Patent Application
20070251706
The instant invention relates to improved systems for extinguishing flammable liquid fire in an industrial scale storage tank having a roof creating a space above the liquid, typically a fixed roof on top of the tank, and in particular to methods and apparatus incorporating use of dry chemical.
Industrial fuel and/or flammable liquid storage tanks frequently have a roof creating a space above the liquid, usually a fixed conically- or geodesically-shaped roof welded to the top of the tank. Such tanks may have a double roof, including an internal floating roof, called a floater, designed to float on top of the fuel/liquid with seals for sealing against the inside tank wall. The fixed cone or geodesic top roof is typically attached by welding. A roof system comprised of either a single fixed top portion or of two portions, a fixed top and a floater, creates and defines a space or cavity between either the surface of the fuel/liquid and/or the floater below and the top roof above.
Fire in an enclosed or fixed roof industrial storage tank can present unique problems. Industrial storage tanks are tanks with a diameter of a 60 feet or greater. Tank walls are typically 50 feet high, and usually 45 feet or higher. The top of the tank wall just below the rim is usually characterized by a series of eyebrow vents around its circumference. The eyebrow vents serve to allow vapor to escape from the tank rather than collect. Collecting of vapor within the tank presents numerous hazards. Thus, vents are typically provided to vent to the atmosphere vapors that collect in the space or cavity between the fuel/liquid (or floater) and a top fixed roof The usual vents are “eyebrow vents” comprising spaced rectangular openings around a top portion of the vertical tank wall, scooper vents provided at the top of the tank and/or roof vents comprising spaced openings around the periphery of the top roof.
When vapor from the liquid within the tank, such as gasoline or crude, catches fire typically the roof is not blown off. The fire manifests itself predominantly at a downwind set of vents. (Air is sucked in through upwind vents.) The liquid within the tank is vaporizing, but the space above the liquid level and below the fixed roof typically offers insufficient oxygen to sustain a full burn. (The vapor mixture in the cavity may partially combust in the cavity.) The full ignition and complete combustion occurs, however, as the vapor exits the vents and comes into contact with the oxygen available in the atmosphere.
In the event of a fire in a fuel or flammable liquid tank having a fixed top roof, it is industry standard procedure, regulated by the NFPA, to extinguish the fire (or at least to attempt to do so) by a foam attack. The attack comprises laying a foam blanket on the fuel/liquid surface typically by discharging foam into the space or cavity between a fixed top roof and the liquid surface and/or a floater. As discussed above, full combustion typically occurs only at the vents where the fuel/liquid vapors meet atmospheric air, and typically only at the downwind vents while air is sucked into the cavity at the upwind vents.
NFPA has guidelines for the rate of foam application and the duration of a foam attack, adjusted for different type fuels or flammable liquids, different foams and different tanks, in order to achieve extinguishment.
It has been discovered that after laying down an adequate foam blanket, adequate by regulatory standards in the covered tank situation, the fire may persist. Observation indicates that significant vapor is yet trapped in the space above the foam blanket and below the fixed roof, and a certain amount of rich incomplete combustion may be taking place on top of the foam blanket within the enclosed space. Furthermore, since foam has a 25% drain time of 2-8 minutes, foam blankets are not permanent. Foam blankets, sooner or later, can permit vapors to pass through, replenishing the vapor supply within the space above the blanket. If the cavity above the liquid level and below the fixed roof is significant, the burn time for just the vapor trapped in the space, without any replenishment, could be long, several hours, which is unacceptably long even if an adequate foam blanket permits no further vapor to pass through. Foam supplies are limited, which dictates that the fire be extinguished within a given period of time.
An experience extinguishing a blended fuel tank fire in Guatemala demonstrated that foam alone may not extinguish a difficult fuel or flammable liquid fire in a storage tank having a fixed top roof, even when foam is placed in the cavity in accordance with NFPA recommended procedures, rates and durations. This appears disturbingly true of the new blended fuels having a high-octane content. It is a disconcerting discovery. Foam alone may not extinguish the fire at all, and quite likely will not do so per current NFPA regulations or guidelines. The instant inventor has further experience of vapor passing through a foam blanket and creating vapor foam bubbles. The bubbles drain out of vents and fall down to the ground burning.
It has been further determined that any flammable liquid, including crude, may produce vapors in a cavity above the surface of a liquid and below a roof that can keep a fire burning (at least at certain vents if not also, to an extent, within the cavity) for a significant time period after establishing a foam blanket. This fire may persist after the establishment of an NFPA adequate foam blanket. If the liquid level in the tank is low enough, and/or if the cavity is sufficiently large, fire from cavity vapor can persist for hours after the establishment of an adequate foam blanket. During this period foam dries out and vapors can traverse the foam blanket, replenishing the vapor in the cavity. Foam supplies may not be adequate to maintain a sufficient fresh foam blanket for hours.
The instant invention teaches, therefore, an improved system designed to cost effectively extinguish not only a “difficult fire” in a tank with a fixed roof, or a roof that creates a space between the roof and the liquid, but also a fire of any flammable liquid. The improved system is designed to cost effectively extinguish a fire of a difficult to extinguish fuel or flammable liquid having a high-octane content as well as a fire of any flammable liquid. The invention teaches a staged and timed discharge of dry chemical into the space between the burning fuel/liquid and the roof. The timing of the staging of the discharge of the dry chemical is selected to follow a pertinent period of foam application. Dry chemical is a limited and rationed resource. Discharging the dry chemical too soon might be ineffective and, thus, waste the resource.
The discharging of the dry chemical can be effected by one of several means or techniques, using portable and/or fixed systems. (A “fixed system” is equipment put in place prior to a fire, fixed prior to an emergency, in anticipation of emergencies. In contrast, portable systems are brought to the locale of the emergency upon notice.) Vents provided to vent vapors that collect under a roof can be advantageously used as an entry means to discharge the dry chemical into the space above the fuel/liquid and below the roof. Both portable and fixed systems could utilize existing vents. Alternately, special ports for fixed foam systems can be utilized for a fixed dry chemical system; as well, special ports for a fixed dry chemical system can be created.
It is the inventor's experience and observation that dry chemicals, timely inserted into a space between burning fuel/liquid and the roof, after a substantial foam attack, chase remaining persistent, pernicious fire or vagrant flames in the cavity and serve to completely extinguish the fire. The movement of air into the cavity through upwind vents or openings, as discussed above, aids a discharged suspended dry chemical in chasing the flames. Foam alone is an inferior and more costly means, if not an inadequate means, to completely extinguish residual flames in such a tank. Furthermore, foam is expensive. The extra time required to secure extinguishment by foam, even if it can be achieved with a continued application of foam alone as compared to the instant invention, is unnecessarily costly.
The instant staged dry chemical methodology and apparatus for extinguishing a “fixed roof” (so to speak) tank fire may be implemented in various forms, including using portable apparatus and/or fixed systems. Fixed systems and/or special portable apparatus could be less risky for firefighters, and as such would be preferred over a portable embodiment requiring firefighters to climb the tank, walk over the roof and insert dry chemical through an existing or created vent or opportune opening with a hand held nozzle.
The term “difficult to extinguish fuel or flammable liquid” or “difficult fuel or flammable liquid fires” is used herein to refer to fluid fuels or flammable liquids that are, at least, in substantial part, low-surface tension fuels/liquids and/or high-vapor pressure fuels/liquids and/or octane-boosted fuels/liquids and/or oxygenated fuels/liquids. The implied comparison in these instances would be recognized by one of skill in the art to be with the historic straight chain fuels or flammable liquids of the mid-20th century.
The instant invention, however, has been determined to also have significant application with the historic straight chain fuels or flammable liquids of the mid-twentieth century.
It should be understood that although a tank may be designed with, and originally exist with, a particular roof system, the initiation of a fire or hazard may have altered or destroyed part or all of the original roof system. Thus, the characterization of a storage tank may have to be reassessed. Original floating roofs, or floating roof portions, may have tilted or partially sunk or totally sunk. Seals may have been destroyed, in whole or in part. Fixed roofs may have been blown awry, or may have been partially dislodged or tilted, or at least their connections, such as a welded connection with a tank wall, may have been partially or totally destroyed. The instant invention relates to a tank that, at the time of the fire, still has at least a significant roof portion creating a substantially enclosed space above the fuel/liquid and below the roof. That is, the invention relates to situations where a difficult fuel or flammable liquid is on fire and there is at least a significant roof portion above the fuel/liquid surface, defining a substantially enclosed space or cavity there between. Although welds may be blown off from an original fixed roof portion, and hatches and vents may be blown apart, the invention applies if there remains a significant space or cavity between a burning fuel/liquid and a roof portion. Note again: the fuel/liquid may be burning only where it secures sufficient oxygen, such as where fuel vapors meet the atmosphere at vents or other open portions. Partial combustion may be taking place in the cavity.
The instant invention discloses a system for extinguishing a fire of a flammable liquid, including difficult to extinguish fuels, in a storage tank having at least a roof portion that creates a substantially enclosed space above a significant portion of the liquid and below the roof, usually a tank fitted with a fixed top roof that remains substantially in place. The invention includes creating a foam blanket on the fuel/liquid surface, such as by discharging foam into a cavity above the fuel/liquid. (A foam blanket should be understood to include foam and/or film.) Preferably after covering at least 90% of the liquid surface with a foam blanket and/or after establishing a foam blanket for a significant period of time under the circumstances, such that at least a minimal blanket of foam is created under the circumstances, most preferably after at least two-thirds of a NFPA recommended application rate/duration procedure guideline for the foam attack, then discharging dry chemical into a cavity above the foam blanket and below a roof portion. Preferably the dry chemical would be discharged during the last ten minutes of a NFPA recommended application rate/duration procedure guideline for a foam attack. Dry chemical would typically be discharged for a period of 10 to 30 seconds. Existing vapor vents offer fortuitous openings for discharging the dry chemical into the cavity between the fuel/liquid and the roof using portable or fixed dry chemical systems. Preferably a dry chemical fixed system could be already in place, having conduits and a nozzle ready to be connected to dry chemical sources, such as wheeled units or a dry chemical skid, and having a discharge orifice or nozzle in the cavity.
Preferable portable systems include a dry chemical wand, preferably attached to a T'ed or cellar-style discharge nozzle. The wand can be hung with the nozzle inserted through an opportune opening and/or vent. “Fish mouths,” where the roof separates from the wall, provide opportune openings. The firefighter can then retreat a safe distance, avoiding the hazards to personnel occasioned from a flashback and/or roof explosion.
Fixed apparatus for extinguishing flammable liquid fire, including a difficult fuel, in a storage tank having a cavity between the fuel/liquid surface and a roof portion could include at least one dry chemical supply pipe or line rising along a portion of a tank wall and having at least one end opening into a tank vent or port, such as through a roof or eyebrow vent, or through a fixed foam system opening into a tank, or through a fixed dry chemical system port. The supply pipe could be placed in fluid communication with a wheeled unit, a skid, or the like, having a source of dry powder. The supply pipe is preferably permanently affixed, but could be portable. Preferably, a dry chemical discharge nozzle located in the cavity is in fluid communication with the supply pipe.
A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiments are considered in conjunction with the following drawings, in which:
The drawings are primarily illustrative. It would be understood that structure may have been simplified and details omitted in order to convey certain aspects of the invention. Scale may be sacrificed to clarity.
In a worst-case scenario, fuel/liquid F is a blended fuel. Blended fuels can have a high-octane content that leads to difficult extinguishment situations. Fuel/liquid F is at least a flammable liquid and may be a difficult fuel/liquid to extinguish.
Tank T in
In a typical embodiment fixed roof portion FXR is a cone roof fixed to the top of the tank wall. Geodesic-shaped fixed top roofs are also known. Floating roof portion FR floats up and down with the surface of the fuel/liquid left in the tank T and has seals to seal against the inner tank wall. Of course, there may be no floating roof, or it may have sunk, totally or partially.
In a situation where no fixed application system for dry chemical exists, offering preinstalled elements such as riser pipes, or pipe extensions, pipe ends and/or nozzles, the methodology can be carried out by firefighters using portable nozzles attached to supply lines. In such cases, however, a firefighter would have to approach (or to create) appropriate vents or openings on the tank or on the roof, proximate a cavity, in order to insert a dry chemical nozzle through the vent or opening.
When portable systems are used it is definitely preferable for the firefighter to hang a dry chemical nozzle on the end of a wand through an opportune opening, as opposed to manually holding a hand held dry chemical nozzle. There is a distinct possibility that the vapor could yet flash back and blow the roof off of the tank.
The methodology for extinguishing a flammable liquid fire in a tank with a fixed roof portion, including a difficult fire, includes an initial foam attack wherein a foam blanket is created. (Again, foam includes film.) Preferably foam is inserted into a cavity between a floating bottom roof portion and/or the fuel/liquid surface and a top roof portion to establish and create a foam blanket. Foam should be inserted or placed in the cavity until the fuel/liquid surface is substantially covered and the fire is substantially abated. Substantial abatement of the fire can be determined to have occurred in most cases when a foam blanket has been laid upon the surface of the fuel/liquid and/or floating roof in accordance with present NFPA guidelines for the foam, fuel/liquid and tank. The period of time this takes varies depending upon the type of foam used, the capacity for discharging foam, the size and complexity of the tank and the nature of the fuel/liquid it contains. Forty-five minutes represents a typical regulatorily approved time period for launching and sustaining a foam attack in a cavity between a floating roof and a top roof In a preferred embodiment, sometime during the last ten minutes of any such foam attack, dry chemical would be inserted through one or more vents, or other available tank openings, into the cavity. If safer or more remotely activatable means are not available, the dry chemical attack can be implemented by a firefighter carrying a hand held nozzle, attached to a line and source of dry chemical, up to a suitable opening into the cavity. Preferably the fire fighter would hang a wand and dry chemical nozzle through a vent, fish mouth or opening of opportunity and then retreat to a safer distance. A ten second application of dry chemical offers a reasonable expectation for extinguishing the remnants of the fire, the vagrant remaining flames associated with the flammable liquid fire, especially those associated with the new blended fuels. It is the experience of the instant inventor that dry chemical timely inserted into such cavities in the above situation, augmented by the drafting air in the cavity, appears to “chase” the remaining fire within the cavity and to extinguish it. Without such dry chemical treatment, maintenance of a foam blanket may have to be extended for two or three times the present regulatorily set time periods, incurring considerable unanticipated expense. Indeed, there is no guarantee or experience conclusively showing that foam alone can extinguish a fire of a difficult flammable liquid in a tank under a fixed roof. Foam reserves may be inadequate to maintain the requisite foam blanket for the period of time required for the vapor in the cavity to cease combustion.
It is recommended in addition to cool tank wall portions above the liquid level and the roof, such as with a water fog, prior to application of the dry chemical. It is believed that dry chemical extinguishes fire primarily by interrupting a chain of oxygenation events. (Dry chemical also produces some amount of C02 that inhibits the fire.) The reignition of the fire in the vapors requires a threshold level of heat and oxygen. The foam blanket has removed heat from the liquid as well as the tank walls. Cooling the tank walls and roof removes additional heat. Access to oxygen is limited in the cavity to the available openings, primarily or typically the eyebrow vents. (Of course, other openings may have been created by the fire, such as blown hatches and/or destroyed seals.) The judicious timed application of dry powder can extinguish the fire with no reignition, given appropriate cooling efforts and augmented by limited access to oxygen.
Dry chemical is a relatively scarce commodity at a fire. The usage of dry chemical is carefully marshaled. Limitations on the supply of dry chemical make discharging dry chemical, even for a period of minutes, essentially unfeasible or impossible. Hence, dry chemical, if it is to be utilized, must be utilized judiciously. As a resource, compared to water and/or foam, in almost all circumstances its availability for use must be considered to be quite limited. Thus, a dry chemical attack is not preferred to be commenced until at least after two-thirds of the time period for a standard recommended NFPA foam attack as per NFPA guidelines. For example, if the foam attack should last over 55 to 60 minutes, the dry chemical attack preferably should not be begun until sometime in approximately the last 20 minutes, preferably not until sometime in the last 10 minutes. If there is no NFPA recommended application rate/duration procedure guideline for a particular foam or tank or fire in a given circumstance, the firefighter should extrapolate a reasonable guideline for the situation based on existing NFPA recommendations in the closest related circumstances, and take that as the NFPA guideline for this case.
In regard to the associated fixed system for the application of dry chemical, a chemical is fed from a source through pipe P, through its own check valve, vapor seal V, and then extending through opening O to a dry chemical discharge tip. The vapor seals or check valves may be of different designs and locations.
The foregoing description of preferred embodiments of the invention is presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form or embodiment disclosed. The description was selected to best explain the principles of the invention and their practical application to enable others skilled in the art to best utilize the invention in various embodiments. Various modifications as are best suited to the particular use are contemplated. It is intended that the scope of the invention is not to be limited by the specification, but to be defined by the claims set forth below. Since the foregoing disclosure and description of the invention are illustrative and explanatory thereof, various changes in the size, shape, and materials, as well as in the details of the illustrated device may be made without departing from the spirit of the invention. The invention is claimed using terminology that depends upon a historic presumption that recitation of a single element covers one or more, and recitation of two elements covers two or more, and the like. Also, the drawings and illustration herein have not necessarily been produced to scale.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/US04/26762 | Aug 2004 | US | national |
This is a continuation-in-part of co-pending application PCT/US2004/026,762, filed Aug. 19, 2004, which in turn claims priority to provisional U.S. applications 60/496,494 filed Aug. 20, 2003 and 60/538,021, filed Jan. 21, 2004.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US05/28946 | 8/12/2005 | WO | 2/9/2007 |
