Plant-wide disturbance assessment with an application on a paper making process Zhang Di, M.Sc, Cheng Hui, M.Sc, Jämsä-Jounela Sirkka-Liisa, Professor 29 Jan, th Nordic Process Control Workshop

2 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

3 1 Introduction:  There are many pieces of process equiment and control loops in one typical industrial plant, and they interact with each other instead of isolating from each other.  So disturbance may propagate through the plant and affect a large number of process variables, evolving into a plant-wide problem.  The widespread nature of the disturbacne then makes it difficult to identify its orgin.

4 1 Introduction:  A plant-wide approach means the distrbution of a distrubance is mapped out, and the location and nature of the cause of the disturbance are determined with a high probability of being right first time.  The alternative is a time consuming procedure of testing each control loop in turn until the root cause is found.

5 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

6 2 Plant-wide disturbance detection More from the single loop disturbance detection, the plant-wide disturbance detection includes the identification of clusters of measurements having similar dynamic behaviour. Thornhill and Horch (2007)

7 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

8 3 Root cause diagnosis Sources of persistent dynamic plant-wide disturbance The non-linear sources include: For example:  Control valves with excessive static friction  On-off and split-range control  Sensors faults  Process non-linearities leading to limit cycles  Hydrodynamic instability such as slugging flows The linear sources include:  Poor controller tuning  Controller interaction  Structural problems involving recycles

9 3 Root cause diagnosis  Diagnosis has two objectives, the identification and the isolation of the disturbance. Thornhill and Horch (2007)

10 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

11 4 Methodology for plant-wide disturbance assessment

12 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

13 Testing environment :APROS interface The basis weight valve is under concern because the poor performance caused by the malfunction of valves is quite common in industry.

14 5 Case study on Paper making process Measurement

15 5 Case study on Paper making process Disturbance Power detection spectra

16 5 Case study on Paper making process Root cause Diagnosis

17 5 Case study on Paper making process  In Choudhury et al. (2004), the existence of non-linearity in a system can be decided by the Gaussian test and the nonlinearity test.  If the signal is non-Gaussian it goes through another test to determine its linearity.  The nonlinearity index (NLI) is provided as shown

18 5 Case study on Paper making process  If the process is identified as linear, the root diagnosis path presented by Bauer and Thornhill (2007) is applied to find the disturbance propagation path and to build the causal diagraph.  Basic idea: To get to know the cause-and-effect relationship through the time delays between process variables.  Procedure: 1 Time delay estimation (cross-correlation function) 2 Build the causality matrix 3 Consistency check It is used to verify and ascertain the results. The limit is N-2.

19 Contents:  1 Introduction  2 Plant-wide disturbance detection  3 Root cause diagnosis  4 Methodology for plant-wide disturbance assessment  5 Case study on Paper making process  6 Results

20 6 Results Disturbance detection Process measurements after mean centering and unit deviation scaling

21 6 Results Disturbance detection ACF of time series for the seven variables

22 6 Results Disturbance detection Power spectra of the seven variables

23 6 Results linear identification Squared Bicoherence calculation for V1 - Basis weight valve opening

24 6 Results

25 6 Results: propagation path 1 Causality matrix: 2 Consistency check: 3 Then: It is used to verify and ascertain the results. The limit is N-2.

26 6 Results: Propagation path 4 The causal map: The root cause is most likely close to the variable 1 - Basis weight valve opening

Thank you for your attention!