March is such a great month. Every year, it gives us the first day of spring, March Madness and all the associated basketball craziness, and of course, the holiday that brings out the Irish in us all (along with green beer in pubs all over the world), St. Patrick’s Day. It is celebrated on March 17 every year, on the anniversary of fifth century death of the patron saint of Ireland.
St. Patrick’s Day represents much more than what appears on the surface to Ireland, which has been observing it as a religious holiday for more than a millennium. Like the Irish culture, the holiday is steeped with folklore and mystic tradition. The history of St. Patrick himself is muddled in exaggerations and myths. For instance, did you know he wasn’t really Irish? He was actually born somewhere in the British Isles around 390 AD and didn’t make it to the Emerald Island until he was kidnapped and enslaved by pirates at age 16. And while legends claim he delivered a sermon on a hillside so powerful that he managed to banish all the snakes from Ireland, most scholars believe the tale is more likely an allegory for eradicating pagan ideology, than actual fact.
Another Irish tradition often associated with St. Patrick’s Day is kissing the Blarney Stone – though it actually has nothing to do with the saint. Built into the battlements high atop of Blarney Castle in Cork, Ireland, legend dictates that kissing the stone will bestow onto the kisser the gift of eloquence and persuasiveness. Though supposedly millions of people have made the effort to kiss the famous stone, the act of doing so is not easily achieved. To kiss it, you must lean backwards over the parapet’s edge while holding onto an iron railing.
Because kissing the Blarney Stone can trigger severe fear of heights (aka, acrophobia) in people, it is not recommended for the faint of heart – yet still thousands and thousands make the pilgrimage to do so every year, despite the risks. The reason why is simple – the stone is significant to them.
As a company specializing in secondary containment for substations and transformers, we also recognize the importance of stone … though for completely different reasons. No, we can’t just kiss a rock and suddenly have the gift of successful oil containment. Our systems are always custom-designed per site-specific variables. While there are a number of different systems available, one common component often seen in substation oil containment is stone.
The difference between the Blarney Stone and containment stone is certain in visual comparison but the importance of the stone has unique similarities. You can’t just go around kissing any stone in Ireland to get the gift of eloquence and persuasiveness. Just like, you can’t use just any stone when installing secondary containment for the gift of reliable oil containment. In order for you to get the result you want from kissing the stone, you have to make sure you’re kissing the correct stone. In order for companies to get the oil containment result they want they have to make sure they are liable for supplying clean, yes let us repeat, clean, washed stone. And that’s not dumb luck – there are reasons why clean, washed stone – are vital to the effectiveness of oil containment.
Three main reasons why stone matters for substation oil containment
1. The right oil containment stone provides a leach field
Before laying a geomembrane liner, a minimum of 6″ washed pea stone (1/8 to 3/8 inch diameter) should be placed to provide additional depth to the oil containment unit. This pea stone floor will function as a leach field assisting in the displacement of water, and should be sloped from the center to the outside walls to promote the flow of water out of the oil containment area. It basically adds additional drainage without increasing the cost, since every square foot of pea stone with a 6” base is capable of draining and containing 1.5 gallons of oil and wastewater runoff in the oil containment area.
(Fun fact: when we first began installing our Oil Filtration Wall systems, we considered using sand for the leach field function, but decided against it for two reasons. First, sand gets everywhere, including within all of the seams! And secondly, while sand will percolate water, it offers almost no capacity. So that’s why we spec pea stone for the leach field, instead of sand.)
2. The right oil containment stone increases void space
Let’s be realistic. To prepare for a catastrophic leak of a large transformer that holds several thousands of gallons of oil, you need to come up with someplace to put the spilled oil, near the oil containment area. Because of the void space it can provide, stone or gravel is a logical choice for oil containment installations. IEEE Std. 980 Guide for Containment and Control of Oil Spills in Substations states the following in regard to considerations that should be given for the stone filled collecting pits used for oil containment:
“The size of the stone used varied in the range of 1.9 cm to 7.6 cm (0.75 in to 3.0 in). Depending on the uniformity of stone size used, the void volume ratio may vary from 20% to 50%.” (IEEE Std. 980 220.127.116.11 Volume Requirements)
While this is straight-forward, we’re amazed at how often it is misinterpreted. We actually advocate for clean, washed stone sized 0.75 to 1.5 inches, screened and free of dirt and fines, to allow for the ideal void space for oil and other wastewater. It’s important to remember, not only do you need to provide the space in the oil containment for the oil to go during a spill, it’s also recommended to prepare for the possibility of a worst-case rain event … aka, a 25-year, 24-hour storm. IEEE Std. 980 18.104.22.168 also states, “In general, an oil-containment system should be sized to contain the volume of oil in the single largest oil-filled piece of equipment, plus any accumulated water from sources such as rainwater, melted snow, and water spray discharge from fire protection systems.” For oil containment design purposes, it’s important to use this scenario to determine maximum volume of short-term water accumulation.
3. The right oil containment stone provides fire suppression
As we detailed in a previous blog post about preventing transformer fires with below-grade secondary containment, a successful fire needs three vital elements – fuel, oxygen, heat – or else it will fizzle out. While above-grade oil containment like concrete moats or earthen berms provide the ideal environment for a blaze to occur, stone does just the opposite, removing two of the three elements, heat and oxygen, from the equation. To best ensure against an oil containment inferno, we recommend at least 12 inches or more of clean, washed and screened stone to provide fire quenching capabilities.
While using the right oil containment stone won’t endow upon a substation any amazing gifts like the legendary Blarney Stone, it can provide something even better – successful, worry-free oil containment worth toasting a glass of green beer! Have a legendary (but safe) St. Patrick’s Day!
For more information about our Geomembrane Liner download the installation manual below:
Set In Stone
As we’ve already stressed, stone is an important aspect of successful oil containment, but not just any stone will do. While we don’t endorse one type of stone over another, what we do promote is stone or gravel of the correct size (0.75 to 1.5 inch in size) that is screened, clean and washed before the installation. (Note: we generally don’t use ASTM D448 standard number sizes for stone, since they vary from state to state. The stone size we use falls into a number of categories and can be referred to as gravel, concrete aggregate, river rock, drain or septic stone, and other names.)
This picture is a good example of the ideal stone, next to stone that is less than ideal. The gravel on the left is clean, free of fines, and perfect for oil containment systems. It is also the right size, approximately 0.75 to 1.5 inch stone that allow a void capacity of around 40 to 45 percent. (For oil containment capacity, this means that 1 cubic foot of stone can contain three gallons of liquid. Normally, 1 cubic foot equals a capacity of 7.5 gallons.) The gravel on the right, while clean, is a little too small for ideal oil containment. The smaller stones offer less area, resulting in less void space.
For more examples of BAD STONE, see below.
Down and Dirty
We’ve stated it over and over – it doesn’t matter what type of stone you use, as long as it’s the correct size, and screened, clean and washed. That means it needs to be free of dirt and fines, and definitely can’t be dense grade (However, we do use dense grade in ramp applications in which there is a need to accommodate a vehicle driving over the oil containment area).
Dense grade aggregate or dirty stone has been such a problem in the past, we’ve been known to turn it away if it shows up on site. The dust and silt produces fine particles that build up and can affect drainage of the oil containment system.
Rip Rap Rubble
While Rip Rap is a great stone when used to protect streambeds, shorelines, bridge abutments and other shoreline structures against water or ice erosion, it should be avoided as back fill for secondary containment installations. The size of the rocks actually creates too much void space, and takes up more space. Plus, because rip rap can be the size of a basketball or larger, back filling will require either large equipment, or it will need to be placed by hand, which is a huge pain.
The only time rip rap would be ideal is in cases of possible washouts or in stabilization situations
Too Small for Comfort
When the stone is too small, there’s not as much void space, which means there will be less storage capacity and less migration of water.
No Mix and Match
A mixture of stones that aren’t uniform in size won’t do the job, either, as there will be even less void space available because the smaller stones will fill voids made by the larger stone. This is why it’s really important for stone to be screened beforehand at a quarry, so it’s consistently sized and free of fines.
Rock and Roll
While some utilities prefer round river rock to gravel, we don’t have a preference, as long as it’s the right size and clean. However, in this particular oil containment install, the river rock was a bad call. It may look pretty, but it is tough to walk on and won’t stay in place, making it a safety hazard. Workers have a hard time walking on it without rolling their ankles. Plus, the rock is made up of a mixture of sizes, ranging from ½” stone to 2 inches – such a large variance greatly reduces the amount of available void space.