Monday, May 26, 2008

When the Steam Doesn't Stay in the Tube: Day 3

It is a little after 5 am and I am at work. Checked in and almost ready to get into the boiler. The night shift has just gone home. They welded in 6 tubes (12 welds) which is very good. It always buoys everyones spirits a little when we stop cutting things apart and start putting them back together.

Annie wondered in her comment on the Day 2 pictures what caused this in the first place. It might take a minute, but I think we have the answer.
Coal is a dirty fuel.... compared to refined oil products, or natural gas. It is a solid fuel, and has dirt in it, basically. To keep all the tubes clean we have air driven lances that blow the ash off the tubes at regular intervals with hot, dry steam. Well, it should be hot and dry, but it takes a bit to warm up the lance and all the connecting piping, so when it gets to the sootblower, some of it has cooled and condensed and when it blows into the tube bundle there is water that mixes with the ash, and steam. This mixture, and to a lesser extent dry steam is like a fine sandblasting mixture, and over time it erodes the tube wall.

In the early days of operation the sootblowers were set to run at about 300# of steam. So some areas got a lot of erosion. This particular sootblower hole is short, and the lance is close to the top tubes. Erosion is always worst above, rather than below the blower. So these tubes took a beating. If nothing is done, pretty soon there isn't any metal between the steam and the metal, and you have a leak. There are thee things that can be done to keep the steam in the tubes:

1. Pad weld the tube. Put more metal on the thin places with lots of weld beads. This can work pretty well, but in this area you have to do a lot of out-of-position welding and the tubes are always somewhat dirty. These two things contribute to POROSITY. Yikes!

2. Put a stainless steel shield around the bottom of the tube. This also works well, and during the last outage we pad welded and shielded hundreds of tubes. More than the usual amount, but we do a lot each year.

3. Put in a dutchman. This is a short segment of new pipe. This is by far the best solution, but the most expensive, the most labor intensive and the slowest. But that is what we are doing now in the boiler in the section that was damaged.

Back to POROSITY. If your tube is dirty or your arc gets long the molten metal get impurities dissolved into it, and they form little gas bubbles. When the metal freezes, they are trapped. You never get rid of it.... well, you can grind it all out and start over, but it is amazing how hard it is to grind it all out. So, for practical purposes, you can't really get rid of it.

All those little bubbles can line up and form a crack, or a passage for the steam to follow to get out of the tube. Once a little steam can get through, it isn't long before the hole is bigger, and you have a leak. Because the tubes are so close together, it isn't long before you have worn a hole in a tube nearby, which send a jet of erosive steam over to another tube, and pretty soon there are holes all over and it looks like there was a Mafia gun battle that went on. You pull all the poor dead bodies out and line them up and look for what could have caused the problem to begin with. This is a picture of the tube that started it all. It was thin, the welder blew through the thin tube wall and a pocket of porosity formed, the welder tried to get rid of it, but it remained, and a crack formed.Then a leak formed and this tube started to leak up at about a 45 degree angle, blasting one of the tubes in yesterday's post that opened up.

That tube failed, and blasted right across. Both tubes opened up at about the same time, blasting each other sideways and getting everyone's attention.

Here are some new tubes, with the end's prepped, cut to length and ready to be welded in.

Home Sweet Home.

Making chill rings.

Chill rings are little devices that help to align and space the tubes so that they are welded just right. Those of higher ideals and purer hearts prefer the more elegant TIG root. This is where you use a tungsten arc with inert shielding gas to weld in the first pass between the tubes. There is normally a 1/8" gap between the tubes, and you delicately fill this with a thin film of molten metal. If conditions are good, then you get the very best weld. But if they are not so good, you just think you get the best weld. These little guys help fix that. You just put them in both tubes and use a normal stick weld. For all intents and purposes, these are the root weld. The ones that you buy commercially are kind of thick and clunky, but we make our own out of the same tube material, and they work very well.


AnnieOfBlueGables said...

That was a very wonderful explanation. I really appreciate the visuals. That makes so much more sense. I know you think I get a glaze over my eyes when you try to tell me about work, but really, I am doing my best to picture it. This is really helpful.
When people ask me what you do at the power plant, I. . . really don't know what to say. From now on, I will just refer them to your wonderful blog.
Love you, sweetheart.
what sounds good for supper?


My America said...

I have been studing your page some today, I am in the powerbusiness as well, QA/QC for the boiler and welding. It looks as if you some huge boilers; the look like B&W units. I wasn’t sure until I noticed the U-strap spacers in your backpass inspection photos. How many pendants do you have across your secondary inlet…man it looks like a lot. I also noticed the corners of them peak out from behind the nose quite a bit; do you get a lot of fouling in that area?

Anyway you have some great posts and super pictures and videos; keep it up.