Lesson 7 - Process Control Mechanisms SAQ
This lesson allows you to test yourself on the material covered in the Process Control Mechanisms lesson and determine the level to which you have met the learning outcomes.
Suggested Response - Q3
** Compare self-acting control, discrete control, and continuous control. What are the advantages and disadvantages of each? For each one, say where in the brewery you would use it.
Response:
You could use a table to answer this question:
Table: Controller types and their qualities
|
Control |
Advantages |
Disadvantages |
|
Self-Acting |
Inexpensive
Robust
Good for safety applications |
No feedback in case of failure
Not possible to change the working point in some cases
Setpoints have to be manually adjusted
Not as precise
|
|
Discrete (On/Off) |
Relatively inexpensive
Can be done with less expensive switches and not continuous transmitters.
|
Will always have a deadband
Wear and tear on control element due to frequent switching |
|
Continuous |
Can deliver very precise control if correctly set up
Possible to change setpoints easily in all cases |
More instruments and control elements required
Controllers are more expensive
Loop tuning is required
|
Here are some examples of where you would find each type of controller:
-
Self-acting: main pressure regulators and pressure relief valves
-
On/off: level control on utilities like water; for discharges from waste tanks
-
Continuous control: most applications; controlling temperature, pressure etc.