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10-9 ampere a t a filling rate of 12 gallons per minute and a capacitance to ground of 750 micromicrofarads, a filling of 15 gallons of gasoline will be required to produce a sufficient charge to raise the car to the minimum sparking potential of 300 volts, even if the car is completely insulated and no charge leaks to ground. The total charge produced by filling with 15 gallons of gasoline will be :
Q
15
= 3 X 10-0 X 60 X - = 225 X
12
lo-* coulomb
The voltage will be:
Vol. 34, No. 6
If an unlimited amount of gasoline is pumped into a car a t a filling station, the resistance of the tires will have to be
greater than 100,000 megohms to charge the car to the minimum sparking potential. Such a high resistance can probably never be obtained with four tires.
Acknowledgment We wish to express our appreciation and thanks to the American Petroleum Institute, Richfield Oil Corporation, Standard Oil Company of California, Shell Oil Company, Inc., and Union Oil Company of California. The expense of this work was covered by a grant from the American Petroleum Institute. The four oil companies cooperated in this work, and the measurements were made on their equipment and trucks.
Guarding against the Flammable Liquid Fire Hazard I
C. L. GRIFFIN Walter Kidde & Company, Inc., Bloomfield,
N.J.
T
HE flammable liquid fire hazard is chief among the threats which menace the process industries. The storage, processing, and testing of flammables present a serious problem in industrial fire protection. The way to extinguish a flammable liquid fire is to smother it, take away its oxygen. I n most cases, it is vital to control the fire in a hurry. One of the answers to this question of fire protection is provided by carbon dioxide gas, stored in cylinders under pressure and released to create a n inert atmosphere in which fire cannot exist. The smothering action which kills flammable liquid fires can be produced most quickly by taking away the oxygen necessary to support combustion. Atmospheric air a t or near sea level contains roughly 21 per cent of oxygen. The action of the carbon dioxide extinguisher is to displace some of this oxygen with inert carbon dioxide gas until the content of the atmosphere surrounding the fire has dropped to 14 or 15 per cent oxygen. Under these conditions fire will usually be snuffed out. Some materials will burn in the presence of smaller quantities of oxygen than will ordinary substances. When we say that fire cannot exist in an atmosphere containing as little as 14 or 15 per cent oxygen, this rule holds for most flammable liquid fire hazards. Gasoline is taken as the standard in formulating this rule. There are certain notable exceptions TI hich 77 ill be discussed later. Khile portable carbon dioxide extinguishers fulfill a n important first-aid function in fire-fighting, this article is concerned entirely ITith the built-in carbon dioxide fire-extinguishing system which is fixed in position.
methods employs a detector for rate of temperature rise, which is part of a pneumatic detecting, actuating, and release system. The fusible link device is one of the simplest forms of thermostatic control for actuating fixed extinguishing systems, Its principal use is for the automatic closing of fire doors but it is also employed for shutting dip tank covers, operating drain valves, etc. The fusible link is purely mechanical in
Detection of Fire The automatic detection of fire may be handled in several effective ways. Quartz bulbs or fusible link detectors may take care of this function. I n some instances thermostatic fixed-temperature devices will set the extinguishing system into operation. One of the fastest and most dependable
Portable carbon dioxide extinguishers are particularly suitable for protection of laboratories; here a 20-pound unit guards a range on which batches of lacquers are being tested.
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Weights attached t o a cable control the automatic release of carbon dioxide from the pressure cylinders; when the cable is released, the weights fall, pull down the levers which operate cutter valves, and pierce the disks which seal each cylinder.
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operation; it consists of two metal parts, Qeld together by a low-melting-point solder. They are designed for operation at various temperatures and have ratings similar to those of automatic sprinkler heads. In the presence of heat the solder melts and allows the two metal parts to separate completely. The link device is usually attached to a cable which, when released, actuates the extinguishing system. Another form of the fusible link is the quartz bulb. I n this device a liquid of known high coefficifnt of expansion is sealed inside a quartz bulb. When subjected to increasing temperatures, the liquid expands until the bulb ruptures and causes the links t o disengage. Quartz bulb installations will operate at predetennined temperatures of 135", 175", 250", 325", 400", and 500" F. The fixed-temperature heat detector is a bimetallic type of thermostat hermetically sealed in a heavy glass tube which is filled with an inert gas. This thermostat may be set for practically any temperature from 0" to 300" F. The most widely used settings are 135", 185", and 260" F. The two bimetallic elements are held apart, forming a Y. In the presence of heat, corresponding to the temperature setting of the thermostat, the two arms are brought together to permit closing of an electrical circuit. The rate-of-rise mechanism is operated by a heat-sensitive detector which is usually located a t ceiling level. This device is operated by the quick rise in temperature which is caused by a fire. In the installation used to protect yachts and motorboats, for example, it is necessary to allow normal increases in temperature without discharging the boat's extinguishing system. A fixed-temperature thermostat in such an installation might be actuated by the extreme heat caused by the sun beating down on a deck. The rate-of-temperaturerise type of detector is not "fooled" by these slow temperature increases. Vents allow these heat rises to occur without actuating the extinguisher releases. I t is possible to adjust the rate-of-rise detector for operation under special conditions of heat. This is accomplished through adjustment of the vents. For example, a rate-of-temperature-rise installation will function perfectly in the highly heated atmosphere of a drying oven. However, this is simply a matter of venting the pressure rise caused by the normal heat-up of the oven. It can be done effectively, and still the mechanism can retain its ability to be actuated by the abnormal heat rise caused by a blaze. However, even in the case of a slow-burning fire, the temperature of a room rises speedily. This rapid rise is accompanied by a corresponding increase in the air pressure within the detecting system. The diaphragm in the detecting device transmits this pressure, which is too strong to be vented, through small-bore tubing to the release mechanism where a pressure trip then discharges the cylinders of carbon dioxide *
gas.
The rate-of-temperature-rise fire detector is completely self-contained. It does not require a separate electrical circuit. It will detect fire promptly, even though other electrical and mechanical facilities in the plant have been disabled.
The Fire-Fighting Gas The high-pressure carbon dioxide system is usually made up of one or more cylinders, each of which has a capacity of 50 pounds of the gas. The amount of carbon dioxide required to guard a process room or storage space may be quickly determined by the following table: Cu. Ft./Lb. of 16 18 20 22
Gas
Cu. Ft. of Space Protected Up t o 1,600 Up t o 4,500 Up t o 50 000 Over 50,600
In any space there will be a certain amount of carbon dioxide gas leakage around doors and windows, but the above table makes adequate allowance for this feature. These figures are based on a room with one door and one window and a theoretical concentration of 45 per cent carbon dioxide. When carbon dioxide is introduced into a given space, air is expelled first. Then a mixture of air and carbon dioxide is expelled. Thus, in addition to the loss of carbon dioxide concentration due to open areas (leakage around doors and windows), the action of forcing the gas into an enclosed space results in a slight loss of concentration. The previous paragraph mentioned a "theoretical" concentration of 45 per cent carbon dioxide. Actually, the fire protection engineer figures that this leakage will give him an atmosphere containing around 35 per cent carbon dioxide. The aim is to reduce the oxygen within the space protected from 21 to approximately 14 per cent, a 33 per cent cut. By introducing enough carbon dioxide into the space t o comprise 35 per cent of the entire atmosphere, this 33 per cent reduction will be accomplished. Now for the exceptions to the rule. MTestated earlier that it is customary to figure the carbon dioxide concentrations from gasoline as the standard flammable liquid. It is not
(Above) Detail of shielded type nozzle, showing wide angle discharge; i n total flooding system, nozzles arc usually placed a t intervals along walls a t one third the floor-to-ceiling height. (Upper right) Cutter valve removed from carbon dioxide cylinder; this valve completely cuts out the sealing disk, as fragment shows.
(Right center) Directional valves permit one central bank of carbon dioxide cylinders to guard more t h a n one hazard area; a 8 1 4 t u r n completely opens this manual valve and directs t h e discharge into t h e space i n distress.
and actuates extinguishing system.
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possible to list all of the exceptions here, but a brief table may be helpful. Figuring gasoline as a basis, the handling of other flammable and volatile materials requires the following increases in carbon dioxide capacity: Gas
% Increase 15 35 76
100
170
In other words, assuming that we have a space to be protected involving 1600 cubic feet, the first table states that we should allow 16 cubic feet per pound of gas. That is, 100 pounds of carbon dioxide are required if the hazard is gasoline. If ether presents the fire hazard within the space to be protected, the second table says that we must allow for a 35 per cent increase, which gives a figure of 135 pounds of carbon dioxide required for proper fire protection. Then, since the 50-pound container is the most common unit for multicylinder installations, 150 pounds of gas, contained in three cylinders, would be used. Carbon dioxide is held in cylinders under a pressure of 850 pounds per square inch at 70” F. Upon opening the valves, this stored pressure causes the gas to move at high speed through the distributing pipes and out through the shielded discharge nozzles. The heat detector usually actuates the extinguishing system by releasing a weight on the end of a lever attached to each cylinder head. Rotation of the levers operates cutter valves and opens all cylinders simultaneously.
Fire Extinguishing A great volume of evidence supports the belief that the fast rate of discharge of a high-pressure carbon dioxide system is more responsible for quick and effective fire control than the use of excessive volumes of gas. This is exemplified by the built-in carbon dioxide system which protects airplane engines from fire. The objective in the airplane system is to flood the motor compartment with carbon dioxide gas a t the fastest possible rate. Thus a relatively small volume of the firefighting gas can snuff out the blaze with split-second speed, despite the slip stream caused by an airplane traveling even at 6 miles a minute. An interesting theory has grown up regarding the airplane fire-extinguishing system. We have been unable to check this with high-speed cameras, but it is worth mentioning since it has a bearing on the question of the relative importance of rate of discharge as against volume of carbon dioxide gas in fire extinguishing. The carbon dioxide is introduced into the airplane motor compartment through a perforated ring
Thinning tank for resins is guarded from fire by the two conical nozzles whioh dimcharge carbon dioxide; this is part of a four-tank process, eaoh of which i a floodedwith carhon dioxide gas the instant fire breaks out.
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fastened to the front side of the engine mountings. Thus, when the gas is discharged, i t forms a rrwall”of carbon dioxide. The general belief is that the gas floods the entire motor compartment; all tests made so far confirm this theory. However, some students of fire protection believe that when a plane is traveling at high speed, this wall of carbon dioxide travels backward through the motor compartment and “erases” fire as i t goes. The high-pressure carbon dioxide system aims to apply the extinguishing agent at the fastest possible rate, to flood the fire area instantly with sufficient carbon dioxide gas to control the fire in the shortest possible time. This is one reason why such strong emphasis is placed upon the fire-fighting nozzles. There should be sufficient nozzle area to distribute the gas quickly to all parts of the fire hazard space and to produce without delay an oxygen-starved atmosphere. The shielded nozzle is an important feature in the efficient distribution of gas. It is slightly smaller than the cheering megaphone in vogue at football games. The nozzle is flattened and oval in shape, and its function is to control the velocity of the carbon dioxide discharge and eliminate the excessive turbulence which would result if the gas were released incorrectly into the threatened space. Carbon dioxide is a liquid in the cylinders under pressure, However, when released through the piping t o the nozzles, it becomes a gas immediately upon contact with outside air. .This change from liquid to gaseous state occurs the instant the gas enters the shielded part of the discharge nozzle. Coincident with the change from liquid to gas, a tremendous expansion occurs-roughly, to 450 times the former stored volume. If this expansion were allowed to go unchecked, the resulting turbulence of discharge would make the gas useless for fire fighting. The burning liquid might be so agitated as to make the blaze nearly impossible to control. The entrainment of air might reduce the smothering effect of the gas. The discharge, in short, must be controlled. A test was made a short time ago which showed the importance of nozzles in fire fighting. Two portable extinguishers, each with a capacity of 10 pounds of carbon dioxide, were tested on flammable liquid fires in tubs 2 feet in diameter. The discharge nozzle had been removed from one of these extinguishers, and the carbon dioxide gas was released without
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Vol. 34, No. 6 Carbon dxoxtde s c r e e n i n g nozzles, indicated by circles, produce an impenetrable curtain in e v e n t of fire in pnclosures not equipped with fire doors; in this booth airplane engines are cleaned by flammable solvents. The shielded nozzle for fire extinguishing ma5 he aeen at left.
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any attempt to control its turbulence and velocity. This estinguisher was completely helpless against the test fire. It never did extinguish the blaze. However, an ideiitical extinguisher, equipped with a nozzle, put the fire out without, difficulty and in fast time-3 or 4 seconds. The design of the shielded nozzle has been carefully engineered to discharge the carbon dioxide gas \\-ithout turbulence. The shielding eliminates high velocity and jet effect. The wide angle construction of the shielded nozzle controls the discharge and releases it gently so that its force will not spread the flames. If a handkerchief is placed along the nozzle and a n end allowed to dangle over the mouth of the horn during discharge, there will be no noticeable tendency for the handkerchief to be sucked slowly into the nozzle. Air eddies have been practically eliminated. It must not be understood that the gas comes floating out of the shielded nozzle in lackadaisical fashion. There is force behind the discharge but it is controlled. The expansion drives the carbon dioxide into every remote part of the protected space-into corners, through grillwork, and into deep recesses where fire may be hiding.’ This penetrating power of carbon dioxide makes it possible to kill running fires and blazes burning at more than one level. Tests have been conducted to determine the uniformity of distribution of carbon dioxide gas when nozzles are mounted in the usual fashion-at intervals around the periphery of a room at one third the floor-to-ceiling height. Readings were taken at dozens of different points during these tests and demonstrated a variation in the carbon dioxide content a t varying heights and locations amounting to less than 1 per cent. There is a n impression that, because carbon dioxide is heavier than air, the distribution of the gas is somewhat spotty. These tests effectively prove a remarkable uniformity. There is another factor in this problem. Carbon dioxide gas discharged into a sealed space has a tendency to settle; also, when fire protection is planned which involves fires in
carbonaceous materials, it is sometimes necessary to continue to dump carbon dioxide gas into the space until all embers have been cxtinguished. This problem arises in ships, for example; cargo fires sometimes cannot be completely ext,inguished down to the last glowing ember until the vessel reaches port and the holds can be opened. Ship operators keep this type of fire safely under control by “bleeding” the gas into the hold and maintaining an inert atmosphere which controls the smoldering action of the fire. K h a t , under these conditions, happens to the tendency of carbon dioxide to settle to the lower portion of the space? Is it not possible for fire to break out above the level to which the gas has settled? These are fair questions and the answer is that, when thermal currents are set up by the smoldering action of the fire, they keep the carbon dioxide gas circulating in sufficient degree to offset the tendency of the gas to settle. If there are no thermal currents, there is no fire and no harm is done when the gas seeks its natural lorn level. When the gas is discharged upon a fire, some particles of carbon dioxide snow will usually be formed a t -110“ F. This figure, plus some dramatic but inaccurate statements of feature writers, has led to the widespread belief that carbon dioxide has unusual cooling properties when used against fires. Since cooling is one of the chief ways by which fires may be extinguished, it is unfortunate that this is not true. Weight for weight, carbon dioxide gas has less than 20 per cent the cooling effect of water. Carbon dioxide ext’inguishes fires by smothering, not by cooling.
Types of Built-in Protection Generally speaking, there are two types of built-in protection. The total-flooding installation is designed to flood a room or enclosure, and the first table above gives the amount of carbon dioxide required to dilute the room’s atmosphere to 5 point where fire will be smothered. The second type of protection is the local application system, in which only part of a space is protected. TOTAL FLOOD~NG. Further refinements of the total flooding system are developed through methods of isolating the fire within a given room. This is accomplished through carbon-dioxide-actuated pressure trips and screening nozzles. The pressure trips can be made to do a variety of tricks, and they utilize the same gas as the extinguishing system itself.
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When the carbon dioxide is released from the cylinders, it is piped to these trips which will shut off agitator motors, close windows and fire doors, turn off fans and ovens, and close dampers. The gas which operates the trips continues its passage through the pipes to the discharge nozzles. The screening nozzles are used to throw a fire-resistant curtain across openings which cannot be closed off. Sometimes they will protect fan outlets or the entrances to cleaning booths, etc. Here we do not use the shielded-type nozzle. Instead, the so-called opposed nozzle is installed to create a fanlike curtain of carbon dioxide which excludes any possible inrush of oxygen that might feed the fire. LOCALAPPLICATION.This second classification of built-in system protection is concerned chiefly with extinguishing fires on liquid surfaces or on process operations which cannot be completely enclosed, Local application covers such hazards as flammable liquids in open tanks, dip tanks and the accompanying drainboards, or paint-spray booths. The supply of carbon dioxide gas should be planned to provide not less than 1 pound of gas for each 0.7 square feet of liquid surface and not less than 1 pound for each square foot of drainboards and other surfaces that may be covered with flammable liquids. The Hand Book of Fire Protection, published by the National FiPe Protection Association, appends this note in referring to local-application carbon dioxide systems: [‘A sufficient supply of gas should be provided to protect all flammable material likely to be involved in the hazard protected.” As in the case of the total flooding system, the localapplication unit may be either manually or automatically actuated. MULTIPLEPROTECTION. Frequently more than one fire hazard is protected by a single group of carbon dioxide cylinders. One large company uses the same cylinder installation to guard both the ducts above its big frying kettles in the plant kitchen and a transformer vault in the basement underneath the kitchen. Multiple protection of this type is accomplished through directional valves by means of which the discharge may be a
One hundred pounds of carbon dioxide gas (two 50-pound oylinders) protect a small-parts washer from fire; circles indicate shielded type nozzles.
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aimed into whichever space is threatened by fire. It is possible to control these directional valves automatically through the same devices that actuate the extinguishing system itself. Manual controls may also be used. One company installed multiple protection for its dye department (benzene and alcohol fire hazard) and a transformer room. This system used fifteen 50-pound-capacity carbon dioxide cylinders with automatic control of the directional valves. The one fire that occurred in the dye department was automatically snuffed out in 30 seconds. SPECIALHAZARDS. Built-in carbon dioxide systems now guard lacquer and thinning departments in many plants, and this type of hazard is also likely to employ carbon dioxide pressure trips for isolating the fire and stopping motors and fans. One chemical company protects four great thinning tanks with a total flooding system which discharges its gas into the closed tanks themselves, since it is not necessary to flood the room which houses them. Four 50-pound carbon dioxide cylinders protect these tanks, and it is interesting to note that the gas is dumped into all four tanks simultaneously. This procedure creates an inert atmosphere inside each tank and eliminates the possibility of fire flashing in any one of these tanks from an outside source of ignition. Dust collectors, too, are a source of danger especially if there is likely to be a flammable liquid residue which is gummy or oily. Often this material is relatively open and porous in character, so that even a stray spark may ignite it. The protection of dust collectors has been greatly simplified. I n most cases it is not essential that the ducts be equipped with nozzles for total flooding in the event of a mishap. The most commonly accepted practice, as recently evolved, is t o damper off both sides of the cyclones and to discharge the carbon dioxide gas into the dust collector itself. This makes it possible to guard these units with small installations, sometimes only one 50-pound cylinder. The cyclone must be shut off instantly when fire is detected, and this, too, can be accomplished by using the carbon dioxide pressure trip t o operate a shut-off device. This installation should not be oversimplified. One large newspaper plant, for example, has a tremendous forced-draft ventilation duct which runs from the pressroom to exhaust vents on the roof of an eight-story building. The entire length of this duct, which is as tall and as wide as an ordinary room, becomes lined with a gummy, lint-filled, oily residue. This plant has a ton of carbon dioxide which is discharged through shielded nozzles a t intervals along the entire eight stories of this huge tunnel. One great advantage of the built-in highMAINTENANCE. pressure carbon dioxide system is the small amount of maintenance required. There are no motors or compressors which can cause the system to fail or which require frequent checking and care. The gas does not deteriorate in the cylinders. The position of levers and weights shows a t a glance whether or not the system has been operated. At regular intervals the cylinders should be weighed to detect possible loss by leakage. Cylinders which show a loss of weight amounting to 10 per cent or more should be recharged. However, it is possible to manage this weighing operation with the cylinders in place and without disconnecting the piping and removing the cylinders from the installation. Thus, carbon dioxide is one of the most familiar of all gases ‘being applied t o free industry of the ever-present threat of fire. These are critical times. Our production must be kept a t top speed, and every unguarded fire hazard is a menace to the uninterrupted flow of materials which we must provide. The built-in carbon dioxide extinguishing system has been developed to a point of unusual efficiency in the complete protection of the flammable liquid fire hazard.
