A mining company has contracted you to provide the design to upgrade their thickener on site.
The thickener has a history of poor performance and causes excessive downtime in the process
plant. The thickener has been operated manually up until this point in time and there is no
instrumentation at all. This upgrade will provide automated control of the flocculant dosing
value and the underflow slurry pump. Some drawings of the thickener are shown in figures 1,
2, and 3.
You have the following deliverables:
1. A new control philosophy for the flocculant dosing valve PLT01CV001. The valve control
should focus on providing the appropriate amount of flocculant based on the feedback
provided by the new instrumentation. The valve is controlled by a 4-20 mA analogue
signal; the valve sends it position back to the PLC also via a 4-20 mA signal.
2. A new control philosophy for the underflow pump PLT01PM001. Again, new instruments
are required to be installed to control the pump. The pump was previously direct-online
(DOL) controlled. The client wants to use a variable speed drive (VSD) to provide variable
speed control to the pump. The pump requires a digital input for the run command, and
a 4- 20 mA analogue input signal for speed control; a 4-20 mA analogue output signal is
sent back to the PLC providing information on the speed.
3. For every instrument used, the control philosophy should include what should happen at
the high and low limits of the feedback. For example, if settling rate is too high what
should the flocculant valve do? If bed-level drops too low, what should the pump do?
Write it in such a way that code can be easily developed from it (so avoid being overly
descriptive, keep it technical and specific).

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