Page 384 - Boiler_Operators_Handbook,_Second_Edition

Controls 369

increases it’s output as process variable increases; a re- really proportional to what we’re measuring, what we
verse acting controller reduces its output as the process call the process value. Let’s modify our float valve and
variable increases. use compressed air instead of water. There are two ad-
Controllers like the one we just described are sel- vantages to using air over water, one is it has very little
dom found today because there are a few problems with weight so the weight of the air doesn’t alter our signal
water; it’s corrosive and contains solids that can eventu- value when the signal is piped up or down two or three
ally plug up the control orifices. In the prior example floors in the building.
dust from the atmosphere could get into the bucket and More importantly nobody complains when it leaks
close the drain hole to prevent the valve opening. We out. Let’s face it, people would complain about our
used to have hydraulic controls (which used oil instead water powered transmitter constantly pissing water
of water in closed systems) but their expense and prob- out but they don’t even notice the air. The air leakage,
lems with corrosion and leaking resulted in their having normally undetected, proved to be a considerably costly
a short period of acceptability. They were replaced by part of control systems.
pneumatic controls which survived several years before We also change the valve and float arrangement so
they were outstripped in price and function by micro- the float arm compresses a spring and the spring force
processor based electronic controls, the current choice is opposed by a bellows that contains our output pres-
as of the writing of this book. Electrical and electronic sure so we get a transmitter that looks like the one in
controls saw some use and a share of the control market Figure 11-3. We have moved the orifice from the bucket
along with pneumatic controls as well. to the air supply and created another one consisting of a
I lived through the era of sophisticated pneumatic nozzle. The nozzle discharging against a baffle becomes
control. It provided more accurate control at lower cost our valve (less expensive than a valve) and the valve
than earlier mechanical and hydraulic systems. We’re moves with the float arm because we want the output to
now living in the era of microprocessor based control. accurately represent the level in the tank. Flow through
Who knows what will follow? the valve doesn’t change based on position of the float, it
The system just described consisted of controller responds to differences between the position of the float
and control valve and is not consistent with modern and the balance of forces of the spring and the bellows.
control systems because the controller measured the This construction is typical of most pneumatic
process variable directly. A typical control system will transmitters. As the level increases the nozzle is moved
have a transmitter which produces a control signal away from the baffle so more air bleeds out at the noz-
proportional to the value of the measured variable, a zle. The pressure in the output bellows decreases so the
separate controller and a final element (control valve). spring pushes down on the float and up on the baffle,
We could relate the level of the water in the tank to the
level in the bucket but that will change as the drain hole
plugs or erodes and is also affected by the pressure drop
through the valve and other factors.
We could convert our float valve controller to a transmit-
ter by drilling a hole in the outlet piping to let the water
drain there and use the bucket as a reservoir. Installing a
pressure gage on the piping feeding the bucket provides
an indication of the output of our transmitter. The prob-
lem is our pressure transmitter can’t produce a control
signal that’s precisely proportional to the level in the
tank. A variation in the water supply pressure, wear in
the valve and drain orifice and friction in the valve pack-
ing will all combine to generate changes in the signal
that produce errors.
A desire for accuracy and, more importantly, re-
peatability resulted in the development of precision
transmitters by introducing feedback. Feedback is the
output and we use it to test or correct our output. In Figure 11-3. Pneumatic level transmitter and control
the case of a transmitter it’s used to ensure the output is valve