Some of you may be aware that an empty grain silo can explode if the dust from the grain is allowed to build up and an ignition source begins a chain reaction that causes the entire grain silo to explode like a bomb. I haven’t heard about a grain explosion for a few years. Maybe that is because a lot of effort is put into keeping the silo clean. Think of how much easier it would be for a coal dust explosion. After all… we know that coal when turned into a fine powder is highly combustible.
When you are covered in coal dust from head-to-toe day after day you seem to forget just how explosive the coal dust you are washing down can be. Our coal-fired Power Plant in North Central Oklahoma was concerned after our downsizing in 1994 that by eliminating the labor crew from the roster of available Power Plant Jobs, that the operators may not be able to keep the entire coal handling system free from coal dust.
The plant had already experienced a major explosion the year before (in 1996) the “Dust Collector Task Force” was formed (See the post: “Destruction of a Power Plant God“). It was clear that the question had been asked by those concerned, “Are there any other areas in the plant that could suddenly explode?” Two electricians were asked to be on the Dust Collection Task Force. Jimmy Moore and myself.
We had a salesman of our Dust Collector come to the plant and train us on the proper maintenance of the dust collectors that were already in place. When he arrived he showed us a video that showed examples of plants that had explosions caused by coal dust. Here is a picture I found on Google of a coal dust explosion at a power plant:
We heard a story about a coal plant where the explosion began at the coal yard, worked its way up the conveyor system, blew up the bowl mills and threw debris onto the main power transformer, which also blew up. Ouch. We thought it would be a good idea to do something about our coal dust problems. Stopping an ounce of coal dust is worth a pound of explosives… as the saying goes.
The Instrument and Controls person on our team was Danny Cain. He had become a Power Plant employee a year before the downsizing and had been at the plant for about four years at this point.
When we began looking at our dust collectors, we found that the dust collectors on the dumper had been rusted out over the past 18 years since they were first put into operation. the reason was that they were located down inside the dumper building below ground where they were constantly exposed to coal and water. I hadn’t seen them actually running for years. They were definitely going to have to be replaced with something.
Okay class… I know this is boring, but you have to learn it!
We had some fairly new dust collectors on the crusher tower and the coal reclaim, but they didn’t seem to be doing their job. They used instrument air (which is clean, dehumidified air) in order to flush the coal off of some bags inside. When they were installed, new instrument air compressors were installed in the coal yard just to handle the extra “instrument air” load for the dust collectors. The very expensive and large dust collectors just didn’t seem to be doing anything to “collect” the dust.
You can see that the dust collector is very large. You actually have to climb on top of them to change out the bags inside.
When the dust collector sales man came to talk to us about dust collection, in the middle of his “Proper Maintenance” speech he happened to mention something about…. “…and of course, if you don’t have the air pulse set at exactly 32 milliseconds, the dust collector isn’t going to work at all.” “Wait! What did he say?” What pulse?”
He explained that Instrument air is puffed through the collector bags with exactly a 32 millisecond pulse at a predetermined interval. If the pulse is longer or shorter, then it doesn’t work as well. The idea is that it creates a ripple down the bag which shakes the dust free. We had been studying our dust collectors in the coal yard, and the interval had been completely turned off and the instrument air was constantly blowing through the dust collectors. This guy was telling us that it was just supposed to be a quick pulse.
Everyone in the room looked at each other with stunned silence. The salesman just looked at us and said…. “It’s right there in the instruction manual….” pointing his finger at the page. We thought (or said)… “Instruction manual? We have an instruction manual?”
We said, “Class dismissed! Let’s go to the coalyard after lunch and see about adjusting the “pulse” on the dust collectors.
In order to measure a pulse of 32 milliseconds, I needed the oscilloscope that I kept out at the precipitator control room to measure the “Back Corona” when trying to adjust the cabinets to their optimal voltage. I ran out to the precipitator and retrieved it and brought it with me to the coal yard along with my tool bucket and my handy dandy little screwdriver in my pocket protector:
When we arrived at the crusher tower where the two long belts sent coal to the Power Plant 1/2 mile away, one of the belts was running. coal dust was puffing around the equipment making the room hazy, which was normal. Water hoses were kept running on the floor trying to wash at least some of the dust down the drain. This was a typical day in the coal handling system. Coal dust everywhere.
I opened the control cabinet for the dust collector and hooked up the oscilloscope.
When we arrived there was no pulsing. The instrument air was on all the time. So, I flipped a switch which put it in a pulse mode. The pulse time was set up to the maximum setting of about a minute (that meant that when the pulse turned on, it stayed on for a minute). As I was playing with the controls, three of the task force members were standing up on the walkway between the two belts watching the discharge from the dust collector (you see, after the dust collector collected the dust, it dropped it back onto the conveyor belt just up the belt from where the coal dropped onto the belt). Nothing was coming out of the chute.
As I adjusted the setting down from one minute to one second, I had to keep changing settings on the oscilloscope to measure how long the air took to turn on and off. When I finally had the pulse down within 1/10 of a second (which is 100 milliseconds), then I could easily measure the 32 millisecond interval that we needed. I was beginning to think that this wasn’t going to really do anything, but I remembered that I had seen stranger things on the precipitator controls where the difference between a couple of milliseconds is like night and day.
When the pulse was down to 35 milliseconds I looked up toward the conveyor system because I heard a couple of people yelling. They were running down the walkway as coal dust came pouring out of the dust collector chute causing a big cloud of dust to puff up. We all ran outside and waited for the dust to settle. We felt like cheering!
We were practically in disbelief that all we had to do was adjust the pulse of air to the right millisecond pulse and the dust collector began working. This meant a lot more than a working dust collector. This also meant that we needed only a fraction of the instrument air (literally about 1/20,000) than we had been using.
In other words. The new Instrument Air Compressors at the coal yard that had been installed to help boost air pressure at the coal yard since the installation of the dust collectors were really never needed. And all this was done by turning a screwdriver on a small potentiometer in a control cabinet. It pays to read the manual.
Along with some rewiring of the controls to the dust collector system, and a redesign of the apron around the dust chutes by Randy Dailey and Tim Crain, the coal handling areas became practically dust free as long as regular preventative maintenance was performed.
That is, everywhere except for the coal dumper. This is where the coal trains dump their coal into a hopper which is then carried on three conveyors out to the coal pile.
You can see the conveyor going up to the building right next to the coal pile. That is from the dumper which is the small off white building next to the fly ash silos. The crusher tower is the tall thin building at the end of the long belts going up to the plant.
We still had a problem with the dumper. The cost of buying new dust collectors and putting them outside where they wouldn’t be so quickly corroded by the harsh environment was “too costly”. Jim Arnold, the maintenance Supervisor made that clear. We had to come up with another solution.
Without a dust collector, the solution was “Dust Suppression”. That is, instead of collecting the dust when it is stirred up, spray the coal with a chemical that keeps the dust down in the first place. This was a good idea, except that it had to be turned off for three months during the winter months when it could freeze up.
A company called Arch Environmental Equipment came and talked to us about their dust suppression system.
They showed us something called: The “Dust Shark”.
The dust shark sprayed the belt on the side with the coal and scraped the bottom side in order to make sure it was clean when it passed through. This was the solution for the dumper. It also worked well at other locations in the plant where you could use it to keep the area clean from coal when the coal was wet from the rain and would stick to the belt.
The task force was considered a success. I have two side stories before I finish with this post.
The first is about Danny Cain.
Danny was a heavy smoker. He had a young look so that he looked somewhat younger than he was. He had been born in July, 1964 (just ask the birthday phantom), so he was 33 during July 1997 when we were working on the task force, but he looked like someone still in college. Whenever he would pull out a cigarette and put it in his mouth, he suddenly looked like he was still in High School.
I told Danny that one day. I was always one to discourage people from smoking…. He seemed a little hurt, and I said I was just calling it like I saw it. He was standing outside the electric shop smoking one day, so I took the air monitor that I used when I had to go in the precipitator and asked Danny if I could borrow his lit cigarette for a moment.
I put the butt of the cigarette up to the intake hose for the monitor about long enough for a puff and then I handed it back to him. The monitor measures the amount of Oxygen in the air, the amount of explosive gases, the amount of Carbon Monoxide and the amount of H2S gas (Hydrogen Sulfide, an extremely toxic gas). The monitor, as expected began beeping…
What we didn’t expect to see was that not only did the Carbon Monoxide peg out at 999 parts per million, but the H2S went out the roof as well. In fact, everything was bad. The Percent explosive was at least 50% and the oxygen level was low. It took about 5 minutes before the meter measured everything clean again. Danny didn’t want to see that.
I said, “Danny? Carbon Monoxide Poisoning! Hello???!!!”
When we were on the Dust Collector Task Force, at one point we had to program “Programmable Logic Controllers” (or PLCs). I had been to an Allen Bradley school a few years earlier where we had learned the basics for this. Here is my certificate from 9 years earlier…
When Danny and I sat down to program the controller, it became clear that he expected the programming task to take a couple of weeks. He started out by drawing some high level logic on the white board. I said… “wait… wait… let’s just start programming the thing.” He told me that wasn’t the way we did things. First we had to figure out the entire program, then we would program it.
The PLCs we were going to program were just some small ones we had bought to run the dust sharks and the dust collectors… Here’s one like it.
I told Danny when I program something I find that its a lot easier and quicker if we just program it as we understand the requirements and then that way we can test it as we go. Then when we figure out what we need, we will be done. In fact… it took us 4 hours and we were done… not two weeks.
End of the Danny Cain Side Story…. On to the second side story… much shorter….
I think it was March 2003 (the power plant men can remind me)…. a year and a half after I had left the plant, the Coal Dumper blew up. It was the middle of the night, a coal train had finished dumping the coal about an hour earlier. No one was in the dumper at the time and the entire dumper exploded. The roof of the dumper, as I was told, was blown off of the building. No injuries or deaths. The “Dust Shark” Dust Suppression system had been turned off because it was winter.
I suppose that the insurance company ended up paying for that one. I don’t know. This is what happens when you say that it is too expensive to replace the dust collectors and instead you buy one of these: