Currently in the U.S., all underground propane tanks are made of steel and conform to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. The cost of steel is increasing and additional costs are incurred for corrosion protection if the tank is buried. The result is a significant rise in the overall cost of underground propane tanks.
Composite materials are used for a variety of structural components because of their lighter weight and higher resistance to chemical attack, including corrosion. Composite materials are used for low-pressure liquid storage tanks and, increasingly, for pressure vessels. Several manufacturers worldwide produce high-pressure composite vessels for compressed gas use, such as the storage of compressed natural gas and hydrogen. At least four international companies and one U.S. company are producing and marketing composite liquefied petroleum gas (LPG) or propane cylinders. Two manufacturers have U.S. Department of Transportation (DOT) approval to market cylinders for sale and use in the U.S.
In this context, the Propane Education & Research Council (PERC) launched a study to determine the feasibility of developing propane storage tanks from materials other than steel. In the initial Phase I study (PERC Docket 11728, Muellerleile, et al., 2005), Battelle determined that alternative materials were sufficiently plausible to warrant a more detailed design study involving a tank manufacturer. For this Phase II effort, Battelle partnered with Lincoln Composites, a manufacturer of high pressure composite cylinders, based in Lincoln, NE. This report is a summary of the Phase II detailed design continuation of the initial Phase I preliminary design work. In this regard, the current report presents the design methodology and design details for a 500-gallon (1900 liter) composite propane tank for underground service. This design is based upon previous experience with the manufacturing of composite pressure vessels for high-pressure use, underground petroleum storage tanks. The design process considered tank static pressure and soil loading.
As a result of this Phase II study, the current preferred design for a composite propane tank is composite tank design and tooling developed for a 41-inch diameter pressure vessel. The composite propane tank consists of a high-density polyethylene (HDPE) copolymer liner fully wrapped with AdvantexTM glass fiber-reinforced epoxy. A boss, made from aluminum alloy 6061-T651X, is in each head of the composite propane tank. The boss at the service end provides a single penetration into the tank for vapor service, filling, liquid level gauging, and the safety relief valve flow path. The boss at the non-service end serves a tail-stock-type function during the fiber winding process as well as providing structural support at confluence point of the fibers over the head. The current composite propane tank designs include nominal 250-, 500-, and 1000-gallon tanks. As a result of the ability to change length with minimal impact on tooling, all three designs are simply length variations on a single composite layup. The baseline composite propane tank is designed for 250 psi with a 5:1 stress ratio and 20 percent margin for internal pressure loads. However, this requirement does not drive the thickness of the design. Rather, the design thickness is driven by external underground loading requirements per AWWA M45 Fiberglass Pipe Design Manual (AWWA, 2005). This thickness results in a stress ratio from
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites