Samson 3372 User Manual page 2

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.
8.5
15
Legend for Fig. 5
6 Nut
7 Stem connector
8.1 Actuator stem
8.2 Rod-type yoke
8.3 Actuator springs
8.4 Rolling diaphragm
8.5 Signal pressure connection
(actuator stem extends)
Fig. 5: Type 3372 Electropneumatic Actuator with integrated i/p converter: functional diagram (left), actuator stem retracts (top right), actuator
stem extends (bottom right)
2
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.
8.3
8.4
10
10.1
i p
i
11
12
13
14
8.2
8.1
8.6 Vent plug
8.7 Stopper (pressure gauge)
8.8 Plug connector
8.9 Cover plate
(span and tight-closing function)
10 i/p converter
10.1 Potentiometer for span
11 Measuring diaphragm
8.7
8.8
8.9
14
7
6
16
12 Lever
13 Range spring
14 Zero adjuster
15 Force switch
16 Rod nut
14
8.7
8.6
T 8313 EN