|Why Carbon Dioxide (CO2) in Fire Suppression Systems?|
by Norb Makowka
GeneralitiesCarbon dioxide is closely related to the life process in both animals and plants. Animal life, including people, produces carbon dioxide internally by combining carbon from their food with oxygen obtained from the air that they breathe. This process provides energy and heat for movement and the other life functions. The carbon dioxide that is thus produced is carried by the hemoglobin in the blood to the lungs where it is exchanged for oxygen and exhaled into the atmosphere.
On the other side of this cycle, plant life absorbs carbon dioxide from the atmosphere and with the help of energy from the sun, reduces the carbon dioxide back into carbon compounds and releases the oxygen back into the atmosphere. This process maintains a balance in the atmosphere and constantly renews the available oxygen.
A Few Physical Properties of Carbon DioxideCarbon dioxide is a relatively inactive compound. With the exception of its ready reaction with water to form carbonic acid, it is rather difficult to promote a reaction between carbon dioxide and other compounds or elements. FIGURE 1 shows the relationship between temperature and pressure and the vapor, liquid and solid phases of carbon dioxide. The solid or dry ice phase can exist only at temperatures less than about -70 degrees F in the pressure region above the solid curve. This solid curve is called the saturation curve and shows the relationship between temperature and pressure in a storage vessel containing both liquid and vapor carbon dioxide. The point labeled Triple Point indicates the temperature pressure condition where all three phases of carbon dioxide can exist in equilibrium with each other. This Triple Point corresponds to a temperature of approximately -69 degrees F and a pressure of about 75 pounds per square inch absolute.
Uses of Carbon DioxideThe physical properties of carbon dioxide permit a variety of interesting uses for this compound. All of us have been exposed to using carbon dioxide in the form of dry ice for refrigerating purposes. Carbon dioxide also has been used a refrigerant in mechanical refrigeration systems. CO2, at one time, was a very common propellant for blasting coal in the mining industry. Other uses of carbon dioxide have included the seeding of clouds to attempt rain-making. There are a multitude of applications of carbon dioxide as a power generator. CO2 has provided the power to operate marine bell buoys, railway signals, elevators for ladders on hook and ladder fire trucks, for propelling torpedoes, for operating paint guns, and of course, it still supplies the power to move beer from the barrel into the glass in millions of establishments around the world.
Carbon Dioxide as a Fire Suppression AgentThe advantages of carbon dioxide gas for fire extinguishing purposes have been long known. As early as 1914, the Bell Telephone Company of Pennsylvania installed a number of seven pound capacity portable CO2 extinguishers for use on electrical wiring and equipment. By the 1920's, automatic systems utilizing carbon dioxide were available. In 1928, work on the NFPA Standard for carbon dioxide extinguishing systems was begun.
Carbon Dioxide and the Fire TriangleThe mechanisms by which carbon dioxide extinguishes fire are rather well known. If we go back to the familiar fire triangle, we realize that an interaction between fuel, oxygen and heat is necessary to produce a fire condition. When these three elements are present in a proper relationship, fire will result. Carbon dioxide extinguishes fire by physically attacking all three points of the fire triangle. The primary attack is on the oxygen content of the atmosphere. The introduction of CO2 into the fire zone displaces sufficient oxygen in the atmosphere to extinguish the open burning. At the same time, the extinguishing process is aided by a reduction in the concentration of gasified fuel in the fire area. And finally, CO2 does provide some cooling in the fire zone to complete the extinguishing process.
With a surface-type fire, that is, a fire which has not heated the fuel to its auto-ignition temperature much beyond the very surface of that fuel, extinguishment is rapid. Such surface fires are usually the case when liquid fuels are involved. Unfortunately, there is no guarantee that all hazards will produce surface fires.
In fact, a great many hazards are more likely to produce fires which will penetrate for some depth into the fuel. Such fires are commonly referred to as deep-seated. When dealing with a so-called deep-seated potential, it is necessary not only to remove the oxygen and decrease the gaseous phase of the fuel in the area, but it is equally important to permit the heat which is built up in the fuel itself to dissipate. If the heat is not dissipated and the inert atmosphere is removed, the fire may very easily reflash. For such hazards, it is necessary to reduce the concentration of oxygen and gaseous fuel to a point where not only is the open flaming stopped, but also any smoldering is eliminated. To accomplish this, the concentration of agent must be held for a sufficiently long time to permit adequate dissipation of built-up heat. The NFPA Standard 12 on carbon dioxide systems has long been a leader in prescribing thorough and conservative fire protection. The standard requires a mandatory 20-minute holding time, or soaking time, for any potentially deep-seated fire hazard. What this means is that the inerting concentration of carbon dioxide shall be maintained in a deep-seated hazard for a minimum of 20 minutes in order to permit cooling and complete extinguishment.
Application Methods of Carbon Dioxide in Fire SuppressionOver the years, two methods of applying carbon dioxide have been developed. The first technique is the total flooding application. The total flooding technique consists of filling an enclosure with carbon dioxide vapor to a prescribed concentration. This technique is applicable both for surface-type fires and potential deep-seated fires. For surface-type fires, such as would be expected with liquid fuels, a minimum concentration of 34 percent carbon dioxide by volume is mandated. Considerable test work has been done with carbon dioxide on liquid fuels and appropriate minimum design concentrations have been arrived at for a large number of common liquid fire hazards.
For deep-seated type hazards, the minimum permitted concentration if 50 percent carbon dioxide by volume. Fifty percent design concentration is used for hazards involving electrical gear, wiring insulation, motors, and the like. For hazards involving record storage, such as bulk paper, a sixty-five percent concentration of carbon dioxide is required. For substances such as fur and bag-house type dust collectors, a 75 percent concentration of CO2 is mandated. It should be noted that most surface burning and open flaming will stop when the concentration of CO2 in the air reaches about 20 percent or less. Thus, it should be apparent that a considerable factor of safety is built in to these minimum CO2 concentrations required by the Standard. Flame extinguishment has never been considered to be sufficient fire protection by those who developed the CO2 Standard. This is in contrast to the guidelines given in standards for other gaseous extinguishing agents. We find that some of these standards mandate agent concentrations which are only sufficient to extinguish open flame but will not produce a truly inert atmosphere.
The other method of application which has been developed for carbon dioxide is referred to as local application. Local application systems are appropriate only for the extinguishment of surface fires in flammable liquids, gases and very shallow solids where the hazard is not enclose or where the enclosure of the hazard is not sufficient to permit total flooding. Hazards such as dip tanks, quench tanks, spray booths, printing presses, rolling mills, and the like can be successfully protected by a local application type system. In this system, the discharge of CO2 is directed at the localized fire hazard. The entire fire hazard area is then blanketed in CO2 without actually filling the enclosure to a predetermined concentration.
Types of Carbon Dioxide Fire Suppression SystemsToday, the NFPA Standard on CO2 extinguishing systems recognizes two types of carbon dioxide systems. The first type is the familiar high pressure CO2 systems, and the second type is the low pressure CO2 system. The basic difference between the two types of systems lies in the method of storing the carbon dioxide.
The high pressure system utilizes DOT spun steel storage cylinders. These cylinders are kept at room temperature and the pressure within the cylinder varies according to temperature. At a 70 degrees F ambient temperature, the internal pressure in such a unit would reach 850 PSI. These cylinders are available in 50, 75 and 100 pound capacities.
On the other hand, the low pressure storage unit maintains the CO2 in a refrigerated pressure vessel. Typical storage temperature is 0 degrees F with a corresponding CO2 vapor pressure of 300 PSI. The refrigerated storage concept uses an ASME coded pressure vessel with a 350 PSI working pressure. Such units are available in standard capacities from 1 1/4 through 60 tons. Larger units have been made for special applications.
From this basic difference in storage configuration, different methods of application and control for the two types of systems are derived. Since the maximum capacity of a high pressure cylinder is 100 pounds of CO2, most systems consist of multiple cylinders manifolded together to provide the required quantity of carbon dioxide. Each cylinder has its own individual discharge valve and once opened, the cylinder contents will completely discharge.
Reserve SystemsIn order to provide a reserve or second shot capability with a high pressure system, a duplicate bank of cylinders must be connected by means of proper valving to a common system discharge pipe. Usually, a manual throw-over switch is provided to put the reserve bank of high pressure cylinders on-line.
With the low pressure concept, a single storage unit will contain as a minimum the quantity required for a single discharge into a hazard. Most often, both a main supply and a reserve supply is incorporated into the low pressure system. When a facility contains several fire hazards, all of these hazards can usually be protected from a single low pressure storage unit. It is simply a matter of sizing the storage capacity to meet the needs of the individual protection requirement.
Low Pressure vs High PressureBefore we enter into a more detailed examination of the low pressure CO2 fire protection concept, let us review some of the comparative features of low pressure and high pressure CO2 systems. In the area of design flexibility and fire fighting capability, we find that with a low pressure system, it is usually impractical to protect many small hazards scattered throughout a facility. The high pressure system does lend itself to covering very small hazards with individual cylinders located through a plant facility. In contrast, low pressure easily can handle many average to large size hazards plus hosereel systems from a single storage unit.
Multiple hazard protection by single cylinder bank of high pressure cylinders is often limited by design complexities as well as hazard to storage distance. The low pressure system can cover hazards at distances of 500 feet or more from the storage unit. In the area of fire fighting capability, we find that 47 percent of a discharge from low pressure storage reaches the hazard as dry ice particles. This provides a greater local application and hand hoseline effectiveness and also greater cooling capacity. With high pressure, only 28 percent of the discharge is dry ice particles and the local application and hoseline effectiveness is somewhat diminished.
With a low pressure system, almost all of the liquid in the storage container is effective for local application fire fighting. When using high pressure for local application, at least 40 percent additional liquid is required in storage. With the low pressure concept, a second discharge into the same or in another hazard is available without any manual manipulation, switch over or time loss. A switch over to a reserve bank is required before a second discharge can be accomplished with a high pressure system. We find that extension of protection to future hazards is more easily accomplished in a properly sized low pressure system than in a high pressure system. It is also possible to design for simultaneous discharge into several inter-exposed hazards with a low pressure system. Simultaneous discharge would require added controls and/or storage capacity when utilizing high pressure CO2.
Hosereel SystemsHosereels are particularly effective and versatile with the low pressure concept. They can be used without requiring recharge of the storage unit and thus, fire protection need not be interrupted. When utilizing high pressure hosereel systems, the system must be serviced and recharged before full protection is again present.
Maintenance ConsiderationsIn the areas of operation and maintenance, we also find some differences. With the low pressure system, it is necessary that electric power be provided for the mechanical refrigeration system. With a high pressure system, no refrigeration is required. With the low pressure system, the contents of the storage unit is read weekly from a liquid level gauge. To determine the contents of the storage in high pressure cylinders, these cylinders must be weighed twice a year as per the NFPA Standard.
With the low pressure storage unit, no retesting of the pressure vessel is required under normal conditions, ASME codes apply. High pressure cylinders must all be hydrostatically tested at least every 12 years. Recharge of a storage unit is another major difference. The low pressure unit is recharged by tank truck at the cost of less than 12 cents a pound including labor. To recharge a high pressure system, you must remove the cylinders from service, transport these cylinders to the site of recharge, hydrostatically test these cylinders if five years have expired since the last hydrotest, recharge the cylinders, and finally, reinstall them in the system.
Installation PeculiaritiesAnd finally, we come to the area of installation. On small systems with up to two tons of storage capacity, the low pressure system hardware will have a higher initial cost. Also, a crane or other type of payloader is required to move the large low pressure storage unit into place. With high pressure cylinders, no heavy machinery is required for installation, but considerable manpower may be needed to install a large number of storage cylinders. With a low pressure system, minimal valving and control equipment must be installed. High pressure valving and controls can be complex. This is particularly so for selector valve systems with main and reserve capacities. The low pressure storage unit can be located outdoors to save floor space. Outdoor location of high pressure storage units requires shelters or special treatment of the cylinders. Standard low pressure systems can be located in areas with ambient temperature ranges from -10 degrees F to +150 degrees F. Special treatment is required for high pressure storage cylinders when ambient temperatures fall below 32 degrees F or rise above 120 degrees F.
In general, the low pressure concept seems worth considering when quantities of CO2, greater than about one ton are required -- or when multiple hazards must be protected within a single facility -- or when discharges are common and frequent recharge of the system will be necessary.