Low-cost, high-return process control improvements

George Buckbee, P.E., is vice president of marketing and product development for ExperTune and he is writing a three-part series on process control improvements for Control Engineering. Following is an excerpt of the first installment of that article; the full version can be found online at www.controleng.

10/01/2009


George Buckbee, P.E., is vice president of marketing and product development for ExperTune and he is writing a three-part series on process control improvements for Control Engineering. Following is an excerpt of the first installment of that article; the full version can be found online at www.controleng.com .

Your plant's control system acts as the nervous system for a process unit or maybe your whole facility. It provides sensing, analysis, and control of the physical process, and sits between the operator and the process. When it's running at peak performance, process variability is reduced, efficiency is maximized, energy costs are minimized, and production rates can be increased.

Unfortunately, most process plants are full of inefficiencies and losses. For example, a typical process plant will have as many as 30% of control loops running in manual. Many of those loops are in manual because of an underlying problem with the instruments, control valve, or controller.

The road to improvement begins when you're able to spot these issues and address them. Let's start with a simple exercise that proves the concept.

Improvement #1 - Compressed air, which is used to actuate control valves (among other things), is actually one of the most expensive utilities in your plant! You use expensive electricity to (inefficiently) compress air, and then transport it all over the plant through leaky tubes. Compressed air leaks not only cost money directly, but these losses can be large enough to require more or larger compressors.

The next time you have a shutdown, leave the compressor online. Take a spray-bottle full of soapy water and a wrench, and walk through the plant listening for the telltale hiss of leaky air lines. Spray some soapy water on the air tubing connections to see which one is leaking, then tighten with the wrench.

Does this really save money? Just how expensive is compressed air? If you know your plant's cost per kWh, you can estimate the cost per year for 1 SCFM (standard cubic foot per minute) of air: 0.25 hp/SCFM x 0.745 kW/hp x 24 hr/day x 365 day/year x cost/kWh. At a typical industrial rate of $0.06/kWh, this is roughly $98/year for each SCFM. And this figure does not include the capital, depreciation, and maintenance costs for the compressor, dryer, and distribution system.





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