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PlantTriage Oscillation Detection

Oscillation in process plants is a common problem that PlantTriage can help you resolve. Oscillation almost always causes lower quality product and causes end product variability. PlantTriage provides several analysis methods for detecting, identifying, diagnosing, and removing the cause of oscillation.


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How Does Oscillation Affect Your Plant?

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PlantTriage Oscillation Analysis

For every loop, during every unit operation assessment interval (that you define per unit operation), PlantTriage analyzes the loop for oscillations. This analysis attempts to determine if an oscillation is present in the loop. If an oscillation is present, a diagnoses is made of the cause of the oscillation. Because loop data is not always representative of "normal operating states" i.e., it may have set point changes, or close to a limit, etc., the analysis is assigned a confidence level. Each analysis is scaled on a 0 to 100% basis, with 100% being fully confident of the diagnosis.

Oscillation Detection

If the oscillation analysis detects that an oscillation is present then the assessment called "Oscillating" will move towards the upper end of the range with the maximum being 100%. If no oscillation is present, "Oscillating" trends towards 0%. If oscillations are detected, the analysis diagnoses the cause. The cause will have 1 of the following 3 diagnoses:

Assessments of Oscillation

It's not enough to know only that the loop is oscillating. You'll want to quantify the problem, and get to the root cause. The sections below show you how PlantTriage points you in the right direction to eliminate the root cause of the oscillation.

Oscillating—Tuning

The oscillation is diagnosed to be caused by aggressive tuning. 0% means the oscillation is not from aggressive tuning. Values towards 100% indicate higher confidence that the oscillation is caused from aggressive tuning.

With an "Oscillating—Tuning" diagnosis you should check the tuning of the loop using a process model. Process models are often obtained automatically using Active Model Capture.

All aspects of loop tuning are handled with PlantTriage, and the fully integrated PID Loop Optimizer.

Oscillating—Load

The oscillation is diagnosed to be from a load or load upsets. The higher the percentage, the more confidence in this diagnosis, up to 100% confidence. This diagnosis can occur from interaction with other loops.

To check the interaction with other loops use the PlantTriage Oscillation Periods and Strengths discussed below.

Oscillating—Valve

If the oscillation is diagnosed to be caused from a valve stiction or hysteresis then this assessment will have a value above 0%. 0% means the oscillation is not from your valve. Values towards 100% indicate higher confidence that the oscillation is caused from the loop's valve.

A value above 50% warrants a stiction and hysteresis check be done on the valve:

  1. From PlantTriage, view the trend of the loop
  2. Click the PID Tuner icon and select the Faceplate window
  3. Follow instruction in PID Tuner for the stiction and hysteresis checks

PlantTriage Oscillation Periods and Strengths

Each loop has a dominant oscillation period. This is Oscillation Period 1 and is calculated from the power spectral density for each loop. If there is a cyclic oscillation in your plant, viewing a list of loops with the oscillation periods as a column can help you quickly group all the loops that exhibit this oscillation - even when it is "hidden" in other information. This will help you quickly pinpoint the cause of the oscillation - usually the most upstream loop. If the cyclic loops are interacting loops then first check to see if the oscillation diagnosis, discussed above is "Oscillating—Valve" for any of the loops. If it is then this is the first loop suspect to check for stiction or hysteresis causing a cycle.

PlantTriage browser list showing Oscillation periods and strengths

The sample report above shows the details of each oscillation, including the periods, strengths, and the likely cause of each oscillation.

For every loop, during every assessment, PlantTriage calculates the relative power at all oscillation periods. The three periods with the most power are recorded. The highest three periods are the most important oscillations of the process variable signal. The three oscillation periods and their corresponding amplitudes are used as performance criteria.

The Oscillation period is in units of seconds. Oscillation strength is in percent of total energy in the spectrum (except the DC component).

Total energy = sum of all amplitudes minus the DC component

Oscillation Strength = Energy at this frequency / Total Energy * 100%

Oscillation Significance

A very powerful PlantTriage Assessment is "Oscillation Significance". This assessment compares oscillations against the other sources of variation in a control loop (noise, SP changes, load upsets), to determine which oscillations are the most significant issues in the plant.

Use the Oscillation Significance assessment to prioritize your efforts.

Long Period Oscillation

PlantTriage searches back through continuous Process Variable data looking for oscillations with long periods. PlantTriage determines both the strength and the period of these long oscillations. Use PlantTriage to understand long-term cycling in your plants.

Root-Cause Oscillation Analysis

List all oscillation components together to quickly pinpoint oscillation sources.

In most plants, there are many oscillations. PlantTriage helps you sort through these oscillations by detecting up to three oscillation periods on each loop.

For example, the primary oscillation on one loop may cause a secondary oscillation on another loop. PlantTriage allows you to compare all these oscillations together, so you can quickly find the root cause of the problems.

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.



 

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