Lesson 8 - Process Control and Instrumentation End of Unit SAQ
This lesson allows you to test yourself on the material covered in the Process Control and Instrumentation unit and determine the level to which you have met the learning outcomes.
Suggested Response - Q3
*** You have a dilution water tank that you would like to keep a minimum of 50% full. This is fed by two D-plants (deaerated water producing plants). Bearing in mind that starting up and shutting down a D-plant leads to energy losses; how would you control the level in the tank if:
a) The D-plants run at a fixed speed?
b) The D-plants can be run at a speed between 50-100%?
Response:
a) As the D-plants run at a fixed speed, we can describe them as either being off or on. The guideline is clear that we need to minimise start-ups and shutdowns. Therefore, we could use multistep on/off control.
We could give the first D-plant an on/off control, which would start it at 55% and stop it at 95%.
The second D-plant could then be used only if the level falls below 50% and it could be stopped at 75% to allow the other D-plant to potentially run continuously.
b) The D-plants should not be ramped up and down continuously, thus we could use cascade control to control the level in the tanks. This would be the primary control, which would output a flowrate setpoint to the D-plants. The D-plants’ flow control would be the secondary control.