Guest commentary: Low-cost, high-return process control improvements, part 1
In the first of a three-part essay, George Buckbee, P.E., vice president of marketing and product development for ExperTune, considers a number of very practical ways you can optimize your plant control systems and improve profitability.
Part 1: Process control improvements can make significant financial gains for your company. In fact, it is estimated that optimizing your control system can deliver savings equivalent to 2% to 6% of total operating cost. This discussion focuses on proven low-cost methods to increase profitability. Some of these methods are incredibly simple, and can be implemented in less than a day while others are somewhat more involved. As you try them out, be sure to document the benefits, and share them with your management.
How control systems deliver results
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 directly between the operator and the process. If you think about it, almost all of the operator's information comes from the control system, and all of the operators commands are carried out by the control system. 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. The process, control system, instrumentation, and infrastructure are all less than perfect. 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. Running that way is only a symptom of something bigger.
The road to improvement begins when you're able to spot those symptoms, and we'll show what to look for. More importantly, we'll show you how to apply some simple low-cost fixes that will drive up profits. Let's start with a simple exercise that proves the concept.
Improvement #1 - Compressed air leaks
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. But that doesn't include the capital, depreciation, and maintenance costs for the compressor, dryer, and distribution system.
If that doesn't convince you, according to the U.S. Department of Energy, air-line leaks waste as much as 20-30% of compressor capacity! It estimates the air leaking from a .062-inch leak can be over 5 SCFM. Fix one of these leaks, and you have saved $500. Not bad, but let's look at what that effort cost. Assuming each of your process loops has a control valve and pneumatic actuator with all its air lines and connections, consider this:
• 100 loops @ 12 loops/hr = 8.5 hours.
• 8.5 hrs @ $60/hr = $510 labor cost.
• If 10% of those 100 loops were leaking x $500/leak = $5,000/year, assuming your repairs hold for at least a year.
Finding and fixing compressed air leaks isn't exactly rocket science, but it's a good start and should help you understand how the whole concept of documenting costs and savings works. When you start looking at your larger control system strategy, you will find there are more sophisticated parts of the process that you can work on and see much larger payoffs. Tackling these requires more specialized tools that we'll begin discussing in the next chapter.
George Buckbee, P.E., is VP of marketing and product development for ExperTune . Reach him at firstname.lastname@example.org. His series Seven habits of highly successful control engineers received very high readership.
-Edited by Peter Welander, process industries editor, PWelander@cfemedia.com
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