MIL-HDBK-1022
4.7.10.1 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.
4.7.10.2 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.
4.7.10.3 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.
4.7.10.4 Remote Operations. Hydraulically operated diaphragm
control valves can be operated remotely. This is accomplished
by installing tubing from the point of pressure sensing to the
valve or by using remote-controlled solenoids within the trim.
4.7.10.5 Materials of Construction. Use stainless steel or
aluminum (in non-contained areas) pilots and stainless steel
tubing. Use bodies, bonnets, and covers made of aluminum,
stainless steel, or internally plated (chromium or nickel)
cast steel. Provide viton or Buna-N diaphragm and disc ring.
Enclose all electrical apparatus according to classification
of the area in which they are installed. Provide a means to
wire seal all adjustable pilots. Do not use aluminum valves
within a contained area.
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