The Mason pump has a valve piston, a main valve, a preliminary valve
and a yoke connected directly to the valve stem, as shown in Fig. 262.
The valve piston is contained in a cylinder above the steam chest, and
moves the main valve by means of a pin, which projects into a pocket in
the top.
[Illustration: FIG. 262.]
The main valve and preliminary valve travel on the same seat, and
receive their motion from the yoke, _E_, Fig. 263, which surrounds
them. This yoke fits the preliminary valve neatly, allowing an
independent movement of the main valve and valve piston.
The preliminary valve is the ordinary ^D^ type and controls three
ports, as shown in Fig. 263, the two steam ports are connected with
either end of the auxiliary cylinder and the exhaust, with the small
port, _A_, in the main valve seat. The duty of the preliminary valve
is to alternately connect each end of the auxiliary cylinder with this
port. The main valve controls four ports, and is also of the ^D^ type,
having an extended cavity connecting the exhaust from the preliminary
valve with the main exhaust. The main valve also controls the main
steam and exhaust ports of the pump cylinder.
Steam enters the chest through the steam pipe, passes around the
central portion of the valve piston and through the passages in the
piston to both ends of the valve cylinder, thus balancing the valve
piston.
Suppose that the yoke and preliminary valve have been carried to the
back end of the chest, and have moved the main valve to bring its
auxiliary cavity over the auxiliary port, _A_, Fig. 263, in the main
valve seat, which connects the back end of the auxiliary cylinder
with the main exhaust. The steam from the end of the chest passes up
through port, _B_, down through port, _C_, and up through port, _A_,
into the exhaust port, _D_, in the main valve. This unbalances the
auxiliary piston, which is driven back by pressure on the opposite
end, and carries the main valve independently of the yoke. The travel
of the main valve causes it to cut off the exhaust from the auxiliary
cylinder, and the remaining steam cushions the valve piston.
[Illustration: FIG. 263.]
The travel of the main valve also opens the main steam port to the
forward end of the pump cylinder, and connects the back end of the
cylinder with the main exhaust, thus reversing the motion of the piston.
This action is repeated at the end of each stroke. Whatever position of
the piston, the pump will start when steam is turned on, as there is
always a connection either directly from the steam inlet to one of the
steam ports, or, if the main valve has covered both steam ports, it is
in a position to connect the auxiliary cavity in the main valve seat
with the main exhaust, which at once releases the steam from one end of
the valve piston, and the pressure on the other end drives the piston,
thus moving the main valve and giving direct communication between the
steam inlet and one of the main steam ports.
Thus it is seen that the piston cannot get into a position where it is
impossible for it to respond to the steam pressure.
_To set the valve_ place the piston in the mid-stroke position and set
the auxiliary valve with rocker arm plumb, and the preliminary valve
covering all its ports equally. These positions may be secured by
adjusting the position of the clamp on the valve stem and move the main
piston sufficiently so that the auxiliary valve will open one of its
ports.