page_title: Firebox (steam engine)
section_title: Sheets and stays
hierarchical_section_title: Firebox (steam engine) / Steam locomotive fire tube firebox / Sheets and stays
caption_attribution_description: Deutsch: Hinteres Ende des Schnittmodells eines Dampfkessels der Lokomotive 99 232 der Bad Doberaner Schmalspurbahn („Molli“). Gebaut von „Orenstein und Koppel“ Berlin-Drewitz 1932. Ausgestellt im „Mollimuseum“ in Kühlungsborn-West. Zu sehen sind: die Feuerbüchse mit Feuertür (rechter Bildrand) eingehängte Roststäbe (1.2) und Schmelzpfropfen (rot, 1.3) Deckenstehbolzen (2.1) Queranker des Stehkessels (2.2) Stabstehbolzen der Stehkesselrückwand (2.3) Bodenanker des Stehkessels (2.4) Flansch des Dampfentnahmestutzens (2.5) Flansch des Sicherheitsventils (2.6) Dampfdom (3.5) Im Langkessel davor liegend die Rauchrohre mit den eingeschobenen Überhitzerrohren. English: Back part from the model of a 1932 steam locomotive boiler, on display in the Molli Museum in the German town of Kühlungsborn at the Baltic Sea coast.
context_page_description: In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name. The hot gases generated in the firebox are pulled through a rack of tubes running through the boiler.
context_section_description: The metal walls of the firebox are normally called sheets, which are separated and supported by stays. The stays support and brace the "sheets" (plates) against pressure. Ideally, they should be located at right angles to the sheets, but since the outer sheet (wrapper sheet) is often rounded and the top of the firebox (crown sheet) is relatively flat by comparison, such a relationship to both sheets is impossible. The actual location of the stays is a compromise.  Since stay breakage is hidden, the stays have longitudinal holes, called tell-tales, drilled in them which will blow water and steam, revealing if they are broken. A boiler with more than 5 broken stays, or two next to each other, must be taken out of service and the stays replaced. The fusible plugs, usually located in the highest part of the crown sheet, have a soft metal alloy core which melts out if the water level in the boiler gets too low. Steam and water blowing into the firebox both alerts the locomotive crew to the low water condition and helps put out the fire. Not all locomotives are equipped with fusible plugs.  Also, fusible plugs should be replaced at regular intervals, about every three months for a locomotive in regular service, because the soft metal alloy core will slowly melt out over time even if the boiler water is carried at proper levels. The "mudholes," or washout plugs, allow access to the interior of the boiler for washing and scraping away boiler mud and scale.
  The sheets on the left and right are called "side sheets" while the sheet in the front of the firebox is the flue sheet. The "front flue sheet" is in the front of the boiler and at the rear of the smokebox. The "rear sheet" is at the back of the firebox and has the door opening in it. The crown sheet is the top of the firebox. The crown sheet must be covered by water at all times. If the water level drops below the crown sheet, it will become overheated and start to melt and deform, usually sagging between the crown stays. If the condition continues, the crown sheet will eventually be forced off the crown stays by the pressure in the boiler, resulting in a boiler explosion. This condition, usually caused by human error or inattention, is the single greatest cause of a locomotive boiler explosion.
caption: Cutaway of locomotive firebox and boiler with radial stays
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