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MAY-JUN 2019

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INTECH MAY/JUNE 2019 43 If APC needs single-loop control to reject un- measured disturbances, then how best to utilize it? APC control objectives The most important APC control objectives are related to production rate and product qual- ity, because these variables are directly related to operating profitability. Production rate is limited by constraints (e.g., maximum temperature and control valve position). Product quality is normally controlled by temperature, analyzers, lab analyses, or indirectly by "soft sensors" (inferred properties). Vessel levels are integrating, inventory-related vari- ables and are almost never included as CVs in an APC controller. The same can be said for vessel pressure, unless it is a constraint variable related to pushing the production rate. (There are some exceptions—fired heater controls often use burner pressure as a substitute for fuel gas flow to control heater temperature.) The lone outlier is flow, which is a true "exten- sive" variable, independent of product quality or operating profitability. In all but truly exceptional cases, flow is always adjusted to achieve some other process control objective. It is almost always the secondary, or slave, in a one-on-one basic cascade. If the flow controller is standalone, then its SP is adjusted by the operator (or an APC controller) to achieve a higher-level control objective. At the same time, the flow controller is a true mitigator of unmeasured disturbances. It is typi- cally characterized as a high-frequency loop, meaning that a change in OP is followed almost immediately by a change in PV. When tuned properly, and when challenged by unmea- sured disturbances, it returns the PV to its SP very quickly and with little overshoot or oscillation. As such, it is normally the vari - able of first choice as an MV for any higher- level control strategy, especially for an APC controller. There are some ex- ceptions. Although flow is a true extensive vari- able, there are instances where it can be used to "create" an intensive variable. Ratio variables are used quite often in process control. For example, reboiler duty on a distillation column can be cal - culated from flow and temperatures and then ratioed to the column charge rate. The column reboiler duty/charge ratio can then be used as an MV in an APC application. Same for treat ratios in absorbers, product yields in fractionators, and so on. The single-loop flow controller rejects un - measured disturbances and thereby stabilizes the created intensive variable. Status quo: Do not disturb Today, even after 50 years, APC continues to rely on the lowly flow control loop, the most basic single-loop control, as the best rejector of unmea- sured disturbances and the most stable platform for the APC/optimization control hierarchy. So, the next time somebody suggests getting rid of single- loop control in an APC application, just ask, "What about unmeasured disturbance variables?" Do not expect a righteous reply. ■ ABOUT THE AUTHOR Jim Ford, PhD (jim.ford@mavtechglobal.com), is a senior consultant at MAVERICK Technologies, a platform-independent automation solutions provider offering industrial automation, strategic manufactur- ing solutions, and enterprise integration services for the process industries. Ford specializes in advanced process control, control system engineering, and ex- ecution of detailed control system modernization and migration front-end loading evaluations. AUTOMATION BASICS Error detector Feedback elements Error signal Feedback signal Actuating signal Single-loop control (shown) is the mainstay of process control, and APC can never be successfully implemented in its absence.

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