We take a look at the risks associated with ATEX/COMAH sites and what the ATEX-approved patented Retractable Grounding Assembly (RGA) does to reduce the risk of tank fires by subduing sustained arcs during the lightning event and other electrical phenomena. If you missed it, you can watch a replay of the webinar here.
Welcome to our guest speaker…
Ian: Morning, guys. Hello and welcome.
Today’s webinar is a little bit special because we’re covering yet another topic that is not totally well understood, but also one that affects sectors and sites that have a ATEX zones or ATEX issues on site.
So, it’s presented by my good friend Joe Lanzoni, from Lightning Eliminators in the States. I have a confession to make. It’s about 3:30am, local time in Denver, Colorado. So, Joe’s presentation is pre-recorded. And we’re going to be standing by to answer and field any of the questions that this might stimulate.
So, the format is same as usual. So, let’s dive in and see what Joe’s got to say about the myths and get some busting of the myth of bypass conductors.
The Necessity of Bypass Conductors
Joe: Hello, I’m Joe Lanzoni, with Lightning Eliminators in Boulder, Colorado. Today, I’m here to talk about floating roof tanks. In particular lightning and how to make your floating roof tanks safer.
So today we’re going to talk about tank fires and how tanks getting ignited by lightning and what is the risk. We’re also going to talk about bonding of the roof and shell and discuss how to do that.
Storage Tank Lightning Facts
- There are over 20 tank fires per year worldwide.
- 1/3 of those are attributed to lightning.
- Lightning is the leading known cause of tank fires.
There’s no such thing as a cheap tank fire. Tank fires are extremely expensive, often measured in the millions and millions of dollars.
Also, some folks believe that covered or dome roof tanks are not at risk. Well, this is simply not true. both external and internal floating roof tanks are at risk. And you need to look no further for proof than this YouTube video, which shows a covered floating move tank being ignited by lightning.
Tank Fire Considerations
There’s several things to consider when we’re thinking about tank fires:
- Size of tanks has increased over time – that means the bigger the tank, the bigger the fire.
- Tank fires are very costly – not just in terms of loss material and tank, but also business interruption, environmental damage and damage to public opinion.
- Controlling tank fires – the more tanks you have and the bigger the tanks, the more firefighting resources you need to have on hand.
So how does lighting cause ignition? And what is my risk?
Well, the next few slides attempt to answer those questions.
Current Flows from Direct Strike:
This slide shows lightning terminating on the top rim of a floating roof tank when the roof is low. As you can see, the current flow shown here in yellow goes down the inside of the shell and across the floating roof and up the other side of the shell.
Current Flows from Nearby Strike:
When there’s a lightning strike to a nearby location, we also have similar current flows, which means there’s current flow up the side of the shell. It then arcs across the seal and across the floating roof and to the other side of the tank shell.
Tank is most at risk when the roof is high. And that’s because the current density tends to concentrate over a very small section of seal. What we’re trying to show here is the current in yellow is concentrated along an area where you will likely have arcs from the roof to the shell. In fact, 80% of all tank fires occur when the tank is more than half full.
Now compare the previous slide to this slide where the floating roof is low. As you can see the current shown here in yellow is spread out or less dense than in the previous slide. And so that means the current density over any section of seal is less and you will be less likely to have sustained arcing along the seal.
Key Lightning Parameters
So, here’s some interesting lightning information. The average lightning strike contains 30,000 amps of current and about 90% of all strikes contain 80,000 amps or less. There’s up to 30 individual lightning strikes in each lightning flash with the average number of strokes per flash being four. That means every time you see lightning there is on average, four distinct current flows in each flash of lightning that you see.
World Lightning Stoke Density
So, here’s a map courtesy of NASA, which shows the world’s lightning strike density. As you could see, certain areas have more lightning than others. The high lighting areas across the world was shown in dark, and that would include Central Africa, Southeast Asia, South-eastern United States, and so forth.
As you can see, areas around Southern Europe have more lightning, then Northern Europe. Although there’s certainly lightning risks throughout Europe and throughout the world.
Does grounding make a tank safe?
Some folks believe that their tank is completely safe as long as it’s well grounded. Well, as a matter of fact, lightning safety for tanks is not dependent on tank grounding. The occurrence of sparks, rim fires is not dependent on grounding resistance.
Furthermore, the presence of a membrane other tank has no impact on lightning related currents. A tank is naturally well grounded. If it’s resting on earth, or a concrete foundation.
Priority #1: Bond Floating Roof Tank Roof and Shell
Most important thing you can do to improve the safety of the tank is to bond its roof and the shell. This location is the worst place to have arcing because vapours may accumulate along the roof shell seal. Therefore, it’s essential to bond the roof and shell of any floating roof tank to reduce or eliminate arcing between the roof and shell.
Methods to Bond Roof-Shell
There’s essentially three ways to bond the roof and shell.
- Shunt – short piece of steel that connect it to the floating roof and presses up against the shell.
- Metallic seals around the roof perimeter.
- Bypass conductor – conductor whose one end is attached to the top of the shell and the other end is attached to the roof.
It’s named a bypass conductor because the conductor literally bypasses the inside of the tank shell, and we’ll have more information on that coming up.
Problems with Shunts/Metallic Seals
So, the problems with number one and number two, shunts and metallic seals are several.
- Dependent on spring tension for contact (meaning that shunts and metallic seals are pressing up against the inside of the shell).
- Petroleum deposits accumulate on the tank inner surface (like wax and tar, and other gunk that’s typically found in crude oil) These materials have high resistance and therefore make the connection a high resistance connection.
- Some tank owners paint the inside of the tank shells, this paint would be an insulator and cause a high resistance.
- Floating roof is not always centered within the shell. This means that you could have gaps between the roof and the shell.
- Regular maintenance is required because the shunts typically have a build-up of tar and wax and other things found in crude.
- Source of arcing between the roof and the shell.
Here we have a typical floating roof tank. And we have rust on the inside of the shell. And that means the shunts between the roof and the shell would have a bad connection. As I said, many customers paint the inside of the tank and that increases resistance. Also, there can be gaps between the floating roof and the shell.
If a tank shells out of round, you could have a case where the shunt is not making contact with the inside of the shell. So, what we have here is a gap between the roof and the shell, which means it’s a poor electrical bond.
Several years ago, the API did some testing and the API testing proved that shunts will arc under all conditions. Doesn’t matter if they’re clean or dirty or well-maintained or whatever. They will arc under all conditions. A test was done with a simulated shunt pressing up against a piece of bare steel. Both the shunt and the steel were clean in this test and simulate lighting current was passed through the shunt and we have a shower of sparks between the shunt and the steel.
What is a ByPass Conductor?
So, a bypass conductor bypasses the seal around the floating roof. There’s basically two categories of bypass conductors, you can have a conventional or retractable.
Conventional bypass conductor is a plain wire cable that’s attached at one end to the top of the shell and at the other end to the roof.
Retractable bypass conductor which is a spring-loaded reel. Very similar to a tape measure.
The problems with conventional bypass conductors are that they’re unnecessarily long and when the roof is high, they will become randomly boiled and splayed on the roof. So, remember from the previous slide that the tank is most at risk when the roof is high. Well, when the roof is high, you would have a situation with a typical conventional bypass conductor. The conductor has to be as long as the tank pipe. But when the roof is high, that cable will boil and splay in a random pattern on the roof and thus create high impedance connection between the roof and the shell.
Retractable Grounding Assembly (RGA)
So, the retractable grounding assembly was invented by LEC in 1999. And we currently have over 13,000 units in service. We have the strongest springs and the most corrosion resistant retractable device on the market. The primary reason why this product is so much better than a conventional bypass conductor.
As you can see, when the roof is high, a retractable reel will be short and tight as compared to the randomly coiled and loose bypass conductor on the left.
Here’s a photo showing that same thing. As you can see in the photo at the top left, a bypass conductor is coiled and loose compared to the RGA conductor, which is shown in the bottom of the slide.
Here’s the math for you folks who like math. Let’s say for example, we have a 150-foot diameter tank that is 50 feet high, and it’s 80% full. Meaning that the roof is 40 feet high, well conventional bypass conductor would still be 50 feet long, and will be randomly coiled on the roof. However, an RGA cable would only be 10 feet long, and it would have about 15% of the impedance of the regular bypass conductor. Less impedance means faster reaction time. Which means faster equalization of the two surfaces, which means a reduction or elimination of any arcing between those two surfaces.
Primary Recommendation from API and NFPA
So, both the API which is the American Petroleum Institute and NFPA National Fire Protection Association, recommend bypass conductors on floating roof tanks. They recommend that bypass conductors be installed no more than every 100 feet or 30 meters around the tank circumference with a minimum of two.
These bypass conductors are easy and inexpensive to install on both new and existing tasks.
Furthermore, the standards say that the bypass conductor should be as short as possible. And please know that a retractable conductor is the shortest possible bypass conductor.
- Tank fires are not uncommon, lightning causes about 1/3 of all tank fires.
- As the weather changes, lightning rates are increasing worldwide.
- Conventional roof to Shell shunts, metallic seals and bypass conductors provides high impedance connections. Which is not what we want, we want a low impedance connection.
- The standards do recommend the installation of bypass conductors on external floating roof tanks. However, it is LEC’s belief and recommendation that they be installed on all floating roof tanks for maximum safety.
- The RGA provides the lowest possible impedance bond between the roof and the shell.
So, while I’ve been talking, some questions have rolled in. So, let’s get to those.
On a bulk floating roof tank, what is the main down conductive path?
So, this person is asking if lightning work to strike a tank. How does that current flow down to earth? Well, lightning current by definition has to reach the Earth and it would get there by flowing along all surfaces among the tank so that means down the shell across the roof to the side of the tank and so forth. Basically, the entire tank would become energized if it were struck by lightning and that lightning current will flow across a tank seeking Earth.
Is there a resistance value that bypass conductor should meet?
Yes, the American Petroleum Institute and the Energy Institute advise a maximum resistance of a bypass conducted to be 30 milliamps.
How do you test an RGA when it’s been installed? And how often?
Well, we don’t recommend testing RGA after they’ve been installed. Because testing them electrically requires them to be tested with a multi-meter, which injects current into the device and current and its own storage don’t mix. So, we do recommend that they be visually inspected once a year to make sure the cables are intact and there’s no corrosion on the on the connections.
What does it usually take to install an RGA? And can it be done on a live tank?
Yes, to men can usually install an RGA in about two hours. It requires holes to be drilled or punched into the top rim and also on the chrome dam on the roof. This can be done on a live tank by using a punch rather than a drill. And it would also be a good idea to have some type of nitrogen bath or some other kinds of non-flammable environment to ensure that no sparks are produced when you’re creating these holes.
Thank you for watching and listening, I hope you found it informative. And again, you can reach me through my contact information. Thank you very much.
Ian: Thank you, Joe, that was very informative.
At this point, we throw it out to the floor to see if there are any more questions. There is one question just come in.
How does the API recommendations compare with 60079-32 as an Example?
That’s a great question. They are two distinct the IEC is a little bit more generic. Whereas the API 54 (I think it is) really zones in on this floating roof tank issue. So, IEC will say you need to take reasonable steps to reduce and mitigate the risk of dangerous sparking by bonding etc. Whereas the API 454 typically talks about bypass conductors. Hope that helps.
Thanks for coming and have a great day. And I’ll see you next time, cheers for now.
On September 17th 2014 the essential need for a retractable grounding assembly (RGA) was revealed, when 1,400 barrels of oil exploded due to a devastating lightning strike. Read more.
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