Heuer adds, A multivariable flow transmitter with an integral flow computer that can communicate via fieldbus avoids the communication issues by performing all functions within the same device. Communication to the control or data logging system can be performed using fieldbus and provide access to the measurements.
In contrast, Wes Meger, an instrument/electrical supervisor in the Operations Reliability Group at Suncor Energy, Fort McMurray, Alberta, Canada, has non-custody applications where non-fieldbus instruments can provide the required performance as transmitters improve and become more stable. The additional accuracy afforded using fieldbus communications might be nice, Meger reports, but the availability of flowmeter diagnostics is a larger driver.
Diagnostics Communicated Here
A part of improving flowmeter performance is increasing the amount of time that the flowmeter produces good measurements. The ability to communicate flowmeter diagnostics to the control system can allow faster detection of failed or failing flowmeters. In addition, diagnostics signals can alert users of impending failures and give them enough time to prevent them before they actually happen.
Joe Incontri, director of sales and marketing for Krohne in Peabody, Mass. says, Fixing the flowmeter prior to complete failure keeps the flowmeter producing good measurements for a larger percent of the time and can reduce the number of process upsets. For example, consider a magnetic flowmeter that has diagnostics that can detect electrode fouling.
Because electrode fouling usually progresses slowly until the flowmeter fails to operate, the user can be alerted to address the problem while the flowmeter is still operating and avoid a sudden failure, Incontri says. Doing so improves flow measurement performance and reliability by keeping the flow measurement online and operating properly for more time, which in turn, can reduce the number of process upset conditions attributable to this flowmeter, concludes Incontri.
This is especially important when the flowmeter is difficult to access, points out Edgar Fajardo, senior instrument and electrical engineer, BASF Wintershall Noordzee, Netherlands, where, for about the last year, fieldbus instruments have been installed on a small unmanned platform that communicates to land via a fiber-optic link. Fajardo says, Diagnosis of an instrument without remote diagnostics requires that the technician take a costly helicopter ride to an off-shore platform to examine the instrument. Remote diagnostics via fieldbus can reduce the number of trips by allowing some problems to be fixed from his office on land. They can also ensure that the technician has the proper equipment and parts should a trip be required.
Back on land, Wes Meger reports, We save time by being able to adjust flowmeter filters from the instrument shop in about 10 min. versus the 30+ minutes it would take to get a permit and send a person to the flowmeter. The work can also be performed in a more comfortable environment considering that the outside temperature can approach -40° C (-40° F.) in the winter. Wes also says that when it comes to diagnostics, you cannot have too much access. While I may not need the information on a daily basis, it should be there when it is needed.
Fieldbus technology can improve flow measurement performance by eliminating signal conversions that introduce error because the flowmeter measurements are communicated digitally. Field control can improve performance by allowing more computations to occur in a given time. In addition, fieldbus-enabled flowmeter diagnostics can be used to increase reliability and maintain good flow measurements for a larger percentage of the time.
David W. Spitzer has more than 25 years of experience in specifying, building, installing, commissioning and troubleshooting process-control instrumentation. Spitzer is a principal in Spitzer and Boyes LLC, which offers consulting services for the process industries in addition to product development, marketing and distribution consulting for manufacturing and automation companies. Contact him by visiting spitzerandboyes.com.