Control loops troubleshooting

Instrument Technician. Start 19 to 31 per hour 39,520 per year. Work on level, fluid flow, pressure, and temperature instruments and control loops troubleshoot maintain operations. 

The cat cracker plays a key role in the overall profitability of the refinery. It must operate reliably and efficiently. It must also operate safely and comply with federal, state, and local environmental requirements. A typical FCC unit circulates tons of catalyst per minute, processes various types of feedstock and uses hundreds of control loops, any of which can make operation difficult. Proper troubleshooting will ensure that the unit operates at maximum reliability and efficiency while complying with environmental concerns. 

The second major troubleshooting use of simulation during startup was on the natural gas feedstock control system. After the trip of one of the methanol plants, severe transients in the feedstock system caused the second methanol plant to trip due to overpressure lifting PSV s and tripping on low steam to carbon ratio. This led to a complex trip and a downtime of days. The problem was found to be in the natural gas letdown controls (see figure 5). Simple as this control loop appears, it caused a number of trips during startip. 

To apply control to a process, one measures the controlled variable and compares it to the setpoint and, based on this comparison, typically uses the actuator to make adjustments to the flow rate of the manipulated variable. The industrial practice of process control is highly dependent upon the performance of the actuator system (final control element) and the sensor system as well as the controller. If either the final control element or the sensor is not performing satisfactorily, it can drastically affect control performance regardless of controller action. Each of these systems (i.e., the actuator, sensor, and controller) is made up of several separate components therefore, the improper design or application of these components, or an electrical or mechanical failure of one of them, can seriously affect the resulting performance of the entire control loop. The present description of these devices focuses on their control-relevant aspects. Later, troubleshooting approaches and control loop component failure modes are discussed. 

The key to effective troubleshooting is expressed in the old adage, divide and conquer. It is important to locate the portion of the control loop hardware that is causing the poor performance the hnal control element, the sensor system, the controller, or the process. The place to start is to test each system separately to determine whether that portion of the control loop is operating properly. The hnal control element can be evalnated by applying a series of input step tests. That is, the input to the hnal control element, which is normally set by the controller, can be manually adjusted. The test allows the determination of the dynamic response and deadband of the actuator system. If the performance in these two areas is satisfactory, there is no need to evaluate the actuator system further. 

Because the microprocessor is so different physically from a series of interconnected analog controllers, a description of troubleshooting a microprocessor control loop will be helpful in understanding the differences. Troubleshooting is still performed on a functional subdivision basis, just as it would be done on a hard-wired interconnected loop but the variations in configuration must be considered. In the microprocessor control system, subdivisions are not separate controllers but rather the Input, Output and Computational... 

The material in this book can be useful for engineers, technicians, and plant operators concerned with the design, operation, and troubleshooting of process control systems for distillation columns. The course can also be useful for students who want to gain insights into the practical approach to distillation process control in industry and tuning control loops in a plant control room. 

Process Instrumentation—course for study of the instruments and instrument systems used in the chemical processing industry includes terminology, primary variables, symbology, control loops, and basic troubleshooting. The purpose of this class is to provide students with an understanding of the basic instrumentation and modem process control used in the chemical processing industry. 

Control loop—system consisting of a collection of instruments that work together to control pressure, temperature, level, flow, and anal3itical variables. Information from control loops is invaluable in the troubleshooting process. 

Explain how control loops are used in process troubleshooting. 

A controller may be left in manual for long periods of time (or indefinitely) if the operator is not satisfied with its performance in the automatic mode. Consequently, if a significant percentage of the controllers in a plant is in manual, it is an indication that the control systems are not performing well or that the plant operators do not have much confidence in them. The topic of troubleshooting poorly performing control loops is considered in Chapter 12. 

This section provides a brief introduction to the basic principles and strategies that are useful in troubleshooting control loops. More detailed analyses that provide useful insights are available elsewhere (Buckley, 1973 Ender, 1992 Riggs and Karim, 2006 Lieberman, 2008). 

An important consideration for troubleshooting activities is to be aware that the control loop consists of a number of individual components sensor/transmitter, controller, final control element, instrument lines, computer-process interface (for digital control), as well as the process itself. Serious control problems can result from a malfunction of any single component. On the other hand, even if each individual component is functioning properly, there is no guarantee that the overall system will perform properly. Thus, a systems approach is required. 

If a control loop is not performing satisfactorily, then troubleshooting is necessary to identify the source of the problem. This diagnostic activity should be based on a systems approach that considers the overall performance of the control loop as well as the performance of individual components. Retuning the controller is not a cure-all, especially if a sensor or a control valve is the source of the problem. Automated monitoring techniques for control loops are currently being developed and are commercially available (see Chapter 21). 

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