Control strategies to improve process capacity and efficiency - Part 1

Many simple additions to PID control loops are presented here that can increase the plant production rate, yield, product quality, and on-stream time. The improvements typically can be done by the selection of PID options, pairing of variables, and the addition of PID controllers and simple standard function blocks. The configuration improvements can often be done and tested in a few days.

My recent AIChE St. Louis Section short course sponsored by MYNAH Technologies “Control Strategies to Improve Process Capacity and Efficiency” is viewable in three parts, each video about one hour in length on www.MYNAH.com by selecting “Videos” at the bottom of the webpage or clicking on the following direct links to each video:

Part 1: Process Dynamics Review, General PID Solutions, Feedforward and Ratio Control, Steam Systems

Part 2: Compressors, Jackets & Heat Exchangers, Valve Position Control, Continuous and Batch Control

Part 3: Reactors, Fermenters, Crystallizers, Evaporators, Distillation Columns, Dryers, and Neutralizers

I am exceptionally proud of this course. It is an updated concise view of what I have learned to be truly important for achieving significant benefits from process control. Here are my Part 1 Key Points that provide a synergistic understanding with the viewing of the video:

1-1: A process can be made to move smoothly in concert to different production rates or deal with disturbances by keeping a series of mass flow rates and/or energy transfer rates in the right ratio.

1-2: The ratios can be seen directly or simply computed from the process operating conditions given on Process Flow Diagram (PFD).

1-3: Most feedforward control systems are addressing this common need to keep flow rates and energy transfer rates in the right ratio.

1-4: Nearly all feedforward systems need feedback correction as shown in the course by a primary PID controller measuring process composition, level, pH, pressure, or temperature.

1-5: When the primary PID controller output directly manipulates a valve signal or power input, the implementations can be simply done via a feedforward summer embedded in the PID controller.

1-6: When the PID controller output is cascaded to a secondary PID flow or speed controller setpoint, the implementation is best done via a Ratio and a Bias block.

1-7: The operator must be able to not only manually set the flow or speed manipulated by the primary PID but also locally set a desired ratio for startup and abnormal operation.

1-8: The actual ratio per measured flow rates or energy transfer rates, the normal ratio set locally by operator, and the corrected ratio set by the primary feedback control must be displayed to operator.

1-9: The feedforward input is designated as the “leader” and the feedforward output as the “follower”.

1-10: Each feedforward input (leader flow or speed) is multiplied by a ratio factor (feedforward gain or ratio setpoint). A bias is then added or subtracted before becoming the feedforward output (follower flow or speed).

1-11: For vessels and columns, the primary PID controller corrects the ratio factor.

1-12: For inline systems and sheet lines, the primary PID controller corrects the bias.

1-13: A ratio or bias not corrected by the primary PID controller can be slowly adapted via the output of a simple integral-only controller that seeks to minimize the feedback correction needed from the primary controller.

1-14: The result of the feedforward output (follower) must arrive at the same point in the process at the same time as the disturbance (leader) as visualized on the block diagram of dynamics for the loop.

1-15: The result of the feedforward output must be equal to and opposite in sign to the disturbance.

1-16: Filtering of the feedforward signal should be just enough to prevent follower valve reaction to feedforward noise.

1-17: A result of the feedforward output that arrives too soon or is too large will cause a response in the opposite direction (inverse response) that is terribly confusing to the primary PID controller.

1-18. Feedforward gain and ratio factor settings are conservatively set to prevent overcorrection.

1-19: Dynamic compensation by an insertion of a deadtime block and lead-lag block on the feedforward signal is needed to achieve the correct timing to insure the arrival of the result is not too early or late and is not too fast or slow.

1-20: The feedforward deadtime is set equal the disturbance (load) path delay minus the feedforward path delay to make sure the result of the feedforward does not start too soon. If the feedforward path delay is larger than the disturbance (load) path delay the ratio factor must be decreased. If the feedforward path delay is larger by more than the total loop deadtime, feedforward be more detrimental than beneficial.

1-21: The feedforward lead time is set equal to the largest lag time (time constant) in the path of the result of feedforward to the same point in the process as the disturbance (load).

1-22: The feedforward lag time is set equal to the largest lag time (time constant) in the path of the disturbance (load) to same point in the process as the result of feedforward.

1-23: For a similar large lag time in both the disturbance (load) and feedforward paths, the lead or lag time can be simply increased to help the feedforward to provide a faster or slower correction, respectively.

1-24: For steam header systems, the feedforward input for each header pressure control output is the summation of the letdown flow to the lower header (acting as a half decoupler besides mitigating letdown flow disturbances) plus the flows of steam users and generators on the respective header. User steam flows have a plus sign and generator steam flows have a minus sign in the feedforward summation.

1-25: The feedforward decoupler does not need dynamic compensation if the letdown valves from upper header and to lower header are in the same relative location.

1-26: The header user and generator feedforward signals need dynamic compensation based on the deadtime and secondary time constant of the integrating pressure response where the input is the user or generator flow measurement and the output is the pressure measurement used for header control.

1-27: For user, generator and letdown steam flows in the same mass flow units the theoretical feedforward gain is 1.0 with the feedforward scale set equal to the linear letdown valve flow capacity.

1-28: A feedforward signal of steam demand when a Cogen high flow override controller output is selected to override a boiler master controller is added to the respective header’s pressure controller output to provide a faster correction of letdown flow to prevent a Cogen steam generation that exceeds permit flow.

I will provide additional synergistic Key Points in future Control Talk Blogs. Until then don’t procrastinate on optimization even though it optimizes living in the past.