Design And Principle Of Operation; Fail-Safe Positions - Samson 3251 Mounting And Operating Instructions

Design and principle of operation

3 Design and principle of oper-
ation
The single-seated Type 3251 Globe Valve is
preferably combined with a SAMSON
Type 3271 or Type 3277 Pneumatic Actuator
(see Fig. 3). It can also be combined with
other actuators.
The seat (4) and plug with plug stem (5) are
installed in the body (1). The plug stem is
connected to the actuator stem (A7) by the
stem connector clamps (A26) and is sealed
by a spring-loaded V-ring packing (15). The
springs in the pneumatic actuator (A) are lo-
cated either above or below the diaphragm
(A4) depending on the selected fail-safe ac-
tion (see section 3.1). A change in the signal
pressure acting on the diaphragm causes the
plug to move. The actuator size is deter-
mined by the diaphragm area.
The medium flows through the valve in the
direction indicated by the arrow. A rise in
signal pressure causes the force acting on
the diaphragm in the actuator to increase.
The springs are compressed. Depending on
the selected direction of action, the actuator
stem retracts or extends. As a result, the plug
position in the seat changes and determines
the flow rate through the valve.

3.1 Fail-safe positions

The fail-safe position depends on the mount-
ed actuator.
Depending on how the compression springs
are arranged in the pneumatic actuator, the
valve has two different fail-safe positions:
14
Actuator stem extends (FA)
When the signal pressure is reduced or the
air supply fails, the springs move the actua-
tor stem downward and close the valve. The
valve opens when the signal pressure is in-
creased enough to overcome the force exert-
ed by the springs.
Actuator stem retracts (FE)
When the signal pressure is reduced or the
air supply fails, the springs move the actua-
tor stem upwards and open the valve. The
valve closes when the signal pressure is in-
creased enough to overcome the force exert-
ed by the springs.
Tip
The actuator's direction of action can be re-
versed, if required. Refer to the mounting
and operating instructions of the pneumatic
actuator:
u EB 8310‑X for Type 3271 and Type 3277
EB 8052 EN