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  wo controllers – the outer primary level controller (LC) and inner secondary feed

flow controller (FC)

  wo measured process variable sensors – the outer primary liquid level (PV1) and

inner secondary feed flow rate (PV2)

 ne final control element (FCE) – the valve in the liquid feed stream

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  The feed flow rate (PV2) responds b e f o r e   the tank level (PV1) when the header

pressure disturbs the process or when the feed valve moves.

  The output of the primary controller, CO1, is wired such that it becomes the set

point of the secondary controller, SP2.

  Ultimately, level measurement, PV1, is our process variable of primary concern.

Protecting PV1 from header pressure disturbances is the goal of the cascade.

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