exceeds the signal passed on by the low-select function, and we know that this controller is directacting, we may conclude that it will be selected if it sees a high level at its PV input. More specifically, it will surely be selected if the measured tank level rises significantly above the setpoint value of 90%. Thus, we may conclude that the purpose of this level controller is to take over control if the tank level reaches or exceeds the 90% mark.

If neither level controller is selected, the signal that gets passed on to the flow controller as a remote setpoint is the 50% fixed signal entering on the left-hand side of the low-select function. Thus, the flow controller tries to maintain a steady flow rate of 50% in the event neither level controller is selected.

Putting all these pieces together, we may conclude that the purpose of this surge tank control system is to maintain as steady a flow rate as possible out of the tank (and on to some other process), letting the liquid level inside the tank rise and fall significantly before any action is taken to change the flow rate. Only if the level drops below 10% will the flow rate be reduced, and only if the level rises above 90% will the flow rate be increased. Otherwise, the flow rate will hold steady at 50%.

To summarize, the recommended technique for analyzing the purpose of an override control system is as follows:

•First, determine the necessary actions of each controller (assume the selector functions are absent, and each controller gets its turn controlling the process).

•Identify the type of selection (high or low) implemented by each selector function.

•Based on the type of selection and the action of the controller, identify what process condition will cause that controller to become selected. This is the condition the controller exists to regulate.

31.9Review of fundamental principles

Shown here is a partial listing of principles applied in the subject matter of this chapter, given for the purpose of expanding the reader’s view of this chapter’s concepts and of their general interrelationships with concepts elsewhere in the book. Your abilities as a problem-solver and as a life-long learner will be greatly enhanced by mastering the applications of these principles to a wide variety of topics, the more varied the better.

•Negative feedback: when the output of a system is degeneratively fed back to the input of that same system, the result is decreased (overall) gain and greater stability. Relevant to loop controller action: in order for a control system to be stable, the feedback must be negative.

•Time constant: (τ ), defined as the amount of time it takes a system to change 63.2% of the way from where it began to where it will eventually stabilize. The system will be within 1% of its final value after 5 time constants’ worth of time has passed (5τ ). Relevant to process control loops, where natural lags contribute to time constants, usually of multiple order.

References

Austin, George T., Shreve’s Chemical Process Industries, McGraw-Hill Book Company, New York, NY, 1984.

Dorf, Richard C., Modern Control Systems, Fifth Edition, Addison-Wesley Publishing Company, Reading, MA, 1989.

Eckman, Donald P., Automatic Process Control, John Wiley & Sons, Inc., New York, NY, 1958.

“FoundationT M Fieldbus Blocks”, document 00809-0100-4783, Rev BA, Rosemount, Inc., Chanhassen, MN, 2000.

“Function Blocks Instruction Manual”, document FBLOC-FFME, Smar Equipamentos Ind. Ltda., Sert˜aozinho, Brazil, 2005.

Lavigne, John R., An Introduction To Paper Industry Instrumentation, Miller Freeman Publications, Inc., San Francisco, CA, 1972.

Lavigne, John R., Instrumentation Applications for the Pulp and Paper Industry, The Foxboro Company, Foxboro, MA, 1979.

Lipt´ak, B´ela G. et al., Instrument Engineers’ Handbook – Process Control Volume II, Third Edition, CRC Press, Boca Raton, FL, 1999.

Mollenkamp, Robert A., Introduction to Automatic Process Control, Instrument Society of America, Research Triangle Park, NC, 1984.

Palm, William J., Control Systems Engineering, John Wiley & Sons, Inc., New York, NY, 1986. Shinskey, Francis G., Energy Conservation through Control, Academic Press, New York, NY, 1978.

Shinskey, Francis G., Process-Control Systems – Application / Design / Adjustment, Second Edition, McGraw-Hill Book Company, New York, NY, 1979.