The Strength of Steel

The Steel Tank Institute has conducted tests that demonstrate steel’s superior mechanical properties. The outcomes disprove the contention that fiberglass reinforced plastic (FRP) is “just as good” as steel for tank applications.

STI conducted the structural performance tests defined in UL 1316, Standard for Fiberglass Tanks, using steel tanks. The results are conclusive.

Steel Tanks Show 22 Times Greater Safety Factor Under Internal Pressure

To test the strength of FRP tanks under internal pressure, UL 1316 specifies that an internal 25 psi hydro load test be conducted in a 10-foot or smaller diameter tank. A safety factor of 5 psi is typically applied in production tests of FRP tanks.

In STI’s steel tank test, however, the load was increased to 110 psi—more than 400% higher than the FRP requirement and 22 times the 5 psi production test.

Steel Tanks Three Times Stronger Than FRP Under External Pressure

UL’s test for FRP tanks under external pressure requires that an empty tank be installed in a standard excavation and the pit filled with water to grade. A vacuum is then applied to the primary tank.

For STI’s test, however, a steel tank was buried to a seven-foot depth and completely submerged in water. Vacuum was then applied until the vacuum equipment could pull no more.

The steel tank withstood a total external pressure, including water and vacuum, of negative 15 psi, three times more than the required performance standard for FRP tanks, and still the steel tank did not fail.

Water Load Test

The UL 1316 test calls for the FRP tank to be placed in a sand bed depth equal to 1/8 of the tank diameter. The tank is then filled with water for one hour.

Steel tanks are routinely installed aboveground, so the water load test is not a challenge. Nevertheless, STI duly completed the test. Extending the test beyond requirements for FRP was not necessary.

Steel’s Proven Performance

Steel’s strength and versatility dominate every aspect of the petroleum supply chain.
Steel can be buried at any depth, even shallow burials.
Steel can be installed in any groundwater situation—even tidal areas.
Steel resists pressures that plastic and FRP cannot tolerate.
Steel tanks perform more than 300% better than FRP in strength tests.
Steel underground tanks are naturally grounded.
Steel is not permeable to vapors or liquids.
Steel is safe and recyclable.