A very dangerous condition in a boiler (in a steam locomotive or other steam engine). If the water level gets too low, the boiler may rupture due to a crown sheet failure.
A low water level generally occurs for one of three reasons:
- Inattention: probably the major cause of low water level conditions just on its own, inattention to maintenance is also a common cause of the other two types of failure as well. A working steam locomotive consumes water constantly; a large steam locomotive working hard consumes water voraciously. If the crew does not keep a constant eye on water level, they may have a low water situation on their hands before they know it. The crew must also be mindful that if the locomotive is climbing, the water level shown in the water glass will be deceptively high, because the water will be tilted higher at the rear, where the gauges are. When level track is reached, or worse, a downgrade, the water level over the crown sheet may drop to dangerous levels.
- Failure of feedwater equipment: Often caused by poor maintenance, but unexpected failure can occur even on the best maintained locomotive. All modern locomotives must have two redundant systems. One is usually an injector while the other may be another injector, an exhaust steam injector, or a water pump, possibly combined with a feedwater heater. Each system should be able to provide water to the boiler at a safe rate. If both should fail, there should be plenty of warning for the crew unless they are inattentive. The recommended practice for the crew under such circumstances is to stop, dump the fire if they can, kill the fire in another way if they can't (generally by covering the fire in a thick layer of damp coal, slowing its burning down), kill the draught by turning off the blower if on, try and dump steam pressure if they can (whistle, safety valve, etc) and then get the hell out of there!
- Failure of gauges: If the gauges don't show the right water level, a low water condition can sneak up on the crew without them being able to know. Modern locomotives have two or even three water gauges, to help prevent this problem. Gauge failure generally occurs through them being blocked by scale or accidentally left isolated -- these are generally due to poor maintenance. Another cause of inaccurate water gauge readings was found to be that if intense boiling was occuring against the backhead (the back of the boiler, where the gauges were mounted), the bubbling and foaming could make the water level appear higher than it actually was. This is addressed in newer gauge designs, such as the water column.