When a steam locomotive was employed as a stationary boiler for use in a factory, how were the tubes kept clean of soot?

Bored out occasionally with a long rod with a brush or sharp tip most likely (if they burned coal).

Compressed air could also assist in tube cleaning and is usually available in a factory environment. Many live steam hobbyists use air shop to clean the ashpan, smokebox, and firetubes after a day of running.

Stationary coal fired boilers typically have steam powered soot blowers. High pressure steam is used to clear the fouling (soot) off the outsides of the watertubes. A sootblowering procedure is performed on a regular basis while the boiler remains hot.

I imagine a sootblower could be designed and built for a steam locomotive in a stationary application; permitting tube cleaning while the boiler remained in service. I have also seen sand poured through the firedoor of oil-fired locomotives during operation. Liquid fuels have a tendency to "stick" to tubes versus the "dry" fouling from coal.

From a 3/28 post about the Maine Narrow Gauge Railroad Engine House Fire:

Some stationary firetube boilers do have soot blowers. They would work in something like an HRT boiler. In a locomotive boiler the intense heat might melt them. Maybe with an arch in place there would be sufficient protection or maybe if the boiler was operated at a very low capacity.

For most firetube boilers you simply "punch the tubes" on a regular basis. This traditionally is done with a long pipe or rod with a round wirebrush attached to the end. There are power tool used today. You can see one here:

http://www.goodway.com/boiler-tube.htm

If you are not sure when it's time to clean your tubes. Watch the stack temperature and if you find it more than 125-150 degrees above the steam temperature (at moderate loads) then you should be cleaning your tubes. Example, at 175PSI steam is about 377 degrees, if the stack temperature is 577 then you should be cleaning your tubes. If your stack temperature remains high after cleaning the fireside of the tubes then it's time to open the boiler and check the waterside for scale. This could cause serious problems if not addressed. This will vary depending on the specifics of each boiler but is a good rule of thumb.

Or you could call one of my favorite Baltimoreans, Mike Rowe!

http://dsc.discovery.com/videos/dirty-j ... eaner.html

OR . . . . many locomotives used as stationary boilers were converted to burn natural gas or furnace gas. So no need to deal with sooted flues.