In this pump (see Figs. 266, 267) the steam is admitted to the center
of the valve chamber which contains the main valve, A, and the
supplemental slide valve, B. The recess in the center of the valve
piston, C, receives the main valve and moves it when steam is supplied
to or exhausted from either end of the valve piston.
In operation live steam passes through the left-hand ports, D and E,
drives the main piston to the right and the exhaust passes out of
the right-hand port, F, into the cavity in the main valve, A, thence
through the exhaust port, G, into the atmosphere.
[Illustration: FIG. 265.]
As the main piston nears the right-hand port, the valve lever H,
attached to and moving with piston rod brings the dog, I, on plate, J,
in contact with the valve arm, K. This moves the supplemental valve, B,
to the right and supplies live steam to the right of the valve piston,
C, and exhausts the steam from the left-hand end through the ports, L.
The main valve, being enclosed by the valve piston, moves with it to
the left. As the steam enters the right hand main port and exhausts
from the left hand port, the main piston commences its return stroke
and the operation just described becomes practically continuous. As
the main piston closes the main port, F, to the right, it cushions on
compressed exhaust steam, because the main valve, A, has then closed
the auxiliary port, M, leading to that end of the main cylinder. The
steam in this case is supplied through the main and auxiliary ports,
but exhausts through the main port only.
[Illustration: FIG. 266.]
[Illustration: FIG. 267.]
_The slow movement of the main piston near the ends of the stroke_
allows the pump valves to seat quietly, and gives the pump cylinder
time to fill with water, thus effectually preventing shock and noise.
The valve piston, C, is cushioned by cushion valves, which are simply
loose valves inserted near the ends in the valve piston. By removing
the steam valve chamber, the main and supplemental valves may be
readily examined or repaired. These valves are the ordinary plain flat
slide with which all engineers are familiar.
_To set the valve of this pump._ Place the piston in the center of its
travel with the valve lever, H, plumb and also valve arm, K, plumb;
adjust the supplemental valve so as to cover equally all the posts,
which is done by lengthening or shortening its valve stem.