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2502 Series
2. Set the percent proportional band control to 200.
3. Set the reset control to .05 minutes per repeat.
4. Slowly open the downstream and upstream manual
control valves in the pipeline and close the manual
bypass valve if one is used.
5. With the controller set at the desired control point,
narrow the proportional band until a cycling condition
exists. Then broaden the proportional band slightly
until stable control is obtained.
6. Adjust the reset control to obtain the highest reset
setting without introducing cycling.
7. To ensure that the optimum proportional band and
reset settings have been obtained, momentarily create
a load upset. If cycling occurs, broaden the proportion-
al band slightly and repeat the load upset until stability
is attained. In general, the narrowest proportional band
and the highest reset setting that will not produce cycl-
ing will provide the best control.
Principle of Operation
All 2502 Series controllers use the same basic pres-
sure-balanced relay with a yoked double-diaphragm
assembly (figure 12). This relay is connected so that
supply pressure is fed to the inlet side of the relay
valve and to the fixed restriction. From this restriction,
the air pressure goes into the relay chamber on the
side of the large diaphragm, and to the nozzle. As long
as there is no pressure change on either diaphragm,
the relay valve remains in equilibrium with both the
inlet and exhaust ends closed.
The area ratio of the large diaphragm to the small dia-
phragm is 3 to 1. A 12 psig (0.8 bar) pressure change
on the small diaphragm need only be balanced by a 4
psig (0.3 bar) change on the large diaphragm.
A change in liquid level, interface level, or density
changes the buoyant force exerted on the sensor dis-
placer, which in turn imparts a rotary motion through
the torque tube shaft. The rotary motion is applied to
the controller, which uses a nozzle, bellows, and pneu-
matic relay to convert the rotary motion to a standard
pneumatic output signal. The output signal is sent to a
final control element. In conjunction with this control
element, 2502-249 Series controller-sensors are capa-
ble of bringing the controlled variable back to a specif-
ic control point all the time.
The following descriptions show how the relay works
in conjunction with the standard proportional-plus-reset
controller, and how the reset relief valve construction
works.
14
Type 2502 Controller
As long as inflow and outflow of the vessel are equal,
the beam and flapper remain motionless and allow sup-
ply pressure to bleed through the nozzle as fast as it
enters the relay through the fixed restriction. A level or
density change either raises or lowers the displacer and
pivots the beam and flapper with respect to the nozzle.
An increase in level or density with direct action, or a
decrease with reverse action, moves the beam and
flapper closer to the nozzle and restricts the escape of
supply pressure. This builds up the loading differential
on the side of the large diaphragm and opens the relay
valve to supply pressure inflow.
On the other hand, a decrease in level or density with
direct action, or an increase with reverse action,
moves the beam and flapper away from the nozzle
and permits supply pressure to bleed through the
nozzle faster than it can enter through the fixed restric-
tion. This builds up the loading differential on the side
of the small diaphragm, and opens the relay valve to
exhaust loading pressure.
The three-way proportional valve can be opened and
adjusted to allow some or all of the output pressure
change to feed back to the proportional bellows in or-
der to change the proportional band of the controller.
This pushes the beam and flapper opposite the way it
is being pivoted by the torque tube shaft, counteract-
ing the pressure change in the nozzle and again stabi-
lizing the relay diaphragm pressure differential. The
relay valve shuts off and maintains a new output pres-
sure according to the change in sensed displacer posi-
tion.
A wide-open proportional valve permits feedback of all
the output change and produces 100 percent propor-
tional response. Closing of this valve produces smaller
proportional responses, since part of the output
change is vented through the valve exhaust and only
the remainder is available to reposition the bellows.
The reset valve can be adjusted to channel some or all
of the proportional pressure into a reset bellows that
opposes proportional bellows action. This automatical-
ly dampens the effect of any proportional overcorrec-
tion by a set amount per time interval, as long as there
is a deviation from the control point.
Figure 12 illustrates these principles at work in a di-
rect-acting right-hand-mounted construction controlling
liquid inflow to a vessel, by means of a direct-acting
diaphragm-actuated control valve. Nozzle positions
and bellows connections would be reversed for direct
action with left-hand mounting or reverse action with
right-hand mounting.
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