A variety of chemicals are added to raw water when treating it for human consumption, and also when treating water for many industrial processes.
In the coagulation stage, chemicals are added to cause suspended solids in the raw water to coalesce into larger particles called ﬂoc, which precipitate out of solution more readily. To maximize eﬀectiveness and to reduce waste, these chemicals need to be added to the water in speciﬁc proportions:
If the raw water inlet ﬂow and composition never change, the ﬂocculant ﬂow control setpoints may be left unchanged as well. Once the correct proportions are set, the ﬂocculant chemicals will continue to be added at just the right amounts.
But what if the raw water inlet ﬂow is subject to change? It would be quite inconvenient for operators to re-adjust the ﬂocculant control setpoints every time a water ﬂow rate change took place. Can you think of a way to implement ratio control in this system to handle such changes automatically?
For those who have studied ﬂowmeter technology, identify the types of ﬂowmeters shown in this diagram and explain how each of them works.
After implementing your control strategy, analyze the eﬀects of the raw water ﬂow transmitter failing with a low signal.
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