Process plants abound with instruments, analyzers and control valves, all of which need calibration to ensure performance as designed. Many plants calibrate these devices at fixed intervals, but that's less than optimal for a number of reasons. First, it's expensive, as many devices can operate within parameters on extended calibration schedules.
Second, it can result in poor operating performance, as some critical instruments should be calibrated more frequently. Third, plant safety can be compromised if safety-related devices drift out of calibration between intervals.
Calibrating each device only as needed is the better method, and that requires automating the calibration process. Smart devices can provide information to an asset management system (AMS) or a calibration management system (CMS) over a digital data link. These systems use this information to determine optimal calibration intervals. They also send data to documenting calibrators, which are used to calibrate the devices. After calibration, these calibrators upload the "as left" condition of the device to the system.
Here's how it works in practice. "Our customer GlaxoSmithKline (GSK) has a pharmaceutical manufacturing plant in Cork, Ireland, with more than 4000 control loops with HART and Foundation fieldbus," says Laura Briggs, the product manager for asset optimization at Emerson Process Management.
Plant personnel were routinely calibrating instruments that did not need the same level of attention as devices that were critical to product quality or safety. To determine which instruments could be moved from the periodic schedule to on-demand calibration, they examined the diagnostics generated by every smart field device and digital valve positioner using Emerson's AMS Suite predictive maintenance software. They began monitoring a select group of less critical instruments, waiting for them to indicate that a change had taken place internally requiring attention. As time progressed, all of the smart devices were migrated to on-demand calibration.
Streamlining regular calibration procedures is based on optimizing the plant's periodic calibration schedules using documenting calibrators and synchronizing instrument data between Beamex's CMX CMS and Emerson's AMS Suite. Calibration data on every instrument is stored and downloaded directly to a portable calibrator for use by a technician in the field. When the scheduled calibrations are completed, the results are uploaded for certification and documentation.
"Savings due to this paperless calibration procedure were 15 minutes cut from each calibration, 21,000 sheets of paper eliminated each year and more than 500 hours per week of manual data entry time eliminated, along with potential errors," notes Briggs.
GSK also extended the interval between calibrations to reduce the overall number of procedures done annually while remaining in compliance with corporate policy and government regulations. The company achieved this through an ongoing, computer-driven analysis of historical data to identify instruments that didn't need to be calibrated as often, resulting in an 8% reduction in scheduled calibration.
Substantial upfront effort is required for any calibration optimization project. "The most time-consuming aspect of getting started isn't the acquisition cost of the equipment or software, but rather populating and setting up the AMS or CMS software with the information from the critical tags and their calibration attributes," explains Jim Shields, process tool marketing manager, Fluke.
"Once the CMS is populated, the data can be mined to make more educated determinations of instances where maintenance intervals can be adjusted," Shields adds. "Most CMS packages have tools for drift plot analysis to help examine the performance of a device, its errors and drift since its last calibration. If these or other tools show a device is performing reliably, then it's a good candidate for an extended maintenance interval."
