Figure 9-27.—Diaphragm control valve, downward-seating
type.
In the diaphragm control valve, operating air
from the pilot acts on the valve diaphragm. The
superstructure, which contains the diaphragm, is
direct acting in some valves and reverse acting in
others. If the superstructure is direct-acting, the
operating air pressure from the control pilot is
applied to the TOP of the valve diaphragm. If
the superstructure is reverse-acting, the operating
air pressure from the pilot is applied to the
UNDERSIDE of the valve diaphragm.
Figure 9-27 shows a very simple type of direct-
acting diaphragm control valve with operating air
pressure from the control pilot applied to the top
of the valve diaphragm. Since the valve in the
figure is a downward-seating valve, any increase
in operating air pressure pushes the valve stem
downward toward the closed position.
Now look at figure 9-28. This is also a direct-
acting valve with operating air pressure from the
control pilot applied to the top of the valve
Figure 9-28.—Diaphragm control valve,
type.
upward-seating
diaphragm. Note that the valve shown in figure
9-28 is more complicated than the one shown in
figure 9-27 because of the added springs under
the seat. The valve shown in figure 9-28 is an
upward-seating valve rather than a downward-
seating valve. Therefore, any increase in operating
air pressure from the control pilot tends to OPEN
this valve rather than to close it.
As you have seen, the air-operated control
pilot may be either direct acting or reverse acting.
The superstructure of the diaphragm control valve
may be either direct acting or reverse acting. And,
the diaphragm control valve may be either upward
seating or downward seating. These three factors,
as well as the purpose of the installation,
determine how the diaphragm control valve and
its air-operated control pilot are installed in
relation to each other.
To see how these factors are related, let’s
consider an installation in which a diaphragm
control valve and its air-operated control pilot are
used to supply controlled steam pressure.
9-19
