Headlines have been describing many types of industrial accidents in months past. Many companies are taking measures to prevent the negative environmental impacts such incidents cause. One such example is an electric utility that serves more than three million customers across the Southeast United States, which fixed an issue at two of their 500-kilovolt (kV) substations.

Electrical distribution

Hobas fittings join easily in the field using the standard FWC coupling.

When power leaves any generating plant, it travels to massive substations that begin the distribution process. Electricity arrives at 500,000 volts and must be stepped down to a lower voltage for its secondary distribution to successive substations in the power grid, where it eventually splits off to various residential neighborhoods and commercial zones.

Such substations are constructed with drainage systems that collect rainwater to be channeled away from the electrical transformer and discharged into a nearby creek or tributary to a river. Because these initial distribution transformers deal with massive amounts of voltage, they are encased in several thousand gallons of oil for cooling.

Preventing oil spills

Hobas pipe can be easily modified in the field. Here a worker cuts a hole to provide a connection for a smaller pipe.

In order to devise the most efficient containment system possible to protect against a transformer leak, the utility provided research funds to several engineering firms to design an interceptor and underground oil containment system for the two existing 500-kV substations. These computer-controlled substations are unmanned, and therefore the challenge of designing the system lay in its automation capability to sense an oil spill on its own. The electric utility also wanted a pipe material that would be impervious to most of the chemicals with which it could come in contact, particularly oil, and they wanted a joining system that would not leak into the subsurface soils.

They chose a design submitted by civil engineers at Davis-Martin-Powell & Associates (DMP) of High Point, N.C. Ben Palmer, P.E., the DMP engineer in charge of the project, worked with Don Joyce of Pomona Pipe Products, Greensboro, N.C., who suggested pipe materials for the project. Both Joyce and Palmer had experience with the joint and chemical resistance of Hobas pipe, and specified it based on that experience.

Palmer's plan to intercept the drainage system involved installing an oil stop valve that would be sensitive to the specific gravity of the liquid in the chamber. Water inside this chamber would cause the valve to float, allowing the water to continue outward to public drainage. If oil were sensed inside this chamber, the valve would not open because oil is less dense and lacks the buoyancy to lift the valve.

Corrosion resistant fittings are custom-made per project requirements.

"The bank of transformers is situated atop a rock field sump," said Palmer. "The sump areas are connected to a storm drain pipe that conveys the stormwater runoff off-site and drains these sumps during a rainfall event."

The oil stop valve was installed inline on the storm drainpipe, leaving the site. Inside of the oil stop valve structure, a precast structure houses the oil stop valve along with a diversion pipe leading into the containment system.

In the event that the valve sensed oil and remained closed, the oil-contaminated water would be funneled into an underground containment reservoir that was constructed of six-foot diameter pipe, where it would temporarily remain until it could be manually pumped out and properly disposed.

"The design capacities for the two sites were based upon containment of a single piece of equipment and the rainfall from a 25-year, 24-hour storm event," said Palmer. "The concept was very similar to an underground stormwater detention facility, just a different application."

The containment vessels are made up of custom-made elbows and tees to create manifolds and 90- to 104-foot long straight runs, according to Matt Swenson of Power Consulting LLC, the on-site inspector for the utility. The straight sections used were 20 feet long and weighed about 7,000 pounds.

One of the straight stretches of Hobas is installed using an excavator. Lightweight Hobas pipe does not require the heavy machinery needed for concrete pipe. As the contractor set each piece, they backfilled a portion to hold the pipe in place and avoid the need for manipulation when tying into the manifold at the end of the run.

One substation's containment vessel covers a footprint of 26 feet by 104 feet and is composed of 328 lineal feet of 72-inch pipe in three straight runs for a capacity of 60,750 gallons. The other 500-kV substation's vessel covers a footprint of 36 feet by 90.25 feet with 383 lineal feet of 72-inch Hobas pipe in four straight runs for a capacity of 81,560 gallons.

According to Joyce, the pipe had been pre-ordered and was onsite when the contractors arrived.

The installation was performed by different contractors at the two different sites, neither of whom had prior experience with this pipe. Quartermaster Environmental, Shelby, N.C., installed one system and Willis Construction Co. installed the other.

Each 72-inch pipe containment reservoir was installed in a 12-foot deep excavation atop a one-foot base of washed stone and topped with five to six feet of cover. There was a learning curve in handling and placing the sections, with correct placement being critical in order to make the straight runs and enable the manifolds to properly join at opposite ends, approximately 100 feet apart, according to Swenson. "As the contractor set each piece, they backfilled a portion to hold the pipe in place so that there would be no need for manipulation when tying into the manifold at the end of the run.

"I feel both projects were a success and I understand that both are operating correctly now a year later, namely that they are dry." PE