Plan ahead for APC maintenance

Advanced process control (APC) has substantial benefit for the process industries. This technology generally provides a 3-5% increase in throughput [1] with little investment respective to other methods for capacity increase. The project returns can have a payout of less than one year, but most within two years [2], and some projects have even been documented with a return above 1,000% [3].

Here, APC refers to any use or combination of the following: model predictive control (MPC), inferential variables, sequential control, fuzzy logic, etc. It's not the classical advanced control approaches of ratio, feedforward, override, etc. However, these techniques are often included in APC applications.

This technology has a downside in that there's a decline in performance over time because the process behavior drifts or changes from the models that described it at installation. The decline in performance is typically very little in the early parts of the project's life, and then substantial as the project ages. For those who don't perform regular maintenance on APC models, about 65% of projects are turned off in the second and third years [4-6].

Based on the overall high rate of return for APC projects, the societal benefit of extending APC life can be large. It may be possible to estimate how much improving that 65% failure rate for APC is worth, globally, to the process industries. One possible way is to look at overall chemical sales, and then estimate how much value could be added by applying APC.

The American Chemistry Council estimated in 2018 that global shipments of chemical products were approximately $4 trillion/year [7]. Assuming that about 35% of the capacity could benefit from APC, but either doesn't have APC or did but the APC benefit fell to effectively zero, and assuming about 4% improvement in production for that percentage of capacity, this estimate for the value of correcting this APC maintenance problem would be about $56 billion/year for the global chemical processing industries (CPI). These figures don't include sectors such as petroleum refining, liquefied natural gas, electrical power generation, pulp and paper manufacturing, mineral processing, computer chip manufacturing, etc. Even if this estimate is an overstatement of the true situation, the amount of value that could be created by correcting this opportunity gap is large, and worth the effort to achieve the potential benefit of APC to society.

The decline in APC performance is now becoming well recognized because of the history of implementations. It wasn't well recognized in earlier eras. Very recently, the decline was reasonably, generically modeled, and can identify an optimal recalibration schedule [8, 9].

Recalibration needs schedule, funding

Periodic recalibration of APC models can extend their useful life, which would also improve acceptability of APC to those who might have been disappointed in an unexpectedly short life on a former project. But the costs of recalibration on an optimal schedule need to be included in the capital project economic evaluation, and in both in the plant's operations budget and planned resource allocation.

We found the functional decline of models over time and essential periodic recalibrations aren't usually included in the economic profitability analysis that leads to APC project acceptance. There's an implicit assumption that the controller will remain wondrously functional for the life of the unit. Though acknowledging functional decline and recalibration costs do affect long-term economic projections, we find APC profitability metrics remain very attractive when those aspects are considered.

Including the need for periodic recalibration in capital planning has four benefits: 1) it properly represents what management can expect, 2) by extending project life, it enhances APC reputation, 3) it permits plant budgeting of expenses and human resource allocation, needed to extend project life, and 4) operations supervisors will be aware of, not surprised by, recalibration need and schedule.

Download the full whitepaper on this topic, which outlines a procedure for including in capital project analysis an estimate of the income over time, and initial and continuing costs for recalibrating the APC models at the optimum interval for performing maintenance [8, 9]. The paper contains examples that show how to apply the analysis, and includes treatments for a variety of options in the APC project (for example, the use of contract or in-company resources to complete projects).

Importantly, the whitepaper emphasizes the need for operations personnel to be involved in project development and maintenance scheduling. Their buy-in and recalibration budgeting and human resource allocation will ensure maintenance is performed in a timely manner.

Our intent for the guide is to extend APC life, which will expand both its societal benefit and management acceptance. We hope to help vendord and userd provide a comprehensive economic evaluation of APC projects, which will ensure projects are sustained long-term.

References

1. M. Bauer and I. K. Craig, "Economic assessment of advanced process control – A survey and framework," Journal of Process Control, vol. 18, no. 1, pp. 2-18, 2008, doi: 10.1016/j.jprocont.2007.05.007.
2. J. Anderson, T. Backx, J. Van Loon, and M. King, "Getting the most from advanced process control," Cost Engineering, vol. 38, no. 11, pp. 31-38, 1996.
3. A. Taylor, "Increasing LNG production and gas plant yields," AspenTech website, Nov. 15, 2018, https://aspentechwebinars.webex.com/aspentechwebinars/lsr.php?RCID=e1b040dd5f572ec894fbe582cb3f39f6.
4. D. Shook, "Best practices improve control system performance," Oil & Gas Journal, vol. 104, no. 38, pp. 52-54,56-58, 2006.
5. L. Heavner, "Private communication between Lou Heavner and Stephen Mayo in 2018," S. Mayo, ed., 2018.
6. T. Fiske, "Best practices for advanced process control," ed., p. 32, ARC Advisory Group, Dedham, Mass. 2008.
7. "2019 guide to the business of chemistry," American Chemistry Council, 2019. Accessed online Apr. 11, 2020, www.americanchemistry.com/GBC2019.pdf
8. S. M. Mayo, R. R. Rhinehart and S. V. Madihally, "APC maintenance scheduling – part 1," Hydrocarbon Processing, . February 2020, pp. 59-63.
9. S. M. Mayo, R. R. Rhinehart and S. V. Madihally, "APC maintenance scheduling – part 2," Hydrocarbon Processing, March 2020, pp. 51-55.

Author Biographies

Stephen M. Mayo has 38 years of industry experience, including process engineering and management roles worldwide in several different countries in ethylene, refining, LNG, and oil and gas production. Mayo is a chemical engineering doctoral candidate, and a registered professional engineer in Oklahoma.

R. Russell Rhinehart has experience in industry (13 years) and academe, including officer positions in several professional societies. He is a fellow of ISA and AIChE, and a Control Process Automation Hall of Fame inductee. Now ‘retired,’ he is working to disseminate techniques with his web site, www.r3eda.com, books, handbook chapters and articles.

Sundararajan V. Madihally is a professor, and recently published the second edition of his textbook, Principles of Biomedical Engineering. Dr. Madihally has received numerous teaching and mentoring awards, including the AIChE Outstanding Advisor Award.