EVALUATING THE POTENTIAL FOR FLASHING DISCHARGE FROM SMALL ENGINE FUEL TANKS Todd M. Hetrick, P.E. Suzanne A. Smyth, Ph.D., P.E. Russell A. Ogle, Ph.D., P.E., CSP Juan C. Ramirez, Ph.D., P.E. Exponent 4580 Weaver Parkway, Suite 100, Warrenville, IL, USA 60555 ABSTRACT This paper explores an infrequently encountered hazard associated with liquid fuel tanks on gasoline-powered equipment using non-vented fuel caps. Depending on the location of fuel reserve tanks, waste heat from the engine or other vehicle systems can warm the fuel during operation. In the event that the fuel cap is not vented and if the fuel is sufficiently heated, the liquid fuel may become superheated and pose a splash hazard if the fuel cap is suddenly removed. Accident reports often describe the ejection of liquid from the fuel tank opening as a geyser. This geyser is a transient, two- phase flow, vertical jet of flashing liquid. This could create a fire hazard as the geyser could result in splashing flammable liquid onto any bystanders. Many fuel tank systems are vented to ambient through the fuel tank cap and, in addition, may incorporate other features that contribute to pressure relief. Venting of the pressurized vapor inhibits the vapor-liquid mixture in the fuel tank from achieving thermodynamic equilibrium, thus preventing the formation of a superheated liquid. It has been empirically determined that flashing two-phase flow can be prevented by keeping the fuel tank pressure below 1.5 psig. However, if the cap is not vented or vents at a lesser rate than the rate of liquid vaporization, pressure in the tank can rise and the flammable liquid can become superheated. This phenomenon is explored here to facilitate a better understanding of how the hazard is created. The nature of the hazard is explained using thermodynamic concepts. The differences in behavior between a closed system and an open system are discussed and then illustrated through experimental results obtained from two sources: experiments with externally heated fuel containers and operation of a gasoline-powered riding lawn mower. The role of the vented fuel cap in preventing the geyser phenomenon is demonstrated. Keywords: superheat, gasoline, flashing, two-phase flow, fuel tank, geyser INTRODUCTION Gasoline powered equipment has remained a centerpiece of the industrialized world since scaled manufacturing began in the early 20 th century. The fundamental makeup of these systems has not changed in that a reserve fuel tank is mounted on the equipment and gradually depleted during operation as fuel is withdrawn for combustion in the engine. To provide for continued operation, the fuel tank must intermittently be refilled, which requires accessing the tank’s interior by removal of a cap or other tank closure. This paper examines a potential hazard associated with superheating gasoline in a closed system. Depending on the location of fuel reserve tanks, waste heat from the engine or other vehicle systems can warm the reserve fuel during operation. In the event that the tank is not vented, and if the fuel is sufficiently heated, the liquid fuel may become superheated and pose a splash hazard if the fuel cap is suddenly removed. Sudden depressurization of the superheated fluid will allow liquid fuel to rapidly flash into vapor. Depending on the fuel type, degree of superheating, tank geometry, and amount of fuel in the tank, a transient two-phase flow of flashing liquid can potentially erupt out of the tank. There is one historical precedent for which it was alleged that this is a reoccurring problem [1]. Research on the Consumer Product Safety Commission (CPSC) website yielded no examples of a recall initiated because of the geyser effect in a fuel tank [2]. A general internet search reveals anecdotal reports involving a variety of types of equipment [3,4,5]. Much of the information available in the public domain about these alleged incidents is substantially incomplete or unverified. In most of the instances reviewed, it could not be determined 1 Copyright © 2014 by ASME Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014 November 14-20, 2014, Montreal, Quebec, Canada IMECE2014-39527