13/12/2016 2 of 23 Content What is fault? Why we detect fault in a control loop? BSc Project What kinds of fault we may confront in a control loop? Fault Detection & Diagnosis What is “stiction”? in Control Valves Comparing “stiction” with other factors Ways we can resolve “stiction” Shahriar Shahram Supervisor: Dr . Nobakhti Contributors : Milad Pooladsanj Shahriar Shahram Sharif University of Technology Fault Detection & Diagnosis in Control Valve 3 4 of of 23 23 Why we detect a fault? What is fault? A. Increasing product quality Controller Actuator Plant SP Output B. Reducing the rate of rejection Sensor C. Fault signals propagation in physical components D. Minimizing risk of instability Controller : poor tuning is not fault Actuator Fault : valve friction Plant Fault : leakage , human error Sensor Fault : calibration Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control valve Fault Detection & Diagnosis in Control Valve 1
13/12/2016 5 6 of of 23 23 CLPM :Control Loop Performance Monitoring CLPM :Control Loop Performance Monitoring CLPM steps are: An important prerequisite for assessing control loops which is based on routine data 1. Diagnosis and closed loop. 2. Fault identification CLPM usually connects to PID controller, because we don’t want to specify additional parameters except the normal controller in order to have an automatic procedure. 3. Isolation CLPM often used in chemical , petrochemical , pulp and paper industry. 4. Root cause CLPM can be grouped according to three different criteria : A. Mathematical Methods B. Nature of Time Trends C. Prevalence of Faults Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 7 8 of of 23 23 Mathematical Method Results Time Trend Properties Saturation Saturation manifests itself at the controller output. Saturation can result from a poorly dimensioned actuators, poor controller tuning or may be caused by integral wind-up of PID controllers after a large setpoint change occurred Sinusoidal Oscillation Generally, as oscillations become sinusoidal as they travel through a process via physical connections since the process acts as a low pass filter Manual Control If the controller is not acting satisfactory the operator or control engineer often switches it off. This is seen in the control system as a switch from automatic to manual mode Reference1 Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 2
13/12/2016 9 10 of of 23 23 Time Trend Properties Time Trend Results Sluggish Behavior This means that the process variable only slowly follows the setpoint. There is no overshoot after a setpoint change. The reason for sluggish behavior lies usually in the tuning settings. Non-Linear Oscillation Some control loops contain a nonlinear element such as a faulty valve. Nonlinear disturbance affects the process variable regularly and repetitively but contains higher frequency components and can therefore be distinguished from a sinusoidal oscillation Quantisation of process variables All signals are digital in amplitude. The resolution of the signal is determined by the sensor. The sensor may have a coarse resolution so that the time trend shows step-like features Reference1 Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 11 12 of of 23 23 Time Trend Results Prevalence of Fault Reference1 Reference1 Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 3
13/12/2016 13 14 of of 23 23 Prevalence of Faults Valve Stiction What is stiction referred to? • According to the Instrument Society of America (ISA) (ISA Subcommittee SP75.05, 1979) ‘‘stiction is the resistance to the start of motion, usually measured as the difference between the driving values required to overcome static friction upscale and downscale’’. • According to EnTech (1998) ‘‘stiction is a tendency to stick-slip due to high static friction. The phenomenon causes a limited resolution of the resulting control valve motion. ISA terminology has not settled on a suitable term yet. Stick-slip is the tendency of a control valve to stick while at rest, and to suddenly slip after force has been applied’’. Reference1 Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 15 16 of of 23 23 Valve Stiction Valve Stiction What is stiction referred to? Why stiction is important ? • According to Horch (2000) Stiction as 1 st frequent fault in control loops ‘‘The control valve is stuck in a certain position due to high static friction. The (integrating) controller then increases the set point to the valve until the static Stiction increases the instability chance in whole system friction can be overcome. Then the valve breaks off and moves to a new position (slip phase) where it sticks again. The new position is usually on the other side of the Stiction reduces control valves lifetime desired set point such that the process starts in the opposite direction again’’. product quality will decrease Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 4
13/12/2016 17 18 of of 23 23 Control Valves Types Resolving Stiction We have three ways to reduce the control valve stiction problem: 1. Test each valve individually In this method we should disconnect the control valve and test it for a specific input .Then we measure the output and we can recognize if it has stiction or not. Advantages You will recognize all of the valves that have stiction Disadvantages • The whole system must be shutdown • Loosing time Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 19 20 of of 23 23 Resolving Stiction Resolving Stiction 3 . Proccesing output signal of the valve 2. Implementing sensors in valve In this method we can approximately recognize the faulty valves by doing some In this method we use several sensors so that we can analyze the output mathematical algorithms on different output signals in control loop. signal of sensors , monitoring them and check if there is stiction. Advantages Advantages You will find faulty valves without shutting down the plant You will find faulty valves without shutting down the plant Time saving Time saving Low cost Disadvantages Disadvantages • Expensive , because we use many sensors • The sensors may be faulty • Some of the methods can be used only in certain conditions (such as availability of • some sensors) (In Iran) even the main sensors of the system are not available! • Most of the methods are not applicable in industrial systems Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 5
13/12/2016 21 22 of of 23 23 Conclusion Challenges 1. We can improve control loop performance by detecting the faulty valves and repair 1. Derive a good modeling for valve stiction them 2. Working on algorithms to process the output signal 2. Valve stiction is very important. The first reason is that valve stiction causes 30% of loops work poorly. Another reason is when a valve has stiction , the risk of instability 3. Increasing the accuracy of results will rise. 3. By proccessing the output signals in control loop we can determine the faulty valve with more advantages comparing to other methods. Shahriar Shahram Shahriar Shahram Sharif University of Technology Sharif University of Technology Fault Detection & Diagnosis in Control Valve Fault Detection & Diagnosis in Control Valve 23 of 23 References 1) Margret Bauera, Alexander Horcha, Lei Xieb, Mohieddine Jelali , Nina Thornhilld . The current state of control loop performance monitoring – A survey of application in industry (2015) 2) T. Hagglund. Department of Automatic Control, Lund Institute of Technology, S-221 O0 Lund, Sweden. A CONTROL-LOOP PERFORMANCE MONITOR(1995) 3) M.A.A. Shoukat Choudhurya, N.F. Thornhillb, S.L. Shaha. Modelling valve stiction(2004) Shahriar Shahram Sharif University of Technology Fault Detection & Diagnosis in Control Valve 6
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