Once a need for an oil spill prevention system is determined, an engineer must weigh the advantages and disadvantages that each oil retention system may have at the facility in question. The oil retention system chosen should balance the cost and sophistication of the system to the risk of an oil spill reaching navigable waters/environment. The risks will depend on such things as soil, terrain, relative closeness to waterways, location with regard to property owners, and potential size of the discharge. Each of the systems described below should be considered based on their relative merits to the facility under consideration. One system will not always be the best choice for all situations and circumstances.
Four to six inches of rock gravel surfacing are normally required in all electrical facility yards. This design feature benefits the operation and maintenance of the facility by providing proper site drainage, reducing step and touch potentials during short circuit faults, eliminating weed growth, improving yard working conditions, and enhancing station aesthetics. In addition to these advantages, the gravel will aid in fire control and in reducing potential oil spill cleanup costs and penalties that may arise from Federal and State environmental laws and regulations.
Yard surfacing should not be considered the only method of secondary oil containment without further study dependent on the volume of oil, yard surface area, and slope of substation. It can also be considered as a backup in limiting the flow of oil in the event that the oil containment system fails.
Soil underlying power facilities usually consists of a nonhomogeneous mass that varies in composition, porosity, and physical properties with depth. The soil’s drainage characteristics -permeability-are of primary concern in the design of an oil containment facility. The permeability coefficient, “k”, is a measure of the capacity of the soil to conduct or pass water under a unit hydraulic gradient. Coarse grained soils are more pervious and have corresponding higher permeability coefficients than fine grained soils. Using the coefficient of permeability, “k” and the hydraulic gradient, “i”, the flow volume “V”, discharged through the soil’s cross sectional area, “A”, during time, “t” may be estimated as follows:
V = volume discharged through the soil, cm3
k = coefficient of permeability, cm/s
i = hydraulic gradient, cm/cm
A = cross sectional area of soil conveying flow, cm2
t = time, seconds
Pits with liners or sealers may be used as part of an oil containment system capable of retaining any discharged oil on-site for an extended period of time. Any collection or containment pit should be constructed with materials having medium to high permeability—above 10-3
cm/sec-and should be sealed in order to prevent migration of spilled oil into underlying soil layers and groundwater. These surfaces may be sealed and/or lined with any of the following materials:
a. Plastic or rubber—Plastic or rubber liners may be purchased in various sizes and thickness. Consideration should be given to the selection of a liner that is not subject to mechanical injury that may occur as a result of construction, installation, equipment, chemical attacks on surrounding media, and oil products.
b. Bentonite (clay)—Clay and bentonite may be used to seal electrical facility yards and containment pits. These materials can be placed directly in four- to six-inch layers or may be mixed with the existing subsoil to obtain a soil permeability of less than 10-3
c. Spray-on fiberglass—Spray-on fiberglass is one of the most expensive pit liners available, but in some cases the costs may be justifiable in areas that are environmentally sensitive. This material offers very good mechanical strength properties and provides excellent oil retention.
d. Reinforced concrete—Four to six inches of reinforced concrete also may be used as a pit liner. The advantage of this material is that it is readily available at the site at the time of initial construction of the facility. The disadvantages of this material are that initial preparation is more extensive and materials are not as easily workable as some of the other materials.
If materials other than those listed above are used for an oil containment liner, careful consideration should be given to not selecting materials-such as asphalt-that may dissolve or become soft with prolonged contact with oil. The final selection of pit liners or sealers should be based on cost, strength, and durability.
One of the simplest methods of providing total substation oil spill control is the construction of a ditch entirely around the outside periphery of the station (Figure below). The ditch should be large enough to contain all surface runoff from rain and insulating oil. Such ditches may be periodically drained by the use of valves. Here again, the method discussed in the previous section should be used in determining the allowance for precipitation.
- Periods of heavy rain may not provide adequate containment.
- Standing pools of water may breed insects.
- May not be feasible in porous soil areas.
COLLECTING POND WITH TRAP
This system consists of a collection pit surrounding the protected equipment, drains connecting the collection pits to an open containment pit, an oil trap that is sometimes referred to as a skimming unit, and the discharge drain. This system is shown in Figure below.
The collection pit surrounding the equipment is filled with rocks and is only deep enough to extinguish burning oil. The bottom of this pit is sloped for good drainage to the drainpipe leading to an open containment pit. This latter pit is sized to handle all the oil of the largest piece of equipment in the station. A cross section of the system is shown schematically in Figure below.
The trap shown in Figures below is designed to contain the total capacity of oil on top of the water. To maintain a dry system in the collecting units, the other side of the intake pipe from the containment pit should be at least the maximum elevation of the oil level. In areas of the country subject to freezing temperatures, the trap, or skimmer, should be encased in concrete, or other such available material, to eliminate heaving as a result of ice action.
Design Guide for Oil Spill Prevention and Control at Substations
UNITED STATES DEPARTMENT OF AGRICULTURE
Rural Development Utilities Programs