MIL-HDBK-1022A
approved by Service Headquarters. Consider using locking tabs
on valves to allow padlocks to be used to lock out valves
during maintenance.
6.5.3
Hydraulically Operated Diaphragm Control Valves.
Diaphragm-operated globe valves are used extensively in
fueling systems as control valves. These valves consist of a
main valve and a pilot control system. The valve is operated
by varying the amount of pressure above the diaphragm. Since
the chamber above the diaphragm exposes a greater area of the
diaphragm to chamber pressure than the area of the disc
exposed to line pressure, an equal pressure in the chamber and
pipeline results in a greater force being applied to the top
of the disc. This forces the disc against the seat, thus
closing the valve. By selecting the proper pilot control
system, these valves can be used in numerous ways to control
flow, pressure, and level within fueling systems. Use extreme
care when including these valves on pipelines as they can
significantly contribute to surge potential, if closing time
is too short. When properly adjusted, they can reduce surges.
6.5.3.1
Open/Close Operation. This is the most basic
operation of hydraulically operated diaphragm control valves.
The operation is accomplished by applying pressure above the
diaphragm to close the valve and relieve that pressure to
allow line pressure to open the valve. The pilot trim used to
perform this operation is a three-way valve which can be
controlled by a solenoid, hand, pressure, pressure
differential, or a float.
6.5.3.2
Throttling. This is the other main method of
controlling the hydraulically operated diaphragm control
valve. In this case, the valve modulates to any degree of
opening, in response to changes in the throttling control.
The throttling control reacts to a pressure differential
across the main valve or across an orifice plate to regulate
the position of the disc in the main valve.
6.5.3.3
Check Valve Function. This is a unique function of
a control valve. In this case, the main valve outlet pressure
is connected to the diaphragm cover. Therefore, if the
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