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To check that the output pressure rises to max in
specified time:
Start the test with sensing pressure 20% below setpoint (i.e.,
if the setpoint is 200 psig lower it to 160 psig). The output pressure
should fall to zero. Allow the PID chamber to empty by waiting
1-2 minutes. Raise the sensing pressure to 3% of the maximum
spring range above the setpoint (i.e., if the setpoint is 200 psig and
the top of the control spring’s range is 32 psig, set your pressure
to 200 + .03 x 320 = 209.6 psig, approximately) Start your timer.
The output pressure should jump up a little and then continue to
rise steadily. Stop your tier when the output pressure has risen to
approximately 90% of its full range (i.e. ˜112.5% for a PID-125).
This should take 10-20 seconds if the integral valve is wide open,
and 1-2 minutes if it is at #7.
4. Dead Band Adjustment Drum
VRP-SB-PID controllers are factory adjusted with “zero” dead
band. “Zero” dead band is defined such that when controller
is at steady state there is a small growing bubble (10-30 seconds)
from exhaust port. For some systems this dead band can be
changed. On the adjusting drum between each whole number
(large markings) are 3 small markings. Turning the adjusting
drum 1 marking to the right (in direction of increasing numbers)
decreases the dead band from “zero” to -1/4 (negative one-fourth)
and introduces a small bleed gas. This action will increase
controller sensitivity. Turning the adjusting drum 1 marking
to the left (in direction of decreasing numbers) increases the
dead band from “zero” to +1/4 (positive one-fourth). Another
example is +1/2 or –1/2 dead band. This simply means that from
the dead band “zero” setting the adjustment drum is moved
2 markings left (+1/2) or 2 markings right (-1/2). This important
terminology is useful when setting different PID models using
the example applications section (pages 15-20). After adjustment
is completed, tighten the set screw to a torque of 1-2 in-lb to lock
the deadband setting.
Changes in the dead band (sensitivity) adjustment affect
setpoint adjustments. Each mark represents a 1/44 rotation
of the adjusting screw. When dead band is decreased, (bleed
gas introduced) setpoint is decreased. When dead band is
increased, setpoint is increased. Using tables 3-5, refer to
setpoint change per revolution. To find out the setpoint
change divide the specified value by 44:
Example Calculation
VRP-600-SB-PID-40, yellow spring = 85 psig per revolution.
Each division on the drum represents 85/44 or 1.9 psig change
in setpoint.
Derivative Orifice
Refer to a recommended derivative orifice and controller gain
for your specific application. Derivative orifice can be adjusted
by turning between numbers 0 and 6. “0” represents the fastest
response of the controller and “6” the slowest. Three orifice sizes
are available: small (not marked), medium (marked “M”) and large
(marked “L”). Small orifice represents the fastest response, large
the slowest.
Integral (Reset) Valve
The reset metering valve can be adjusted between 4 and 15 on
the outer barrel. Four represents the smallest reset value (the
slowest correction rate).
Proportional Band Gain
Each model is available with three gains. The feedback module
can be used without rings for the highest proportional band or
with sets of rings. Middle gain controllers use large rings in the
feedback chamber and high gain controllers use small rings.
By referring to the spring range charts on pages 7 - 9, along
with initial settings from table 8, the desired proportional band
is selected. This selection determines which gain rings are
needed if any. Controllers are normally shipped with high
gain rings.
Applications
The Becker VRP-SB-PID configurations and their operative
principles have been explained in detail. At this point we can
summarize three advantages in the application of the
VRP-SB-PID:
1) VRP-SB-PID is a three mode controller (proportional-
integral-derivative) which complements the VRP-SB-CH. It is
available in four configurations: a full direct or reverse acting
PID, and a direct or reverse acting proportional plus derivative
(PD) controller.
2) VRP-SB-PID is used for short systems (power plants, double
stage cuts, etc.) where proportional feedbackfunction (P) is
necessary to avoid cycling. For large systems VRP-SB-CH
should be used.
3) A VRP-SB-CH controller can be converted to a VRP-SB-PID
easily in the field simply by adding a feedback module.
The next question is, “What are typical applications for
the controller, and how do the different configurations
come into play?”
To illustrate this very important question, 6 examples of typical
applications will be given. In addition to the total schematic
layout of the example application, critical initial orifice/metering
valve adjustments are given to assist with installation. Before
the application examples, however, a general view for selection
of initial gain and size of the derivative orifice for specific valve
applications can be seen in Table below.
Table 8 - Initial Recommendations for Control Valve
Application
Initial Gain
Recommended
Derivative
Orifice
Reset
Metering
Value
Globe valve
w/spring and
diaphragm
(w/o positioner)
High
(Rings
w/small hole)
S=3
7
Globe valve
w/positioner
Low
(No Rings)
L=3
7
Ball valve
w/spring and piston
(w/o positioner)
w/volume booster
High
(Rings
w/small hole)
S=3
7
Ball valve
w/positioner
High
(Rings
w/small hole)
L=3
7
