Page 388 - Boiler_Operators_Handbook,_Second_Edition

Controls 373

to set the gain to much more than two in this case or the
output may not change enough to fully stroke the valve.
Creating a setpoint controller allows us to use
something more than the level control range for the
transmitter so we can use the transmitter for instru-
mentation as well as control. We can put a long arm on
the float and produce an output signal proportional to
almost the full height of the tank so we can tell where
the level is even when it’s not in the control range. For
example, our level transmitter could be set to indicate
levels from zero to 60 inches in the tank. We select our
control range and adjust the controller gain accordingly.
If we want the level to control within ten inches we set Figure 11-6. Totalizer with reset accessories
the gain of the controller to six. If we establish a set-
point at fifty inches the control valve will be fully closed If this looks like an unmanageable concept don’t
when the level reaches 55 inches and closed at 45 inches. quit yet. Reset control does some great things and after
We can also adjust the setpoint to anywhere from five we get through this discussion you should be able to ap-
inches to 55. We have to reserve half the control range to preciate what it does.
have control, that’s why the setpoint can’t be anywhere Operation of reset control is difficult to compre-
within the range of the transmitter when we’re using hend and I’ve discovered many technicians have an
proportional control. If we raise the setpoint to, say 58 inappropriate perception of it because they think in
inches, then we will not be able to stroke the output terms of speed, not response to an error. The operation
valve completely. of the ratio totalizer provides a basis of understanding
By now you’re asking how good is a controller because the dynamic effects are apparent. Let’s start
that needs ten inches to work when it comes to con- with a steady state condition where the pressure in the
trolling the water level in a boiler. If we had to use the output bellows matches the pressure in the reset bellows
system described we would have to set the gain to sixty and the error is zero. Assume the process variable drops
in order to keep the level within an inch of setpoint. a little so an error is generated. The proportional func-
There’s two answers to that question, first we normally tion of the controller responds immediately, changing
use a maximum of twenty inches for the range of the the output an amount equal to the error times the gain.
boiler water level transmitter (even if the boiler is over Also assume the error isn’t corrected immediately by the
a hundred feet high) and we’ll add reset to our control- output change and holds. Since there is now a difference
ler. There are practical limits to an instrument’s range between the output and reset bellows control air bleeds
when it is used for control but reset control is a refine- through the needle valve to (or from) the volume cham-
ment that can only be described as beautiful; it makes ber and reset bellows. Since the error persists the output
the setpoint realistic. will have to continue to change to balance the error. If
To convert our controller to a reset controller we the error continued to exist the output would continue
add (Figure 11-6) some tubing, a needle valve, and a to change until it reached its practical limit (zero psig or
small volume chamber. It’s these reset accessories that supply pressure).
make our controller a reset controller. The controller has That doesn’t happen often, usually the controller
now acquired dynamic properties. The only time a reset action results in the process variable returning to the set-
controller will be in balance is when the setpoint and the point. That’s the beauty of reset control, it always works
process variable are precisely the same and the output to return the process variable to the setpoint, not some
has stopped changing. With our proportional controller value that’s offset by the proportional value. It’s real
the system could be stable with the level holding at a control. You can see that the only time the controller isn’t
value below or above the setpoint. Now the left side of changing its output is when the process variable and set-
the controller is in balance only when the process vari- point are exactly the same and the pressure in the output
able and setpoint are precisely the same, when the error and reset bellows equal each other. You can also see that
is zero. Even then the controller output can be changing, the controller can be in balance with any pressure at the
when the pressures inside the output and reset bellows output. The output signal can be anything from zero to
are different. supply pressure balanced by the same pressure in the re-