Samson Type 3766 Mounting And Operating Instructions - Design and Principle of
1
Design and principle of
operation
The positioners ensure a fixed assignment
between the valve stem position (controlled
variable x) and the pneumatic or electric in-
put signal (reference variable w). They com-
pare the input signal received from the con-
trol unit with the travel of the control valve
and, issue the corresponding output signal
pressure p
st
(output variable).
The positioner consists of a lever for travel
pick-up, a measuring diaphragm and the
pneumatic control system with nozzle, dia-
phragm lever (flapper plate) and booster.
The positioner is designed either for direct
attachment to SAMSON Type 3277 Actua-
tors or for attachment according to NAMUR
(IEC 60534-6-1) with an adapter housing.
The positioner can be additionally equipped
with either inductive limit switches and/or a
solenoid valve or a position transmitter.
The positioner operates according to the
force-balance principle. The valve travel, i.e.
the valve position, is transmitted to the
pick-up lever (1) over the pin (1.1) and de-
termines the force of the measuring spring
(4). This force is compared to the positioning
force generated by the pressure p
e
at the
measuring diaphragm (5).
If either the control signal or the valve posi-
tion changes, the diaphragm lever (3)
moves, altering the distance to the nozzle
(2.1 or 2.2), depending on the set operating
direction of the positioner.
The air is supplied to the booster (10) and
the pressure regulator (9). The controlled
supply air flows through the X
p
restriction
(8) and the nozzle (2.1, 2.2) to finally
stream on the diaphragm lever (flapper
plate). Any change in the reference variable
or the valve stem position cause the pressure
to change upstream or downstream of the
booster.
The air controlled by the booster (signal
pressure p
st
) flows through the volume re-
striction (11) to the pneumatic actuator,
causing the plug stem to move to a position
corresponding to the reference variable.
The adjustable X
p
restriction (8) and volume
restriction Q (11) are used to optimize the
positioner control loop.
The pick-up lever (1) and the range spring
(4) must be selected to match the rated valve
travel and the nominal span of the reference
variable.
Positioner with inductive limit switches
In this version, the rotary shaft of the
positioner carries two adjustable tags which
actuate the built-in proximity switches.
Positioner with solenoid valve
When the positioner is equipped with a so-
lenoid valve, the valve can be moved to the
fail-safe position regardless of the
positioner's output signal. If a control signal
corresponding to the binary signal '0' (off)
is applied to the input, the signal pressure
p
st
is shut off and the actuator is vented. The
actuator springs move the valve to its
fail-safe position. If a control signal corre-
sponding to the binary signal '1' (on) is ap-
plied to the input, the signal pressure p
st
is
supplied to the actuator. The valve is in
closed-loop operation.
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EB 8355-1 EN
Design and principle of operation