For Robust, Maintainable Advanced Control, Keep It Simple
- Type: Conference Presentation
- Conference Type:
AIChE Spring Meeting and Global Congress on Process Safety
- Presentation Date:
April 1, 2019
- Skill Level:
KEEP IT SIMPLE
(âSimpleâ does not mean easy!)
by Lou Ann Perry, Sr. Process Control Engineer
Chevron Phillips Chemical Company LP
The trend in Advanced Process Control (APC) implementation over the past several decades has been toward more automated project implementation and complex controllers. In addition, demographic changes have resulted in fewer experienced process control engineers available to implement and maintain APC in many operating facilities. These trends have contributed to the proliferation of advanced control solutions that are challenging to operate, troubleshoot, and maintain.
1. Keep it simple!
â¢ Simplify the problem:
o Give your APC software the simplest possible control problem to solve and the result will be repeatable, predictable, stable control.
o Often, a robust 80% solution will provide more benefits than a complicated 95% solution.
o Regulatory control evaluation and adjustments:
â¢ Resolve problems with valves, instruments, and equipment before beginning work on an APC controller.
â¢ Tune all regulatory controllers affecting the target area, not just the manipulated variables (MVs).
â¢ Consider reconfiguring the regulatory control strategy or break cascades, as appropriate. Often, the regulatory controls can be simplified because APC will reduce the need for non-critical cascades, overrides, feed-forwards, etc.
â¢ These steps often take much longer than APC implementation.
â¢ Simplify the solution:
o Limit the number of instruments used in Inferred property control variables (CVs). Additional inputs reduce robustness and can result in complex dynamic responses.
o Use transformed CVs sparingly!
â¢ Transforms are often misapplied, and they are challenging to troubleshoot.
â¢ Many non-linear variables (valve positions, column dPs) can be modeled linearly within the normal operating range or near the constraint limit.
â¢ Simplify the hardware/software solution:
o Implement supporting calculations on robust platforms (DCS, APC software) at the lowest practical level.
o Limit the number of data paths; each one is a potential source of failure.
o Use gain multipliers and calculations instead of multiple configuration files for mode or feed type changes.
o For multiple similar APC applications (such as ethylene cracking furnaces), develop a procedure to implement modifications consistently, such as text substitution to create configuration files.
2. Bigger is not always better!
â¢ The larger the controller, the more difficult it is to operate, troubleshoot and maintain. Reduce the size of the APC control matrix, where possible.
o Itâs far easier to modify or troubleshoot several small controllers than a large one.
o Often, a controller only needs to be large enough to encompass the feed system up to the primary constraint to feed maximization (where applied).
â¢ Limit CVs to necessary targets and current process constraints. Avoid the mindset of âWe may need this in the future.â
â¢ As processes are modified or debottlenecked, revisit the process constraints and remove any unnecessary CVs.
3. Leave a trail of breadcrumbs!
â¢ Documentation doesnât have to be extensive or complex, just an overview of APC goals and a map pointing to more detailed notes.
â¢ Put detailed comments where the calculations reside (DCS code, configuration file code).
â¢ Include revision history with calculations (date, reviser, short description of modification).
4. The customer is always right!
â¢ Poor APC online metrics are an indication that your customers (operators) are not finding your product useful.
â¢ Clamping MV limits to prevent poor control action is usually indicative of a design issue or model matrix issue (poor sub-model, missing sub-model), not poor tuning.
â¢ Develop customized operator displays, if needed, but keep this to a minimum.
â¢ Add custom features to reduce operator error (add toggle on/off switches for redundant CVs, send messages/alarms to the DCS on APC shed or control give-up, initialize operator limits according to feed type, etc.).
â¢ Training, training, training! Create a training program for incoming operators and engineers. Training is an ongoing process due to staff turnover.
5. Donât buy a Ferrari if you donât know a good mechanic!
â¢ Complex APC solutions should be maintained by experienced staff who plan to be in the position long-term.
o Cater the complexity of the APC solution to the expected level of support staff: current and future.
â¢ Advocate for a technical mentorship program to provide expertise where more complex controllers exist.
â¢ Try to limit reliance on vendors â work in-house where possible. Save company dollars and build in-house expertise.
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