«Portable Plastic Gasoline Container Explosions And Their Prevention Glen Stevick, Ph.D., P.E., David Rondinone, Ph.D., P.E., Allan Sagle, Ph.D. of ...»
Portable Plastic Gasoline Container Explosions
Glen Stevick, Ph.D., P.E., David Rondinone, Ph.D., P.E., Allan Sagle, Ph.D. of
Berkeley Engineering And Research, Inc. and Joseph Zicherman, Ph.D. of Fire
The work described in this report was undertaken to determine probable causes of
portable plastic gasoline container explosions and to consider and demonstrate
preventive technologies for such explosions. Dozens of explosions with this class of containers have been reported in newspapers, legal briefs as well as in fire incidence and peer-reviewed engineering literature.
Fresh conventional gasoline stored in portable plastic containers has an associated headspace which is sufficiently rich in hydrocarbon vapors to prevent ignition from random ignition sources. However, since fresh gasoline evaporates readily, and the lighter ends of these gasoline blends of common hydrocarbons evaporate preferentially, a working hypothesis considered in this work has been that given adequate time, such a vapor space filled by vapors from commercial gasoline blends will eventually fall into the explosive range as more volatile fractions evaporate.
No comprehensive test program demonstrating how and when such processes occur has been published to date. This report provides preliminary results for such a test program pending their further dissemination.
Results presented here clearly show that when gasoline aging occurs [evaporation and diffusion of lighter ends through openings of portable plastic gasoline containers], ambient temperature and volume of aged gasoline in the container are primary variables controlling when explosions can occur.
Additional data is provided demonstrating how simple inexpensive flame arrester designs fitted to gasoline containers can prevent such accidents.
1. Background Explosions and explosion-like events associated with pouring gasoline from a partially filled, portable, plastic gasoline container have been reported as discussed in this paper. More frequently than not, when such events are reported, they have resulted in serious injuries to individuals participating and nearby. Most of the incidents have involved the burning of trash and more specifically pouring of gasoline from the can onto fire debris in an attempt to restart a fire thought to have burned out.1 Similar mishaps resulting in injuries are reported in the accident and fire incidence literature and in reports from Federal agencies.2 It is the objective of this paper to review the occurrence of such incidents, characterize the physics associated with their occurrence and consider possible mitigation features such as flame arresters in the containers to reduce or eliminate these occurrences.
A review of peer-reviewed articles in engineering and fire science journals revealed one on-point article, published in the Journal of Hazardous Materials, by Lori Hasselbring.3 Hasselbring’s research and testing indicated that such phenomena are real, but her research was limited in terms of explaining how and why gasoline container explosions occur. Air entrainment during pouring was suggested as one possible explanation.
More general comments can be found in fire science texts suggesting both that portable plastic gasoline container explosion events do occur and that they do not occur. The Ignition Handbook is one example, with the book suggesting 1 Wibbenmeyer, Lucy A. MD, FACS; Kealey, Gerald P. MD, FACS; Young, Tracy L.
MS; Newell, Ingrid M. MD, BS; Lewis, Robert W. II PA-C; Miller, Benjamin R. BS;
Peek-Asa, Corrine PhD, “A Prospective Analysis of Trash, Brush and Grass Burning Behaviors,” Journal of Burn Care & Research, May/June 2008 – Volume 29, Issue 3, pp 441-445.
2 Consumer Product Safety Commission, National Electronic Injury Surveillance System, CPSC Document #3002, http://www.cpsc.gov/cpscpub/pubs/3002.html.
National Fire Incident Reporting System, Department of Homeland Security, http://nfirs.fema.gov.
3 Lori Hasselbring, “Case Study: Flame Arresters and Exploding Gasoline Containers,” Journal of Hazardous Materials 17 Mar. 2006: 64–68.
gasoline container explosions can happen while a Corrigenda to the original volume provided by the author on the internet states otherwise.4 However, no references are provided to any significant testing or analysis for either condition, and as such, neither claim should be considered persuasive.
The National Bureau of Standards (NBS) has commented in its reporting that gasoline can explosions can occur and the use of flame arresters in the spout can prevent such explosions.5,6 The National Bureau of Standards (NBS) is the predecessor to the National Institute of Standards and Technology (NIST).
Consideration of such explosions in anecdotal forums is common. For example, the Consumer Reports has commented on this subject on two occasions. 7 Newspaper and television reports often quote witnesses describing an explosion sound and in at least one case a plastic gasoline can was produced clearly showing material tearing behavior consistent with an explosion.8 (See figure 1.) This investigation began by developing a method to measure the hydrocarbon concentration in the head space of a gasoline can to allow the assessment of various causes of an explosion. The gasoline used in the testing described in this paper was commercial grade 87 octane, California winter blend. It should be noted that winter blend gasoline in a plastic can, with few openings and closings, quickly ages to and its properties asymptotically approach those of summer blend gasoline. Thus, the authors would expect no significant differences with summer gasoline.
4 Vytenis Brabrauskas Ph.D., Ignition Handbook, (Issaquah, WA: Fire Science Publishers, 2003); Corrigenda to Ignition Handbook 2008, http://www.doctorfire.com/corrigenda.pdf.
5 Tyrell, E., 1975. “Gasoline and Gasoline Container Fire Incidents,” NBS Technical Note 850.
6 Jones, C.E., 1977. “Standards for Gasoline and Kerosene Cans,” NBSIR 78-1414.
7 “Gasoline Cans,” Consumer Reports May 1973: 332-335; “Gasoline Containers,” Consumer Reports March 1981: 168-171 8 Fox 25 News, Boston, “Fire pit explosion critically burns man”, 20 Jun 2009, 3 Nov 2009 http://topics.myfoxboston.com/m/23142009/fire-pit-explosion-criticallyburns-man.html.
Figure 1. Subject gasoline can showing clear signs of explosion and rupture.
2. Measurement of Hydrocarbon Vapor Concentrations
Oxygen sensors are used in many applications to measure volumetric percent hydrocarbon in a vapor solution, such as a container headspace. Such a methodology is described by Shirvill, et. al9 and the same basic method was used in this study. The concentration of gasoline vapors is obtained from a pair of readings using a City Technologies oxygen sensor which provides a 0-10 volt output corresponding to 0 to 30% oxygen. The first reading, V0, measures the partial pressure of oxygen in air. The second reading, V, measures the partial pressure of oxygen in the mouth of the gasoline can. The percent
volumetric concentration of gasoline, Pg, in the mouth of the can is given by:
Pg = 100 (V- V0)/ V0.
Use of this method allowed hydrocarbon concentrations in the headspace to be directly measured in the gasoline cans used. Example measurements of fresh gasoline and lightly aged gasoline (1 liter of gasoline left for 8 hours in a 30.5 cm diameter bucket, outdoors, 10 oC average temperature, and 8 oC 9 L.C Shirvill, P. Roberts, C.J. Butler, T.A. Roberts and M. Royle, “Characterization of the Hazards from Jet Releases of Hydrogen,” International Conference on Hydrogen Safety (Pisa, Italy, September 8-10, 2005.) temperature range) are shown in figure 2. The light “bucket” aging was roughly equivalent to aging for 5 days in a typical 18.9 liter (5 gallon) plastic gasoline container with a 1.9 cm diameter open nozzle at an average 10 oC with a 16 oC range. These results illustrate the interrelationship between aging, temperature, and amount stored in a given volume, as well as the relationship of these variables to the commonly accepted UEL for common gasoline blends.
The results clearly show that the percent hydrocarbon in a gasoline can and propensity to explode, is a function of aging, temperature and quantity.
Figure 2. Hydrocarbon vapor concentration in a 18.
9 liter (5 gallon) gasoline can for fresh and lightly aged gasoline (aged in an open bucket equivalent to 5 days at 10 oC in a typical 18.9 liter gasoline container with an open spout).
The data shows that the hydrocarbon vapor concentration of lightly aged gasoline can easily fall below the upper explosive limit for lower temperatures and smaller amounts of liquid fuel in the can. It should be noted that it is common in explosion incidents investigated by the authors that the subject gasoline can was left open for weeks or months prior to the incident. The data also shows that smaller amounts promote accelerated aging as the ratio of liquid fuel surface area to volume is increased, resulting in an increased evaporation and diffusion rate per volume.
3. Gasoline Aging Aging of gasoline is a relevant and important process in these discussions because aging of fresh gasoline is a precondition for a gasoline container explosion. If there is no aging, the hydrocarbon concentration in the vapor space will be above the upper explosive limit and neither ignition nor explosion can occur.
Like most liquids, gasoline will evaporate if left in an open container. Gasoline contains a range of lighter and heavier hydrocarbon constituents, typically with between 4 and 12 carbon atoms per molecule.10 The so-called “lighter ends,” which are lower molecular weight fractions, evaporate preferentially from such an open container, lowering the gasoline's hydrocarbon vapor pressure with time. This process is what we refer to here as “aging.” It is also well known, based on research related to air pollution considerations, that component fractions of gasoline will diffuse across the walls of a standard plastic gas can. In such cases, the rate of this diffusion is inversely proportional to the molecular weight of the hydrocarbon fraction involved, and directly proportional to the flux gradient of any given fraction; i.e. the greater the difference in concentration of a given fraction across the container wall, the greater the diffusion rate.3 The authors performed aging tests with gasoline blends stored in plastic cans with their spouts left open. The resulting changes in vapor concentrations in the headspace of these cans over time were measured and are shown in figure 3 below. The data show a relatively sharp drop in vapor concentration in the headspace followed by a decreasing rate of change in concentrations.
10 Chris Collins, “Implementing Phytoremediation of Petroleum Hydrocarbons,” Methods in Biotechnology 23, ed. Neil Willey (Totowa, NJ: Humana Press, 2007).
Figure 3. Aging results for commercially obtained 87-octane gasoline stored outdoors, average temperature of approximately 10C, 16F average daily range, in a Blitz 18.
9 liter gasoline can (19 cm by 38 cm base, 63.5 cm tall). Starting volumes ranged from 100 ml to 10 liters.
The gasoline aging data presented above is quite similar to data published recently by Japanese researchers in the Fire Safety Journal.11 The light ends evaporate preferentially and the headspace hydrocarbon vapor pressure and volumetric percent hydrocarbon drops with time.
As shown in figure 3, gasoline vapor pressure percent hydrocarbon is lower for decreasing temperature and decreasing amounts in a container. As an example, for 1.5 liters of gasoline stored in an open 18.9 liter can at an average 11 Katsuhiro Okamoto, Norimichi Watanabe, Yasuaki Hagimoto, Koji Miwa, Hideo Ohtani, “Changes in Evaporation Rate and Vapor Pressure of Gasoline with Progress of Evaporation,” Fire Safety Journal 44, July 2009: 756-763.
temperature of 10 oC (16 oC average temperature range) the percent hydrocarbon in the vapor headspace falls into the explosive range in less than 20 days. The Blitz 18.9 liter can was used for the aging studies.
4. Computational Fluid Dynamics Analysis
It was theorized by Hasselbring1 that air entrainment during pouring might be responsible for the vapor headspace dropping below the upper explosive limit (UEL) and thereby creating an explosive hydrocarbon/air mixture. In order to test this potential contributing factor, pour tests were conducted initially with measurement of the vacuum produced in the can. This simple “monometer testing” was followed by a computational fluid dynamics (CFD) analysis using the multi-physics finite element program, COMSOL.12 The apparatus for monometer testing and the results are shown in figure 4a
18.9 liter plastic gasoline storage can was filled with liquid, placed on a scale, and tipped such that the liquid poured out while the manometer and scale were both captured on video to document the negative internal pressure simultaneously with the change in mass (which relates directly to the liquid flow rate). These tests (can tipped 90 degrees) show that a significant negative internal pressure can occur in a gasoline can in the range of 15-18 cm of water column (1.5-1.7 kPa) during pouring. Interestingly, this depression is of approximately the same magnitude as the positive pressure in a typical gas stove or heater burner manifold. It is also more than adequate to cause airflow into such a can during pouring.
12 COMSOL Multiphysics Modeling and Simulation Software, http://www.comsol.com/.
Figure 4. Gas can connected to a monometer (left) and typical time vacuum pressure as a function of time curve (right).