When the containment area surrounding storage tanks is properly constructed with the right materials, terminal operators have a head start should a catastrophic spill occur, said Tarik Hadj-Hamou of GeoSyntec Consultants, Huntington Beach, California.
"What we want is a system that works," he said. "The goal is to minimize contamination." Hadj-Hamou discussed secondary containment at the Independent Liquid Terminals Association meeting June 12 in Houston, Texas.
The appropriate containment design may vary from terminal to terminal, based on many different factors, including product stored, maintenance considerations, materials available locally, soil type, and the site's relationship to the environment. "We need to use the best available technology for the site-specific conditions," he said.
Traditionally, containment dams have been earthen and generally constructed with local soils to reduce the cost of hauling them to the building site. Concrete and sandbags are other examples of materials used in the dams, and each has its advantages and disadvantages.
Hadj-Hamou estimated that there are 1.8 million storage tanks in the US - 80% without low-permeability secondary containment. Even when the dams and containment area are made of a high permeability material, the walls may not be strong enough to withstand the force from a sudden tank rupture. He pointed out that the force can be intensified by the shape of the hole made from the rupture.
Manholes have blown out and become missiles, penetrating the dam. Welds and pipe connections can rupture, and seams can unzip, releasing huge amounts of product. In addition, the angle of the dam wall will either return the product to the retaining area, or move it over the wall. The type of product will dictate how it spreads if it is released. Some fluids are absorbed quickly into the soil while others will spread outward on top of the ground.
"There can be tremendous force caused by a tank rupture," he said. "Sudden releases are rare, but those that have occurred often broke the dike, so we have to consider this product velocity."
At the same time, a slow leak can be insidious, particularly if it is undetected underneath the tank. "The tank bottom itself is the most likely to leak product into the aquifer," he said.
To insure that the containment area will retain enough spilled product, the Environmental Protection Agency (EPA) requires the area be capable of holding the capacity of the largest tank in the terminal, plus a certain amount of calculated rainfall.
For engineering guidelines, planners should refer to the American Petroleum Institute. Other sources are the EPA and the National Fire Code.
Choice of materials to be used in the containment area are based on many variables. Clay can be impenetrable but only when it is hydrated. While liners may be effective in some cases, they may interfere with day-to-day operations. "You can't drive on a plastic liner," he pointed out.
At the same time, with products such as asphalt or tar that are slow to permeate the soil, a liner is not needed. "Soil itself can be a liner," he added. "Enhanced soil can be used and is less costly than a liner."
Whatever the materials chosen for the containment area, they should be sufficiently impervious to the product that is stored. Walls should have erosion protection. Steel and concrete walls are especially susceptible to erosion.
Just as emergency preparation measures are important, so is prevention, Hadj-Hamou said. He recommended installation of overfill prevention devices, leak detectors, backflow prevention equipment, and double tank bottoms.