MIL-HDBK-1022A
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.
3.6.10.1 Open/Close Operation. This is the most basic
function 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.
3.6.10.2 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. For proper
operation these valves should be installed with straight pipe
on both sides of the valve. Ten pipe diameters on the
upstream side and five diameters on the downstream side is
sufficient.
3.6.10.3 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
downstream outlet pressure exceeds the inlet pressure (which
normally holds the valve open), the valve will close and
prevent backflow. Note: In order for the valve to close it
must backflow, sometimes for a substantial amount of time.
Consider putting a regular check valve in series with this
valve in cases where this is a concern.
3.6.10.4 Two-Stage Flow Control Valves. Use flange-
connected, two-stage flow control valve of the desired
pressure and capable of slowing and stopping the flow
delivered by connection to a preset device, in accordance with
the applicable service requirements. Use a diaphragm-type
with a high flow and a low flow pilot valve. Equip the high
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