Advanced Diagnostics to Detect Plugged AnnubarAdvanced diagnostics is the name given to a class of transmitters that provide additional information about the condition of itself and the process.  In addition to internal electronics problems, advance diagnostics can alert you to power supply issues and condensation in the housing.  Another feature is statistical process monitoring.  Rather than just measuring and reporting the primary variable (PV) of a process, statistical process monitoring also calculates the mean and standard deviation of a process variable.  The sensor in a process is able to measure and record much faster than the transmitter reports.  Any noise in a process is detected by the sensor, but not necessarily reported by the transmitter due to internal dampening.  While this is good for loop tuning and process control, it may not be good for process reliability.

Consider a car driving down a road.  The speed of the car is controlled via the cruise control.  The car measures the rotation of the tires and correlates this to speed.  The control then adjusts the fuel system to maintain this speed.  If the car were to leave the pavement and continue on gravel, the driver would not notice any change in the speed.  If only the speed were being monitored, there would not be any alert or alarm.  Using statistical process monitoring an alert would be created for this event.  The tires would report a much different profile between pavement and gravel.

Orifice plates with a differential pressure (DP) transmitter are used to measure flow in a process.  An obstruction in a process line creates a differential press which is correlated to flow.  The orifice plate is connected to the DP transmitter via impulse lines.  Plugged impulse lines caused by dirt and debris is a common problem in a process plant.  Detecting plugged impulse lines with advanced diagnostics is a common application.   While the PV of the transmitter may not have changed, the statical profile will be much different.  By monitoring the mean and standard deviation of the sensor signal an alert can be created for this condition.

An averaging pitot tube (APT) is another type of primary element used to measure flow in a process.  While orifice plates are the most common primary element, they create a significant amount of permanent pressure loss (PPL).  Pitot tubes create very little pressure loss, but do account for variations in the flow profile across a section of pipe.  An averaging pitot tube does account for flow provide and maintains low PPL.  Unlike an orifice plate, the openings in an APT are quite small compared to the pipe size.  As with impulse lines, the slots and holes can become plugged.  Using a transmitter with advanced diagnostics can alert you to plugging.

There are many more applications where advanced diagnostics and statistical process monitoring can benefit your process.  Included are pump cavitation, flooded columns and steam trap failures.  These will be presented in greater detail soon.

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