Control Valve Assessments
Control Valves are the workhorses of the process control world. If the control valve is not working properly, the entire control loop suffers. Before tuning a control loop, it is important to determine if the control loop is functioning properly.
PlantTriage monitors the control loops in your plant 24 hours a day. When the valve starts to show signs of mechanical wear, PlantTriage tells you about it.
Control Valve Assessments
With the accurate, targeted information from PlantTriage, you can improve Maintenance planning. This pages shows some of the control valve assessments you can get from PlantTriage.
PlantTriage's Valve Health Assessment provides an overall assessment of the performance of each control valve. "Valve Health" incorporates the results of several aspects of control valve performance, to give you the best summary assessment. This is a powerful way to evaluate the overall performance of control valves.
Another PlantTriage Assessment quantifies the effect of stiction on control loop performance. With "Sources of Variation", you can determine how much process variation can be attributed directly to valve stiction.
Often, plant personnel are asked to "tune the loop" to stop cycling. If the cycling comes from stiction, then you can't "tune" the problem. Focus your maintenance and process improvement efforts on fixing the right things, for the right reasons.
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 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.
PlantTriage can automatically diagnose instrument, control, and valve problems. It does this using the real-time data and 80+ Assessments available on each loop.
PlantTriage contains an expert system, using engineering rules developed over decades of experience. In a way, it is like a tireless engineer, with over 100 years of experience.
The end result is a specific diagnostic from PlantTriage, telling you exactly where to focus on each loop.
Like a human engineer, PlantTriage can draw conclusions by combining data from a variety of metrics. Now, PlantTriage can also assign a degree of confidence in each diagnosis.
Many control loops have more than one problem. There may be a problem with the controller, and another with the instrument or valve. With PlantTriage's Loop Diagnosis-Full, you get a complete summary of the problems within a control loop.
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.
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.
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.