Do you know what you're looking at? Are you really seeing what you think you're seeing? How can you be sure? How fast can you do it?
These are some of the unsettling questions that characterize the need for calibration in process control. The reason they're disturbing, of course, is that control, automation and manufacturing in general depend on functional certainties delivered fast, which enable operators and engineers to make decisions and complete all their tasks successfully and on time or better. So when information comes along that questions those absolutes and throws doubt on them, well, it can be pretty unnerving and drain time.
This is also the reason calibration is so crucial in the process control field—it restores accuracy to instruments and confidence and timeliness to users. However, the relativism that comes along with calibration can still be pretty spooky, which is why most users approach it with traditional caution. Fortunately, many calibration methods and tools are getting increasingly easier to use, so there's less need to worry and much better accuracy, improved optimization, speedier throughput and other benefits to be gained.
For instance, Cabot Microelectronics Corp. in Aurora, Illinois, is the world's leading supplier of slurries and polishing pads used to remove excess material from silicon wafers in the semiconductor production process, and master electrician Michael Schlegel and his colleagues use meters, calibrators and other devices to maintain and troubleshoot Cabot's mixers, blenders, shipping line conveyors and robots.
"I'm usually called on to look at anything electrical, so about 10 to 20% of my job is troubleshooting, and the rest is preventive or predictive maintenance and working on capital projects," says Schlegel. "I may jump from electrical to mechanical to pneumatics to plumbing, so I use a lot of Fluke (www.fluke.com) tools in the process, including the 725 process calibrators, 381 remote-displays, true RMS, an AC/DC clamp meter, and a Fluke 87 DMM that I have with me most of the time."
Schlegel reports that Cabot calibrates most of its pressure and temperature transmitters in-house, so he frequently uses his 725 calibrator. "I introduced it to Cabot not long after I got here because it's more adaptable to our business needs than what they had before," explains Schlegel. "The previous calibrator had a lot of modules that would drift and had to be constantly calibrated, but the Fluke process calibrator is more efficient and easier to use. My 725 tells me everything about the process, and it covers RTDs, thermocouples and process loops. As a result, our calibration time dropped from about a month with the previous system to about four to five days with the Fluke 725."
Because calibrating instruments usually means taking them at least partially offline, it's often viewed as an unwelcome interruption in crucial processes. Conversely, any effort or capability that can shorten calibration time is more than welcome.
For example, British Sugar reports its plant in Wissington, U.K., is the world's largest beet sugar manufacturer, which processes more than 3 million tons during peak campaign periods and produces 420,000 tons of sugar per year—and can't waste time doing it (Figure 1).
Because making sugar requires huge amounts of steam, the Wissington facility's combined heat and power (CHP) plant produces 500,000 megawatt/hours (MWh) of electricity annually, but it can only be shut down for maintenance for 10 days per year. During this period, all maintenance tasks have to be completed, including statutory and mandatory testing, repairs and inspections.
The CHP plant includes a LM6000 gas turbine, waste heat recovery boiler, a 34-megavolt ampere (MVA) steam turbine, a water treatment plant, two small shell boilers and a back-up plant consisting of three water-tube boilers and a 20-MVA steam turbine. The plant supplies heat and power to the sugar operation and a bioethanol application, and even delivers waste heat and carbon dioxide to 46 acres of greenhouses on site producing 140 million tomatoes annually. The CHP plant also exports 45 MW of power back to the U.K.'s National Grid, which is enough for a 120,000 residential consumers.
These varied operations enable British Sugar's Wissington plant to also annually manufacture 140,000 tons of animal feed, 6,000 tons of betaine, 55,000 tons of bio-ethanol, 120,000 tons of limex, 15,000 tons of tomatoes, 150,000 tons of topsoil, and 5,000 tons of stone that's cleaned and sold as aggregate. Finally, an associated carbon dioxide recovery and liquefaction plant recovers up to 70,000 tons of carbon dioxide per year from the bio-ethanol fermentation processes.
"It's essential that we all work together, so there's no interruption of steam supply to our clients," says Trevor Wolfe, EC&I engineer at British Sugar. "Any interruption to the steam supply would shut the sugar factory down, causing much inconvenience and expensive downtime, potentially destroying a multi-million pound tomato crop, and causing us financial penalties through loss of export revenue."
To avoid these disasters, British Sugar recently introduced a new boiler house standard that required all 400 of its operationally critical and safety instruments to be calibrated every year. These devices include a mix of temperature, pressure, flow, pH and conductivity transmitters, as well as associated pressure gauges and switches. Unfortunately, completing all these calibrations didn't look like it was going to be possible in the plant's 10-day window.
To reduce the time required for each calibration, Wolfe and his team adopted a MC5 multifunction calibrator and CMX calibration software from Beamex. This allowed them to perform more calibrations in the field, instead of taking instruments back to their workshop for calibration, and also minimized the risk of impulse line leaks. They also created a CMX calibration database that further reduced calibration time. The plant's instrument technicians also adopted the concept of combining a loop test with each calibration by working in pairs via radio.
"Less time is wasted with technicians returning to the shop to swap equipment because MC5 can carry out most calibrations with just the one calibrator" explains Wolfe. "Being able to download multiple jobs to MC5 also means a day's worth of calibrations can be given out at the start of a shift, so the instrument technicians can plan their day better. Using the Beamex setup helped us successfully halve the time needed to complete outage calibrations, and enabled us to comply with company standards without increasing labor costs."
Wolfe adds his team's solution has also been rolled out to the Wissington plant's bio-ethanol application, and CMX software is being used to automatically transfer work orders and other data to British Sugar's CMMS system. Their solution is also in the process of being implemented at the other British Sugar plants.
Schlegel adds that Cabot's improved calibration methods have also helped it minimize downtime. "We had a situation not long ago where the Fluke 381 amp meter helped us find a persistent intermittent problem that really had us theorizing," adds Schlegel. "One of our palletizer machines kept tripping out every three or four hours. We'd shut it down and take some ohm readings using the 87 DMM, but nothing showed up. So we reset the machine, and the main circuit breaker (MCB) would trip again later.
"Thinking that inrush current was probably the problem, we attached the 381 clamp meter to our MCB, closed the cabinet and moved away from the machine with the remote display. Then we ran the machine until it tripped and saw on the display that constant current wasn't the problem—it was the inrush current from a chattering master contactor. With the help of the 381's min/max feature to help identify a continual inrush current, it turned out that the problem was a loose connection on one of the terminals to the coil of the contactor. The contactor energized the hydraulic pump circuit, which helped us rule out the pump's solenoid circuitry. Without the 381, we wouldn't have been able to rule out the other possibilities so quickly because we were able to troubleshoot the machine with the main enclosure door closed, and not have to suit up in our category-rated electrical PPE gear."