FRP Tank Specifications

Fiberglass Water Storage Tanks Specifications

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This document is offered to assist in the purchasing and proper installation of underground direct bury fiberglass water storage systems. Use the following guidelines to communicate with potential tank suppliers and installing contractors. Please note that Darco does provide a comprehensive installation manual and knowledgeable verbal assistance to all contractors installing our products.


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


  • Total F.O.B. delivered price and the estimated lead time necessary for manufacturing.
  • A preliminary concept drawing indicating the location and size of all fittings and appurtenances.
  • Cutsheets for accessories and special equipment depicted in the concept drawing.
  • Backfill material requirements and the proper backfill placement procedure.
  • Anti-buoyancy information with suggested anchoring details.
  • Lifting lug locations and rigging procedure for safely unloading the tank delivery trailer.
  • Shop construction approval drawings to be provided during formal order placement.


The supplier shall provide tanks with Materials, Tolerances, and Workmanship generally conforming to the applicable sections of the following national standards:

  • ASTM / D4097 – Fiberglass Contact Molded Corrosion Resistant Chemical Storage Tanks.
  • ANSI / AWWA D120 – Fiberglass Corrosion Resistant Vertical Chemical Storage Tanks and Underground Horizontal Water Tanks.
  • NFPA 22 / 1142 – General structural guidelines, storage capacity, and fire flow requirements.
  • NSF 61 – Resin listing and post cure for potable water applications


The supplier shall have no less than 20 years of experience with direct bury underground fiberglass tanks and have at least 500 tanks currently 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 when modeled by Finite Element Analysis.
  • Traffic Loading: When installed according to the instructions, the vessel shall meet H-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 degrees F.  Higher temperature designs shall be available for Thermal Storage applications.
  • Insulation: Rigid foam insulation applied to the tank shell between the ribs and covered with an outer fiberglass jacket shall provide a waterproof R12 value.  The application of in situ 4’x 8’ rigid foam board above the tank is also approved.
  • Composite Flexural Modulus: The Type 1, Grade 1, Class 1 cylindrical shell shall have a certifiable minimum flexural modulus of 1,500,000 PSI and the contact molded heads no less than 1,000,000 PSI.
  • Fiberglass Content by Weight: The contact molded shell shall contain no less than 50% glass and the contact molded heads no less than 35% glass
  • External Ribbing: both the shell and heads may be bolstered with ribs at approximately 24 inch center to center.
  • Venting: Design for internal atmospheric pressure at all times, including fill and discharge cycles.
  • Bury Depth: D5 vessels are designed for typical applications requiring 2 to 5 feet of cover depth.  D10 vessels have additional wall thickness allowing a cover depth up to 10 feet. Vessels for bury depth beyond 10 feet shall be available upon request


  • Fit and Finish: All internal and external seams and penetrations shall be filled and overlaid with glass mat leaving no crevices, exposed edges, or protruding fibers and coated for protection against ground water.
  • Potable Water Applications: The tank internal water contact surface resin shall be listed under NSF 61 as suitable for short term drinking water storage.
  • Tank Fittings: Standard fittings shall be of fiberglass construction with a flat faced ANSI flange, female tapered pipe thread, fiberglass stub pipe, or PVC stub pipe. PVC stub pipes are only acceptable when located above the water level.
  • 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 air pressure tested for pinhole leak detection at all joints and fittings. Recorded test values taken during construction at numerous locations for thickness and cure shall be available upon request.
  • On Site Field Testing: Installing contractor may perform only hydrostatic leak-down testing as specified in the installation instructions. Potentially dangerous on site air pressure testing is PROHIBITED.
  • Deflection Monitoring: Tanks shall have 2 or more deflection measuring ports to ensure proper backfill procedure is followed


  • Tank Installation: A complete installation manual shall be provided outlining the excavation process, rigging for delivery truck offloading and placement, anchoring, 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 their technical representative.  Factory approved anchor straps and anchoring hardware shall be available at the time of tank purchase. Not all tanks require anchoring.


  • Equipment Design and Selection:  Selection and approval of integral project equipment such as pumps, level telemetry instruments, and firefighting accessories shall be the responsibility of the owner and their technical representative.  The tank supplier may assist in project design efforts by suggesting the most cost-effective fabrication approach and to offer historical or precedential knowledge.


  • Labels and tags: Each vessel shall be placarded with a confined space warning, marked externally to state the empty lifting weight, and labeled inside the manway extension per the American Water Works Association vessel identification guidelines


  • Tank Protection: Tank supplier 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 their 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 the shipper’s control.  Delivery times are approximate and subject to minor revisions and adjustments.


Tank supplier warrants that their tankage products will not leak, corrode, or fail structurally due to inappropriate design, improper construction, defects in materials, or poor workmanship for a period of 2 years from date of delivery.  A copy of the supplier’s full warranty shall be included in the owner information and installation packet supplied upon ordering.


DARCO INCORPORATED – Post Office Box 68 – Cottonwood, AZ   86326  www.DarcoInc.com ~ 800-232-8660