Samson 3321CT, Mounting And Operating Instructions

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EB 8115 EN 3-1
Design and principle of operation
3 Design and principle of
operation
The single-seated Type 3321CT Globe Valve
is preferably combined with a SAMSON
Type 3379 Pneumatic Actuator.
The seat (2) and plug with plug stem (3) are
installed in the body (1). The plug stem has a
thread to screw it onto the actuator stem and
is sealed by the spring-loaded packing (8).
The process medium flows through the valve
in the direction indicated by the arrow in the
flow-to-open direction. The plug position de-
termines the cross-sectional area between
the seat and plug.
The position of the plug is changed by the
control signal acting on the actuator.
3.1 Fail-safe action
Depending on how the compression springs
are arranged in the actuator, the control
valve with the SAMSON Type 3379 Pneu-
matic Actuator has one of two fail-safe posi-
tions that become effective when the supply
air fails:
− Actuator stem extends (FA)
When the signal pressure is reduced or
the air supply fails, the springs move the
actuator stem downward and close the
valve. The valve opens when the signal
pressure is increased enough to over
-
come the force exerted by the springs.
− Actuator stem retracts (FE)
When the signal pressure is reduced or
the air supply fails, the springs move the
actuator stem upwards and open the
valve. The valve closes when the signal
pressure is increased enough to over
-
come the force exerted by the springs.
3.2 Additional fittings
Strainers
We recommend installing a SAMSON
strainer upstream of the valve. It prevents sol -
id particles in the process medium from
damaging the valve.
Bypass and shut-off valves
We recommend installing a shut-off valve
both upstream of the strainer and down -
stream of the valve and installing a bypass
line. The bypass ensures that the plant does
not need to be shut down for service and re
-
pair work on the valve.