Hazards and Hazard Testing

10 Hazards and Hazard Testing HENRY M. SHUEY

Downloaded by COLUMBIA UNIV on June 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0088.ch010

Rohm and Haas Co., Redstone Research Laboratories, Kuntsville, Ala. 35807

The identification and evaluation of hazards in manufacturing solid propellants are being brought to a semiquantitative state of the art. Indiscriminate use of routine "approved" tests is being supplanted by analysis of the processes and operations to be used. By identifying the principal stresses involved (as thermal, friction, impact, electrostatic), one can design specific "use" tests, resulting in numerical values broad enough to distinguish discrete differences in stimuli necessary to ignite materials tested. Consideration of the consequences of such ignition allows tests to assess the worst catastrophe probable and suggests modifications of process conditions or plant construction to minimize risk to personnel, facilities, and product.

' T h r o u g h o u t the history of industrial development man has generally adopted a new invention, process, or product and attempted quickly to adapt it to a particular need for which adequate data are unavailable. In aerospace propulsion many of the prior arts and processes were those of the explosives industry, an industry characterized by intensive employee training and considerable reluctance toward change. In the expansion caused by utilization of many of the processes i n solid propellant manufacture, adaptation was necessarily performed b y personnel not experienced i n the rationale of the prior art, occasionally leading to disastrous results. The operations involved i n the manufacture and handling of solid propellants involve grinding of oxidants, blending of fuel and additives, loading of mixers, incorporation, discharge of the blended material, casting, curing, disassembly of tooling, and trimming of the final product. The materials are exposed at the various stages to mechanical stimuli, such as impact and friction, electrostatic discharge, thermal environment, and—under some conditions—shock. Considerable experience has been obtained as to the probability of occurrence and the results of ignitions A

296 In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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i n these various stages of manufacture with subsequent modification of the procedure to reduce both the ignition probability and its effect.


Recently, attempts have been made to analyze hazards on a sound, predictive basis, not only to eliminate the risk involved i n handling a new material or changing a process but to allow the process itself to be designed to produce the desired material at the lowest risk to personnel, property, and product—a sort of before-the-fact value engineering. Attempts to estimate the probable hazard of handling a new compound were generally based on correlation of available test data with that of materials which had been processed successfully. Unfortunately, many of the test techniques used were designed for reproducibility of result rather than for interpretation of hazard, and they applied primarily to correlation within the industry where they were developed. Hazards Analysis Although the basic principles of explosives safety—the minimization of personnel exposure, of material quantity, and of possible ignition sources—could be transferred from the explosives industry, only minimization of personnel exposure could be accomplished efficiently without additional knowledge. A n understanding of the physical chemistry of ignition and propagation of combustion and of the physics of heat generation as they relate to the processing and handling of propellants was necessary. The application of this knowledge as quantitatively as possible is regarded as "hazards analysis." Solid propellants are formulated to contain both a fuel and an oxidizer so that on ignition they w i l l deflagrate uniformly and efficiently. They must also possess adequate physical properties to maintain structural integrity over a wide range of stress and strain rates. H i g h explosives are generally formulated of materials which contain both fuel and oxidant moieties so that upon shock stimulus the material w i l l detonate uniformly and efficiently. Herein lies a primary difference between a solid propellant and a high explosive: one must be cohesive and maintain essentially theoretical density under fairly high stresses; the other w i l l fracture easily and is purposely manufactured at less than theoretical density i n a prestressed state so that the shock stimulus necessary for initiation is not too great. The reaction of the two systems to an ignition stimulus is markedly dissimilar; the cohesive material presents a combustible surface wherein the rate of regression is controlled b y thermal diffusivity into the propellant; the non-cohesive system offers flame paths of convection into the explosive bulk, which if ignited produce gases to fracture the ma-

In Propellants Manufacture, Hazards, and Testing; Boyars, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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terial further and i n many cases cause transition from a deflagrative reaction to detonation, often referred to as D D T .


Hazard Tests Tests made on explosives for "sensitivity" by impact or friction, a l though in reality only ignition tests, should correlate well with the hazard of detonating the material. O n the other hand, the same tests on a co hesive solid propellant would indicate only its ignitability under that particular stimulus and would not correlate prior experience i n the ex plosives industry. A typical example is interpretation of impact testing, generally expressed as the height of fall of a hammer onto a specially prepared sample which should produce ignition 50% of the time. Composition B , a high explosive often taken as a reference for a "safe" material, has a test height of 25 cm. on a particular machine. The same test run on a rubbery binder, ammonium perchlorate-oxidized propellant, yields the value of 11 cm., indicating at first that the propellant was much more hazardous than the high explosive. Yet the propellant in conventional test size shows little reaction i n response to a high energy shock, while Com position Β may be initiated b y a detonator. Actually, the test only indi cates that i n thin films under impact loading, ignition temperature is reached i n the solid propellant at a lower height of fall, and even this may be misleading since the test sample size enhances the effect of con verting mechanical impact into thermal stress b y the hardness of the ammonium perchlorate and the closeness of approximation of the film thickness to the particle size of the ammonium perchlorate. O n the other hand, if we consider the processing steps i n making an ammonium perchlorate-oxidized solid propellant, the same test data would indicate a significant hazard of explosion or detonation when the propellant was unconsolidated and porous, particularly from impact or frictional stimulus i n thin films such as might happen i n shear-mixing. The test results would also indicate an ignition and fire hazard greater than the high explosive i n machining or trimming operations of the final consolidated product where mechanical stimulus is applied to thin films. Hence, normal safety practices dictate remote operations during mixing and trimming, good hazards analysis practice would require conversion of the impact fall into kinetic energy units to compare with possible measured and predicted stresses which may be encountered i n processing. Explosive Liquids Handling of explosive liquids has for years been based on a com parison of various test data with that obtained for nitroglycerine since



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that compound has had the most production experience. The results have not correlated well when standard impact or thermal tests were used. More recently work at the Bureau of Mines (I, 3, 4) has shown that initiation of liquids to a low velocity reaction at low stimulus is a function of the material properties of both the liquid and its container and allows a more rational assessment of hazards to be made. Instead of providing stimulus only to thin films of liquid in an i m pact test (which still has predictive use), testing may be done at levels and conditions comparable with that which may be accomplished i n practice. Analysis of probable stimuli in the handling of discrete quantities of liquids shows a highly probability of dropping. Tests may therefore be designed to drop the material in question i n the proper containers from various heights until an initiation occurs and until there is some statistical certainty of non-initiation at some finite height below this. Since heights may be compared, an assessment of the safety margin may be obtained for this handling operation. More in line with the predictive use of hazards analysis, however, is the experimental and theoretical assessment that the viscosity of the liquid significantly affects this mode of initiation. Such information allows redesign of the process to eliminate handling of low viscosity liquid explosives, and quantitative measurement of the sensitivity of the system to mild shocks as a function of viscosity may allow the optimum level to be selected. This is not necessarily a new concept, only quantified in a different manner. Thirty years ago transporters of neat nitroglycerine in the o i l fields were paid $25 a day. The stipend for transporting jellied nitroglycerine was seven dollars, a practical comment on the understood difference in hazard. Another type of testing, that of electrostatic sensitivity, has been demonstrated in some cases to be more properly a delicate test of the ignitability of the material under localized thermal stress which correlates best to the friction sensitivity of the system under test rather than to electrostatic hazards. O n the other hand, electrostatic tests done i n the supposed atmosphere above a propellant mixer were reduced in absolute value by more than an order of magnitude when the ammonium perchlorate dust actually present was introduced in the test since this altered the potential path for spark discharge within the system. Another favored test for the past few years has been a form of gap testing, usually cards, whereby the explosive stimulus necessary to initiate the material to detonation is determined. Again, as for impact testing, the results are presented i n terms of inches or numbers of cards which attenuate the donor shock wave to non-initiation. The results are much more valuable if expressed in terms of the minimum initiation pressure necessary to initiate the system since a quantitative assessment



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of hazards i n terms of flight malfunction or high speed impact may be made. I n many cases it is also important to determine the minimum donor which can initiate the material since the potential hazard of handling can be shown to be small if insufficient energy is available to initiate it. The previous examples demonstrate the need for an over-all under standing of manufacturing operations, ignition and combustion, and ana lytical test methods to perform a hazards analysis on a new material or process or even to define more accurately the safety margins of existing systems. T h e investigator should determine, i n order and before the fact, the stresses imposed upon materials during processing, their reaction to such stresses and subsequent propagation at that degree of consolida tion, altering the process or the physical state of the material such that a reasonably positive margin of safety exists. W h e n this is not plausible, the investigator should provide such data to management that a logical decision may be made regarding the economic aspects of taking certain negative margins of safety. A n excellent attempt to formalize such an investigative procedure has been presented by Richardson ( 2 ) who summarized hazards analysis as "essentially an accident investigation before it happens." Literature Cited (1) Hay, J. E., Watson, R. W. et al., Ν.Y. Acad. Sci. Conf. Prevention Protec tion Against Accidental Explosions Munitions, Fuels, Other Hazardous Mixtures (Oct. 13, 1966). (2) Richardson, R. H. et al., Ν.Y. Acad. Sci. Conf. Prevention, Protection Against Accidental Explosions Munitions, Fuels, Other Hazardous Mix tures (Oct. 13, 1966). (3) Symp. Detonation, 4th, Naval Ordnance Lab., White Oak, Md., 1, A121 (1965). (4) Van Dolah, R. W., ASESB, Explosives Safety Seminar High-Energy Solid Propellants, 5th, Santa Monica, Calif., Minutes (S), 1963, 344-360. RECEIVED May

4,

1967.


Hazards and Hazard Testing

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