Injector

Injector is a device found on steam engines , used to feed water to the boiler from storage , in order to maintain nominal water level . It is usually operated by a valve, and can be opened or closed.

Stored water is cold. As it is injected into the boiler, it may drop temperature of the water already there, causing steam to condense back into water. This can result in significant pressure drop, so water injection is recommended to be done at small amounts and well timed .

There is no way to monitor water temperature, other than indirectly by looking at the speed the boiler pressure gauge needle is moving.

Injectors normally rely on steam pressure to inject water, but in Derail Valley this limitation is omitted to avoid complications when starting a cold engine.