Principle Of Operation; Overall Operation - Emerson Fisher KC Series Instruction Manual

Instruction Manual
Form 5207
April 2007
AUTOMATIC
POSITION
OUTPUT PRESSURE
TO FINAL CONTROL
ELEMENT
RELAY OUTPUT PRESSURE
SUPPLY PRESSURE
MANUAL LOADER OUTPUT PRESSURE
48A5230-A
A2999-1 / IL

Principle of Operation

Overall Operation

Refer to the schematic diagram in figure 5-4.
The input element is connected to the process
pointer and to the flapper by connecting links. As the
process pressure increases (in a direct-acting
controller). This movement restricts the flow through
the nozzle and increases nozzle pressure. When
this occurs, relay action increases the output
pressure (delivery) of the controller. Output pressure
is fed back to the positive feedback bellows. The
action of this bellows is a positive feedback action
that moves the flapper closer to the nozzle,
increasing nozzle pressure, which in turn, increases
the relay output. Output pressure to the final control
element switches to full supply pressure.
As the process pressure decreases, approaching
the lower switching point, the flapper moves away
from the nozzle (in a direct-acting controller)
reducing nozzle pressure. Through relay action,
pressure to the positive feedback bellows is
4195KA, KB, KC, and KS Controllers
AUTO/MANUAL
SWITCH
RELAY
SUPPLY
PRESSURE
Figure 5-5. 4195KS Series Auto/Manual Schematic
MANUAL LOADER
PLASTIC TUBE
METAL BALL
OUTPUT PRESSURE
TO FINAL CONTROL
ELEMENT
reduced, moving the flapper further away from the
nozzle, and further reducing nozzle pressure. Output
pressure to the final control element switches to
zero.
The set point adjustment changes the proximity of
the nozzle and flapper as does a change in process
pressure except that, when the set point is changed,
the nozzle moves with respect to the flapper. The
set point adjustment moves both the upper and
lower switching points.
The proportional band knob positions the nozzle on
the flapper. Increasing (widening) the proportional
band moves the nozzle away from the input
connection. When the proportional band adjustment
moves the nozzle across the feedback connection,
the controller action changes between direct and
reverse. On a direct-acting controller, changing the
proportional band adjustment will widen or narrow
the differential gap between the two switching
points. This is accomplished by moving the position
of the lower switching point. On a reverse-acting
controller, changing the proportional band
adjustment will widen or narrow the differential gap
MANUAL
LOADER
KNOB
MANUAL
POSITION
AUTO/MANUAL
SWITCH
5-7