WATER MIST FIRE SUPPRESSION FOR GAS TURBINES
The first automatic sprinklers were invented in the mid 1800’s, and consisted of fusible link actuated sprinkler heads installed on pipes. Formal specifications were being developed to standardize the design and installation of these sprinkler systems by the end of century. As known, fire sprinklers utilize water by direct application onto flames and heat, which causes cooling of the combustion process and prevents ignition of adjacent combustibles. Conventional sprinklers remain the main technique for fixed firefighting installations around the world during two centuries. Moreover the use of systems that apply water fog called water mist system at the end of the 20th century has been commercially viable even they developed at the beginning of same century. Water mist refers to fine water droplets in which 99% of the volume of the spray is in drops with diameters less than 1000 microns [1]. These pulverized small water droplet increases water’s fire suppression capacity and that is why water mist system can suppress same fire with less water. In these systems where water is used in small particles, the diameter of the distribution pipes, the capacities of the pumps to be used and the water tank requirement are much less than sprinkler system depending on the decreasing amount of water applied. Furthermore, it is ensured that the damage caused by water is minimized in the place where it is protected.
System designs; (1: 1 scale fire test) conducted by the manufacturer. The Fire Protection Subcommittee of the International Maritime Organization (IMO), has developed a fire test protocol [2] for water mist systems that are intended to replace sprinkler systems in marine machinery compartments. There are many tests for prototype machine rooms or turbine booths and the design parameters are very reliable for these systems which have widespread use in the maritime industry. The approval of an internationally validated system for the systems to be applied and the design of appropriate design criteria are necessary for proper application of the extinguishing system. It is necessary for the correct application, the extinguishing system must be accepted by an institution that has proven validity in the international arena and design criteria have been determined clearly.
Nowadays; gas turbines are widely used in industrial plants, pipelines and power plants. As already mentioned, water mist system which finds its use in internal combustion engines or in spaces containing gas turbines, the DURAQUENCH ™ that is an environmentally-sensitive and sustainable system approved by FM for this system, design criteria is examined below.
First of all, the primary fire hazard associated with the gas turbine compartments is failure of the fuel or lubrication systems. If either of these systems leaks during turbine operation, the flammable liquid may come into contact with a surface that has a temperature well above the auto-ignition temperature of the fuel and cause a fire. In most cases, the fire threat is identified as the ignition of Class B fuels in the following configurations:
- Pools of fuel leak on equipment surfaces and/or the floor of the enclosure
- Fuel sprays due to a rupture of pressurized fuel line
- Potential Class “A” involvement caused by ignition of fuel soaked materials
Historically, gas turbine generators were protected against fire with either CO₂ or Halon 1301. In a study on gas turbine fires conducted by R.E. Dundas, a 49% failure rate for total flooding Halon 1301 or carbon dioxide systems was reported. Thirty-seven percent of these failures were attributed to the gaseous agent leaking from the protected enclosure through open doors or ventilation in the article ASME-90-GT-375
The DURAQUENCH ™ water system eliminate both risk of failure because it is not affected by impermeability.
It is known that during the operation of the gas turbines, ventilation is required in order to keep the equipment relatively cool. On the other hand, during this operation the wall temperature of the combustion chamber is increased up to 900 ° F (482 ° C). When the water mist is discharged, it cools these outer surfaces by heat transfer, turning the water droplets into steam. As mentioned above, the DURAQUENCH ™ water system capable to extinguish fire in the cabin when the cabin is not fully closed and ventilation is applied.
Predominantly, the system to be designed for the protection of the entire turbine cabinet depend on the FM-approved design criteria given in Table 1.
The extinguishing head must be placed as shown in Figure 1. The extinguishing head must not be placed on the turbine centerline to prevent damage to the turbine casing from thermal stress.
The flow rate of each nozzle is given by;
Q = K √ (P)
Where:
Q = Flow rate (US gpm or l/min)
K =Flow rate of unit pressure (√ (gpm/psi) or √ (l/min/bar))
The total flow rate can be find with integration of each nozzle.
The above calculation method is theoretical and practical calculation solutions should be confirmed with the hydraulic calculation software approved by the international independent audit institutions.
REFERENCES
1. NFPA 750, "Standard for the Installation of Water Mist Fire Protection Systems," 1996 Edition, National Fire Protection Association, Quincy, MA, 1996.
2. International Maritime Organization (IMO), "MSC/Circ. 668, ANNEX, APPENDIX A (Component manufacturing standards of equivalent water-based fire extinguishing systems) and APPENDIX B (Interim test method for equivalent water-based fire-extinguishing systems for machinery spaces of Category A and cargo pump rooms),î London, 1994.
