Recent enhancements to the EDDL (Electronic Device Description Language) standard have helped make advanced setup and diagnostics of pressure transmitters easy.
Pressure transmitters are not used only in pressure applications, but also for level and flow. In addition to measurement, today’s pressure transmitters diagnose their own health as well as that of the sensor module, along with checking the integrity of impulse lines, and detecting abnormal process condition. The diagnostic functions enable more effective maintenance schemes, keeping loop and plants up and running with minimum downtime.
Easy pressure calibration trim
For the plant to perform at its best, the calibration must be done correctly. Yet, the required procedure may vary depending on the sensing technology used, meaning technicians require proper guidance.
Early versions of configuration and calibration software did not support ‘wizards’ to guide the technician’s work. Sensor trim was difficult, particularly for early fieldbus transmitters requiring ‘mode’ and other such parameters to be set, after which the mode had to be returned for the transmitter to be operational. This required expert knowledge.
Wizards take the mystery out of transmitter calibration by enforcing the correct sequence. Not only do these wizards remind the technicians to inform operators to put the loop in manual and provide step-by-step instruction on when to isolate, equalise, and vent manifolds, but also hide the complexity of fieldbus by taking care of mode switching.
The wizards are provided by the transmitter manufacturer incorporated in the EDDL file that comes with the transmitter from the factory. That is, manufacturer know-how is built-in to enforce proper steps unique to the transmitter are taken in the right order.
EDDL wizards work not only on Windows computers, but also on handheld field communicators that can be brought to the field for in-situ sensor trim. These rugged yet light-weight tools are highly valued by technicians, as they reduce mistakes, ensure correct setup of the transmitter, and make field work easier. EDDL is the only device integration technology supported on universal handheld communicators.
Advanced pressure diagnostics
Several application problems incurring costs to plants have eluded a solution while using regular transmitters and detectors. For instance, furnace burner flameout due to varying BTU content results in costly downtime and has an element of risk.
An industry-wide problem is plugged impulse line to the pressure transmitter, a serious issue because the actual pressure cannot be measured, resulting in wrong or non-existent control and safety action.
Statistical process monitoring
In the past, transmitters were limited to only providing rudimentary diagnostics able to diagnose if the sensor was dead or alive. Today, some transmitters have sophisticated diagnostics able to detect abnormal process conditions in a wide variety of applications based on statistical process monitoring. The sensor samples pressure at high frequency (more than 20 times per second) and the transmitter internally computes the statistical mean of the pressure and standard deviation of the process noise. For a significant change in noise with no change in pressure the transmitter alerts the technician to the impending problem.
Abnormal condition detection using statistical process monitoring is already proven for plugged impulse line, furnace flame instability, and catalyst ‘stick slip flow’.
Extensive diagnostics are particularly capable for transmitters using Foundation fieldbus as these are allowed to draw more current than their analog counterparts and therefore have more powerful microprocessors to run more sophisticated software. Thus abnormal situations cannot only be monitored, but prevented.
Effective set-up of statistical process monitoring is simplified by EDDL graphics designed by the device manufacturer and displayed as intended on any EDDL-based system.
Because DCS typically only registers input updates once a second or slower, followed by damping to filter out the noise and transients, statistical analysis of noise or capturing intermittent events in software is not effective. Data from historians is even less frequent. The statistical process monitoring and transient capture must be done close to the sensor to enjoy the necessary update rate.
Graphics unplugged
Set-up of advanced diagnostics such as plugged impulse line detection previously required special plug-in software components because the original Device Description technology from 1992 did not support the required graphics. The new EDDL technology in the IEC 61804-3 standard jointly developed by the Fieldbus Foundation, HART Communication Foundation, Profibus Nutzerorganisation e.V. (PNO), and OPC Foundation supports trend charts display, signal waveform plots, needle gauges, table grids, and images, enabling sophisticated devices to be set-up without specialised software.
Pressure expert know-how
Nobody knows the transmitter better than the manufacturer. Using EDDL the manufacturer can impart their expert knowledge to the user using several different ways, and this becomes accessible from any system based on EDDL. First, information is organised logically in a menu system such that related information appears on the same page. Text and images are included to illustrate installation, applications, or other concepts.
Lastly, the transmitter manufacturer provides help text for all parameters and wizard, which appears at a click of the help button. Together, the know-how embedded using EDDL makes use of the transmitter intuitive.
Requirements
Therefore it now makes sense to upgrade the device management software in control systems to support the new standard IEC 61804-3 to take full advantage of smart capabilities in intelligent devices. Also make sure to specify EDDL for new projects and devices. This technology is already supported by leading systems and instrument manufacturers and will be demonstrated in the multi-vendor showcase at the ISA Expo in Houston in October 2008.
Jonas Berge is director of PlantWeb Consulting at Emerson Process Management.