Page 387 - Boiler_Operators_Handbook,_Second_Edition

372 Boiler Operator’s Handbook

will open to admit air if the beam is rotated clockwise change of the signals where the output has to change
or close off the air supply and stop while the beam twice as much as the input. That’s the concept of propor-
continues to rotate counterclockwise. Further counter- tional control and in this case the controller has a gain
clockwise rotation will open the bleed end of the valve of two which means the output has to change twice as
to dump air to atmosphere. much as the input. Now the controller will run the water
Let’s start with proportional control. That’s where valve from closed to open with half the change in the
the output of a controller is proportional to the differ- output of the level transmitter, between 25% and 75% of
ence between the process value and the setpoint. As- the signal range.
sume we’re using the level transmitter covered earlier to We could increase the gain until there was very
produce the process value so, in this case, our controller little change in process value to produce a full stroke
will be used for level control. We’ll also assume the level of the water control valve so the water level would not
control valve is reverse acting so an increase in control- vary much. If we did that it wouldn’t work too well be-
ler output will close the valve. When the water level in cause any little ripple in the water level would produce
our tank increases the control signal decreases. To make a dramatic change in the valve position and we would
the system work any increase in process value should have a lot of valve wear. We would also have controller
result in a increase in output to close the valve. Now “noise” where the output is jumping around with little
we can look at the ratio totalizer to see how to connect relationship to the actual level in the tank.
the process variable. The output bellows pushes up on Conversely we could reduce the gain to something
the right side of the beam so any increase in output will less than one which would create another problem; the
tend to rotate the beam around the pivot in a counter- water valve would never close. It might work during
clockwise direction. It’s a pressure balance system so the normal plant operation but when the plant is shut down
process variable has to create a tendency to rotate the the controller output couldn’t increase enough to close
beam in the opposite direction to balance the forces. the valve. Too much gain produces a lot of noise and er-
If the beam tends to rotate clockwise more air is ratic operation while too little gain can result in failure
supplied to the output and output bellows to counter to operate at the extremes of load.
that rotation. If it tends to rotate counterclockwise the One problem with this controller arrangement is
vent valve opens to decrease the output. Connecting the we have no way to adjust the setpoint. For all practical
signal from the transmitter to the bottom bellows (A) purposes the setpoint is the center of the transmitter’s
does the job. Now a change in the level will produce a position. In order to have an adjustable setpoint we use
change in the output of the controller to open or close the B bellows of the controller and supply it with a con-
the control valve. As shown the controller acts pretty trol signal that is adjustable. The setpoint signal in this
much like a signal booster because it produces a change case is produced by a simple air pressure regulator. By
in output that precisely matches a change in input. As connecting the regulator to the bellows opposite the one
the level transmitter output changes from minimum sensing the signal from the transmitter we have devel-
to maximum the controller output produces the same oped a setpoint controller.
value because the bellows areas are identical. It works Now the output of the controller is proportional to
pretty much like our float controller, requiring the level the difference, what we call the error, between the set-
change over the full range of the float to position the point and the transmitted level signal. Instead of acting
control valve between open and closed. only on the pressure from the float transmitter the action
The whole reason for using a control system is to is dependent on the difference between the setpoint and
improve on the operation we get with a float control the process variable. The setpoint pressure acts on the
valve so let’s see what we can do with it. We can reduce diaphragm at B pushing down on the right end of the
the change in level by moving the pivot on the controller beam opposite the process variable signal coming in at
closer to the output end. It works just like a teeter totter. A. The force tending to rotate the beam is equal to the
Let’s adjust the controller so the distance from the center difference between the two pressures times the area of
of the process input to the pivot is twice the distance one diaphragm.
from the output bellows to the pivot (two thirds of the All modern controllers operate on the error, not
beam length). Now, if the level varies to produce a 1 psi the actual signal value. Now changes in output are
change in the transmitter output the controller output proportional to changes in the error, not changes in the
has to change by 2 psi to maintain the force balance in level. An important part of this to understand is you can
the controller. There is a proportional difference in the introduce an error by changing the setpoint. We’ll need