Don't Over Look PID in APC

That Old Stalwart PID Can Do a Lot of Heavy Lifting in Your Process Control Efforts

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Additional override controllers may be added to the mix to prevent flooding by keeping the differential pressure from getting too high across a key section of the column. Alternatively, use the VPC controllers to lower column temperature setpoint to a permissible limit, increasing distillation rate.

Minimizing Reagents

A VPC can maximize waste reagent use by forcing the purchased reagent manipulated by the pH PID to a minimum throttle position. For several stages of neutralization, a VPC can minimize the pH setpoint of the first stage for acidic waste until the final stage pH base trim valve reaches a maximum throttle position. For basic waste, the VPC would maximize the first stage setpoint. A VPC process variable can be computed to prevent cross-neutralization of reagents from multiple-stage, split-ranged operation.

Minimizing Reactants

Composition controllers can maximize yield by adjusting the ratio of reactants to be closer to the stoichiometric ratio. The at-line analyzer PID provides a bias correction to the ratio of reactants to account for bias errors in the flow measurements. A VPC can adjust the input bias to the flow feed-forward to move the analyzer PID output to 50%, which is the point of zero feedback correction of the reactant flow measurement. 

Maximizing Flexibility

Plant-wide, feed-forward control can move flows in unison per ratios established by the process flow diagram (PFD). Flow feed-forward enables the process controllers to stay close to setpoint for changes in production rate or product grade. The result is fast and smooth changes in flows with minimal disruption to process variables. The production rates and grades can be optimized based on market demand, day-to-night utility rates, and waste fuel and reagent supply. The flexibility translates to minimum inventory and maximum efficiency. Feed-forward summers are generally used because of their robustness and simplicity. When the process controller output is 50%, there is no bias correction to the feed-forward. A VPC can correct the flow input bias or ratio factor or feed-forward gain to return the process controller output to 50%.

Key PID Features

Several features of the PID increase the functionality of the VPC and make the tuning easier and more robust. Directional velocity limits on setpoints manipulated by the VPC combined with dynamic reset limits in the VPC PID enable faster correction by the VPC when a process PID valve is going open to prevent running out of valve. When the process PID valve is decreased to a less efficient setting, a more gradual approach can be used. The result is a cautious approach to optimization with a fast getaway to prevent violating a constraint. The controller can be tuned for fastest correction. The dynamic reset limits in the VPC PID prevent the VPC output from changing faster than the process PID can respond, greatly simplifying the tuning.

The tuning of the VPC can be automatically identified and scheduled by an adaptive tuner. It increases VPC control action by a higher VPC gain for a higher PV corresponding to the process PID valve moving to a flatter portion of the installed characteristic. This works in conjunction with the directional velocity limits to prevent excessively exceeding a valve position constraint. Feed-forward of unmeasured disturbances can be used to assist the VPC.

An enhanced PID developed for wireless doesn’t change its output unless there is an update in the setpoint, feed-forward or measurement that exceeds a threshold sensitivity limit. By suspending integral action until there is an update, limit cycles from valve stiction and backlash are eliminated. The interaction between the process PID and VPC PID is also suppressed by setting the threshold sensitivity limit to ignore inconsequential changes in the process PID output. If a VPC execution time is chosen that exceeds the process response time, the VPC gain can be set equal to the inverse of the product of the process gains, enabling a single correction for a deviation in optimum valve position.

The enhanced PID can also suppress oscillations from the sample time, cycle time and multiplex time of at-line analyzers. By waiting for an update in the communicated analysis, the enhanced PID does not cycle due to an excessive update delay. If the total delay in the analysis result is much greater than the process response time, the enhanced PID gain can be set equal to the inverse of the process gain, enabling a single correction for setpoint and composition changes. The elimination of cycling and the simplification of tuning make the job of the VPC optimizing the at-line analyzer PID valve much easier. 

Greg McMillan is a retired Senior Fellow from Solutia/Monsanto, an ISA Fellow, and a member of the Process Automation Hall of Fame.

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