Standards Certification Education & Training Publishing Conferences & Exhibits Process Control & Safety Symposium November Houston, Texas USA Tutorial: Diagnosing the Root Cause of Oscillations

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Presenter – Steve Obermann A Chemical Engineer with emphasis in process automation. Expertise in advanced process control designs for refinery and petrochemical process units, process modeling, inferential properties, real time optimization and cost/benefit estimation. Metso ExperTune develops and markets pre-packaged industrial software for the processing and manufacturing industries which maximizes productivity, efficiency and reduces waste. 2

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Agenda Find oscillation Fix oscillation Questions & Answers 3

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Find Oscillations 4 Observe! Amplitude matters - may not be a problem on the immediate loop….others? Cycles within cycles – which one are we concerned about? Don’t forget the Controller Output – affecting another loop?

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA The new way! Signal analysis Period and strength Automatic and always on Historical record 5

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Isolate the problem Put loop in manual—cycle gone? 6 YES Problem is local to this controller Valve or Tuning? NO Problem is someplace else Don’t waste time with ‘tuning adjustments’. Don’t calibrate the positioner. Start searching…. more to follow!

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Problem is local to this controller Try to eliminate the valve as the cause. Look at the closed loop trend of the measurement and controller output. 7 Valve Sticking Valve Hysteresis Per 1 Per 2 Per 3 Plot measurement vs controller output

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Closed loop Pattern not clear? Perform open loop testing 8 Hysteresis Stiction Sum up steps

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA So its not the Valve…. Sometimes it is not the valve…. (but most often it is) Good News: –you have already checked the valve operation and may have some bump test data for a model and tuning analysis. 9

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA The problem is someplace else… Start Searching –Look at trends of other loops that are along the process flow path. –Modern tools make a big impact in this situation. (frequency analysis) –A neat trick using PowerPoint… –Find a trend that appears related. –Set both trends time window to have the same amount of time. –Take a snapshot (snagit) of the trend region of each trend and paste image into a powerpoint page. 10

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Neat Trick 11

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Power Spectrum Pick cycling out of noisy measurement Use identified period to compare to other loops 12 Modern Way to find interacting loops Related?

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Modern Way to find interacting loops Multiple power spectrums Use the historical record of the period and strength Find loops with similar oscillation periods 13

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Modern Way to find interacting loops Apply mathematics….. (get tools to do it for you….) Correlation analysis 14

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Interaction: who is driving? Time shifting and correlation Strongest correlation on the leading time shift 15 “Neat Trick” performed with mathematics

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA Summarizing Oscillations come from 3 places –Valves, Tuning, External Identify the correct root cause Modern tools can speed this process up 16

Process Control & Safety Symposium November Houston, Texas USA Process Control & Safety Symposium November Houston, Texas USA 17 Questions? Thank You