Jason K. Firth, C.E.T.

Instrumentation, Control, and Automation

Possibly Apocryphal

Dec 102014

December 10, 2014

I'm Jason Firth.

 

A quote attributed to Mark Twain goes "History never repeats itself but it rhymes".

I started my career going to college as an instrumentation engineering technologist. Over 20 years before that, my father was going to trade school as an instrument mechanic just a couple hundred kilometers away in Brandon.

Instrumentation is a fast moving trade: New computer control products are constantly coming out, there's new technologies, new devices, new trains of thought, and tomes filled with the new ideas in control that come out every month. However, a lot of the fundamentals stay the same. A lot of the curriculum we both learned could be taught in either classroom. Pneumatics, electronics, fluid mechanics, op-amps, PID controllers, final control elements.

There's one story in particular that both of us were taught, 20 years apart. It's a story about why you should have qualified people working as instrument techs.

I'll start with the process, and move on from there.

The Kraft paper making process starts with wood chips, which are then placed in a "digester". A powerful caustic called "white liquor" is added to the digester, and the whole unit is heated and put under pressure. The white liquor dissolves the stuff keeping the wood fibres bound together, and once the digesting process is complete, you've got a combination of wood pulp, and spent white liquor, which is called "black liquor", because it becomes filled with all the sugars and lignates and such from the wood. From the digesters, the result is placed on a giant drum called a washer, and the black liquor is washed out of the fiber, which then heads off to your paper machine or pulp machine or whatever you're going to use the fiber for. The black liquor then pumped to the recovery process.

The recovery process takes that black liquor and "recovers" it into white liquor. The first step is that the black liquor is pumped into a giant boiler (we're talking 8-10 stories tall, with a cross-section of a small house), called a "Recovery boiler", where it burns. The sugars and lignates from the wood burn, producing heat. Once the liquor is burned, it drops into a chamber below the boiler, at which point it is now "green liquor". From there, it gets sent to the recausticizing plant, where it is clarified and strengthened, and it becomes white liquor once again.

Recovery boilers are huge, as we've established, and they're also quite high pressure. Plants I've worked at had steam of 800psi, but I've heard of plants as high as 1500psi. Besides producing enough steam to run the process, there is often enough steam left over to run a turbogenerator to offset the huge amount of electricity involved in the paper making process. I've seen turbogenerators of 20MW, but 100MW or more of electricity generating capacity is definitely out there. That's enough energy to power a small city. When paper prices collapsed, some plants remained operational only from profits made by selling electricity back to the grid!

So you have these boilers that are dangerous by themselves simply by virtue of being massive pressure vessels containing enough energy to power a small city, but recovery boilers have an additional danger: The caustic which drops into the chamber below the boiler is called "slag", and it reacts violently with water. Getting water into your recovery boiler is a great way to not have a recovery boiler any longer.

So finally, on with the story. Fort Frances is a town in northwestern Ontario, which for a long time had a pulp & paper mill. In recent years the mill has come on hard times, but before that it was in operation for decades.

The story goes, that on two separate occasions, they literally blew up their recovery boiler, because they weren't using qualified people to handle their instrumentation and controls.

The first story goes like this: The union plant allowed someone from operations to work as an instrument technician without going through an apprenticeship first. One day, they installed a fail open control valve on the fuel line into the boiler. The first time the valve lost air pressure, the fuel valve opened 100%. The huge excess of fuel caused a boiler explosion. Fail safety is one of the fundamentals of instrumentation, so any qualified instrument tech should have caught the problem before it became a problem.

The second story goes like this: Apparently not learning from their first episode, the plant allowed someone who wasn't qualified as an instrument technician to work as one. One day, the boiler was running dry to do some testing. This is an extremely unusual situation, and generally it isn't something you'd ever do, because you can damage the boiler. No tag out was employed to explain that the boiler was being run dry intentionally, so when an operator noticed the low boiler water level, they panicked and started adding water. When you add cold water to a superheated empty boiler, the water immediately boils, turning to steam. The shock can cause an explosion. They ended up blowing up their boiler again. Lock out and Tag out are another one of the fundamentals of instrumentation, so any qualified instrument tech should have tagged the controller, preventing the problem.

In the age of the Internet, I haven't been able to find any articles supporting the idea that either of these things happened. However, 20 years apart, at two different colleges, in two different programs, the same stories were told about the same mill in Northern Ontario, in both cases a cautionary tale of using unqualified instrument techs.

Thanks for reading!

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