Samson 3251-E, Mounting And Operating Instructions

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EB 8052-E EN 3-1
Design and principle of operation
3 Design and principle of oper -
ation
The Type 3251-E Valve is a single-seated
globe valve. This valve is preferably com-
bined with a SAMSON Type 3271 or
Type 3277 Pneumatic Actuator. It can also
be combined with other actuators.
The body can be fitted with three different
trims:
− With clamped-in seat and plug (see
Fig. 3-1)
− With screwed-in seat and plug (see
Fig. 3-2)
− With piston and cage trim (see Fig. 3-3)
All the trims are interchangeable in the field
without any revisions necessary to the pres-
sure-bearing or pressure-retaining parts.
The plug or piston stem (5.1) is connected to
the actuator stem (A7) by the stem connector
clamps (A26/A27) and sealed by a
spring-loaded V-ring packing (15) or a
cord-ring packing (15). The springs (A10) in
the pneumatic actuator are located either
above or below the diaphragm (A4) de-
pending on the selected fail-safe action. A
change in the signal pressure acting on the
diaphragm causes the plug (5) or piston (5)
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 po-
sition of the plug (5) in the seat (4) or the po-
sition of the piston (5) in the cage (124)
changes and determines the flow rate
through the valve.
We recommend the use of positioners with
integrated diagnostic firmware (see
section 3.5) for valves used for on/off
service. The partial stroke test included in this
software 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 (A10) move the
actuator stem (A7) downward and close the
valve. The valve opens when the signal pres-
sure is increased enough to overcome the
force exerted by the springs.
Tip