Spring has sprung, sort of, and thoughts of outages come unbidden to young and old men's minds. Mostly the come because we dread it so much and there is so much to get ready for. This weekend we had a short outage on Unit 1 to fix a few things that we could run with now, but probably would get worse and force and outage later.
Yesterday afternoon we did our back pass inspection. The boiler consists of two sections. The boiler cavity is where the fire is. The mills grind the coal to a flour like fineness and big fans blow it through specially designed burners into the cavity of the boiler.
It is a pretty big structure, and mostly you can't see anything but a huge, blinding ball of fire when it is on line. A lot of heat is created here, and a lot of it is absorbed into the boiler walls which are made of steel tubing and filled with boiler water. The water boils in these tubes and rises to where it is guided into the drum. The drum separates the water from the steam. The water falls back down some large pipes called downcomers and is recycled back into the wall tubes. The steam is cool - for steam, and needs to get more heat energy before it goes to the turbine. So, in the gas path there are hundreds of more tubes that absorb more of the heat from the gas, and as they do, they heat up the steam that travels through them.
Here are a couple of pictures of the back pass. The whole steam/gas path interaction is complex but can be summed up in the coolest steam is warmed by the coolest gas in the path, and as the steam travels up and forward it moves against the gas path and is in contact with increasingly warmer gas.
In a backpass inspection, you crawl through on the tubes and look for broken soot blowers (horizontal pipe in lefthand picture), or places where the soot blower has eroded the tubing and it needs to be shielded or pad welded.
One last picture. These are the platen pendants and they hang over the fireball and pick up a lot of heat. They aren't the last stage in steam warming, but pretty close. Just a little part of what it takes to keep the lights on.