Little General Store Propane Explosion - US Chemical Safety and ...

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Little General Store September 2008 4.0 Propane System Fundamentals 4.1 Propane Properties Propane is used extensively as a fuel for cooking and heating in residential, retail, and commercial applications. 20 It is a gas at ambient temperature and pressure but is transported and stored as a liquid. Propane-air mixtures can be explosive when the atmospheric concentration of propane is between 2.4 and 9.5 percent by volume. Because propane is odorless and colorless, small quantities of an odorizing chemical (ethyl mercaptan) are added so leaks can be detected by smell. Unlike natural gas, propane is heavier than air. Consequently, propane releases tend to accumulate at ground level or in other low points such as pits and basements. 4.2 System Features The propane system at Little General consisted of a stationary 500-gallon storage tank (Figure 6) and two supply regulators to reduce the pressure of the propane for pizza ovens used in the store. The storage tank had five connections: 1. A fill connection fitted with an internal check valve; 2. A liquid level gage to measure the quantity of propane in the tank; 3. A relief valve (safety device) to prevent over-pressurization of the tank; 4. A service valve to supply gaseous propane to the regulators; and 22
Little General Store September 2008 5. A liquid withdrawal valve fitted to an internal dip tube for removing liquid propane when necessary. Figure 6. Typical ASME propane tank. The liquid withdrawal valve incorporated several safety features: 1. A threaded valve plug to protect the valve from foreign material and provide a secondary seal if the primary valve were to leak. 2. A telltale in the threaded area on the valve plug. The telltale is exposed before the plug is completely removed. The telltale releases propane if the primary valve leaks allowing a propane service technician to check for valve leaks before completely removing the plug. 3. A special connector that, when screwed on, opens the valve. 20 According to PERC, about 17.5 million U.S. households use propane as a heating or cooking fuel, excluding grills. About 3,500 companies market propane around the country. 23
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