Page 381 - Boiler_Operators_Handbook,_Second_Edition

366 Boiler Operator’s Handbook

rent, or a bunch of electrical charges in a tiny microchip we’re trying to control (process variable) is sensed by
that we relate to as ones and zeros. If they don’t know the controller which compares that value to the setpoint
any more than that we shouldn’t have any problem then adjusts its output accordingly. The change in out-
understanding them. Understanding controls isn’t that put produces a change in the process variable and the
difficult, our controls can be used in any application, not controller compares that new value to the setpoint to
just boiler plants. The really wise boiler operator will be change its output again. A control loop contains a con-
able to relate to how the controls work with the boiler troller, a device to measure the process variable, a source
and its auxiliaries. of setpoint, an output device that controls the flow and
Let’s start with parameter, it’s a quantity, value, anything else that changes the value of the process vari-
or constant whose value varies with the circumstances able or the setpoint.
of the system. The controller doesn’t know what the A loop can be as simple as a level controller con-
parameter is and it doesn’t care. It can be pressure, tem- sisting of controller with internal setpoint adjustment, a
perature, level, count, pH, oxygen content in percent, level transmitter and control valve to similar devices in
differential pressure, a flow of any fluid, a weight, etc. combination with a large number of computers located
The controller basically deals with parameters that are in different parts of the plant. The practical limit of a
called inputs and they are used to create an output, or loop is at the devices that affect the process variable and
outputs. Inputs are assigned names that indicate what any one of those devices can be part of another control
they are in relation to the controller with the two most loop.
important ones being process variable and setpoint. There is always a control range. The values the
The process variable is a value representing the controllers use have an upper and lower limit. The range
measurement of whatever it is you are trying to main- of transmitters has to be established to permit reason-
tain. If it’s a pressure controller it’s the pressure. If it’s able control and allow for the normal variations in the
a level controller it’s the level. It’s the control system’s measured parameter. A range is selected by the applica-
representation of the actual value of the parameter tions engineer (the gal or guy that selects and specifies
you’re trying to control. the controls to be used on a job) to ensure the system will
The setpoint is a value representing what you want control properly. What’s the big deal? It’s a question of
the process variable to be. If you want the boiler pres- accuracy and stability.
sure to hold at 100 psig you adjust the setpoint until the If you are operating a low pressure steam plant,
parameter represents 100 psig. When properly applied then a transmitter producing a signal in the range of 0 to
the controller will indicate it is set at 100 psig and you 30 psig can maintain a pressure of 10 psig plus or minus
don’t even have to know what the actual value is. Set- 0.15 psi because the transmitter (which typically has an
points are not always set by you; a setpoint can be the accuracy of ± 1/2 %) will produce a signal that accurate.
output of another controller. On a plant operating at 3,000 psig a 0 to 4,000 psi trans-
We normally describe a setpoint as being “local” mitter would be accurate to ± 20 psi and that wouldn’t
when you can adjust it and “remote” when it’s the out- necessarily be considered accurate control. So the engi-
put of another controller. Note that the terms don’t relate neer might use a transmitter that works in the range of
to what you would consider as local and remote. If you 2,500 to 3,500 psig to get a transmitter accurate within 5
have to leave the boiler plant and go around the main psi.
building to the shed under the water tower to adjust the Control signals also have a range and each system
setpoint of the tower’s water level controller it’s still a normally uses the same signal range for all the devices
local setpoint even though it’s remote from the boiler in the system. There are many standard ranges of con-
plant. If it was a pneumatic setpoint you could install a trol signals with the most common ones being 3-15 psig
little regulator and tubing in the boiler plant (local) and (pneumatic), 0 to 5 volts (electric, electronic), 4 to 20
extend the tubing out to the shed under the water tower milliamps (electronic). Several other signal ranges were
to produce a… well, I’m sorry to say it but it’s still a local used and it’s not uncommon to encounter a mix of these
setpoint. As you’ll see later, a remote setpoint can come ranges within systems that are a mix of old and new in-
from a controller right beside the next one in the control struments and controllers. Other signal ranges you may
panel so don’t confuse local and remote with location. encounter are 0 to 30 psig, 0 to 60 psig, and 3 to 27 psig
Now we can define a loop. We use the term loop to pneumatic, 0 to 10 volt, -5 to + 5 volt, 0 to 12 volt, and
describe parts of a control system because each control 0 to 24 volt values on electrical and electronic systems.
loop is like a circle; there’s no end to it. The parameter There are others but their use is industry specific and