I’ve featured quite a number of experts around Emerson over these past three years. Some categories, like Process Optimization, have more than 50 posts. There is quite a bit of wisdom mixed in all those posts.
A few months ago, I was asked by Plant Engineering magazine managing editor, Jack Smith, if I’d be willing to write a process optimization article highlighting some of the ideas offered in these posts. It was a thrill to be asked and I quickly agreed.
I wrote a draft and went back to some of the experts highlighted, such as Mark Coughran and Pete Sharpe, for their improvement suggestions. The resulting article, Downturn a good time to review, improve process optimization went live on PlantEngineering.com website last Friday and is printed in the March 2009 magazine edition.
I tried to stress things you could do as a plant engineer to improve your process without having to spend a lot of capital, which is an issue for many process manufacturers in this global economic slowdown. Finding ways to reduce process variability is a good first step. Sources of variability that our variability consultants have tabulated over the years include:
- Control valve performance – 30%
- Improper tuning – 30%
- Improper process and/or control scheme design – 20%
- Other – 20%. The 20% of other causes are not necessarily design- or control-scheme related, but more operational issues that occur over time.
I distilled down five ways to reduce this variability: size control valves properly, minimize loop dead time, measure process dynamics and compensate for them, tune the loops, and apply advanced process control. I won’t spill all the secrets divulged in the article but instead highlight a couple of points.
My Fisher valve colleagues often remind me of the importance of the control valve since it directly touches the process.
Control valves, being variable in gain, must be correctly sized and characterized for the application’s flow to be sufficiently linear to stay within specified gain limits over the operating range of the process.
Other parts of your control loops to check for:
…sources of dead time include inadequate signal conditioning on transmitters, incorrect transmitter range/resolution, poor physical location of transmitters and measurement lags from applied filters and dampeners.
Without the proper process dynamic measurement applications, many plant engineers have had to rely on rules of thumb and guesswork to loop tuning parameters. With the process dynamics understood, you can tune the loops with linear responses and try to reduce the non-linearities in the others in several ways including:
…changing any master loop configuration to prevent interaction with the slave loops. See if you can remove unnecessary interlocks that may disturb the control loop. If you uncover extremely high process gains, adding upstream control loops can help. Other advanced regulatory control strategies such as feedforward, cascade, override and split-range control can compensate for different process conditions.
With the basics addressed, you can look for areas to apply advanced process control, especially in big, energy-consuming units.
I hope some of the ideas excerpted from the article help you find ways to improve your process and help your business through these challenging times.
Update:I heard from Jack that this article is currently the most popular one on the PlantEngineering.com website. Thanks for stopping by to read it!