SYSTEM PROVIDES SECONDARY CONTAINMENT FOR ABOVEGROUND STORAGE TANKS

Eddie K. Rangel, J. Douglas Lindsay
Marmac Inc.
Cypress, Calif.

Although a new California law which regulates and requires leak-detection systems on aboveground petroleum storage tanks takes effect this year, some oil companies are already ahead of the lawmakers in taking action.

For the past 3 years, a major oil company has been using a secondary containment and leak-detection system on several of its new and existing aboveground storage tanks at marketing distribution terminals. The leak prevention and detection system is used to detect and contain floor leaks prior to a potential release into the environment.

WHY SECONDARY CONTAINMENT

Releases from aboveground tanks became a sensitive public issue in 1988 following two major oil spills. The first involved Ashland Oil Inc.'s 95,000-bbl storage tank at Floreffe, Pa. (OGJ, June 6, 1988, p. 16).

The 40-year old tank had been rebuilt and was being filled with No. 2 diesel fuel when it collapsed. About 18,000 bbl escaped into the Monongahela and Ohio Rivers in January, posing a contamination threat to the water supplies of thousands of Pennsylvania, Ohio, and West Virginia residents.

The second major oil spill occurred in April 1988 at the Shell Oil Co. refinery in Martinez, Calif. A rainwater collection drain line ruptured inside the storage tank, sending an estimated 9,500 bbl of crude oil down a storm drain and into Peyton Slough.

The oil spill fouled more than 150 acres of wetlands and 11 miles of shoreline, killing fish, birds, and other wildlife. Fines and damages levied against Shell Oil amounted to $19.7 million.

The environmental impact of these disasters caused swift and severe legislative and regulatory reaction. Both California and Pennsylvania passed aboveground storage tank laws in 1989 which impose requirements for storage lists, inspections, and secondary containment in some instances. They also require release monitoring and reporting, and the development of spill-prevention control and countermeasure plans.

Although these laws were enacted as a reaction to catastrophic tank failures, their impact was to prevent insidious leakage into soil and groundwater.

Even though the oil spills focused public attention on the problem, many industry experts had already concluded that more stringent regulations for aboveground tanks were inevitable.

In recent years, there has been a growing concern about the liquids contamination of ground and surface water as well as solids contamination due to storage tank leaks and spills.

Attention was focused first on underground storage tanks (USTs) because of their high number, an estimated 2 million according to the Environmental Business Journal, March 1989. Federal laws now require the testing of underground tanks by 1993, and the replacement of all of those found to be leaking.

All new underground storage tanks, according to U.S. Environmental Protection Agency (EPA) rules, must be installed to industry codes and equipped with spill and overfill protection. And both the tank and piping must be equipped with corrosion protection and leak detection. Secondary containment is mandatory for petroleum and chemical USTs.

Following the same philosophy of cleaning up and mitigating potential water and soil contamination problems, lawmakers and environmental protection agencies have now shifted their attention to the estimated 650,000 aboveground storage tanks located throughout the United States. According to General Accounting Office estimates, some 2,000-3,000 spills from aboveground storage tanks have occurred since 1983 (OGJ, April 3, 1989, p. 30).

ABOVEGROUND TANK LEAKAGE

Leakage can pose a substantial problem with aboveground tanks, and the leaks can be difficult to detect. While 30 years generally is the upper end of a tank's life, leakage can occur long before that point, particularly in tanks without proper cathodic protection and in tanks placed in highly corrosive soils. Because of their immense size, it is difficult to test the integrity of most aboveground tanks. Typically, they range from 30 to 220 ft in diameter, 16-60 ft in height, and 2,000-400,000 bbl in capacity.

Current leak-detection methods for these aboveground tanks consist of either placing monitoring wells for the tank facility in the ground and down to groundwater or installing sophisticated tank-gauging systems. Each method has its limitations.

Creating a monitoring system for the entire tank facility may reduce initial costs but the disadvantages are that tracing the leak's origin can be an arduous process, and the leak is detected only after release into the ground. Often, the proximity of tank farms and terminals to each other complicates the leak-tracing process.

Highly sensitive, automated tank-gauging systems provide continuous monitoring and information on product level, temperature, and gross and net volume. They conceivably can alert a tank owner or operator to losses due to leaks. But because of the size of the tanks and corrections for the degree of uncertainty, a minute leak would be extremely difficult to detect.

SECONDARY CONTAINMENT AND LEAK DETECTION

To complement monitoring and gauging methods, Marmac Inc. has worked with a large oil company to provide secondary containment for new and existing tanks and to develop an easily accessible leak-detection system. The involvement included engineering, design, permitting, procurement, construction management and support, and inspection services.

The secondary containment consists of a high density polyethylene (HDPE) liner placed below the tank floor and sloped toward a lined collection sump (Figs. 1 and 2).

The sump is drained by a line to a standpipe placed outside the tank's perimeter. The standpipe provides personnel a point from which to detect leaks.

The liner is manufactured to National Sanitation Standard 54 and is composed of sheets of 80-mil HDPE welded together by an extrusion welder. The sheets are available in 16-22 ft rolls and are laid with a 3-in. overlap between adjacent sheets.

Where the liner crosses the edge of the tank floor or concrete ringwall, two layers of geotextile fabric are installed to protect it. At the shell, the HDPE is reinforced with multiple layers, attached with studs, and covered with mastic.

The leak-detection system involves having a 2-in. pipe encased in a 4-in. steel sleeve going from the sump to an 8-in. standpipe complete with a cap on the bottom and a flange and lockdown flange cover on the top. Generally, the standpipe is located at least 3 ft from the tank's foundation. The piping and standpipe are fabricated from HDPE.

Once the liner and leak detection are in place, they are covered with 3-4 in. of concrete. Low-strength concrete is used because its longer curing time makes it easier to work with.

Grooves are created in the concrete subfloor to promote drainage from any tank floor leak to the collection sump to expedite leak detection. Also, 3/4-in. polyethylene conductor piping is installed in the tank foundation ringwall forms for new construction and inserted into the tank shell prior to installation of the new tank floor. The piping is aligned with the concrete grooves and serves two purposes:

The piping is used to test floor integrity prior to hydrotesting or to locate floor leaks. Air is injected through the piping while the tank is initially filled with water. Personnel then inspect the tank floor for air bubbles.
The piping is also used in the event that a leak is detected when the tank is in service. After the tank is drained, piping caps are removed to facilitate drainage of residual product and can be used as water detection points during concrete washing operations. Water can be added to the leak detector as well, and the water can be observed from the piping.

The conductor piping is used for testing the tank floor and to flush the secondary containment area if a leak is detected. A new 1/4-in. (minimum) cone-down steel floor is installed above the concrete, according to API 650 construction standards.

NEW TANK CONSTRUCTION

In new tanks, the excavation of the tank sump and installation of the leak detection piping is done prior to pouring the tank ringwall forms (Fig. 1). The secondary liner is attached to the side or top of the ringwall foundation.

The incremental cost of adding secondary containment and leak detection to a new tank system is estimated at $40,000 for a 97-ft tank.

RETROFITS

Retrofits for existing aboveground welded and riveted storage tanks generally take 4-8 weeks to complete, although the entire process of engineering, permitting, bidding, scheduling, and retrofitting may take from 6 to 9 months. The cost of retrofitting an existing 70-ft tank is estimated at $200,000 and includes engineering, permitting, labor, and materials for gas-freeing, tank floor removal grading, preparation, liner and new floor installation, and nozzle relocation (Fig. 2). Even before retrofitting begins, a tank must be taken out of service and cleaned to a gas-free condition. On the tank is safe, workers cut a door in the shell and begin removing the existing tank floor.

The objectives during retrofitting are to maintain tank shell structural integrity and to minimize removal of the existing tank floor. As little of the existing floor is removed as possible to minimize waste disposal and subgrade recompaction costs.

The existing tank floor is cut with an air hammer and cold chisel. Personnel use breathing apparatuses until the underlying surface is checked for combustible vapors and the area is cleaned for breathing efficacy. Then the center sump area is excavated along with the trench for the detector piping. The size of the center sump can vary depending upon the size, type, and service of the tank.

In a retrofit for a 132,000 bbl gasoline storage tank, the center sump has an outside diameter of 34 in. and an inside diameter of 24 in. The depth is 12 in.

The tank sump is fabricated from 3/8 in. steel plate in accordance with the American Petroleum Institute's (API) 650 standards. The containment sump is fabricated from 1/2 in. steel plate.

API 650 requires that the new tank floor must penetrate the tank shell rather than just butt up against it. To facilitate the floor installation, temporary "C" clamps are welded to the shell at least every 5 ft around the periphery of the shell to support it before slots are cut for the new floor. When the retrofit is completed, there is a predictable loss of tank capacity due to the raised floor. As an example, for a 40-ft high, 144-ft diameter tank, the loss of capacity would be estimated at about 5%.

Retrofitting existing tanks is preferred over replacing the tanks with new construction with secondary containment systems because retrofitting costs less, requires fewer permits, and takes less time for installation.

TESTING AND INSPECTION

In the construction of secondary containment and leak-detection systems for new and existing tanks, it is recommended that qualified inspectors be on site to ensure the job is being done properly. The line to the sump, for example, must be perfectly sealed.

Throughout the installation, the welded seams in the HDPE sheets must be checked by vacuum box and tensile strength tests. After the tank floor is installed, vacuum box and air tests are conducted to check the integrity of the tank floor.

Because of the size of aboveground tanks, it is critical that all tanks be hydrotested according to API requirements. Pennsylvania's new law, for instance, directs the Department of Environmental Resources to develop new methods and procedures for testing new or substantially modified aboveground storage tanks and specifically mentions hydrostatic tests.

Hydrotesting makes sense from an economic point of view as well. Hydrotesting allows the tank operator to check the tank's integrity prior to filling it with product. Any leak would drain to the sump and be observed at the leak detector well.

This process eliminates the chance of an undetected leak when the tank is returned to service, thus saving the expense of emptying and gas-freeing the tank for repairs.

PRIORITY SYSTEMS

Practically speaking, because of the time and expense involved, tank owners and operators need to establish a priority system for installing secondary containment and leak-detection systems. Obviously, top priority would be any tanks known to be leaking.

The risk of substantial fines and penalties is high for any tank facility owner or operator who fails to act. Secondary containment and leak-detection systems also make sense for any new construction. The economic expense is minimal compared to the overall construction cost of the tank.

If a tank is being taken out of service for any reason, that is a good time to do a retrofit because the tank must be cleaned and made gas free anyway. Also high on the priority list should be older tanks which have not had cathodic protection for an extended period of time, and tanks which do not conform to the latest API standards.

Given the increasing scrutiny of aboveground tank storage by regulatory agencies and state legislatures, installing secondary containment and leak detection systems on aboveground storage tanks can be a prudent business decision.