DE69028119T2 - FIRE EXTINGUISHING PROCEDURE - Google Patents

Description

Field of the invention

This invention relates to compositions for use in preventing and extinguishing fires resulting from the combustion of combustible materials. It specifically relates to such compositions which are highly effective and "environmentally friendly". In particular, the compositions of the invention have little or no effect on the ozone layer depletion process and have little or no contribution to the global warming process known as the "greenhouse effect". Although these compositions have minimal effect in this area, they are extremely effective in preventing and extinguishing fires, particularly fires in enclosed spaces.

Background of the invention and prior art

When preventing or extinguishing fires, two important elements must be considered for success:

(1) the separation of combustible substances from the air and

(2) the prevention or reduction of the temperature necessary for combustion to take place. Thus, small fires can be extinguished by using blankets or foams to cover the burning surfaces to isolate the combustible materials from the oxygen in the air. In the usual method of pouring water on the burning surfaces to extinguish the fire, the main element is to reduce the temperature to a point where combustion cannot take place. Obviously, in the water situation, some extinguishing or separation of the combustible materials from the air also occurs.

The particular method used to extinguish fires depends on several items, such as the location of the fire, the combustible materials involved, the size of the fire, etc. In fixed enclosed spaces such as computer rooms, warehouses, rare book library rooms, petroleum pipeline pumping stations, and the like, halogenated hydrocarbon fire extinguishing agents are currently used. These halogenated hydrocarbon fire extinguishing agents are not only effective for such fires, but also cause little, if any, damage to the space or its contents. This is in contrast to the well-known "water damage" which can sometimes exceed the fire damage when the usual water-pouring method is used.

The halogenated hydrocarbon fire extinguishing agents that are currently most popular are brominated hydrocarbons, e.g. bromotrifluoromethane (CF3Br, Halon 1301) and bromochlorodifluoromethane (CF2ClBr, Halon 1211). These brominated fire extinguishing agents are believed to be highly effective in extinguishing incipient fires because these compounds decompose at the high temperatures involved in combustion to form products containing bromine atoms that effectively disrupt the self-sustaining free radical combustion process and thereby extinguish the fire. These brominated halocarbons can be delivered from a portable device or from an automatic room flooding system activated by a fire detector.

In many situations, enclosed spaces are affected. Fires can occur in rooms, halls, enclosed machines, furnaces, containers, storage tanks, boxes and similar areas.

The use of an effective amount of fire extinguishing agent in a confined space involves two situations. In one situation, the fire extinguishing agent is introduced into the confined space to extinguish an existing fire. extinguish. The second situation is to provide a permanent atmosphere containing the fire "extinguishing" or, more precisely, "preventing" agent in such an amount that the fire can neither break out nor be sustained. Thus, in U.S. Patent 3,844,354, Larsen proposes the use of chloropentafluoroethane (CF3-CF2Cl) in a total flooding system (TFS) to extinguish fires in a fixed confined space, the chloropentafluoroethane being introduced into the fixed confined space so that its concentration is maintained at less than 15%. On the other hand, in U.S. Patent 3,715,438, Huggett discloses the creation of an atmosphere in a fixed confined space which does not sustain combustion. Huggett provides an atmosphere consisting essentially of air, a perfluorocarbon selected from carbon tetrachloride, hexafluoroethane, octafluoropropane, and mixtures thereof.

It is also already known that brominated halocarbons such as Halon 1211 can be used to provide an atmosphere that does not support combustion. However, the high cost due to the bromine content and the toxicity to humans, i.e. cardiac sensitization at relatively low concentrations (e.g. Halon 1211 cannot be used above 1 to 2%), make the brominated materials unattractive for long-term use.

In recent years, even more serious concerns have arisen about the use of brominated halocarbon fire extinguishing agents. The depletion of the stratospheric ozone layer and in particular the role of chlorofluorocarbons (CFCs) have led to a great deal of interest in the development of alternative coolants, solvents, blowing agents, etc. It is now believed that brominated halocarbons such as Halon 1301 and Halon 1211 are at least as active as chlorofluorocarbons in the ozone layer depletion process.

Although the perfluorocarbons proposed by Huggett, cited above, are not believed to have as great an effect on the ozone depletion process as chlorofluorocarbons, their exceptionally high stability makes them suspect in another environmental domain, namely the "greenhouse effect". This effect is based on the accumulation of gases which provide a shield against heat exchange and leads to an undesirable warming of the earth's surface.

US-A-1,926,395 discloses in detail the use of halogenated methanes and ethanes to prevent combustion, special mention is made of CF4, and concentrations of 10% and 30% CF4 are disclosed. A large number of halogenated methanes and ethanes are disclosed by means of two tables. No method of preparation is given for any of the compounds. The properties given in the tables are in many cases incorrect, and a number of the compounds given in the tables were unknown at the time of publication, including the three fluorinated ethanes cited above. No precise information is given on the concentrations at which the halogenated hydrocarbons could be used.

US-A-4 954 271 describes fire extinguishing compositions comprising a chlorofluorocarbon and a terpene. The purpose of the terpene is to combine with and neutralize toxic combustion products of the fluorocarbon. A fluorocarbon such as pentafluoroethane or tetrafluoroethane may also be present in the fire extinguishing composition.

US-A-4 459 213 discloses foamable fire extinguishing compositions comprising a protein, a polyhydroxy compound and at least one halogenated hydrocarbon. Pentafluoroethane and tetrafluoroethane are mentioned in a long list of halogenated hydrocarbons.

EP-A-0 460 990 was published after the filing date of the application and describes the use of hydrofluoroalkanes as fire extinguishing agents. Compositions are disclosed which are mixtures of halogenated alkanes of two general formulae. There is no disclosure of introducing the composition into an enclosed space at a concentration level sufficient to suppress combustion while maintaining environmentally friendly conditions.

Therefore, there is a need for an effective fire extinguishing composition and process that has little or no contribution to the stratospheric ozone depletion process or the "greenhouse effect".

The object of the invention is to provide such a fire extinguishing composition and to provide a method for preventing and controlling a fire in a fixed, enclosed space by introducing into the fixed, enclosed space an effective amount of the composition.

Summary of the invention

The invention is based on the discovery that an effective amount of a composition comprising at least one partially fluorine-substituted ethane selected from the group consisting of pentafluoroethane (CF3-CHF2), also known as HFC-125, the tetrafluoroethanes (CHF2-CHF2 and CF3-CH2F), also known as HFC-134 and HFC-134a, prevents and/or extinguishes a fire due to the combustion of combustible materials, particularly in a confined space, without adversely affecting the atmosphere in terms of ozone depletion or the "greenhouse effect".

According to the invention, a method is provided for preventing, controlling and extinguishing a fire in a closed air-containing area containing combustible Materials of the non-self-sustaining type which comprises introducing into the air of the enclosed space an amount of at least one fluoro-substituted ethane selected from the group CF₃-CHF₂, CHF₂CHF₂ and CF₃-CH₂F sufficient to impart a heat capacity per mole of total oxygen which suppresses combustion of the combustible materials in the enclosed space while maintaining environmentally friendly conditions in the enclosed space.

The above partially fluoro-substituted ethanes can be used in conjunction with as little as 1% of at least one halogenated hydrocarbon selected from the group of difluoromethane (HFC-32), chlorodifluoromethane (HCFC-22), 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123), 1,2-dichloro-1,1,2-trifluoroethane (HCFC-123a), 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124), 1-chloro-1,1,2,2-tetrafluoroethane (HCFC-124a), pentafluoroethane (HFC-125), 1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1,2-tetrafluoroethane (HFC-134a), 3,3-Dichloro-1,1,1,2,2-pentafluoroethane (HCFC-225ca), 1,3-Dichloro-1,1,2,2,3-pentafluoroethane (HCFC-225cb), 2,2-Dichloro-1,1,1,3,3-pentaflupropane (HCFC-225aa), 2,3-Dichloro-1,1,1,3,3-pentafluoropropane (HCFC-225da), 1,1,1,2,2,3,3-Heptafluoropropane (HFC-227ca), 1,1,1,2,3,3,3-Heptafluoropropane (HFC-227ea), 1,1,1,2,3,3-Hexafluoropropane (HFC-236ea), 1,1,1,3,3,3-Hexafluoropropane (HFC-236fa), 1,1,1,2,2,3-Hexafluoropropane (HFC-236cb), 1,1,2,2,3,3-Hexafluoropropane (HFC-236ca), 1,2-Dichloro-1,2-difluoroethane (HFC-132), 1,1-Dichloro-1,2-difluoroethane (HFC-132c), 3-Chloro-1,1,2,2,3-pentafluoropropane (HCFC-235ca), 3-Chloro-1,1,1,2 ,2-pentafluoropropane (HCFC-235cb), 1-Chloro-1,1,2,2,3-pentafluoropropane (HCFC-235cc), 3-Chloro-1,1,1,3,3-pentafluoropropane (HCFC-225fa), 3-Chloro-1,1,1,2,2,3-hexafluoropropane (HCFC-226ca), 1-Chloro-1,1,2,2,3,3-hexafluoropropane (HCFC-226cb), 2-chloro-1,1,1,3,3,3-hexafluoropropane (HCFC-226da), 3-chloro-1,1,1,2,3,3-hexafluoropropane (HCFC-226ea), and 2-chloro-1,1,1,2,3,3-hexafluoropropane (HCFC-226ba)

Preferred embodiments

When added in appropriate amounts to the air in a enclosed space, the partially fluorine-substituted ethanes eliminate the combustion-sustaining properties of the air and suppress the combustion of flammable materials such as paper, fabric, wood, flammable liquids and plastic items that may be present in the enclosed compartment.

The fluorinated ethane is probably used in an amount sufficient to impart a heat capacity of at least 40 cal/ºC, e.g. from 40 to 55 cal/ºC, per mole of total oxygen.

The fluorinated ethane is preferably maintained in the enclosed area at an extinguishing concentration of at least 10 and less than 80 vol.%. The amount of fluorinated ethane in the enclosed area can be controlled at about 20 vol.%.

These fluoroethanes are extremely stable and chemically inert. They do not decompose at temperatures as high as 350ºC to produce corrosive or toxic products and cannot even be ignited in pure oxygen, so they remain effective as flame suppressants at the high ignition temperatures of the combustible items present in the compartment.

The fluoroethanes HFC-125, HFC-134 and HFC-134a are also advantageous due to their low boiling points, e.g. boiling points at normal atmospheric pressure of less than -12 ºC. Thus, these gases liquefy at any low ambient temperature likely to be encountered and do not thereby diminish the fire-preventing properties of the modified air. In fact, any material possessing such a low boiling point would be suitable as a coolant.

The fluoroethane HFC-125 is also characterized by an extremely low boiling point and high vapor pressure, i.e., over 164 psig (1.23 mPa) at 21ºC. This allows HFC-135 to act as its own propellant in hand-held fire extinguishers. Pentafluoroethane (HFC-125) can also be used with other materials, such as those disclosed on pages 5 and 6 of this specification, to act as a propellant and auxiliary fire extinguishing agent for these lower vapor pressure materials. Alternatively, these other lower vapor pressure materials can be expelled from a portable fire extinguisher by the usual propellants, i.e., nitrogen or carbon dioxide. Their relatively low toxicity and short atmospheric lifetime (with little impact on global warming potential) compared to the perfluoroalkanes (lifetime of over 500 years) make these fluoroethanes ideal for this fire extinguishing application.

To eliminate the combustion sustaining properties of the air in the confined space situation, the gas or gases should be added in an amount that will impart to the modified air a heat capacity per mole of total oxygen present sufficient to suppress or reduce the combustion of the non-sustaining materials present in the confined space.

The minimum heat capacity required to suppress combustion varies with the combustibility of the particular flammable materials present in the confined space. It is well known that the combustibility of materials, namely their ability to ignite and Sustaining long-term combustion, under a given set of environmental conditions, varies according to chemical composition and certain physical properties such as specific surface area relative to volume, heat capacity, porosity, and the like. For example, thin porous paper such as wood pulp paper is considerably more combustible than a block of wood.

In general, a heat capacity of about 40 cal/ºC and a constant pressure per mole of oxygen is more than sufficient to prevent or suppress combustion of materials of relatively moderate combustibility such as wood and plastics. More combustible materials such as paper, cloth and some volatile flammable liquids generally require that the fluoroethane be added in an amount sufficient to impart a greater heat capacity. It is also desirable to provide an extra margin of safety by imparting a heat capacity in excess of the minimum requirements for the particular flammable materials. A minimum heat capacity of about 45 cal/ºC per mole of oxygen is generally adequate for moderately combustible materials and a minimum of about 50 cal/ºC per mole of oxygen for highly flammable materials. More may be added if desired, but an amount that imparts a heat capacity greater than about 55 cal/ºC per mole of total oxygen adds significantly to the cost without any significant further increase in the fire safety factor.

The heat capacity per mole of total oxygen can be determined by the formula:

wherein:

Cp* = total heat capacity per mole of oxygen at constant pressure;

Po2 = partial pressure of oxygen;

Pz = partial pressure of the other gas;

(Cp)z = heat capacity of the other gas at constant pressure.

The boiling points of the fluoroethanes used in the invention and the mole percentages required to impart heat capacities (Cp) to air of 40 to 50 cal/ºC at a temperature of 25 ºC and a constant pressure while maintaining an oxygen content of 20% and 16% are tabulated below:

The introduction of the appropriate gaseous fluoroethanes is easily achieved by dosing the appropriate amounts of the gas or gases into the closed air-containing compartment.

The air in the compartment may be treated at any time that is deemed desirable. The modified air may be used continuously if a fire threat is constantly present or if the particular environment is such that the fire hazard must be kept to an absolute minimum, or the modified air may be used as an emergency measure if a fire threat develops.

The invention will be better understood by reference to the following Examples. The unexpected effects of the fluoroethane compositions in fire suppression and fighting, as well as their compatibility with the ozone layer and their relatively low "greenhouse effect" compared to the other fire suppression gases, particularly the perfluoroalkanes and Halon 1211, are demonstrated in the Examples.

Example 1 - Fire extinguishing concentrations

The fire-extinguishing concentrations of the fluoroethane compositions compared to several controls were determined by the ICI cup burner method. This method is described in "Measurements of Flame-Extinguishing Concentrations," R. Hirst and K. Booth, Fire Technology, Vol. 13(4): 296-315 (1977).

Specifically, an air stream at 40 lpm is passed through an external chimney (8.5 cm I.D. by 53 cm high) from a glass bead diffuser at its base. A fuel bowl burner (3.1 cm O.D. and 2.15 cm I.D.) is located in the chimney 30.5 cm below the top of the chimney. The fire suppressant is added to the air stream prior to its entry into the glass bead diffuser while the air flow rate is maintained at 40 lpm for all tests. The air and agent flow rates are measured using calibrated rotameters.

Each test is conducted by adjusting the fuel level in the reservoir so that the level of liquid fuel in the bowl burner is just level with the bottom glass lip on the burner bowl. While maintaining the air flow rate at 40 l/min, the fuel in the bowl burner is ignited. The fire extinguishing agent is added in measured portions until the flame is extinguished. The fire extinguishing concentration is determined from the following equation:

Extinguishing concentration = F�1; / F1; + F2; x100

where F₁ = mean flow velocity

F2; = Air flow velocity

Two different fuels, heptane and methanol, are used, and the average of several values of mean flow velocity during extinguishing is used for the following table. Table 1 Extinguishing concentrations of certain fluoroethane compositions compared to other agents

Example 2 - cardiac sensitivity

The cardiac sensitivity or toxicity of the fluoroethanes in comparison with several controls was determined using the procedures described in "Relative Effects of Haloforms and Epinephrine on Cardiac Automaticity," R.M. Hopkins and J.C. Krantz, Jr., Anesthesia and Analgesia, Vol. 47, No. 1 (1968) and "Cardiac Arrhythmias and Aerosol 'Sniffing'," C.F. Reinhardt et al., Arch. Environ. Health, Vol. 22 (February 1971).

Specifically, cardiac sensitivity is measured using unanesthetized healthy dogs using the general protocol outlined in the article by Reinhardt et al. First, the dog is exposed for a limited period of time to a flow of air through a semi-closed inhalation system pre-tied to the dog with a cylindrical face mask. Then, epinephrine hydrochloride (adrenaline) diluted with saline is administered intravenously and the electrocardiogram is recorded. Air containing varying concentrations of the agent to be tested is administered, followed by a second injection of epinephrine. The concentrations of the agent necessary to produce a disturbance in the normal conduction of an electrical impulse through the heart, as characterized by a severe cardiac arrhythmia, are shown in the table below. Table 2

* CF3Br

** CF2Clbr

Example 3

The ozone depletion potential (ODP) of the fluoroethanes and various mixtures thereof, compared to various controls, was calculated using the procedure described in "The Relative Efficiency of a Number of Halocarbon for Destroying Stratospheric Ozone", D.J. Wuebles, Lawrence Livermore Laboratory Report UCID-18924 (January 1981) and "Chlorocarbon Emission Scenarios: "Potential Impact on Stratospheric Ozone", D.J. Wuebles, Journal Geophysics Research, 88, 1433-1443 (1983).

Basically, the ODP is the ratio of the calculated ozone depletion in the stratosphere resulting from the emission of a particular agent compared to the ODP resulting from the same rate of emission of FC-11 (CFC13), which is set as 1.0. It is believed that the ozone depletion of the compounds containing chlorine or bromine is due to migration through the troposphere to the stratosphere where these compounds are photolyzed by UV radiation to chlorine or bromine atoms. These atoms destroy the ozone (O₃) molecules in a cyclic reaction in which molecular oxygen (O₂) and [ClO] or [Br] radicals are formed, these radicals react with the oxygen atoms formed by UV irradiation of O₂ to reform chlorine or bromine atoms and oxygen molecules, and the reformed chlorine or bromine atoms then destroy additional ozone, etc., until the radicals are finally captured from the stratosphere. It is estimated that 1 chlorine atom destroys 10,000 ozone molecules and 1 bromine atom destroys 100,000 ozone molecules.

The ozone depletion potential is also discussed in "Ultraviolet Absorption Cross-Sections of Several Brominated Methanes and Ethanes", L.T. Molina, M.J. Molina and F.S. Rowland, J. Phys. Chem. 86, 2672-2676 (1982), in Bivens et al., U.S. Patent 4,810,403, and in "Scientific Assessment of Stratospheric Ozone: 1989", U.N. Environment Programme (21 August 1989)

The following table shows the ozone depletion potentials for the fluoroethanes used in the invention and for various controls. Table 3

Example 4

The global warming potentials (GWP) of fluoroethane and various mixtures thereof compared to several controls were determined using the procedure described in "Scientific Assessment of Stratospheric Ozone: 1989," funded by the U.N. Environmental Program.

The GWP, also known as the "greenhouse effect", is a phenomenon that occurs in the troposphere. It is calculated using a model that calculates parameters based on the atmospheric lifetime of the agent and its infrared cross section or infrared absorption strength per molecule, measured with an infrared spectrophotometer.

The general definition is:

GWP = calculated IR strength based on the agent / emission rate (equilibrium state of the agent)

divided by the same ratio of parameters for CFCl₃.

The following table shows the GWPs for the fluoroethanes and the controls. Table 4

Claims (4)

1. A method of preventing, controlling and extinguishing a fire in an enclosed air-containing area containing combustible materials of the non- self-sustaining type, which comprises introducing into the air in the enclosed area an amount of at least one fluorine-substituted ethane selected from the group of CF₃-CHF₂, CHF₂-CHF₂ and CF₃-CH₂F sufficient to impart a heat capacity per mole of total oxygen which suppresses combustion of the combustible materials in the enclosed area while maintaining environmentally friendly conditions in the enclosed area. 2. The process of claim 1, wherein the amount of fluoro-substituted ethane in the enclosed region is maintained at a concentration of at least 10 and less than 80 vol.%. 3. The method of claim 2, wherein the amount of fluoro-substituted ethane in the enclosed region is maintained at about 20 vol.%. 4. A process according to claim 1 or 2, wherein the fluoro-substituted ethane is present in a concentration sufficient to impart a heat capacity of at least 40 cal/°C per mole of oxygen.