All of these variables and more play a role in the design of alternative fire suppression systems. In the wake of the Montreal Protocol, the Clean Air Act (ACC) and other environmental measures, the market was flooded with new products, vying to fill the void left by Halon 1301.
The Frontrunners Emerge
Ultimately the market has settled down and substandard products have been eliminated, leaving a core group of solutions that meet these environmental regulations and individually offer their own benefits for varying applications. Of the suppression products that have materialized over the past few decades, clean agents, hypoxic (oxygen deprivation) systems and water mist have solidified the strongest positions in the marketplace.
Although these methods have the strongest presence in power generating and chemical processing plants, there are other alternatives that are fitting for these applications. Carbon dioxide systems have a long history in these environments but must be applied only in very specific conditions in order to preserve life safety. Aerosols are another suppression alternative that is currently on the rise with a growing presence in industrial applications. As is demonstrated in the Alternative Suppression Agents Matrix at the end of this article, there are an abundance of suppression options available to facilities, but careful evaluation is often needed to find the optimal solution for the specific hazard.
In terms of functionality, clean agents are the alternative that most closely mirror the operation of Halon 1301. By using a mix of cooling effects and inhibiting the chemical interaction of the free radicals of the heat chain reaction and the oxygen/fuel, fires are effectively suppressed.
The category of clean agents generally covers HFC (hydrofluorocarbons) style agents such as FM-200 and ECARO-25 (HFC-125). The early emergence of FM-200 on the market as a replacement, but not a direct drop-in replacement substitute to Halon 1301, has allowed it to gain a massive share of the special hazard systems market, ranging from 75% to 85% of the systems installed.
Another prevalent clean agent that is not of the HFC family is the Novec 1230 Fire Protection Fluid, manufactured by 3M. Because it has a much higher boiling point than FM-200 & ECARO-25, its advent into the market came with a high degree of visibility because it could be displayed in open containers. The boiling point for FM-200 is 2.48 oF and ECARO-25 is -55.3 oF, whereas Novec 1230 has a boiling point of 120 oF.
The initial price of clean agents compared to inert agents can be a deterrent for facilities when selecting a solution, but the higher cost is offset by economical storage options. Inert agents themselves are comparatively inexpensive but the 360psig stored pressure vessels in which clean agents are housed more than compensates for the cost of the gas, making clean agents the most economical alternative to traditional sprinkler systems.
A caveat with hydrochlorofluorocarbon (HCFC) based agents such as FM-200, HFC-125 and the fluoroketone, NOVEC 1230, is that when exposed to high temperatures, one of the byproducts of thermal decomposition is hydrogen fluoride (HF). This is a caustic acid that can have toxic effects on people and destructive effects on equipment. Factors such as the size of the fire and temperatures involved has a direct bearing on the amount of hydrogen fluoride produced. With the vast majority of installations the level of HF will not reach dangerous toxic loads (DTL).
This class of agents utilizes three primary inert gases in varying quantities: nitrogen, argon and carbon dioxide. Utilizing these gases for the purpose of fire suppression involves depriving a fire of oxygen by inserting inert agents, which effectively displaces a significant amount of the room's atmosphere, lowering the level of oxygen to the threshold at which combustion is not supported. Although an effective method of suppression, there are several considerations that must be regarded to determine if inert agents are a viable solution for a particular environment.
Because of the substantial shift in the concentration of atmospheric gases, human exposure must be severely restricted. The general standard for all systems is a maximum exposure time of five minutes per NFPA, although this time interval decreases as concentrations of the agent rise. NFPA sets these limits not only because of the dangers associated with the inert or clean agent themselves, but also because of the undesirable particulates inherent to the fire itself and the risks associated with possible thermal decomposition. Another essential variable that must be assessed is the environment itself.
Proper design of the room in which an inert agent could potentially be deployed is critical to its effectiveness. A typical atmospheric composition is approximately 21% oxygen, 78% nitrogen and a 1% amalgamation of CO2, methane, helium and trace amounts of other miscellaneous gases. To successfully control a fire through inert agents, the oxygen level in the room must be reduced to 15% or less. For this to be achieved, anywhere from 35% to 50% of the room volume will be replaced with the inert agent in the span of 60 seconds, making it essential that proper venting exists to exhaust the ambient atmosphere of the room. Failure to provide venting could result in collapsed walls or blown out doors, putting facility occupants at risk. Inert agent hydraulic calculations can provide enclosure minimum strength requirements and required minimum venting to ensure the structural aspects of the room are sufficient.
Beyond the prerequisites of the area that is being protected, plants also need to be cognizant of the requirements associated with storage of the gases. Inert agents, without refrigeration, must be stored as a gas under pressure. To maximize the amount of inert agent available, systems are designed to provide storage in pressures up to 300 bar or 4351 pounds per square inch gauge (PSIG). Most systems are based on 200 Bar (2900 PSIG), making the most expensive component of the system the storage tanks. Due to their limited capacity, many tanks must typically be assembled and manifolded together to protect a space, driving up the cost of the system. A manifold assembly of schedule 80 or 160 piping is required to handle the pressure until an engineered pressure reducer orifice is reached. These orifices reduce the pressure and the flow to levels that schedule 40 piping can sustain for the balance of the system piping to the discharge nozzles.
This group of systems relies chiefly on the most traditional medium for suppressing fires- water. There is a very diverse and wide-ranging product offering of water mist systems available, based on pumped or "twin fluid" systems, giving facilities the flexibility they need for their specific environment. A good fit for mechanical spaces, turbine areas/enclosures and machinery spaces, water mist systems are suitable for environments that present a primarily Class B hazard with limited Class A combustibles.
The foundation of water mist systems is miniscule water droplets that are many times smaller than those created by a typical sprinkler head. With a size range of 10-400 microns, (for reference, a sheet of copy paper is approximately 100 microns thick) these droplets are extremely buoyant and have an overall elevated surface area. When a water droplet impinges on a fire the fire is cooled and the water droplet is converted to steam, expanding at a 1 to 1700 ratio, which also deprives the fire of oxygen.
Some water mist systems also utilize nitrogen to generate a smaller water droplet size through specially engineered nozzles or a distribution system, creating a more robust suppression solution. Nitrogen is used in the piping leading to or at the specially engineered nozzles, displacing the room volume. The amount of nitrogen that is introduced into the room is not as substantial as an inert agent system but it still provides significant aid in the fire suppression effort.
Another solution that gained popularity following the halt of Halon 1301 production was carbon dioxide systems, which have maintained a presence in specific applications. A potentially lethal agent, the levels of concentration that are required to control a fire also diminish the level of oxygen to a degree that the atmosphere can no longer support life. Because CO2 is heavier than air, there is also a risk in any low lying areas immediately adjacent or underneath the hazard where the gas may "pool". In recent years NFPA has moved to add significant safety features to CO2 systems when installed in normally occupied enclosures. All other methods of fire suppression must be exhaustively researched (and documented) as viable alternatives prior to allowing CO2 to be used. Lock out valves, pneumatic time delays, signage and pneumatic audible signals must be included in the system design. In rare cases these safety devices can be eliminated when hazards to personnel and protected equipment present too much of a danger by adding the safety equipment to the system.
Selecting the Best Option for Your Application
The advancement of fire suppression systems that fall outside the traditional realm of sprinkler systems is essential to ensuring the protection of power generating plants and chemical processing facilities, especially as the hazards themselves continue to evolve. The disadvantage of a market saturated with new products and features is that the decision making process for facilities has also become more convoluted.
When assessing the various suppression options, the first crucial step is to evaluate the actual hazard to the asset that is being protected. The fire classification must be determined so that the facility can narrow their choices as to which suppression method is the most effective for the application.