should be provided at least 40 feet from buildings/structures and should be spaced so that any building/structure can be reached with a 250-foot maximum radius. It is acceptable to use non-indicating underground valves for individual hydrants. p
Oil-filled transformer fire caused by turbine disintegration. A waterspray fire suppression system was provided but damaged by turbine part projectiles. Note the effectiveness of the fire walls, but openings in the wall above the transformer could expose inside equipment and cause smoke damage.
Simulated cable tray fire at a test facility. Even "fire retardant" cable is still highly combustible once ignited. Minimum required duration of water supply is two hours.
Dominique Dieken, P.gif., CFPS, is a senior engineer with HSB Professional Loss Control. He provides field engineering for electric power generation facilities in the Western U.S. and Europe. He is a graduate of Cal Poly San Luis Obispo, is a registered fire protection engineer and holds a National Board Commission.
1 Dieken, D., "Gas Turbine Fire Protection," Power Engineering, April 1998.
2 NFPA Alert Bulletin No. 93:1, National Fire Protection Association, February 1993.