Measure First to Improve Control System Performance

Basic steps, such as measuring the percentage of the plant running in manual mode, are a good place to start. A typical process plant runs with between 20 and 30 percent of controls in manual mode. Consider that a typical control loop costs $10,000 or more. A typical oil refinery may have 3,000 control loops, with 600 to 900 in manual at any given point, representing a lost investment of over $6 million. And that doesn't even count the annual process losses, which are at least as high.

Additional simple measures, such as the percentage of control valves at a limit, and overall variability, can give you a better idea of the opportunity for savings at your plant. Start by taking a random sample of 100 control loops. In one hour sitting with the operator, you can confirm whether your plant has a significant opportunity to improve.

下一步是定期测量和监视控制性能。软件工具可以监视各种措施，提供问题的报告和通知。此步骤对于认识到任何一天的植物中存在问题至关重要。

Automated monitoring also allows for more sophisticated measures and diagnostics. Modern software tools can find failed instruments, broken control valves, poorly tuned controllers and process problems. These analytics are needed for the next step: resolving the root cause of the problem.

如果30％的控制器以手动模式运行，则该解决方案并不像将所有内容放入自动模式一样简单。运营商可能是因为某种原因将这些控制器放入手动。任何良好的控制绩效项目都可以使用您的流程历史学家的数据来帮助查明和解决问题的真正根本原因。

Some problems, such as valve stiction, have a unique signature pattern, as shown in Figure 1. The square wave pattern on the process measurement and sawtooth movement of the control output are a sure sign of valve stiction. The ladder pattern in the PV/CO plot shown in Figure 2 further confirms valve stiction.

Other problems, such as oscillations and interactions between various parts of the process, need more sophisticated analysis. Tools such as Fourier Transforms and the resulting power spectrum chart, shown in Figure 3, can find the cause of oscillation problems.

Process Interaction analysis, as shown in Figure 4, can also be used to pinpoint the cause of a problem that may be far upstream in the plant.

With the problems identified and prioritized, your focus can now turn to managing the required repairs, tuning changes and process adjustments. This work should be managed and organized like any project: with specific plans, assignment of responsibilities and expected completion dates. Some repairs may need to wait until planned shutdowns, but others can be completed on the fly.

Finally, it is important to track the effect of these improvements. A simple before/after trend can be very useful to show the results. Whenever possible, you should also identify the economic impact of your work, and share it with your management team.

如果您的工厂有足够的员工，则可以在内部完成这些步骤。经验丰富的软件和服务供应商还可以提供这些活动的全部或部分服务，从网站评估开始，并从捕获福利并维持结果一直持续发展。

What kind of results can you expect? That depends, of course, on your starting situation. But it is common to see reductions in variability that lead to energy savings, production increases and quality improvement. Because automation system improvements have a direct effect on process results, the return on investment is typically measured in months, not years.

This article was adapted from content provided by George Buckbee, P.E., ExperTune

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