What Makes Steam Engines So Dangerous?
In the main story (and the original film) I did not go into a deep explanation of how boilers were made and how they work. The firebox and ash pan are actually one large box that is suspended inside the rear of the boiler. It is designed to be completely submerged in water. The crown sheet is the top of the firebox. The water level in the boiler should NEVER be allowed to drop below the crown sheet.
The water in the boiler absorbs a lot of heat from the metal surfaces of the firebox and as long as those surfaces are submerged the boiler can be operated safely. If the water level ever drops below the crown sheet the temperature of this part will climb quickly. If the operator adds more cold water to the boiler and it touches the crown sheet, steam will be generated so quickly that the pressure will build uncontrollably fast and a catastrophic rupture is likely. Steam engine explosions are almost always a result of operator error.
The Boiler Shell
The boiler's shell is made of large pieces of cast iron sheet metal that are 0.375 inches (9.6 mm) thick. One piece of this was curved around on itself to make a large cylinder.
The place where the two edges of the sheet of metal meet form a seam that must be riveted together (arc welders had not been invented when these engines were constructed). You can see this seam on Kay-Gee 1875 if you look closely at its right side at the double row of rivets running the length of the cylindrical part of the boiler. If you look at the front of Kay-Gee 1875 this seam is located at the 4 o' clock position.
When engine makers first started making boilers they used a lap seam. With this type of seam the two edges of the sheet metal shell of the boiler are lapped over each other and riveted together.
In time this design was changed to use the butt strap seam. This seam was constructed by taking the two edges of the sheet metal shell and butting them together. The joint was then sandwiched by two straps of sheet metal, one inside the boiler and one outside, that run the length of the seam. Two rows of rivets were used to hold all this together.
The butt strap seam is considered to be a stronger joinery method than the lap seam. In 1988 the Tennessee Boiler Board issued the regulations governing antique boilers. All boilers constructed with lap seams were immediately condemned. This one decision automatically eliminated many of the engines from participating in the Threshing Show except as static displays. Engines with butt strap seams, like Kay-Gee 1875, must be inspected each year and the boiler must undergo an ultrasound exam every few years, all at the owner's expense. Even then, the pressure at which the passing engines are allowed to run is minimal.
The Firebox
A rear end view and cross-section view of a firebox.
The firebox is a large, sheet metal box. The top or roof of the firebox is called the crown sheet. The large opening in the rear of the firebox is the door where the fuel is fed to the fire. The smaller opening is the damper. This door gives access to the ash pan for cleaning out ashes and allows the engineer to regulate the airflow to the fire.
The firebox is divided into two sections by a set of grates. The top section is where the fuel and fire are and the bottom section is the ash pan. Each grate is a specially shaped rod. The spacing between the grates allows room for the ashes to fall to the ash pan and air to get to the fire from below.
The grates rest on rails that are built on either side of the firebox. There is a lever on the outside of the firebox that the engineer can move to rock the grates to allow bigger chunks of material that will not burn (e.g. a coal clinker) to fall through to the ash pan.
The Firebox Inside the Boiler
The firebox is actually mounted inside the boiler so that it can be surrounded on all sides by water. A special section of the boiler was built around the firebox and riveted to the rear of the cylindrical section of the boiler to accomplish this.
The firebox was suspended inside the boiler with some 200 or so stay bolts. These bolts are rods that are threaded on each end and screwed into holes drilled into the sides of the boiler and the firebox. Without these stay bolts the firebox would collapse and the sides of the boiler would bulge as steam pressure increased. The rear wall of the boiler was braced to the top of the boiler as stay bolts were impractical here. The openings for the firebox door, damper, and the flues also helped the boiler and firebox hold its shape.
The Flues, Smoke Box, and Steam Dome
A flue sheet was riveted near the front to the cylindrical part of the boiler creating the smoke box. The flues connect the firebox to the smoke box to provide an exit route for the smoke. The flues also get hot and help to heat the water in the boiler.
The steam dome was built onto the top of the boiler to provide a place for steam to collect and be tapped for use. The plumbing that goes to the safety valve, steam cylinder(s), injectors, and whistle is attached here.
There is a drain plug in the front of the non-cylindrical part of the boiler near the bottom. There are also a couple of hand holes in this part of the boiler which can be opened to allow someone to inspect the inside of the boiler.
The firebox door is also large enough for a man to squeeze through in case one of the flues needs to be replaced or some other maintenance needs to be done. During a wheat threshing, when time was money, the fire would be put out and the water drained and someone would go inside the firebox to fix a leak while the firebox and boiler were still hot.
Maintaining the Water Level
A cross-section of the upper left part of the boiler looking forward.
One of the most critical parts of running a steam engine is maintaining the proper water level in the boiler. There is a sight glass mounted on the side of the engine for the engineer to check the water level (Kay-Gee 1875's is on the left side). The water is clear (unlike the diagram above) so it can be hard to see exactly where the water level is.
The goal is to keep the water level visible in the glass. If there is too much water in the boiler (above the top of the glass), water can be sucked into the cylinder(s) along with the steam which would decrease the power provided by the engine and possibly damage the cylinder(s).
The bigger concern is not to let the water get too low (below the bottom of the glass). If this happens the crown sheet (the top of the firebox) is no longer under water. Every steam engine has water tanks which holds enough water to run for a few hours. The water can be transferred from the tanks to the boiler with an injector. Kay-Gee 1875 has two of these, a primary and a backup.
