Transformer oil containment practices vary throughout the industry, and different jurisdictions may require oil containment for station permitting. Utilities have environmental standards dictating what levels of oil containment are required at their stations for various types of oil-filled equipment.
Avista Utilities in Spokane, WA, has developed a new standard for transformer oil containment with the introduction of a new, minimum-maintenance, oil-containment design that incorporates the transformer foundation.
The challenge with most oil containment systems that do not use oil-water separating systems (with tanks and pumps) is how to deal with water filling the containment basin. Obviously, if the basin is full of water when an oil leak occurs, there is no longer a contained, controlled system in the event of an oil spill.
Additionally, typical oil-water separator systems are expensive and involve long-term maintenance. Other systems that have little to no maintenance may not meet newer spill requirements as they become more stringent over time. Finding a system that meets requirements, while not producing additional maintenance costs and resource demand, is a problem faced by most utilities.
For years, Avista used a below-grade, impermeable-lined basin around its transformer foundations to contain oil leaks and spills. This lining for a typical distribution station power transformer was 36-by-24 feet in size, and 2.5-feet deep, around a 9-by-12-foot transformer pad, which is offset inside the liner area. Most of the liner area was on one side of the transformer.
The system had a reglet cast into the concrete foundation of the transformer pad, which sealed the liner tight around the foundation. The lining was then filled with 1.5 inches of rounded, washed gravel. A monitoring well pipe made of 12-inch PVC was installed off one of the foundation corners, which provided a means of pumping out any excess water that is caught in the basin. To protect the lining from damage, the entire containment area was enclosed within a plastic barrier chain and warning signs to keep equipment out.
Unfortunately, this system was hard to inspect to ensure the integrity of the lining. Over the years, the monitoring wells have become full of water and fine sandy debris, which does not filter through the fabric.
In 2007, Avista hired Aaron Henson, P.E., as a civil and structural engineer in its substation engineering department. Avista had not a civil engineer in the department for as long as anyone could remember. One of Henson’s first tasks was to research ideas for an above-ground, oil containment system that would not require an oil-water separator tank and pump system.
To contain a transformer oil leak, a new basin had to be designed. Henson immediately gravitated toward a moat-style design; a simple transformer foundation with a concrete moat around it. He could easily design it to capture all the transformer oil, but how could it work without the moat filling up with water?
Research led him to SPI’s patented technology. This technology consists of filters with three different layers of filtration, which essentially form a plug in the filter once the oil is “soaked” up into the filtration media. The media only absorbs the oil, so water passes through the filters. Avista found that the filter would allow the moat to drain water and plug up when oil was detected. Next, the challenge was designing and building the new containment system.
Avista selected the Petro-Pipe® filter from Solidification Products International. The filter consists of a sleeve that can be cast into the concrete moat wall at a 25-degree angle. The Petro-Pipe® filter cartridge inserts into the sleeve and is replaceable as needed.
Next, the foundation was designed to include filter drains near the four corners of the moat. Inside the moat at each filter drain, a pre-filter, basket-type cover is installed to keep larger debris from entering and plugging the filters.
Three separate concrete pours are needed. The first is a “slab on grade” pour on which the transformer pedestal (foundation) will be poured. Next, the moat walls are built around the perimeter of the slab on grade. To form a seal, waterstop is placed in the cold joint between the slab on grade and the moat walls. The waterstop is made by Synko-Flex and it is designed specifically for exposure to hydrocarbon liquids.
Channels are inserted to support the removable, galvanized steel-perforated grating that covers the moat. This allows electricians to safely work on the transformer while standing above the moat. Ground conductors are run along the outside and over the top of the moat from the ground grid in at least two separate locations. They are bonded to the channels to provide a solid, ground connection to the steel-grating. In addition, accommodations for removable stanchions are included on the outside of the moat, so that warning lines can be quickly installed to prevent workers from falling.
On average, an Avista crew can complete a foundation for a distribution power transformer and oil containment system in about seven days. For a 230-115 kV auto transformer, that time frame increases to about 10 days. Forming the moat walls was the largest challenge initially, but now being equipped with the forms and the experience, the construction goes much faster.
To use this moat-style foundation, modifications were needed for connections on both the high and low voltage sides of the transformer. This resulted in the transformer being at least 2.5 feet higher than transformers on a typical pad-style foundation.
Employing this new moat-style transformer pad and oil leaks or spills will be appropriately contained according to regulations. It offers assurance in using a standardized, long-term transformer oil containment system solution for generations to come.