Technology assists in testing, improving storage tank cathodic protection

Horizontal boring allowed Amoco Pipeline Co. easily and inexpensively to retrofit cathodic protection under storage tanks at its Wyco, Colo., terminal.

With the Bore Logic system, developed by Corrocon Inc., Broomfield, Colo., operators insert slotted PVC piping at very shallow depths, 18-24 in. under existing tank bottoms.

The piping networks are then used for developing accurate tank-to-soil potential profiles, placing reference cells beneath tank centers and, when necessary, retrofitting tanks with undertank cathodic protection systems, says Corrocon.

Colorado site

Shallow bed perimeter cathodic protection does not always provide adequate current to protect the soil-side bottoms of large-diameter aboveground storage tanks (ASTs), says Amoco.

Until recently, says the company, accurately measuring current flow beneath tanks has proven difficult. Annual cathodic-protection surveys generally relied on perimeter-to-soil measurements, which provided only a rough idea of how much current was actually getting to tank undersides.

Also, these measurements did not always reveal which areas of the tank bottom were receiving sufficient current. For example, at large ASTs, a measurement of 1,300 mv at the perimeter does not indicate adequate coverage at the center.

Five shallow anode ground beds were installed around the perimeter of Amoco's Dupont, Colo., tank farm to provide corrosion control for eighteen 50-120 ft tanks and their associated piping.

Routine monitoring of the shallow bed cathodic-protection systems showed that structure-to-soil potentials at the perimeter of a 120-ft diameter tank were 900 mv.

This suggested that potentials toward the center might be lower than the 850-mv threshold specified by NACE. Although Amoco increased the rectifier output, potential measurements increased only marginally, causing concern that the tank bottoms were underprotected.

Amoco used Corrocon's system at Dupont in October 1993 to drill beneath a tank and develop a tank-to-soil potential profile with a copper/copper sulfate reference cell at 5-ft intervals from the tank center to its perimeter at depths varying from 18 in. to 2 ft.

Instant off-potential readings at the tank center were less than 500 mv, verifying that the tank was inadequately protected.

Corrosion specialists installed a stationary reference cell beneath the tank center at a depth of 2 ft so that potential readings can be taken at any time. When a full profile is necessary, workers can withdraw the reference cell through the PVC pipe and take a new tank-to-soil potential profile.

Amoco retrofitted the tank with an undertank anode ground bed installed in a crow's foot pattern about 5 ft beneath the tank's bottom (Fig. 1 [23782 bytes]).

When the on-potentials from the new cathodic-protection were compared with on-potential readings from the perimeter system, the improvement was dramatic (Table 1 [9385 bytes]).

Amoco hired Corrocon to develop tank-to-soil potential profiles and install reference cells for five other tanks at the site. Corrocon determined that of the six tanks tested, four failed, a 67% failure rate.

Retrofitting

Amoco retrofitted three tanks with an undertank anode ground system, and reference cells were placed under three additional tanks. The tank farm was sold before any more work could be undertaken.

Although the cost of shallow bed cathodic-protection systems is relatively low, interference can prevent them from providing adequate current to tank bottoms. Bringing current closer to the tank bottom's surface makes the current more likely to flow directly to the tank and not to piping or other nearby metals.

Corrocon's method for horizontally boring underneath tanks provides an easy testing method for perimeter cathodic protection systems, says Amoco. If the systems prove inadequate, tanks can be easily retrofitted with undertank anodes.

Horizontal piping networks installed beneath tanks also provide the opportunity to add external leak detection and monitoring systems that utilize tracer testing or hydrocarbon vapor/liquid testing technologies. These act as early warning to protect against leaks and ensuing liability.

Amoco says increased undertank protection and monitoring means that the intervals between NACE 653 inspections can be lengthened with minimal risk of leakage caused by corrosion.

That protection and monitoring provide attractive and lower-cost alternatives to retrofitting tanks with double bottoms, which costs about $450,000 for a 120-ft diameter tank, not including the time the tank is out of service, according to Amoco.

In contrast, the company says retrofitting with undertank anodes and a reference cell costs about $25,000. The cost of an external leak detection system, added at the same time, is as low as $8,000.

Also, even a double-bottom tank can fail. An anode cathodic-protection system is usually installed between the new and old floors, and the space is filled with sand. If the system fails, it is impossible to extract it or add a new system.

If a defect develops in the new bottom, and product leaks into the annular space, the sand will become saturated. In some cases, the upper tank floor may have to be disassembled to remove the contaminated sand.

The cost of such action is high, according to Amoco. By comparison, a single-bottom tank with an effective leak-detection system can be inexpensive to repair: The early warning can result in relatively low repair costs, minimal environmental damage, and little chance of endangering beneficial groundwater.

Horizontal piping networks within 2 ft of tank bottoms allow corrosion engineers to determine whether shallow-bed or deep-well cathodic-protection systems provide adequate current to tank bottoms.