Clack valve

The connecting pipe between a locomotive injector and the boiler, through which the water is forced into the boiler against the steam pressure within, is provided with a "clack-valve," which allows the water to pass in the inward direction only and not the outward; this is housed in a "clack-box". No question of "advantages or disadvantages" attaches to the use of a clack-box; it is a necessary part of locomotive equipment.

The feed water enters the back and cool end of the preheater and passes forward to the front and hot end, after which it enters the boiler through a clack valve.