101. Q. Explain the operation of the feed valve.

A. The feed valve consists of two portions, the supply and regulating
portions. The supply portion consists of a slide valve 9 and a piston 8
(see Fig. 6). The supply valve 9 opens and closes communication between
the main reservoir and the feed valve pipe and is moved by the piston 8
which is operated by main reservoir air entering through passage "a" on
one side or by the pressure of the spring 6 on the other side. The
regulating portion consists of a brass diaphragm 14, on one side of
which is the diaphragm spindle 16, held against the diaphragm by the
regulating spring 17, and on the other side a regulating valve 12, held
against the diaphragm or its seat, as the case may be, by the spring 13.
Chamber "L" at the left of the diaphragm is open to the feed valve pipe
through the passage "e" and "d". The feed valve is adjusted by turning
the hand wheel 22 in or out, thus increasing or decreasing the pressure
exerted by the spring on the diaphragm. The same results are obtained
in turning the hand wheel 22 as when turning the adjusting screw in the
older types of feed valves.

[Illustration: Fig. 6. Diagram of B-6 Feed Valve, Closed.
Connections: MR, Main Reservoir Pipe; FVP, Feed Valve Pipe.]

Air from the main reservoir flowing through passage "a" into chamber "B"
will force the piston 8 to the left against the tension of the spring 6;
the piston in moving will take with it the supply valve 9, opening the
supply port in the valve to port "c" in its seat as shown in Fig. 7.
Main reservoir air will now be free to flow through passage "a", chamber
"B", port "c" and passage "d" to the feed valve pipe. Air coming through
port "c" also flows through passage "e" to chamber "L" at the left of
the diaphragm 14, and this pressure tends toward forcing the diaphragm
to the right; but the diaphragm being supported by the regulating spring
17, will remain in its position at the left, holding the regulating
valve 12 off its seat, until the pressure in chamber "L" exceeds the
tension of the regulating spring 17. Air, therefore, continues to flow
from the main reservoir through a, B, c, d and e to the feed valve pipe
and chamber "L", increasing the pressure, until the pressure on the
diaphragm 14 overcomes the tension of the regulating spring 17, when the
diaphragm will move to the right, allowing the spring 13 to force the
regulating valve 12 to its seat, closing port "K". Chambers "