pushed downward and the valve in the control
valve diaphragm. Note that the valve shown in
pilot is opened. This increases the operating air
figure 6-11 is more complicated than the one
pressure from the control pilot to the top of the
shown in figure 6-10 because of the added springs
diaphragm control valve. The increased operating
under the seat. The valve shown in figure 6-11 is
air pressure acting on the diaphragm of the valve
an upward seating valve rather than a downward
pushes the stem downward, and since this is an
seating valve. Therefore, any increase in operating
upward seating valve, this action OPENS the
air pressure from the control pilot tends to OPEN
diaphragm control valve still wider. Obviously,
this valve rather than to close it.
this won't work for this application. An
As you have seen, the air-operated control
INCREASE in controlled pressure must result
pilot may be either direct acting or reverse acting.
in a DECREASE in operating air pressure.
The superstructure of the diaphragm control valve
Therefore, we made a mistake in choosing the
may be either direct acting or reverse acting. And,
direct-acting control pilot. For this particular
the diaphragm control valve may be either upward
pressure-reducing application, you should choose
seating or downward seating. These three factors,
a REVERSE-ACTING control pilot.
as well as the purpose of the installation,
It is not likely that you will be required to
determine how the diaphragm control valve and
decide which type of control pilot and diaphragm
its air-operated control pilot are installed in
control valve is needed in any particular
relation to each other.
installation. But you must know how and why
To see how these factors are related, let's
they are selected so you do not make mistakes in
consider an installation in which a diaphragm
repairing or replacing these units.
control valve and its air-operated control pilot are
used to supply controlled-steam pressure.
PRIORITY VALVES.--In systems with two
Figure 6-12 shows one arrangement that you
or more circuits, it is sometimes necessary to have
might use. Assume that the service requirements
some means of supplying all available fluid to one
indicate the need for a direct-acting, upward-
particular circuit in case of a pressure drop in the
seating diaphragm control valve. Can you figure
system. A priority valve is often incorporated in
out which kind of a control pilot-direct acting
the system to ensure a supply of fluid to the
or reverse acting-should be used in this
critical/vital circuit. The components of the
installation?
system are arranged so the fluid to operate each
Try it first with a direct-acting control pilot.
circuit, except the one critical/vital circuit, must
As the controlled pressure (discharge pressure
from the diaphragm control valve) increases,
increased pressure is applied to the diaphragm of
the direct-acting control pilot. The valve stem is
Figure 6-12.--Arrangement of control pilot and diaphragm
Figure 6-13.--Priority valve.
control valve for supplying reduced-steam pressure.
6-8
