m f e t y in the chemical laboratory
edited bv MALCOLM M. RENFREW University of Idaho MOSCOW. Idaho 83843
The Safety of Small Containers for Flammable Fluids Edward S. S h a n l e y Arthur D. Linle, Inc., Acorn Park, Cambridge, MA 02140 Each time we open a flammable fluid container, perhaps a stove fuel or paint thinner container a t home or a solvent can a t the laboratory, we provrde nn access path hetween any nearhg ignition WUTCL. and the vapor space in the container. This vapor space, in general, is in equilibrium with the liquid contents. If the equilibrium vapor is flammable, a nearby ignition source can initiate a flashback, that is, a combustion reaction inside the container. Fires, injuries, and fatalities have resulted from such flashbacks. Since all chemistry students and most people a t large encounter flammable fluid containen, wide awareness of flashhack hazards is desirable. The study described herein has highlighted aspects that are unfamiliar to most laymen and to many practicing chemists as well. The findings may lend themselves to presentation in introductory chemistry courses. Any flammable fluid can, in principle, generate equilibrium vapor that is t w lean to hurn, flammable, or t w rich to hurn, depending on the fluid temperature. Behavior a t ordinary ambient temperatures is ohviously most important and is the focus of this study. Note that thevapor in the head space of any conventional liquid container will remain in near-equilibrium with the contained liquid, even if the neck or spout is left open for a relatively long period. This follows from the fact that the vapor generated by volatile liquids is generally heavier than air, usually much heavier. Emission from the (upright) container is limited by the slowness of diffusion and convection. Maximum energy release in a flashback, and maximum rate of energy release as well, are realized when the vapor-air mixture is a t or near the composition corresponding t o
E. S. Shanlsy is a retired Vice President of Amur 0. Linie. inc., now serving as a consultant to the company. His professional interests are in applied physlcai chemistry and thermochemistry. He has led many studies of Chemical fire and exposion hazards. Shaniey's Interest in hazard manage men1 stems hom long association with peroxy compounds. He is the authw of numerous technical papes and review articies on the chemistry and applications of hydrogen peroxide and other peroxy corn pounds.
A6
Journal
of Chemical Education
oxygen balance. At this composition there is just enough fuel in vapor form to combine with all of the oxveen in the head soace t o form csrhm dm&; and water. (Othirreac. tion products are relatively unusual since must common flammahk flu~dsare either hydrovarbms or rarhgln-hydrogen-oxygen compound, I Standard handhooks contain values for the "flammalh range" of flammable vapors, that is, the vapor coneentration expressed as volume percent in air, corresponding to the lower and upper flammahilitv limits for the comoound in auestion. These data are useful. for example, in ralculnt~ngthe vent~latwnrate necessary to prevent accumulat~onof flammahlc vapor in a n oven or a processing area but provide no direct information on the properties of the equilibrium vapors over any given fluid. Flammability limit data and calculated values for oxygen balance can, however, be interoreted in terms of the temoerature of the flu;d in question hy way i,f readily available vapor pressure vs. temperature data. In the case of relatively pure chemical comp~mnds. the simplest means for relating temperature t o vapor concentration is to equate vapor pressure in atmospheres t o mole fraction. This imputes ideal behavior to the vapor. Since the dataof most interest refer to relativelv dilute vanor. the uncertaintv so introduced i* arcepcable for the purpose at hand. T a h k 1 contains Handhmk valuesfor the lower and upper flammability limits of 18 common solvents, as well as calculated values for the vapor concentrations corresponding to oxygen balance. Also included are calculated values for the temperatures a t which the eauilibrium vanors carresoond to the lower flakmability limit, ux& hnlance, and upper flammability limit fur each compound. It is not possible to calculate the vapor concentration over fluid mixtures such as gasoline or kerosene in the manner described above because the molecular eamposition of these materials is not determinant. Physical properties data do, however, provide guidance. Far example, kerosene is required by regulation t o have a flash point above 100 OF (32.8 OC). The minimum temperature required to produce a flammable mixture in air cannot he calculated explicitly but is clearly higher than 100 OF, well above the level of ordinary ambient temperatures. Similarly, gasoline is manufactured to vanor nressure soecificatians that provide ambieni tempera&re vapor eoncentrations far above the flammable range. The
equilibrium vapor over gasoline can become flammable if the fluid is cooled, say to the reeion of 0 OF (-17.8 "C). yhe temper&ure-related data in T a b k 1 constitute prrdirtions regarding the flnmmability behavior of the equrlihrrum vapor over each of the listed fluids
Flammability Tests Equilibrium vapors a t 23 O C over most of the solvents noted in Table 1have been tested for flammability. Each test was carried out by placing 5 mL of the selected solvent in a l-quart "F'-type can. This is about 20fold the amount needed t o saturate the can volume. The fueled can was dosed tiehtlv hv, ,. means of the rrrea cap, then d l w t d w equilibrate for two hours with periodic turning and shaking a t laboratory temperature. A small gas flame ignition source was then presented a t the newly opened can spout. The container walls were flexed to draw hot gas into the vapor space. Findings are noted in Table 1. "Flashback" indicates that presentation of the ignition source was followed by a strong, noisy burst of blue flame, leaving the can hot to the touch. "External eomhustion" indicates that combustion was limited to quiet burning of vapor diffusingfrom the can with no internal burning. Note the agreement between the experimental observations and the calculated temperature limits of flammability. Among the flammable fluids tested. dionane and the common alcohulu are exceptiunal in forming flammable equilibrium vapurr at ordinary temperature.
.
Vapor S p a c e Hazards Flammability of the vapor in a liquid container is of little practical concern as long as the closure is in place. As noted above, a flame or other ignition source near the open spout may initiate a flashback. The resulting internal combustion will raise the pressure in the head soace to a level deoendine upon the total energy released and the ratio of ventinl: area w head space volume. It turns out that c2 >2 -8
Remarks no defamation
cans slightly deformed cans severely deformed cans severely deformed cans burst cans burst calculated Pat constant volume
Volume 65
Number 1
January 1988
A9
Table 3.
Slze of Ooenlna Varlous Fluld Contalners
Container Type Onlv gasoline outside the container will be involved, even if the container opening is wreathed in flame. I n Tahlc 3 &have noted the vent area ratios of scvcral container types other than those rramined in this study. I t is not ohvi. ous, however, that the safe vent area ratit, we have established for I -gallon containers wnuld be adequate for larger wlumes. Summary We have documented the range of temperatures required to provide flammable equilibrium vapors over a number of commonly used combustible solvents. Among the fluids considered, the alcohols and dioxm e stand out as providing highly flammable equilibrium vapor a t ordinary temperatures. Any ignition source in the neighborhood of the open mouth of an alcohol or dioxane container may lead to a flashback into the container head space. If adequately vented, this internal reaction will not burst the container. Our study has shown that the normal spout in quart- and gallon-size rectangular metal cans and in gallon plastic containers will vent the worst case internal combustion reaction without endangering the container. However, an internal combustion may cause the ejection of burning fuel or cause the user to spill a quantity of fuel. If the container opening is restricted, as hy an inner seal that has not been com-
A12
Journal of Chemical Education
metal l-quarl can metal l-gallon "Vcan camplng fuel l-gallon can cyllndrlcal 5-gallon solvent can Plastic pull-out style Metal pull-out style 50-gallon solvent drum recfangularplastic l-galion paint minner container pletely removed, or in any other way, it is possible for an internal combustion to burst the container. We have provided experimental data relating vent area per unit of container volume to internal pressure rise during the worst case internal ignition. These data may also be useful in considering the hazard potential of containers larger than those used in this study.
The Special C a s e of Gasoline Containers The common perception that gasoline containers are subject to explosion remains to be explained. In fact, gasoline-type hydrocarbon mixtures a t ordinary temperatures furnish equilibrium vapor that is far too rich to burn. The vapor over gasoline does become flammable if the liquid is cooled to, say, the neighborhood of -20 'C but is definitely not flammable if the liquid is atordinaryroom temperature or higher. I t is true, of course, that dosed gasoline con-
Spoui Dimenslam Area Diameter lcm) (crn2)
cm2/gal
2.4 3.57 1.91
4.52 10.01 2.86
18.09 10.01 2.86
3.51 3.81 5.41
9.85 11.40 23.00 4.91
1.93 2.28 0.46 4.01
2.50
tainers may burst if healed very strongly, as in a fire. Thia is due to the high vapor pressure of the heated liquid or to thermal iailure of the container-and is not specific to gasoline. The had reputation of gasoline may also relatem thenumerour accidents withMempty" containen. A very small liquid residue may, indeed, make the vapor space flammable, even though the equilibrium vapor over an appreciable quantity of liquid would be too rich to hum. Furthermore, small resior h e a w ends mav exist in dues of eums " gasoline containers for years only to be volatilized under the heat of a welding torch and create a flammable atmosphere. Welding or other heating of used gasoline containers is extremely hazardous but not hecause gasoline per se generates flammable equilibrium vapor at room temperature or a t higher temperature.
