A. With an inside admission valve (usually a piston valve), the live
steam comes between the piston valve heads, the outside end of the heads
being connected with and exposed to exhaust pressure, it admits steam
past the inside edges of the valves. An outside admission valve has the
space between the ends connected to the exhaust and a space at the ends
connected with the live steam. It admits steam past its outside edges. A
piston valve can be either inside or outside admission, while a slide
valve is always outside admission.
29. Q. What is the relative motion of the main piston and the steam
valves for inside admission, and, on the other hand, for outside
admission?
A. If the piston is in the front end of the cylinder, an inside
admission valve must move forward in order to connect the inside of the
valve with the front live steam port to admit steam against the piston.
The outside end of the valve opens the exhaust port for the back end of
the cylinder. In the same position of the piston an outside admission
valve must move backwards to open the steam port or in the same
direction as the steam piston when commencing its stroke.