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API RP 1639-2003 (2012) – Installation of Underground FRP Tanks: Best Practices and Technical Guidelines
A detailed technical overview of the recommended practice for installing fiberglass-reinforced plastic underground storage tanks, covering scope, technical requirements, and compliance considerations.
API RP 1639-2003 (2012) provides industry-recognized recommended practices for the installation of underground fiberglass-reinforced plastic (FRP) tanks. These tanks are widely used for the storage of petroleum products, chemicals, and wastewater due to their corrosion resistance and long service life. This article examines the scope, technical requirements, and compliance notes of this standard to assist engineers, contractors, and inspectors in achieving reliable and compliant installations.
1. Scope and Application
API RP 1639-2003 (2012) – Recommended Practice for the Installation of Underground Fiberglass Reinforced Plastic Tanks – applies to the installation of factory-fabricated, cylindrical, horizontal FRP tanks intended for underground service. The standard covers site preparation, tank handling and placement, bedding and backfill, anchoring systems, leak testing, and manhole assembly. It is referenced by other standards such as NFPA 30 and EPA UST regulations.
The document is intended for tanks storing non-aggressive liquids at service temperatures below 120°F (49°C) and for tank capacities up to 50,000 gallons (189 m³). Larger tanks may require additional engineering analysis.
Tip: Always consult the tank manufacturer’s installation manual in conjunction with API RP 1639-2003 (2012). Many manufacturers provide site-specific parameters that supplement or exceed this recommended practice.
2. Technical Requirements for Installation
2.1 Site Preparation and Excavation
The excavation must be free of sharp objects, debris, and frost. The bottom of the excavation must be level and compacted to provide uniform support over the tank’s length. API RP 1639 recommends a minimum of 6 inches (150 mm) of compacted granular bedding below the tank bottom. Excavation width should allow access for backfill placement and compaction (typically 2 feet or 0.6 m beyond the tank diameter).
2.2 Bedding and Backfill Materials
Selection of bedding and backfill material is critical to tank integrity. The standard specifies free-draining, granular materials that are free from silt, clay, organic matter, and other corrosive substances. Common materials include washed pea gravel or crushed stone in the range of ¼ to ¾ inch (6–19 mm). Angular particles achieve better compaction. The following table summarizes typical backfill requirements:
| Parameter | Recommendation |
|---|---|
| Bedding depth below tank | 6 inches (150 mm) minimum, compacted to 95% maximum dry density (ASTM D698) |
| Backfill material type | Clean, crushed angular stone / pea gravel; no fines |
| Maximum particle size | 1 inch (25 mm) |
| Compaction requirement | 90% relative density or 95% Modified Proctor (ASTM D1557) for load-bearing layers |
| Placement method | Mechanical compaction in lifts not exceeding 8 inches (200 mm) |
Warning: Do not use clay, silt, or organic soils as backfill. These materials can lead to uneven support, moisture retention, and accelerated corrosion of fittings.
2.3 Tank Placement and Alignment
Before lowering the tank, all lifting lugs, straps, and reinforcements must be in good condition. The tank should be centered in the excavation and checked for level (within ¼ inch per 10 feet or 2 mm per 3 m). API RP 1639 stresses that the tank must not be dropped; controlled lowering with spreader bars is required. After placement, install temporary supports as needed until backfill is placed.
2.4 Anchoring Systems
In areas where groundwater may create buoyancy forces, anchoring is mandatory. The standard describes the installation of flexible straps, I-beam anchors, or cable-based systems. Anchor design must account for tank size, groundwater level, and worst-case buoyancy (tank empty backfill saturated). Anchor straps should be tensioned to the manufacturer’s torque specification (typically 20–30 ft-lb; 27–41 N·m).
Note: In high water table zones, consider installing a drain layer and sump pump to lower groundwater during backfill and testing operations. Do not over-tighten straps to avoid deforming the FRP tank.
2.5 Hydrostatic and Leak Testing
Before backfilling above the tank, the standard requires a hydrostatic pressure test at 1.5 times the specified operating pressure or 5 psi (34.5 kPa) gauge, whichever is lower. The test must be maintained for a minimum of 30 minutes. The tank must also be monitored for leaks after backfill and before commissioning often using interstitial monitoring or automatic tank gauging (ATG). API RP 1639 emphasizes that all vents, pipes, and fittings must be sealed during testing.
3. Compliance and Regulatory Alignment
API RP 1639-2003 (2012) serves as a recognized industry practice for compliance with U.S. EPA regulations (40 CFR Part 280), which mandate proper installation of underground storage tanks. It also aligns with NFPA 30, ASTM D4021 (glass-fiber-reinforced polyester tanks), and local building codes. Many jurisdictions require that installations be performed by certified contractors following this recommended practice.
Success: Installations that adhere to API RP 1639-2003 (2012) have demonstrated service lives exceeding 30 years, with minimal need for corrosion repairs when paired with proper cathodic protection for metal components.
4. Quality Assurance and Inspection
Key inspection points during installation include:
- Verification of excavation depth and bedding material before tank placement.
- Check of tank alignment, levelness, and anchorage assembly.
- Witnessing of hydrostatic test and documentation of pressure readings.
- Monitoring backfill compaction tests using nuclear gauges or sand cone density tests.
The standard recommends that all phases be documented in a “tank installation record” that may be required by the regulatory authority.
Frequently Asked Questions (FAQ)
Q: What is the purpose of API RP 1639?
A: API RP 1639-2003 (2012) provides recommended practices for the installation of underground fiberglass reinforced plastic (FRP) tanks. It addresses site preparation, tank handling, bedding, backfill, anchoring, leak testing, and final commissioning to ensure safe and durable installations that meet industry regulations.
A: API RP 1639-2003 (2012) provides recommended practices for the installation of underground fiberglass reinforced plastic (FRP) tanks. It addresses site preparation, tank handling, bedding, backfill, anchoring, leak testing, and final commissioning to ensure safe and durable installations that meet industry regulations.
Q: What type of backfill material is recommended for FRP tanks?
A: The standard recommends clean, angular, free-draining granular materials such as crushed stone or pea gravel with particle sizes of ¼ to ¾ inch (6–19 mm). The material should be free of fines (silt, clay) and organic matter. Angular particles achieve better compaction and provide uniform support.
A: The standard recommends clean, angular, free-draining granular materials such as crushed stone or pea gravel with particle sizes of ¼ to ¾ inch (6–19 mm). The material should be free of fines (silt, clay) and organic matter. Angular particles achieve better compaction and provide uniform support.
Q: Are anchoring systems always required?
A: Anchoring is required when groundwater conditions could cause the tank to float (i.e., when the tank is empty and backfill is saturated). API RP 1639 specifies that anchorage must be designed for the maximum buoyant force. In low groundwater areas, anchoring may be omitted per manufacturer guidance, but a risk assessment is typically performed.
A: Anchoring is required when groundwater conditions could cause the tank to float (i.e., when the tank is empty and backfill is saturated). API RP 1639 specifies that anchorage must be designed for the maximum buoyant force. In low groundwater areas, anchoring may be omitted per manufacturer guidance, but a risk assessment is typically performed.
Q: What testing must be performed before completing the installation?
A: A hydrostatic test is required before final backfilling. The test pressure is typically 1.5 times the operating pressure or 5 psi (34.5 kPa), held for a minimum of 30 minutes. After backfill and prior to commissioning, the tank should also be tested for leaks using an interstitial monitoring system or automatic tank gauge (ATG) in accordance with EPA regulations.
A: A hydrostatic test is required before final backfilling. The test pressure is typically 1.5 times the operating pressure or 5 psi (34.5 kPa), held for a minimum of 30 minutes. After backfill and prior to commissioning, the tank should also be tested for leaks using an interstitial monitoring system or automatic tank gauge (ATG) in accordance with EPA regulations.
© 2026 – Technical Article on API RP 1639-2003 (2012). This content is provided for informational purposes and does not substitute for the full text of the standard.