Protection of Gas Turbine from Fire
Carbon dioxide and halon 1301 traditionally have been the extinguishing agents of choice for the turbine, auxiliary components and generator in packaged units. Preaction waterspray systems have been used for turbine units in buildings, but care must be exercised so water does not directly impinge on hot turbine parts. For preaction waterspray systems to discharge, one or more fire detectors must actuate, and heat must be present to melt the fusible links in the nozzles; as such, they are less susceptible to inadvertent discharge. The National Fire Protection Association (NFPA) publishes standards covering the design and installation requirements for fire protection systems.
For many years, gas turbine operators preferred halon over CO2 because of halon`s non-toxic properties. The required halon concentration for gas turbine applications is 7 percent per Hartford Steam Boiler (HSB) and other accepted standards. For normal applications such as control and switchgear rooms, the required concentration is 5 percent. There are, however, some drawbacks to halon. After halon has acted on a fire or is exposed to surface temperatures above 900 F, it breaks down into corrosive and toxic byproducts (hydrogen fluoride, hydrogen bromide and free bromine).
Carbon dioxide is inexpensive and nondestructive to equipment, but is lethal at the concentrations required for fire extinguishment. According to NFPA 12, Carbon Dioxide Extinguishing Systems, the minimum required CO2 concentration for fire extinguishment is 34 percent, which is adequate for fuel and lube oils. For natural gas and rotating electrical equipment, however, the CO2 concentration must be increased to 37 percent and 50 percent, respectively.
The halocarbon FM-200 has gained the most use as a halon alternative, primarily because it has no ozone depletion potential. Although it is not a `drop-in` replacement for halon, it does act on fire at the molecular level by the release of free radicals to inhibit the combustion chain reaction, similarly to halon. FM-200 shares many of the same advantages as halon, except that about 60 percent more volume is required. Other disadvantages include high price (comparable to halon) and decomposition at high temperatures. The minimum design concentration for FM-200 is 7 percent for flame extinguishment and 8 to 12 percent for explosion inerting.
Inergen, a mixture of various inert gases that acts on fires through oxygen depletion, has also been used as a halon alternative in occupied areas. Inergen`s advantages are similar to those of CO2 with the additional benefit that fire extinguishment concentrations are not lethal. Drawbacks include high volume requirements (about 10 times those of halon) and potential enclosure overpressurization. The minimum design concentration is 38 percent.
Although not a new technology, a highly promising agent for the protection of combustion turbine generators is watermist, where the extinguishing mechanisms are heat removal, oxygen depletion and steam expansion. Watermist is safe for occupied areas and provides a better cooling effect than gaseous suppression agents. Unlike gaseous systems, which are engineered for a specific application, watermist systems are pre-engineered and approved for a compartment of certain maximum volume. Watermist systems also are not as sensitive to enclosure tightness as gaseous systems. NFPA 750, Watermist Fire Protection Systems, governs the design, installation, maintenance and testing of these systems. There are currently two watermist systems, manufactured by Securiplex and Marioff, that are approved for use in combustion turbine enclosures. These systems use single or dual piping and can deliver suppression volumes greater than 9,000 cubic feet.
Since fires in combustion turbines are deep-seated and can reflash in the presence of hot surfaces, a retention time needs to be provided for all gaseous agents. The standards for suppression agents do not address the length of the retention time, but NFPA 850, Electric Generating Plants and High Voltage Direct Current Converter Stations, suggests a minimum of 20 minutes. Aeroderivative turbines generally have rundown times of around 5 minutes, while industrial turbines require rundown times of around 20 minutes due to increased mass. During rundown, additional lube oil release can be expected, making continued suppression necessary.
Combustion turbine manufacturers have typically equipped existing units with halon or CO2 fire suppression systems, in one of three discharge configurations: "one-shot" single discharge, connected reserve and extended discharge. Experience has shown that a 20-minute retention time generally will not be maintained by a single discharge system; therefore, these types of systems are the most suspect and should be the first candidates for an enclosure tightness test. A connected reserve system will usually discharge if a fire reflashes, thereby accomplishing the same effect as an extended discharge system. It would be advisable, however, to have the connected reserve system discharge automatically after the initial discharge by installing a timer. For turbines with extended discharge systems, the length of the extended discharge should be verified to at least meet the rundown time of the protected equipment.
Many combustion turbine manufacturers do not recommend a full discharge and concentration acceptance test. HSB Professional Loss Control has experienced an 80 percent failure rate for acceptance tests conducted at insured sites. These failures were mainly attributable to insufficient agent concentration (for the specified duration) and faulty installation/hardware.
A full acceptance test consists of a full discharge of the gaseous agent while measuring the inside of the compartments with an analyzer for proper agent concentration. This is the best and most complete method of acceptance testing and is a requirement of NFPA 12. `Dry` and `puff` tests are not adequate substitutes for a full acceptance test. NFPA 12A, Halon 1301 Fire Extinguishing Systems, and NFPA 2001, Clean Agent Fire Extinguishing Systems, do allow the use of an enclosure integrity test (i.e., door fan test) as a substitute for a discharge test.
What about halon systems in older units? Obviously, halon concentration tests can no longer be conducted. If plant personnel performed an acceptance test before start-up, original test documentation needs to be verified for proper agent concentration and retention time. This should include an evaluation of strip charts showing the concentration over a length of time. If this information is not available, or if a visual inspection of the unit reveals a questionable enclosure tightness, a door fan test (see sidebar, below) should be conducted. As an alternative to the door fan test, an actual discharge test can be conducted with a test gas such as halon 121, sulfur hexafluoride, which has similar physical characteristics as halon 1301. Even if the unit passed the concentration test initially, many factors such as deterioration of door seals, malfunctioning dampers and the removal of enclosure panels during outages will negatively affect the agent concentration. Therefore, a discharge/enclosure integrity test should be conducted periodically, such as every 5 years. Since high-pressure storage cylinders need to be hydrostatically tested at least every 12 years, this is a good time to perform a discharge/concentration test (mainly on CO2 systems).
Maintenance and Use
NFPA codes and standards outline the testing and inspection requirements for fire suppression systems:
CO2 and Halon Systems
Monthly inspections should include visual checks of the cylinders, actuation valves, hoses, liquid level (low-pressure systems), manual actuators, detection systems and the control panel.
Semiannual testing should include the weighing of all high-pressure cylinders and notation of last hydrostatic test.
Annual testing should include a complete test of the system, including manual and automatic releases, detection, control panel functions, auxiliary interlocks, alarms and piping/nozzles.
Weekly water level and air pressure checks and churn testing of pump (if provided).
Quarterly alarm and main drain test (if equipped).
Annual complete system inspection of components and testing of actuation components.
Human element factors are equally important. Procedures should be in place regarding operation of the unit with respect to the fire protection system:
Plant operators should be trained to keep enclosure doors closed at all times. In hot climates, personnel sometimes keep the enclosure doors open to improve cooling of the unit as a way to get a few more megawatts. This severely compromises the effectiveness of the fire suppression system.
Under no circumstances should the unit be run without the fire suppression system in place. If no connected reserve is provided and cylinders cannot be recharged in a short time, a spare bank of cylinders may be necessary.
Procedures should be in place regarding entering the enclosure during operation of the unit. Although undesirable, it is acceptable for safety reasons that the automatic operation of the fire suppression system be temporarily deactivated when personnel enter the enclosure for routine inspections.
If additional vents are installed for cooling, automatic closing systems need to be provided. The installation of additional vents can also change the enclosure integrity and a new concentration test may be necessary.
Equipment maintenance is foremost in the prevention of fires. All hoses and fittings should be in good condition and the floor of the enclosure should be free of oil accumulations and rags. Any obvious openings in the enclosure should be promptly sealed.
Although turbine damage is typically insignificant when a fire is successfully controlled and extinguished by a well designed fire suppression system, there still can be fire and smoke damage to the turbine and the compartment. Flame impingement on an aeroderivative turbine casing can cause thermal shock or slight deformation, which will cause rubbing of the blades. Damage can also be expected to insulation, steam, fuel and instrumentation lines. It could take several weeks to return the unit to operational conditions, assuming that the turbine must be removed for minor repairs.
When an inadequately designed and/or tested fire suppression system fails during a fire, severe turbine and compartment damage can result, amounting to 35 to 45 percent of the installed unit cost. An LM5000 gas turbine system, for example, could incur losses up to $5 million. If the plant is part of a lease pool, or has a spare turbine on site, the expected business interruption can be 3 months for the repair of the compartment until a replacement turbine can be installed. If the plant does not belong to a lease pool or a spare is not available, it can take up to 6 months to restore the unit to operational condition. p
Discharge testing of a watermist system on a combustion turbine. Photo courtesy of Securiplex Inc.