10
EB 8139 EN
Design and principle of operation
3 Design and principle of oper-
ation
The pneumatic control valve consists of an
angle seat valve with a soft-seated plug and
a pneumatic piston actuator. Depending on
the version, the actuator can be equipped
with a mechanical manual override or an
electric limit switch.
The medium flows through the valve in the
direction indicated by the arrow. The signal
pressure applied to the piston actuator deter-
mines the position of the plug and thus the
cross-sectional area of flow between the seat
and plug.
The plug/actuator stem (14) is sealed by a
self-adjusting PTFE V-ring packing (22) at
the valve and by a radial shaft seal (25) at
the actuator.
3.1 Fail-safe action
The fail-safe position of the valve upon sup-
ply air (signal pressure) failure is determined
by how the piston and actuator spring are
arranged in the pneumatic actuator.
Fail-close (FA)
The actuator spring closes the valve upon air
supply failure. The valve opens when the sig-
nal pressure increases.
Fail-open (FE)
The actuator spring opens the valve upon air
supply failure. The valve closes when the sig-
nal pressure increases.
