Just as it happens in your humidifier, as you vaporize water, the impurities that cannot vaproize become ever more increasingly concentrated in what is not yet vaporized. So there is a problem in boilers when new water is added that has its own load of dissolved solids (carbonates, sulfates, etc). The dissolved solids adhere to the boiler via electrolysis due to the differences in electrical charge between various ions in the water and the metal boiler and tubes. This problem also happens in your hot water tanks and in those in RVs. You can treat the water with chemicals (adds a cost to operating locomotives), but you still get a chemical sludge lingering at the bottom of the boiler in an ever-growing blob. This blob takes up boiler volume and reduces efficiency. It eventually must be purged, and I believe that a “blow-down” could do this with the right pipes…not sure about that. Certainly you can “blow-down” our water heater with the valve at the bottom…carefully, of course.
Now I wonder how the Western Maryland Scenic Railway deals with this?
or other Mondern Steam operartions
One of the ways is with a daily “blow down” of the boiler first thing when it is up to steam. Before it is coupled to its train it releases a dramatic and potentially dangerous amount of water. This why it is done before it leaves the shop area. The cost of treating the water is very little comparred to boiler repairs. And i do not mean that it is dangerous to the engine but to people if they came into contact with very hot water at pressure.[2c]
This is one reason that boiler check valves went to the top of the boiler, where before they had been on the sides. If the check valve is on the side, any impurities will form scale, while if the check valves are above the water line, the impurities will form a mud which can be blown out. Koodos to whatever steam-era genius figured this out. Top mounted check valves are apparently also more efficient.
I was riding the Hill Country Flyer (Austin to Burnet TX) one day. They had refilled the tender in Burnet prior to the return to Austin. The first couple of miles out of Burnet have a steep grade. As the train went up the grade, it gradually slowed to a stop, blocking the main highway into town. They did a boiler blow-down right there. Seems like the water/chemicals added in Burnet had formed a layer of scum on the water in the boiler - significantly reducing steaming ability and pressure dropped below what was needed to pull the hill. So they blew the scum out of the boiler, rebuilt pressure and was able to continue the trip. Glad I was on the train - not on the highway in a hurry.
dd
ps - the Durango-Silverton steam trains tend to do their blow downs while on trestles to avoid injury to bystanders.
At Steamtown, we test the water in the boiler every day the engine operates, then add chemicals to the tender water after it’s filled. The water comes from Scranton’s regular water system. The sample is drawn out of the trycocks on the backhead of the boiler.