The injector forces water into the boiler, using steam in a "venturi tube" to create a partial vacuum which pushes the water toward the tube's nozzle. Without the injector, the boiler's pressure would reverse the flow of water, going out to the tender instead of in. A check valve in front of the steam dome prevents steam from escaping down the delivery pipe, but when injector pressure becomes as high as the boiler's internal pressure, the check valve opens to admit fresh feedwater.
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Multiple uses
Injectors in England
Injectors in United States