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HART Communications and Advanced Diagnostics Work Together to Maximize the Productivity of Instrument Technicians

Instrument maintenance has become increasingly difficult in today's climate of budget constraints and downsizing. Most facilities must accomplish more with fewer personnel and senior instrument technicians' abilities must be utilized to the maximum.

Senior instrument technicians can better manage their time by accessing process information such as flow, temperature, pressure and composition remotely. Flue gas oxygen (O2) measurements are particularly well-suited to remote communications for the following reasons:
  1. The O2 measurement is important to the safe, efficient operation of any combustion process.
  2. Oxygen probes are usually mounted in hot, inaccessible locations where instrument diagnostics are difficult to perform. Personnel safety is an important consideration.
  3. Analyzers offer more diagnostic information than transmitters that measure physical quantities such as pressure, flow, temperature, etc. A typical oxygen analyzer will offer 12 to 15 diagnostic alarm indications.
  4. In situ oxygen sensing cells are in direct contact with process flue gases and degrade over time due to the effects of sulfur and other flue gas components. Consequently, calibration is required more frequently than with a pressure/flow transmitter. Unlike these instruments, a calibration standard (bottled gases) can be applied to the analyzer automatically using solenoids, eliminating the need for technicians to perform calibrations at the installation site.
Calibration intervals for zirconium oxide (ZrO2) cells vary greatly depending on the application and the preferences of operating personnel. Some in situ ZrO2 oxygen analyzer users report that they calibrate once a week. However, depending on the application conditions and requirements, many in situ ZrO2 analyzers are calibrated only once per year. Typically, a new ZrO2 cell operating in a low-sulfur flue gas environment should maintain accuracy for at least a year without requiring recalibration. However, as the ZrO2 sensor cell ages, cell degradation accelerates and calibration intervals will likely become shorter and shorter. A ZrO2 cell near the end of its life may require calibration every few weeks.

Rosemount Analytical has developed a diagnostic feature that measures cell impedance on-line once every hour. This feature has been incorporated into the Oxymitter In Situ Oxygen Flue Gas Transmitter. The Oxymitter's 4-20 mA signal output representing oxygen is unaffected during this instantaneous measurement of cell quality. When cell impedance increases to a point where a need to calibrate is indicated, an automatic calibration can be initiated. Contact closures are provided to notify the plant operators of the calibration. If an automatic oxygen trim control loop is used, the loop can automatically be placed into manual until the calibration is complete.

With HART communications, a single senior technician can now diagnose problems and manually initiate calibrations for many analyzers from a convenient location. When a trip to the analyzer is required, the problem is identified and the necessary tools and spare parts can be carried along.

Emerson Process Management's Asset Management Solutions (AMS) software provides all of the HART communications functionality described plus keeps a record of calibrations and alarms over-time, providing a predictive maintenance tool to indicate impending failures before they occur.