Brian,
I'm sure others will respond to your post. My response is this:
1. Follow what instructions you have been given.
2. The method I mentioned works with little or no stresses to the boiler sheets. It was designed from practice that allows a cooling at a rate that is not injurious and it is there to help when the needed because of service requirements.
It was developed over a number of years on a railroad that suffered both hot and extreme cold, with varied service requirements and both old and new steam power.
3. But, anytime you can take a day or two to let the boiler cool down, it will be that much easier on the sheets and joints. This is a given. Why force the issue if you don't have to.
4. The roads that are operating on a daily or close to it basis, will come up against this problem eventually. While it would be nice to have enough power to do power swaps that allow complete maintenance to take place, it doesn't happen that way in real life.
5. The 5lb reduction method is fine. It is a time consumer. Again, if those are your instructions, then that must be followed.
6. It is not what you can get away with, it is what are the procedures to do the work that provide the least amount of (boiler) stress during an allowed time frame.
7. The business of cold water can be a touchy one. How it is dealt with in a hot boiler is the problem. That is why the top checks were a big help in reducing sheet cracking and other problems as mixing the cold with the hot was at a much easier rate.
8. Remember, the cold water supply doesn't have to be run wide open, thats probably why God sparked someone to invent a valve to control the flow. This goes for using an injector or water supply hose.
Your questions are reasonable. Underkill can be as bad as overkill. And who is to say what long term effects take place even with the most benign procedure.
Imagine, if one had the best blowdown and fireup practices, but no water treatment in a bad water area. The boiler(s) would have no stress problems with sheet cracking or broken staybolts, but they were ate away by errosion in a just a few years.
Sometimes, it depends on the situation, location and personnel. What works for some, doesn't always work for others. On the other hand, there are those that don't want to be confused by the facts.
What it comes down to is, ain't nobody perfect except the 'Big Guy' upstairs. But, it shouldn't be a sin to ask - why (do we do it this way)?
Chris
