Design And Principle Of Operation; Fail-Safe Action - Samson 3254 Mounting And Operating Instructions

3 Design and principle of oper-
ation
The Type 3254 Valve is a single-seated globe
valve. This valve is preferably combined with
a SAMSON Type 3271 or Type 3277 Pneu-
matic Actuator (see Fig. 3-1). It can also be
combined with other actuators.
The seat (4) and plug with plug stem (5) are
installed in the body (1). An additional plug
stem guide exists in the bottom body flange
(100), which prevents the plug stem from
oscillating. The plug stem is connected to the
actuator stem (A7) by the stem connector
clamps (A26/27) and is sealed by a spring-
loaded V-ring packing (15). The springs in
the pneumatic actuator are located either
above or below the diaphragm (A4)
depending on the selected fail-safe action. A
change in the signal pressure acting on the
diaphragm causes the plug to move. The
actuator size is determined 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.
Tip
We recommend the use of positioners with
integrated diagnostic firmware (see sec-
tion 3.4) for valves used for on/off service.
The partial stroke test included in this soft-
EB 8060 EN

Design and principle of operation

ware helps prevent a shut-off valve normally
in its end position from seizing up or getting
jammed.

3.1 Fail-safe action

The fail-safe position of the control valve up-
on air supply or control signal failure de-
pends on the actuator used (see associated
actuator documentation).
Depending on how the compression springs
are arranged in the SAMSON Type 3271
and Type 3277 Pneumatic Actuator, the
valve has one of two different fail-safe posi-
tions:
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
3-1