Control logic---Describe based on the diagram
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Dear experts, I would like to ask about the relationship between the elements within the green boxes in the diagram. Also, how should one approach analyzing a diagram like this? Thank you.
The reactor bottom liquid level LT14006 and flow rate FT14007 are controlled in cascade by valve FCV14007 (main loop LIC14006, secondary loop FIC14007); the reactor bottom temperature TT14013 and flow rate FIC14006 are controlled in cascade by valve FCV14006 (main loop TIC14013A, secondary loop FIC14006). Generally, steam or water is used for this control. The PV value of TIC14013A and that from TIC14013B down to valve FIC14007 are indeed not visible; it’s best to ask the designer ； I personally guess that the PV value of TIC14013A should be compared with the PV of the output regulated by FIC14007, and the lower value should be used to control the valve FCV14007. FIC13003 is not visible; it is assumed that the PV value of TIC14013A is compared with the PV of the control output from FIC-13003, and the lower value determines the operation of control valve FCV13003.
Thank you very much. Does the low-level selection for PV in TIC and FIC involve comparing 4-20mA values?
There is an SP (setpoint) from TIC 14013A to FIC 14006. What does this mean?
This should be the PV of the cascade main loop, and the SP of the secondary loop; it’s marked this way I guess so as to distinguish it from the PV that goes to TIC14013B
Guess: The lower value is the minimum setpoint for the opening degree of the final control valve of TIC/FIC; for example, if the FIC opening degree is 20% and that of TIC is 25%, then 20% is taken. But it’s best to ask the design team or find the process package documentation; there should be instructions regarding special controls.
I don’t understand why the temperature signal for the flow control of FCV14006 is an SP value; it should be calibrated using a PV value. Take a look at the instrument symbol schematic on the home page; what does the LTO signal mean?