Samson 3271, Монтаж и эксплуатация, Страница: 12 / 36

12 EB 8310-8 EN
Design and principle of operation
3 Design and principle of oper-
ation
The Type 3271 Pneumatic Actuators with
1400-250 cm², actuator area contain a roll
-
ing diaphragm (A4) and internal springs
(A10). They are mounted on SAMSON Se
-
ries 240, 250, 280, 290, and 590 Valves.
The signal pressure p
st
creates the force F =
p
st
· A at the diaphragm surface A which is
opposed by the springs (A10) in the actua -
tor. The bench range is determined by the
number of springs used and their compres
-
sion, taking into account the rated travel. The
travel is proportional to the signal pressure
p
st
. The direction of action of the actuator
stem (A7) depends on how the springs are
installed in the actuator.
The stem connector clamps (A26/27) con
-
nect the actuator stem (A7) with the plug
stem of the valve. A support stem (A174)
and support bearing (A173) are screwed
onto the actuator stem.
The actuators are fitted with an anti-rotation
fixture (A88, A127). Guide rods (A170) are
installed to stabilize the springs.
3.1 Direction of action
The direction of action is determined by how
the springs and diaphragm plate are ar
-
ranged in the actuator. With direction of ac -
tion "actuator stem retracts", the compressed
air is applied to the signal pressure connec
-
tion on the top diaphragm case. With direc -
tion of action "actuator stem extends", the
compressed air is applied to the signal pres
-
sure connection on the bottom diaphragm
case.
3.2 Signal pressure routing
In the "actuator stem extends" version, the
signal pressure is routed through the bottom
signal pressure connection (S) to the bottom
diaphragm chamber and moves the actuator
stem (A7) upward opposing the spring force.
In the "actuator stem retracts" version, the
signal pressure is routed through the top sig
-
nal pressure connection (S) to the top dia -
phragm chamber and moves the actuator
stem (A7) downward opposing the spring
force.
3.3 Fail-safe action
When the signal pressure is reduced or the
control signal fails, the fail-safe action of the
actuator depends on whether the springs are
installed in the top or bottom diaphragm
chamber.
3.3.1 Version with direction of
action "actuator stem
extends" (FA)
When the signal pressure is reduced or the
control signal fails, the springs move the ac
-
tuator stem downward and close the globe
valve. The valve opens when the signal pres
-
sure is increased enough to overcome the
spring force.