PlantTriage Field Instrument Diagnostics
PlantTriage realtime performance management is continually performing valve diagnostics, control loop assessment, and controller performance assessment. Most field instrument diagnostics are available using your web browser.
Comparison of Field Diagnostics Capabilities
ISA Paper Compares PlantTriage Diagnostics to Fieldbus Diagnostics
Paper on this topic originally presented at ISA 2005.
Click here to read the paper.
PlantTriage Advantages
Works with every type of field connections or bus
From any DCS connection to the field - works with 4-20mA signals, fieldbus, profibus, or HART. Since PlantTriage works with plant data collected realtime via OPC or from your historian, no special connection protocol is required. Diagnostics are done on-line, on a running plant. No need to take the plant off-line to do the diagnostics. Extraordinarily rich set of diagnostics with zero process intrusion. See sample diagnostic comparison with Fieldbus.
Allows remote diagnostics
Most field instrument diagnostics are available using your web browser using the PlantTriage web-server on your LAN. Perform diagnostics without a trip to the plant.Uses normal plant data
Performs diagnostics from normal plant operating data. Does not require specific tests and all loops stay in automatic mode.Field Diagnostic Assessments
Valve Travel
This is the total distance the controller output moves during the assessment period normalized to be on a 24 hour basis.
It is found by summing the absolute value of the controller output change between each sample. This number is then divided by controller output span and normalized to be on a 24 hour basis.
Valve travel is a direct indication of valve wear. For many processes it can be an indication of an opportunity to achieve the same or higher performance with less valve movement, reducing maintenance costs. ExperTune's experience with thousands of clients shows that a reduction in valve travel of a factor of 5, roughly doubles valve life. Benefits include extended equipment life, reduced cost and higher performance from the controls and process.
Valve Reversals
Valve Reversals is the number of times the controller output changes direction above a threshold value. This threshold is automatically determined by PlantTriage based on the process noise. Valve Reversals is normalized to be on a 24 hour basis. More reversals result in more valve wear. Benefits are similar to valve travel in that valve reversals can be reduced while achieving higher performance and reduced maintenance costs.
CO Noise band
This is the statistical analysis of the percent noise in the controller output (CO) during the assessment period. PlantTriage finds the noise band by finding the standard deviation (sigma) of CO during the quietest time.
If the CO noise band is large, then valve travel and valve reversals should be checked to see if there is a problem with the valve or PID tuning. Alternatively, measurement noise could be coming through the controller. If the CO noise band is large, this may be an opportunity to add a PV filter, or perhaps check the derivative value for proper setting.
Noise band
This is the statistical analysis of the percent noise during the assessment period. PlantTriage finds the noise band by finding the standard deviation (sigma) of PV during the quietest time.
Normal process noise is minor variations in the process variable not caused by the control system, setpoint changes or load upsets. Normal process noise may come from electrical interference, magnetic fields, turbulence in flow controllers, waves in a tank in level controllers, etc. Generally the larger the noise band the poorer the performance of the controller. A very fast cycle in the controller will appear as noise.
On a bell shaped curve of all the quiet time data points, 3 standard deviations above and 3 standard deviations below the mean (for a total of 6) represent the noise band. 99.7% of all data points in the quiet time fall within the noise band.
PV noise band can be baselined to indicate problems with field transmitters. A shift in the PV noise band indicates a possible problem in the transmitter or measurement system. Possible problems indicated could include a partially plugged orifice plate, transmission lines, fouled sensor, or failure of the transmitter itself.
PV Availability
The "PV Availability" is the percentage of time that the PV was available for analysis during the assessment period. It is a measure of the quality of connection for data flowing into PlantTriage.
Output Standard Deviation
The "Output Standard Dev" is the standard deviation of the controller output.
For many processes it can be an indication of an opportunity to achieve the same performance with less valve movement, reducing maintenance costs. It also is an indicator of valve movement or movement of inner controls. Lower values are generally better.
Output At Limit
Output At Limit is the percentage of time during an assessment that the output was saturated at a limit.
Output At Limit indicates that the control valve is up against a hard constraint and automatic control has ceased. This may be indicative of a valve incorrectly sized or an unachievable process target sometimes referred to as a hard constraint. Additional process problems that could be indicated with "Output At Limit" include insufficient pump head, poor heat transfer, damaged or missing valve trim, etc.
High "Output at limit" values could indicate a bottleneck in the process. This could occur after increased plant throughput and quickly pinpoints the bottleneck of future increases.
Time in Normal
The Time in Normal is the percent amount of time the controller spends in Normal mode. Typically normal would be Automatic, but it could be any mode specified.
In general, the more time in normal mode, the better. If the controller is not in normal mode, then the effectiveness of this controller and the process is likely compromised. A simple campaign to raise the "Time in Normal" of a population of controllers will result in significant performance improvement. A larger population of controls will be in a higher mode, consistent with the designers intention, thereby raising adherence to process objectives.
Get a Demonstration of PlantTriage
There is so much to see in PlantTriage. Get a private guided tour. Demo can be accomplished over the web, and takes about 1 hour.
Request a demonstration on-line or Contact ExperTune at +1 (262) 369 7711.