A steam injector is a type of nozzle which is used to top-up the water in the boiler of a steam engine while the engine is working and the boiler is under pressure. The injector uses steam, either directly from the boiler, or (more efficiently) exhaust steam from the cylinders, to force water into the boiler against the pressure of the water and steam in the boiler. Given that the water is initially at atmospheric pressure and the boiler can be at more than 200 pounds per square inch (1400 kPa), a mechanical pump is difficult to design; early engines had force pumps driven from the valve gear but these had the obvious disadvantage that they only worked when the engine was in motion. Nevertheless, injectors are complex devices and most engines will have at least two installed in case of failure. If water cannot be injected there is a risk that the level will expose the crown of the firebox and melt the emergency fusible plug, stopping the engine.
The principle of the device is that steam is passed through a series of cones which reduce its velocity and temperature while increasing its pressure. Fresh water from the tender or tank is entrained in the fast-moving initial steam jet by Bernoulli's principle, and the mixture passes into a convergent "combining cone" which mixes them thoroughly. The steam and water mixture then enters a divergent "delivery cone" which slows down the jet, condenses most of the steam and builds up the pressure of the mixture to above that of the boiler. There is a one-way ball valve at the exit of the injector which only allows the water to enter the boiler when its pressure is high enough.
Injectors are thermally efficient because the heat energy contained in the steam is recycled, apart from some heat losses from the body of the injector.
One disadvantage of an injector is that it cannot be used with pre-heated feedwater, as cool water is required to condense the steam and make the device work. Because of this, some late 20th century locomotives with pre-heaters reverted to mechanical force pumps.