Samson 3767, Translation Of Original Instructions

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EB 8355-2 EN 3-1
Design and principle of operation
3 Design and principle of oper -
ation
Î
See Fig. 3-1
The positioner consists of an electropneumat-
ic converter and a pneumatic unit equipped
with a lever for travel pick-off, a measuring
diaphragm and the pneumatic control system
with nozzle, diaphragm lever (flapper plate)
and booster.
The positioner is designed either for direct
attachment to SAMSON Type 3277 Actua -
tors or for attachment to actuators according
to NAMUR (
IEC 60534-6 ) using an adapter
housing.
The positioner can be additionally equipped
with either inductive limit switches and/or a
solenoid valve or position transmitter.
The control signal, e.g. 4 to 20 mA, issued
by the controller is transmitted to the electro-
pneumatic converter (13) where it is convert-
ed into a proportional pressure signal p
e
.
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 deter-
mines the force of the range spring (4). This
force is compared to the positioning force
generated by the pressure p
e
at the measur-
ing 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 adjusted di-
rection of action of the positioner.
The supply 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)
and hits the diaphragm lever (flapper plate).
Any change in the set point or the valve po-
sition causes the pressure to change up-
stream or downstream of the booster.
The air controlled by the booster (signal
pressure p
st
) flows through the volume restric -
tion (11) to the pneumatic actuator, causing
the plug stem to move to a position corre-
sponding to the set point.
The adjustable X
p
restriction (8) and volume
restriction (11) are used to optimize the posi -
tioner 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 set point.
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 sole-
noid valve, the valve can be moved to the
fail-safe position, regardless of the position-
er's output signal. If a control signal corre-
sponding 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 actu-
ator springs move the valve to its fail-safe
position.
When a control signal corresponding to the
binary signal '1' (ON) is applied to the in-
put, the signal pressure p
st
is applied to the
actuator, allowing the valve to move accord-