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3 Design and principle of oper-
ation
The Type 3253 is a three-way valve which
can be used either as a mixing or diverting
valve. The design of the mixing and divert-
ing valves in sizes DN 15 to 25 is identical.
The Type 3253 Valve is preferably combined
with a SAMSON Type 3271 or Type 3277
Pneumatic Actuator (see Fig. 3-1). It can also
be combined with other actuators.
The two seats (4), plug (5) and plug stem
(36) are installed in the body (1). The plug
stem is connected to the actuator stem (A7)
by the stem connector clamps (A26/27) and
is sealed by a V-ring packing (15). Alterna-
tively, an adjustable high-temperature pack-
ing can be used.
The springs in the pneumatic actuator are lo-
cated either above or below the diaphragm
depending on the selected fail-safe action
(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
Legend for Fig. 3-1 and Fig. 3-2
1
Body
2
Bonnet
4
Seat
5
Plug
8
Threaded bushing
(packing nut)
10
Lock nut
14
Nuts
EB 8055 EN
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 seats changes and determines
the flow rate through the valve.
In mixing valves, the process media to be
mixed enter at valve ports A and B. The
combined flow exits the valve at port AB (see
Fig. 3-1).
In diverting valves, the process medium en-
ters at the valve port AB and the partial
flows exit at ports A and B (see Fig. 3-2).
3.1 Fail-safe positions
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:
15
Packing
17
Body gasket
20
Seat body
36
Plug stem
60
Yoke assembly
92
Castellated nut
225
Nut
Design and principle of operation
A4
Diaphragm
A7
Actuator stem
A8
Ring nut
A10
Spring
A26/27 Stem connector
clamps
3-1
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