FRP Tank Specifications

Fiberglass Underground Water Tanks

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This information is offered to assist in the purchasing and proper installation of underground direct bury fiberglass tankage. Use the following guideline to communicate with tank suppliers and installing contractors. Please note that Darco Inc. does provide a comprehensive installation manual and verbal assistance to all contractors installing our products.


Provide and /or install the following underground direct bury fiberglass tank as directed by the water storage system Owner, Agent, Architect, or Engineer of record


  • Manufacturer’s quotation drawing including the location and size of all appurtenances
  • Support illustrations for accessories and special equipment depicted in the drawing
  • Manufacturer’s backfill material requirements and proper placement procedure
  • Anti-buoyancy information with the manufacturer’s suggested anchoring details
  • Lifting lug locations and rigging procedure for safely unloading the tank delivery trailer
  • Total F.O.B. delivered price and the estimated lead time necessary for manufacturing
  • Shop construction approval drawing set provided after formal order placement


The manufacturer shall be in the business of building tanks with Materials, Tolerances, and Workmanship conforming to the corresponding sections of the following national standards:

  • ANSI / AWWA D120 – Thermosetting Fiberglass Reinforced Plastic Tanks.
  • ASTM / D3299 – Filament Wound Fiberglass Tanks.
  • ASTM / D4097 – Contact Molded Fiberglass Tanks.


The manufacturer shall have no less than 20 years of experience manufacturing direct bury tanks and have at least 500 tanks in active water storage service.


  • External Hydrostatic Pressure: In empty condition, anchored, backfilled, and totally submerged below 5 feet of gravel flooded to grade; the tank shall remain water tight and structurally unaffected during a 24 hour test period.
  • Traffic Loading: When installed according to the manufacturer’s instructions, the vessel shall meet HS-20 vehicle axle load carrying requirements.
  • Specific Gravity: Design for potable water or domestic sewage at 1.0 S.G.
  • Temperature: Standard design is for a maximum direct water impingement temperature of 100 F.  Higher temperature designs shall be available for thermal energy storage tanks.
  • Insulation: standard 2 inch thick rigid foam insulation covered with an outer fiberglass jacket shall provide a waterproof R12 value.  Additional insulation thickness shall be available.
  • Composite Flexural Modulus: The chop / hoop wound glass cylindrical shell shall have a certifiable minimum flexural modulus of 2,000,000 PSI and the chopped spray heads no less than 1,000,000 PSI.
  • External Ribbing: both the shell and heads shall be bolstered with hemispherical profile ribs at no greater than 24 inch center to center spacing.
  • Venting: Design for internal atmospheric pressure, including fill and discharge cycles.
  • Bury Depth: Design for direct bury to a maximum soil cover depth of 5 feet with deeper bury designs available upon request.


  • Fit and Finish: All internal seams and penetrations shall be filled and overlaid with glass mat leaving no crevices, exposed edges, or protruding fibers.  All external surfaces shall be smooth and coated for resistance to ground water.
  • Potable Water Applications: The tank internal water contact resin layer shall meet FDA food industry standards and be listed under NSF 61 as suitable for potable water storage.
  • Tank Fittings: Standard fittings shall be of fiberglass compatible construction with a flat faced flange or female tapered pipe thread design.  Fiberglass or PVC stub pipes shall also be available.
  • Lifting Lugs: The combined capacity of designated lifting lugs shall be 2 times the empty tank weight.
  • Fasteners: All bolts shall be stainless steel with gall resistant brass nuts


  • Factory Testing: Tanks shall be vacuum proof tested by the manufacturer for external buckling resistance to 10 inches of mercury and then low air pressure tested for pinhole leak detection under a soap / water spray solution
  • On Site Field Testing: Installing contractor may only perform hydrostatic leak-down testing as specified in the manufacturer’s installation instructions


  • Tank Installation: Tank manufacturer shall provide an installation manual outlining the excavation process, rigging, delivery truck offloading procedure, and backfill details.


  • Anchoring Against Uplift: Both dead man and slab anchor designs shall be available for installer review.  The choice to use either anchor style or sub drain piping is the responsibility of the owner and his technical representative.  Factory approved anchor straps and anchoring hardware shall be available at the time of tank purchase.


  • Integral Equipment Design and Selection: Selection and approval of integral project equipment such as heat exchange devices, pumps, telemetry instruments and diffuser piping shall be the responsibility of the owner and his technical representative.  Tank manufacturer shall assist in project design efforts by suggesting the most cost effective fabrication approach and to offer manufacturing proprietary, historical, or precedential knowledge


  • Labels and tags: Each vessel shall be placarded with a confined space warning, stenciled externally to note the empty lifting weight, and labeled inside the manway collar per the American Water Works Association vessel identification requirements


  • Tank Protection: Tank manufacturer shall include transportation to the job site on specialized trailers specifically fitted for the safe delivery of large fiberglass vessels.  All fittings and openings shall be taped or covered to exclude road splash and other avoidable internal contamination.
  • Tank Offloading: Offloading equipment and rigging on site is the responsibility of the owner and his installation contractor.
  • Special Freight Guidelines: Wide load freight quotations are best estimates and subject to demurrage and other additional charges based on the actual mileage traveled as effected by weather, highway detours, and other factors beyond our control.  Delivery times are approximate and subject to minor revisions and adjustments.


Tank manufacturer warrants that their tankage products will not leak, corrode, or fail structurally due to inappropriate design, improper use of materials, defects in materials, or poor workmanship for a period of 2 years from date of delivery.  The manufacturer’s full warranty documents must be included in the installation and owner information packet supplied upon order placement.


DARCO INCORPORATED – Post Office Box 68 – Cottonwood, AZ   86326