Hi there, and welcome to this lecture on process flow diagrams: simple control loops.
My name is Marina Miletic.
Let's discuss adding simple control loops to process flow diagrams.
Properly controlling a process is critical for making sure it's running safely, efficiently,
and effectively.
It's impossible to run a complex process safely and effectively without process control.
This is because for robust control you can maintain predictable and consistent conditions
within major pieces of equipment.
Having proper control means the process will yield a predictable quantity and concentration
of final products.
Any variability from the desired amount and composition of the final streams means money
is being lost.
Because either a product is not meeting specifications or resources are being wasted creating a product
which is above specifications.
Both of these situations, especially at the extremes need to be avoided because they result
in wasted money.
Adding simple control loops to a process flow diagram is not absolutely necessary, but it
is useful for identifying important areas where controllable variables should be properly
maintained in their correct range.
These areas of the process have the potential to create runaway reactions, unwanted side
reactions, or change the composition of the process stream considerably.
Units like reactors and separators are particularly influential to the process, since one upset
can lead to dramatic changes in the product.
This is why we must identify these important and potentially vulnerable areas in a process
flow diagram.
So we're aware of the likely origins of unexpected problems.
There are four main variables which are usually controlled in a process: temperature, pressure,
level, and flow rate.
These four variables are very important, because when they're properly controlled in a well-designed
process, with correctly sized equipment, hundreds of tons of product can consistently be made
24 hours a day, 7 days a week.
Many students like to point out that concentration is likely the most important variable which
needs to be controlled in a process, yet it's not listed here.
They're correct in saying that achieving the desired concentration in a process is instrumental,
and a product that does not meet specifications cannot be sold.
However, in a chemical process, concentration is almost always a dependent variable.
What this means is that the desired concentration is reached by designing a process correctly,
and by controlling temperature, pressure, level, and flow rate.
While concentration is certainly measured sometimes throughout the process, it is oftentimes
not controlled directly.
Let's next look at some examples of simple temperature, pressure, level, and flow rate
control loops.
You'll notice that these examples feature feedback control, and that feedforward systems
are not usually shown on a process flow diagram.
These are simplified control loops, and to see more detail, you'll need to look at a
piping and instrumentation diagram of this process.
Since we're specifying simple control loops, we'll only be indicating three things, what
kind of controller is being used, where is the controller is receiving its signal, and
how the controller is controlling the variable, whether it's by opening and closing a valve,
varying the power to a pump or compressor motor, or changing the flow rate of an inlet
stream.
Let's look at at least one example of each of the four types of simple control loops
so you can see how these are officially depicted and where you would most likely place them
in a process flow diagram.
Let's first look at a simple temperature control loop for a distillation column.
Temperature is a critical variable to control to avoid explosions, runaway reactions, and
unwanted reactions.
We'll see how we can set up E-104, so it can be controlled to deliver a specific inlet
temperature for the column T-101.
So the purpose of this feedback loop is to control the feed temperature going to T-101
to help the distillation column achieve consistent separation.
Let's take a closer look at E-104 and T-101, and we'll start by taking away some labels
so you can see the streams more easily.
This stream's temperature must be controlled.
So we'll draw a line signifying that we're measuring the temperature and sending a signal
to a temperature indicator controller, abbreviated TIC.
The indicator is not absolutely necessary, but it's used to indicate or display the temperature
and the setpoint.
We could have just as easily named this a temperature controller, which means it's a
controller with no indicator.
The controller's job is to process a signal and decide if the temperature is too high,
too low, or in an acceptable range.
If it's too high or too low, the controller will send a signal which will open or close
a control valve slightly.
Let's talk for a moment about this valve and what it's doing.
This control valve's job is to increase the flow of Freon if the distillation column inlet
temperature is too high, and decrease the Freon flow rate if it's too low.
Keep in mind that, because of the size of this process and the high flow rate of the
refrigerant in the process stream, so long as this process is in operation, Freon will
always be flowing through this heat exchanger.
All the valve is doing most of the time is slightly modifying or regulating this flow
based on the downstream temperature, to ensure it remains in the specified range.
Notice also, this valve is placed on the Freon stream going into the heat exchanger, this
is because the flow of Freon can be easily adjusted.
It's important to remember that control valves should be placed on streams that can be independently
adjusted.
They should not be placed on streams that cannot be adjusted, such as this process stream.
Because the Freon comes from a refrigeration loop, independently operated from this process,
its flow can be increased or decreased without adversely affecting the rest of the process.
In other words, avoid putting control valves on process streams, unless they're at the
very beginning or end of a process.
As we'll see in the next two lectures, most of the time you'll be putting control valves
on utility streams or at the bottom of vessels to control the level.
Let's next look at this line.
This hash mark means that this valve is being opened and closed using air pressure.
Many valves use air pressure or a pneumatic actuator to rapidly and precisely open and
close the valve.
This solid line means that a signal is being measured or transferred from a liquid, in
other words the temperature is being read from a liquid stream, which is considered
a capillary signal.
Sometimes the signal transfer line is dotted, which means that the temperature is being
read or transmitted via an electrical signal.
Since this is a simple control loop, a solid or dotted line are both okay to use.
It's far more important to correctly specify these signal lines in detail only within more
complex control loops in a piping and instrumentation diagram.
In the next lecture we'll continue looking at simple control loops by examining control
of pressure, level, and flow rate.
Thanks for watching.