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MAR-APR 2018

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42 INTECH MARCH/APRIL 2018 WWW.ISA.ORG By Jacques Smuts, PhD, PE E very plant in the process industry most likely has several control loops that do not perform well. Although there are many factors that may negatively affect control loop performance, a few of these factors are most common. With some skill and knowledge, it is reasonably easy to identify which of these com- mon factors is the root cause of poor control loop performance. With closed-loop control, poor performance typically manifests itself as deviations from set point. By analyzing these deviations, one can determine if they are caused by suboptimal controller tuning settings, a poorly perform- ing control valve, insufficient filtering of the mea surement signal, or simply process dis- turbanc es where the frequency and magnitude exceed the regulatory capabilities of a control loop. The diagram (figure 1) can be used as a troubleshooting tool to help determine the root cause of poor control loop performance. Much of the troubleshooting that follows requires looking at process trends. Normally these are time trends of the process variable (PV ), set point (SP), and controller output (CO). The time span of the trends depends on the dy namic nature of the process—fast-respond- ing processes, such as flow and liquid pressure, require only a few minutes of data for proper analysis, while slow-responding processes may require data spanning several hours. Control sys - tems have built-in or add-on process historians that can be used for viewing process trends. Oscillations The first step in troubleshooting poor control is to determine if the deviations of the PV from its SP are random or cyclical in nature. With cyclical deviations, or oscillations, the PV peaks at fairly regular intervals. Random deviations do not have this regularity (figure 2). A visual inspection of the PV trend is all that is required to make this determination. Once it has been determined that the control loop is oscillating, one should determine the ori- gin of the oscillation. A control loop can be caus- ing its own oscillation, or it could be affected by an interacting oscillation somewhere else in the process. A simple way to determine if the origin of the oscillation lies within the control loop or has an external source is to put the control loop in manual to see if the oscillation stops or contin- ues. If the oscillation continues, it is likely caused by an external source. The exception here is if the control valve's positioner oscillates by itself. This can be determined by leaving the controller in manual mode, going out to the valve in the field, and seeing if the valve stem is oscillating. Closed-loop control troubleshooting

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