2
T 8313 EN
Principle of operation (actuator with integrated i/p converter)
The electric control signal issued by the controller as a refer
-
ence variable from 4 to 20 mA is transmitted to the i/p con
-
verter where it is converted into a proportional pressure sig
-
nal. The pressure signal creates a force that acts on the mea
-
suring diaphragm. This force is then compared with the force
of the range spring. The movement of the measuring dia
-
phragm is transmitted by the lever to the force switch, and fi
-
nally a corresponding signal pressure is produced.
A change in input signal causes the actuator stem to move to
the position determined by the reference variable. The flow
rate through the valve changes accordingly.
Principle of operation (actuator with mounted positioner)
Details on principle of operation are described in the Data
Sheet
u
T 8394 for Type 3725 Positioner.
15
i p
10
10.1
11
12
13
14
i
8.4
8.3
8.1
8.2
8.5
8.7
8.6
14
14
8.9
8.8
8.7
7
6
16
Legend for Fig. 5
6
Nut
8.6
Vent plug
12
Lever
7
Stem connector
8.7
Stopper (pressure gauge)
13
Range spring
8.1
Actuator stem
8.8
Plug connector
14
Zero adjuster
8.2
Rod-type yoke
8.9
Cover plate
(span and tight-closing function)
15
Force switch
8.3
Actuator springs
10
i/p converter
16
Rod nut
8.4
Rolling diaphragm
10.1 Potentiometer for span
8.5
Signal pressure connection
(actuator stem extends)
11
Measuring diaphragm
Fig. 5:
Type 3372 Electropneumatic Actuator with integrated i/p converter: functional diagram (left), actuator stem retracts (top right), actuator
stem extends (bottom right)
Tight‑closing function
The electropneumatic actuator is completely filled with air or
vented as soon as the reference variable falls below or ex
-
ceeds a certain value.
Actuator stem extends (FA)
When the reference variable falls below the switching point of
4.08 mA, the actuator is fully vented. This causes a mounted
globe valve to close. In three-way valves, port
B
is closed
when the valve is used for mixing service and port
A
is closed
when the valve is used for diverting service.
Actuator stem retracts (FE)
When the reference variable exceeds the switching point of
19.95 mA, the actuator is filled with air. This causes a mount
-
ed globe valve to close. In three-way valves, port
A
is closed
when the valve is used for mixing service and port
B
is closed
when the valve is used for diverting service.
