This article was originally published in Turbomachinery International Magazine’s Blog
The two main fire hazards affecting gas turbine generators are fuel and lubrication, and hydraulic oil. Even the best protected fuel arrangement can cause an external ignition of the fuel, resulting in a fire. Similarly, mineral lube and hydraulic oils can result in a fire in the event of a leak or an oil spray which ignites on hot turbine parts.
The most popular fire suppression agent used for gas turbines today is carbon dioxide (CO2). In general, CO2 systems have a good track record of effective operation. CO2 works primarily by removing the oxygen component from the fire triangle and its advantages comprise the lack of any residue, low cost and its electrical non-conduction properties. One of the main disadvantages is its asphyxiation hazard.
The current design and installation standard for CO2 systems is the 2011 edition of National Fire Protection Association (NFPA) 12, Carbon Dioxide Extinguishing Systems. The CO2 supply can be either from individual high-pressure cylinders or from a refrigerated low-pressure storage tank. Cylinders offer the advantage of off-site filling, and also cost less. One of the major storage tank advantages is that there is no need for hydrostatic pressure testing, and there is the availability of a larger CO2 supply. As the need for larger CO2 volumes increases, the price difference between cylinders and storage tank decreases.
Total-flooding mode for gas turbines
For gas turbines, CO2 is typically applied in a “total-flooding” mode, which requires a tight enclosure around the turbine to build up the necessary concentration. Although CO2 can also be applied using a “local application” method without an enclosure, this is not a preferred approach because there is the risk of the CO2 getting re-ignited after dissipation. Per NFPA 12, the minimum acceptable design concentration for liquid fuels is 34 percent by volume.
If the fuel is natural gas, the minimum concentration is raised to 37 percent to provide inerting, in addition to extinguishing the flame. NFPA 12 further breaks down total-flooding systems into two sub-categories of surface fires or deep-seated fires. From here on, the requirements are more difficult to find, mainly because they are contained in standards other than NFPA 12. Turbine fires can either be surface fires which assume prompt extinguishment (i.e. pools of burning fuel or oil) or deep-seated fires which assume that the fire is not immediately extinguished (i.e. insulation materials, smoldering conditions). Therefore, the more conservative approach should be used from both sub-categories.
The requirement for surface fires is that the design concentration must be reached within one minute from the start of discharge. In accordance with NFPA 12 for total-flooding systems, the concentration “…shall be achieved and maintained for a period of time to allow effective emergency action by trained personnel.” The following standards specifically address the duration of the retention time for total-flooding systems as they relate to gas turbines.
In accordance with NFPA 850, the concentration “…should be held as long as the hazards of hot metal surfaces above the auto-ignition temperature [of fuel and/or oil] and uncontrolled combustible liquid flow exist.” Unless this information is known and published by the manufacturer, which is unlikely, this performance-based statement is of little use to fire protection system designers.
Minimum retention time
NFPA 850 further suggests that this duration is at least 30 minutes for large industrial type turbines, but does not give a quantitative value for aeroderivative turbines. However, NFPA 37 requires a minimum duration of 20 minutes for any type of turbine. Similarly, Data Sheet 7-79 establishes that the duration should be either 20 minutes, or the turbine rundown time plus 10 minutes, whichever is more.
Therefore, the retention time should be at least 20 minutes for the lighter aeroderivative turbines and 30 minutes for frame turbines. The minimum required concentration at the end of the retention time varies according to the source. In accordance with NFPA 12 and NFPA 37, the design concentration must be maintained for the required duration, which can be estimated to be either 34 or 37 percent, from the discussion above. FM Global, meanwhile, requires that the concentration should be at least 30 percent for the extinguishing period after the initial design concentration has been achieved.
A system that discharges a predetermined quantity of CO2 and then counts on an initial higher concentration to decrease gradually (hopefully not less than the minimum concentration at the end of the required duration) is adequate for short retention times (i.e. 10 minutes or less). This is referred to as a “single-shot”. However, experience has shown that most turbine enclosures do not offer the level of “tightness” required for a single-shot concentration to last 20 minutes or more. Therefore, an extended discharge is a prudent method. This consists of a smaller secondary CO2 system that continues to “trickle” a smaller quantity of CO2 into the enclosure after the initial discharge is exhausted. The quantity of CO2 provided for the extended discharge should take into account the required duration.
As mentioned above, CO2 carries a unique personnel hazard – the potential for asphyxiation, or death due to lack of oxygen. NFPA 12 has taken an aggressive stance, beginning with the 2005 edition, by making stringent personnel safeguards mandatory for both new and for existing systems. These include a proliferation of ANSI Z535-compliant warning signs, manual lockout valves, and audible and visual alarms.