Samson 3372 - Page 2

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 -
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 ow
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 lled 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 lled 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.