External Corrosion Protection Systems for Steel Underground Storage Tanks
UL 1746 was first published in November 1989. It provides requirements for corrosion protection of steel underground tanks. There are three types of pro-tection systems covered by the Standard: cathodically protected tanks, composite tanks and jacketed tanks. The base tank is constructed in accordance with Stan-dard UL 58 and more corrosive protection is added.
Part I of UL 1746 covers pre-engineered cathodic pro-tection systems. The tanks are provided with a gal-vanic anode. The material specifications are in the Standard. It also specifies requirements for the back-fill around the anode, insulating bushings for electri-cally isolating the tank from the piping, and wire connectors for cored anodes.
The cathodic protection system shall be con-structed to provide a minimum system design based on a 4,000 ohm-cm soil resistivity. The system shall consist of the following:
• Dielectric coating applied to all exposed exterior tank surfaces;
• Electrical insulating devices (gaskets and bushings) installed in each tank opening prior to shipment;
• At least two galvanic anodes per tank; and
• Test station provisions.
The Standard identifies how to determine the cur-rent requirements of the current system; the soil resis-tivity; the available galvanic anode current; and the galvanic anode weight.
Performance tests include:
1. Anode pull test—Lead wire shall withstand a direct pull of 70 pounds for a period of one minute.
2. Anode assembly durability test—A representative anode assembly is soaked in water for four hours, allowed to dry for at least 48 hours, then mounted on a vibration table. The sample shall be subjected to a 1/16 inch displacement from the table. The sample is to be vibrated for one hour. After this test the anode assembly shall not become inoper-able nor shall the anode shift to a position closer than 0.50 inches from the container wall.
3. Pressure wire connectors tightening torque test—The connection between the wire and the connector shall be subjected to the tightening torque specified in the Standard for five seconds. There shall be no breakage of the conductor, stripping of threads, shearing of parts, separation of the conductor from the connector or other damage to the connector.
4. Pullout test for pressure wire connectors—A connector wire assembly shall withstand a direct pull of 70 pounds for a period of one minute without sepa-ration or movement of parts relative to one another.
5. Pullout test for test lead—A test lead shall be sub-jected to a direct pull of 70 pounds for one minute without separation or loosening.
6. Dielectric coating tests—Sample coupons coated with the dielectric coating shall be subjected to impact tests both “as received” and after condi-tioning. The conditioning processes are air-oven aging, light and water exposure; and -29 degree C exposure. The sample shall be affected to the extent that holidays form less than one inch from the point of impact.
• Resistance to Environmental Fluids Test. Addi-tional coupons shall be immersed for 270 days in environmental fluids representing various soil conditions. The coatings shall not crack or flake, nor shall the coating dissolve from the surface to the extent that the base material becomes visi-ble. The coating shall not disbond more than 1.5 square inches.
• Flexibility Test. Sample coupons shall be placed on supports and a load applied until the deflec-tion equals 9.7 x 10-2/6t, where “t” equals sample thickness. After the load is removed, the samples shall be subjected to the holiday test in ASTM G62-87. There shall be no evidence of hol-idays on the sample coupons.
• Abrasion Resistance Test. Sample coupons shall be subjected to three drops in accordance with ASTM G13-89. After the drops, the samples shall be subjected to the holiday test in ASTM G62-87. There shall be no evidence of holidays on the sample coupons.
• Cathodic Dis bondment Test. Flat sampl e coupons are to be subjected to Method B of ASTM G8-90. Half of the coupons are to be per-forated per ASTM G8-90; the electrolyte is to be maintained at 23 +/- three degrees C; and the samples immersed for 28 days. The disbonded area of the perforated samples shall not exceed 1.5 square inches. The coating shall not disbond from the uncoated samples.
|This worker is placing pea gravel under the lower quadrant of FRP jacketed tanks. Courtesy of UL.|
7. Insulated bushings test—Samples of the insulat-ing bushing are to be installed in a test fixture sim-ulating a tank fitting. The assembl y is to be partially filled with regular unleaded gasoline and sealed. The assembly is to be inverted and sub-jected to four temperature cycles of 16 hours at minus 0 degrees F, followed by eight hours at 120 degrees F; 72 hours at 0 degrees F; and 16 hours at 0 degrees F. After conditioning, the samples are to be checked for leakage by aerostatically pressuriz-ing to 5 psi.
8. Resistance to automotive fluids—The insulating device shall be exposed to fluids representing auto-motive fluids. The volume change and extraction shall be determined after the exposure. The maxi-mum volume change is minus one to plus 40% for fluids with ASTM Fuel C mixtures and minus one to plus 25% for samples exposed to ASTM Fuel A. The maximum extraction permitted is 10%.
9. Dielectric strength test—Insulating devices shall not break down electrically when subjected to a 500 volt dc (direct current) potential. Samples are to be conditioned for 24 hours in distilled water prior to applying the test potential.
10. Aging test—Insulating devices shall not crack when subjected to seven day air-oven aging at 87 degrees C. Following the conditioning, the sam-ples are to be installed on a test fixture and sub-jected to a 5 psi aerostatic pressure test.
11. Tensile strength test—Insulating device material shall have a tensile strength of at least 8,700 psi “as received” and retain at least 95% after seven days of air-oven aging at 87 degrees C.
12. The tank assembly shall be subjected to the fol-lowing tests: strength of pipe fitting tests, includ-ing torque and bending moment; strength of lift fittings test; and the leakage tests noted below under UL 1316 tests.
|UL performs a strength-of-lift fittings test on a FRP UST.|
Courtesy of UL.
Part II—Composite Tanks
Part II of UL 1746 covers composite tanks. These are steel tanks complying with UL 58 that are coated with a nonmetallic coating at least 0.100 inches thick. The steel thickness of the tank may be reduced if the earth load test, water load test and external pressure test are conducted on a sample tank. These tests are noted below under UL 1316 tests.
Samples of the composite tank-cladding system are subjected to the air-oven conditioning and to environ-mental fluids immersion for 30, 90 and 180 days. The samples shall retain 80% of the “as received” flexural strength value after aging; 50% after Type A fluid immersion and 30% after Type B fluid immersions. Additional samples shall be subjected to the light and water exposure test and the impact and cold exposure test noted below under UL 1316.
The coating shall also be evaluated with respect to corrosion protection when exposed to fluids representing group oil conditions. There shall be no evidence of blistering, softening, crazing or other damage that could impair the performance of the cladding after 270-day exposure to the test fluids at 38 degrees C.
The composite tank assembly shall be subjected to the following tests: Strength of pipe fittings tests, including tests for torque and bending moment; and strength of lift fittings test noted below under UL 1316; and the external pressure test noted above under UL 58.
In addition, the tank assembly shall be subjected to the leakage test, tank impact test and holiday test. The tank shall be subjected to a 5 psi aerostatic leakage test and checked for leakage using a leak detection solu-tion. The tank impact test consists of impacting the tank with a 12 pound steel ball. The complete tank assembly is subjected to a holiday test using a 35,000 volt holiday tester. There shall be no evidence of holi-days after the above tests.
|These FRP doublewall tanks are subjected to a rigorous series of 16 tests under UL 1316. Courtesy of Fluid Containment.|
Part III—Jacketed Tanks
Part III of UL 1746 covers jacketed tanks. These tanks consist of a steel tank complying with UL 58 with a nonmetallic external FRP or thermoplastic jacket. The tank and jacket are separated by an interstitial space with the jacket acting as a secondary containment barrier. The jacket shall be fabricated using a material at least 0.100 inches thick.
The tank as sembl y shall be subj ected to the strength of pipe fitting test; the bending moment, and strength of lift fittings test; the tank impact test; the leakage test; the holiday test; and the annulus proof pressure tests noted below.
In addition, a jacketed tank shall also be subjected to an interstitial communication test. During the com-munication test, a sample is installed underground and filled to capacity with water. At the farther point from the interstitial monitoring point, water, and vacuum-created pressure shall be added to the inter-stitial space. There shall be measurable communica-tion to the monitoring point within 24 hours.
If a jacketed tank is constructed with reduced steel thickness, the water load test, the earth load test and the external pressure test (noted below) shall be con-ducted on a sample tank assembly.
Part IV—Urethane-Coated Tanks
Part IV of UL 1746 is currently being developed and will cover urethane-coated tanks. As part of normal production, each tank shall be subjected to the leak-age test. Each cathodically protected tank is subjected to the anode continuity test to verify that there is con-tinuity between the anode lead wire and the tank con-nection. Each composi te tank and areas of FRP jacketed tanks where the FRP is bonded to the steel tank shall be subjected to the holiday test.
|A typical UL test pit.|