Split-Second Fire Protection for Solid Propellants
Previous articles in FIRE ENGINEERING have discussed the subject of rocket fuels and the attendant hazards. Liquid propellants were described in the August and September 1958 issues, and the looming challenge of the missile age was reported in the April 1959 issue. Another aspect of the problem—the solid-type propellant—is considered in this article, together with a practical approach which has been used successfully.
WITHIN RECENT MONTHS, a new word has been coined for a challenge which has stirred man’s imagination for centuries. The birth of “Astronautics” symbolizes our position at the threshold of the conquest of space. The first Astronauts, pioneers of the universe, have been selected and are scheduled to be launched into space within two years.
In the April 1959 issue of FIRE ENGINEERING, Lester Eggleston chronicles the development of the missle age and the challenge that the fire protection field must meet. This article discusses one practical approach to the problem which has been used successfully at the U. S. Naval Propellant Plant, Indian Head, Md. This plant is doing research and development on propellant fuels, and is manufacturing solid propellants and rocket motors for many operational missiles.
Nature of solid propellants
A comparison with liquid propellants illustrates the basic nature of solid propellants. Liquid propellants are of two main types. One, a liquid monopropellant, is a single liquid with its own oxidizing component which is capable of sustained autodecomposition upon injection into a rocket power plant because of a catalyst or high temperature within the plant. The other, a liquid bipropellant, has two distinctive components, the fuel and the oxidizer, which are stored separately and injected separately into the rocket power plant. A hypergolic bipropellant system is one in which the fuel and oxidizer ignite spontaneously upon combination. Solid propellants, on the other hand, are solid combinations of a fuel and oxidizer which are capable of sustained combustion once initiated. The fuel-oxidizer combinations may be either homogeneous or heterogeneous, depending on the nature of the composition. Of the two general types presently used, the double-base type is homogeneous, while the composite type is heterogeneous.
Solid propellants generally have lower energy yields than liquid propellants. Although their firing is not controllable like that of liquid propellants, which can be stopped, resumed, or modulated, solid propellants present advantages in lower cost and simpler design requirements. Their ease of storage and handling, and relatively simple firing procedures, assure optimum readiness for use, and explain their selection for use on board ships or submarines.
Solid propellants are the oldest form of rocket fuel. Black powder, which has been used for hundreds of years, is the forerunner of today’s more sophisticated propellants. Black powder, then as now, is a composite type, with oxidizer of potassium nitrate and fuel of charcoal and sulfur. The sulfur also serves as a binder for the mix.
When smokeless powder was developed, it first was as a single base with nitrocellulose as the basic explosive, and then successively as double base and triple base as other explosive material was added. The early basic propellants joined the fuel and oxidizer chemically by nitration, as do the homogeneous double-base types of today, such as that with cellulose nitrate and nitroglycerine composition, among others.
Modern composite types use oxidizers such as ammonium perchlorate, ammonium nitrate, or others, in combination with fuels such as powdered metals, or plastics or synthetic rubbers of various sorts which also serve as the binder for the heterogeneous mix. A typical composite propellant might contain 75 per cent ammonium perchlorate oxidizer, 20 per cent polymer binder and fuel, and 5 per cent of other materials such as plasticizers, deterioration stabilizers, and burning modifiers.
—Official photograph Naval Propellant Plant
Hazards of solid propellants
The increasing hazard potential of solid propellants may be better understood by a short discussion of the differences between the terms “explosion,” “detonation,” and “deflagration,” as defined by the National Board of Fire Underwriters. An explosion may be described briefly as sudden release of a large amount of energy at a rate sufficient to exert enormous pressures on the surroundings, but relatively slow compared to a detonation. An explosion has velocities ranging to upwards of 1,500 feet per second and develops pressures up to 50,000 pounds per square inch.
A detonation is a violent, practically instantaneous release of chemical energy accompanied by a severe shock wave and characterized by extensive shattering and cratering. Velocities of a detonation range from less than 2,000 feet per second to over 25,000 feet per second, and pressures range from 50,000
to 4,000,000 pounds per square inch.
A deflagration is an extremely rapid and destructive burning action. Low explosives function by deflagration. Many materials capable of explosion may not be subject to detonation, but in some cases, a deflagration under confinement can change to a detonation.
The earlier propellants, black and smokeless powder, are considered low explosives which operate by “pushing” and cannot shift from a deflagration to a detonation. Many modern solid propellants, however, are capable of undergoing a transition from deflagration to detonation by confinement. The heating values, combustion temperatures, and burning rates are increasing as new propellant compositions are developed. Also, larger sizes and more complex configurations of propellant units are adding to their destructive capacity.
The manufacturing processes present a number of hazards. Included are the grinding of oxidizers and other components, mixing of ingredients, extrusion or casting to grain size, cutting, machining, curing, and others. All of these operations, with the possible exception of curing, warrant consideration for installed fire protection since they involve exposure of the propellant material to ignition sources such as friction heat, static electricity, shock, or compression.
Continued on page 792
for Solid Propellants
—Official photograph Naval Propellant Plant
—Official photograph U. S. Navy
Continued from page 663
Choice of extinguishing media
Considering the fire triangle with its elements of fuel, air, and heat, the extinguishment principle most suitable for solid propellants is determined by their peculiar characteristics. Since their combustion cannot be stopped or restarted like that of liquid propellants, elimination of the fuel is not practicable. Removal of air is not effective because they contain their own oxidizers, and many actually are intended for burning in an airless atmosphere. The only means of extinguishment which appears feasible is reduction of the temperature. Cooling is also effective for prevention of explosion or detonation, because when propellant compositions are heated, such as by exposure to radiant heat from an adjacent fire, the chemical reactions of the material can be greatly exaggerated—possibly to a rate where detonation can occur.
Where cooling is required, it seems only natural to think of our old friend, water. With but few exceptions, water has the highest heat-absorptive capacity of liquids. One gallon of water applied at 50° F. and evaporated into steam at 212° F. absorbs nearly 9,500 Btu. It is interesting to note that maximum effectiveness in the use of water for cooling requires the vaporization of the water into steam, because over 8,000 of the 9,500 Btu are required to convert the water to steam after its temperature has reached the boiling point. Other obvious advantages of water, its comparatively ready availability, low cost, and suitability for continuous supply, lead to its preference for fire protection use.
A possible disadvantage of water, its incompatibility with some of the propellant ingredients such as powdered metals, should be considered. The use of water on powdered metal fires would not generally be recommended because of its agitating action and explosion potential. However, the advantages of water outweigh this disadvantage, in the case of propellant material in which the powdered metal is a part of the heterogeneous mix.
The hazard potential dictates the method of utilizing the water. With such extreme heating values, it is necessary that efficient application methods be used in order to get the maximum cooling benefit from the water. Split-second operation is essential because of the rapid burning rates, early development of the heating values, and the need for preventing explosion or detonation. Manual application of water would not be adequate for these reasons, in addition to the severe hazard to personnel. Therefore, fixed, permanent, installations are necessary for the proper use of the water.
Turning to historical record, and the proven success of automatic sprinkler systems since their introduction in the latter part of the 19th century, it is only logical that this work-horse of fire protection be adapted to the modern propellant hazard. With modifications to meet water demands and unusually fast operating requirements, the deluge type sprinkler system has come forth as a “high-speed, preprimed,” deluge system to cope with the hazards of propellants.
System design features
There are several unique features of high-speed, preprimed, deluge systems which distinguish them from ordinary deluge systems. These may be classified as general features, detection and actuation methods, and application techniques.
The most important general design feature is the subdivision of the systems to the smallest practicable size. A separate system for each “heat-influence” area is preferable. A heat-influence area may be considered as that which would be immediately subject to the heat from a fire, and is relatively small in size. Since a heat-influence area is not necessarily defined by fire walls or other physical fire cut-off facilities, its boundaries are determined by judgment, based on the nature of the propellant material. Subdividing the systems increases their costs, hut it is necessary in order to reduce the operating time to a level allowing adequate fire protection. Also, the cost increase from the larger number of deluge valves is somewhat offset by the reductions in the size of the system piping and the length of the detection circuits.
Another general feature is the arrangement of the system for minimum blast damage, in order to allow continued operation after a deflagration. The ability of a system to continue to discharge water is of vital importance for aiding life safety, protecting the structure and equipment, and cooling down other propellant material in the area. For example, in one deflagration which was experienced, the sprinkler system was extensively damaged but was able to discharge enough water from the broken piping to protect an injured employee from the fire and to prevent the involvement of several hundred pounds of propellant material.
Blast-resistance may be improved by supporting the piping from structural elements likely to withstand the deflagration, such as main columns and beams or floors. Since these areas in the plants concerned have little personnel traffic, running the piping along the floor is usually not objectionable from the tripping standpoint. Designing the building with adequate explosion relief facilities to minimize structural damage will greatly increase the blast resistance of the sprinkler systems. Other factors include eliminating weak piping elements such as cast-iron fittings, and providing flexibility in the piping, such as by swing joints or flexible couplings where practicable. In the example described above, the reason a portion of the sprinkler system continued to function after the deflagration is that a swing joint at the top of the main riser allowed the cross main to deflect with the roof. Although the branch lines were separated from the cross main, water was discharged from the broken riser nipples on the cross main.
Another general feature worthy of note is the provision of adequately sized floor drains or wall openings to prevent deep pooling of the water in areas occupied by people. Although remote, there is a potential drowning hazard to an injured or unconscious person.
Special detection and actuation methods are necessary in order for the system to operate within a required 0.5 second. A supervised thermo-pneumatic type detection system is used, based on the rateof-rise principle with fixed temperature back-up. This system uses heat responsive devices, which are the heat collectors, arranged on circuits of small-diameter metal tubing. A supervisory air pressure of about 24 ounces is retained in the tubing, and when the heat collectors are exposed to heat, the resulting increase in the internal pressure actuates the system by a release mechanism.
Several methods are used to attain high speed operation. The number of heatresponsive device circuits is minimized. (The optimum situation would be one deluge system with a single circuit, for each heat-influence area.) Heat-responsive devices per circuit are limited to two, or if all are in the same heat-influence area, three are allowed. Individual circuits are arranged to protect only one heat-influence area. In a small heat-influence area, or “spot” hazard, where one heat-responsive device would normally suffice, an additional device is provided. This increases the sensitivity by exposing a greater heat-collecting area. Also, the heat-responsive devices are positioned as close as possible to the hazard, located so as not to interfere with the water discharge, and they have heat baffles to increase their effectiveness.
Unique sprinkler system
The nature of the hazard calls for application techniques that are also different from the ordinary deluge system. A discharge density of at least 0.5 gallons per minute per square foot of area is required in the room, and greater densities are necessary for concentrated quantities of propellant. The process equipment is protected by nozzles of proper flow capacity and discharge pattern to provide a “cone of protection” of water spray at the required density. The structure is protected by sprinkler heads spaced for the density needed. Although the desire is to extinguish the propellant fire, as was actually accomplished for the earlier compositions, the confinement of the fire to the propellant itself, and cooling of the propellant material to prevent detonation, are considered satisfactory goals for the newer combinations. The systems are designed by hydraulic calculations to meet the water demands.
For split-second application of the water, several techniques are used. The entire system is primed at a low pressure with water, to avoid the delay which would result from filling of the piping. Since water is not compressible, admission of the higher pressure water from the supply by operation of the deluge valve immediately discharges water from the system. The nozzles and sprinkler heads are the open type, but are fitted with small plugs which are specially designed to relieve at a pressure slightly above the priming pressure. With the piping water-filled, it is necessary to arrange the water motor alarm line to retain the priming, and the deluge valve must be a type factory-equipped with a closed drain for the space above the clapper. Other high-speed features include locating the deluge valve as close as possible to the hazard, but preferably protected or at least beyond the heatinfluence area, and locating the nozzles as close as possible to the propellant material, allowing adequate throw for the development of the cone of protection.
It generally is not practical to test the systems by creating a propellant fire. Simulation of such fires by tests with small quantities of propellant material are of questionable reliability because of the difficulty to extrapolate the data or to correlate the results with full-size potentials. Since the rate-of-rise principle is based on a rate of temperature increase rather than a fixed temperature, testing does not require duplication of the expected combustion temperatures. Sudden immersion of the heat-responsive devices in boiling water is adequate to test the rate-of rise operation.
Timing of the tests, however, presents problems. There are inherent delays in the equipment, such as approximately one-fourth of a second for the deluge valve weight to fall, and there are errors and reaction times on the part of the test operators. Use of a stop-watch calibrated to tenths of a second does not allow the degree of accuracy desired, but after one becomes experienced in the testing procedure it will serve as a ready and easy field test method of gauging the adequacy of a system. Some success has been obtained with an electric timer calibrated in milliseconds, but there is need for further improvements in the test methods.
One illustration shows an older type propellant grain which was ignited accidentally during its processing. The fire was completely extinguished by the sprinkler system. On present propellants, there has been a great deal of success in confining the fire to the grain in which it originates, so the amount of loss is limited to the value of the grain itself. As mentioned earlier, even in the case of a severe deflagration which caused major structural damage, the sprinkler system was effective in protecting personnel and preventing involvement of other propellant material. By the proper design of the sprinkler system and the provision of adequate explosion relief facilities for the buildings, the degree of protection afforded will be further improved.
Challenge of the future
It is probable that the hazard potential of solid propellants will increase as continued efforts are made to improve their energy yields. Already planned are individual propellant grains which are measured in numbers of tons, and further developments are expected which will create fire protection problems because of the large grain size and complex configuration of the burning surfaces. Likewise, heating values, combustion temperatures, and burning rates will probably increase.
These changes will challenge the fire protection field. There will be greater need for research in many areas. The use of other extinguishing media must be investigated further. If water continues to be the best method, realistic water demands and operating requirements must be determined for the new hazards. Improved application methods will be needed for the larger and more complex propellant grain shapes. Faster detection methods and elimination of equipment delays are prime targets, with possible solutions including the use of infra-red sensing devices to operate powder-actuated deluge valves or other near-instantaneous discharge devices.
The fire protection engineering profession will have to meet these problems and overcome them. The extreme hazards, severe loss potential, and strategic importance of the solid propellants will demand it.