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3 Design and principle of oper-
ation
See Fig. 3‑1
The seat (4) and plug with plug stem (5) are
installed in the body (1). The plug stem is
screwed to the plug stem with bellows seal
(37). The plug stem is sealed by the metal
bellows and the packing (15). The stem con‑
nector clamps (A26/27) connect the actua‑
tor stem (A7) of the mounted actuator.
The bellows seal (22) is fitted with a test con‑
nection (42). It can be used to monitor the
sealing ability of the bellows.
A strainer installed upstream of the valve
body. It prevents solid particles in the pro‑
cess medium from damaging the valve. The
strainer is not required when a central strain‑
er facility is installed.
The springs in the pneumatic actuator are lo‑
cated either above or below the diaphragm
depending on the selected fail‑safe action. A
change in the signal pressure acting on the
diaphragm causes the plug to move. The ac‑
tuator size is determined by the diaphragm
area.
The process medium flows through the strain‑
er and the valve in the direction indicated by
the arrow. A rise in signal pressure causes
the force acting on the diaphragm in the ac‑
tuator to increase. The springs are com‑
pressed. Depending on the selected direction
of action, the actuator stem retracts or ex‑
tends. As a result, the plug position in the
seat changes and determines the flow rate
through the valve.
EB 8020 EN
Design and principle of operation
The signal pressure p
valve whose coil is connected to the safety
interlock circuit (switch 14, Fig. 3‑2 and
Fig. 3‑3). During operation, the coil is ener‑
gized and the signal pressure p
diaphragm. In the event of a malfunction,
the pilot valve is activated and the pressure
acting on the diaphragm is relieved, causing
the control valve to quickly close.
is applied to the pilot
st
acts on the
st
3-1
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