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This report presents the design methodology and design details for a 500-gallon (1900-liter) composite tank for underground LPG service. This design, developed in conjunction with Lincoln Composites, Inc. of Lincoln, NE, is based upon previous experience with underground petroleum storage tanks, considering tank static pressure, cyclic pressure caused by temperature and filling, and soil loading. Commercial product cost estimates were also discussed.

The current preferred design for a composite propane tank is a polymer-lined composite pressure vessel design and tooling developed for a 41-inch pressure vessel. The composite propane tank consists of an HDPE copolymer liner that is fully wrapped with Type E-CR glass fiber- reinforced epoxy. 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 current design. Rather, the design thickness is driven by the external underground loading requirements of AWWA M45 Fiberglass Pipe Design Manual. This thickness results in a stress ratio from internal pressure near 7.4:1. In addition to the pressure vessel, the development effort includes a riser design. The proposed riser extends to the ground surface from one of the composite propane tank end openings. This riser includes a vertical pipe housing a tube to connect the vapor space of the vessel and riser, and space for a fluid level sensing system. The riser pipe is topped with a conventional combination valve typically used for this application. A gusset system is included for stabilizing the riser to the vessel during transport.

Preliminary marketers’ cost estimates for the tanks are provided for three nominal tank sizes: (a) 250 gallons, (b) 500 gallons, and (c) 1000 gallons. As was shown, there is a price premium for the cost of composite propane tanks compared to that for steel tanks. These prices are based on current equipment and tooling used by Lincoln Composites. Through a combination of increased production rate, specialized tooling, and increased development effort, it is anticipated that the gap between these projected and realized costs will be reduced once serial production is reached. An estimate of the final composite tank prices after significant cost reductions still show a price premium over standard steel tanks ($470 for a 250 gallon, $680 for a 500 gallon, and $790 for a 1000 gallon tank) even after significant cost reductions. For steel tanks, these costs do not include the costs of periodic monitoring or inspections of tanks and/or the cathodic protection system. The cost premium of the composite tanks would be substantially reduced or even eliminated if these periodic inspection costs are added to the costs of the steel tanks.

Depending upon the final cost estimate for the composite propane tank, proposed additional work would include further optimization of the propane tank design in order to reduce the tank cost. This could include, for example, consideration of consumable versus non-consumable mandrels for fabrication. The current proposed design uses the HDPE copolymer as a consumable liner, which also adds to the overall expense of the propane tank. If the finalized cost estimate is practical, then building a prototype based upon the detailed design is also proposed.

Alternative Underground Propane Tank Materials, Phase II—Final Report


September 2009 Battelle and Lincoln Composites

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