10.2.6.1 Aboveground Installation
Aboveground containers shall be placed on concrete foundations. If the location could be flooded the container shall be fixed and the foundation shall provide adequate anchors or weighting such that the container would not float, even if empty. Product identification and safety signs shall be installed at the container. The “NO SMOKING” sign shall be clearly legible at the safety distance applicable and from points of access to the storage site. Grounding requirements are defined in “Grounding Connections for Tanks” in Chapter 3. Large customer tanks (>10 tons) need two grounding connections, smaller tanks may have only one. A conductive lug, free of rust or color will serve as an attachment point for the truck bonding cable.
10.2.6.2 Underground Installation
Underground containers shall be protected from superimposed loads, e.g. vehicular
traffic loads, either by fencing or protecting them with a reinforced concrete slab or other load-bearing means. If the area for the container is not fenced off, the container manhole cover and container fittings shall be protected against damage and tampering.
Mounded containers shall be protected either by fencing off the area around the mound
Underground containers shall be surrounded by sand that is not aggressive in terms of corrosion. Minimum coverage on top is 0.3 m of sand. A concrete slab may be added. If traffic moves over the slab, no load shall be conveyed to the tank. Mounded tanks shall be covered similar to the requirements (0.9 m) mentioned in Chapter 3. For smaller tanks the cover may be less provided the cover cannot be eroded by rain or firewater.
Underground installation greatly reduces fire risk and security problems. Where spacing is tight or not available, underground installation may be chosen.
Figure 10.2.6.1: Typical container
Underground containers and their fittings, valves, pressure relief valves, gauging devices and regulators shall be adequately protected against corrosion. Proper drainage shall be provided for the housing dome to eliminate accumulation of water. The following guidelines shall be observed when installing underground containers:
1. Only ASME containers constructed for underground service and marked accordingly shall be installed underground.
2. All fittings, including any plugged openings, are plugged tight and free from leaks. Containers may be pressurized with air or LP-gas vapor to make certain there are no leaks.
3. The container is purged in accordance with generally accepted industry practices. (See NPGA Safety Bulletin 133 and the LPG Safe Operations Guide).
4. Rust, dirt and other foreign matter have been cleaned from the surface of the container, and the container has been visually inspected for gouges, dents, pits or other defects.
5. The external surface of underground tanks shall be Grit blasted to SA 2½ standard or chemically treated and coated with an adequate paint system with specialist advice. Shop applied coatings are preferred, but field applications are also acceptable. Irrespective of type and application method of coatings, a holiday test on the coating shall be carried out immediately prior to installation on site.
6. All points of contact shall be protected while the container is being loaded and transported. Damage to the protective coating shall be prevented. 7. All underground containers and piping shall be cathodically protected unless
written tests of soil samples indicate that it is not required. Failure to provide cathodic protection can cause hidden corrosion and leaks as well as weakening of the tank wall over time.
8. The bottom of the hole shall be level and free of rock. If rocks are present, a 150 mm bed of sand shall be used. For completely buried tanks, the hole shall be dug to a proper depth to provide for the housing dome to extend far enough above ground level to prevent entrance of water (50 to 150 mm is common practice), allowing for grading away from the dome.
Figure 10.2.6.2: Underground installation
9. The top of the container shall be at least 150 mm below grade, unless the container might be subject to abrasive action of physical damage from vehicular traffic or from other causes such as in LP-gas service stations. In this case, it shall be placed not less than 600 mm below grade or equivalent protection shall be otherwise provided (such as by the use of a concrete slab) to prevent imposing the weight of a loaded vehicle directly on the container shell (NFPA 58- Par.3.2.4.8 a).
10. For mounded systems, the same general procedure shall be followed, except that the housing dome would be above ground, and the above ground surface area of the tank shall be covered with at least 300 mm of earth or sand. 11. In the flood plain and high water level areas, provisions shall be made to
adequately secure the container to the ground, or to a concrete slab, to prevent flotation. Local soil conditions may require other provisions to allow proper drainage from within the housing dome.
12. Precautions shall be taken to prevent damage to the tank coating while lowering the tank into the hole and while back filling. Any damage to the coating shall be carefully repaired. Any small unprotected areas of a coated, wrapped and cathodically protected tank and piping system will be subject to concentrated corrosive action resulting in the possibility of severe metal loss and ultimately a leak. A conductive lug, free of rust or color will serve as an attachment point for the truck bonding cable.
13. Back fill shall be free from rocks or similar abrasives. Clean, dry sand is preferred. See Figure 10.2.6.2.
14. Because the installed container should be adequately anchored if flotation is a possibility and shall be leak tight with a slight positive nitrogen pressure, it could be maintained empty. However, it is preferred that the container be filled immediately after installation is complete.
15. Where underground containers are installed in locations subject to infrequent vehicular movement, sufficient provision shall be made to prevent the weight of such vehicular traffic from damaging the container or appurtenances. The top of the tank shall be at least 600 mm below grade or be protected by a concrete slab or equivalent.
16. Barriers shall be provided to protect the housing dome, relief valve discharge stacks, filling risers and any appurtenances that extend above grade level. 17. Product identification and safety signs shall be installed adjacent to the
housing dome and filling risers. The “NO SMOKING” sign shall be clearly legible at the safety distance applicable and from points of access to the storage site.
18. For underground tanks that are embedded in watertight concrete casings and sand-bed, observation wells with PVC casing of at least diameter 50 mm with 0.5 mm slots shall be installed. Observation wells shall be installed at two diagonal corners of the underground tank(s) and shall extend to a depth of 600 mm below the bottom of the tank(s).