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| Home > Articles > Contemporary diagnostic and control devices |
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Contemporary diagnostic and control devices |
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Early detection of anomalies, either process related or device related, is the key to improving plant availability. Fault detection through field devices, i.e. transmitters, valves, motors would improve the diagnostics capability and possibility of early anomaly detection. Once an anomaly is detected, it must be isolated and related to the most probable source, i.e. valves, motors, pumps or transmitters. Isolation of the proper source will enable a diagnostics system to perform more advanced diagnostics to determine the root cause of the problem. Determining the root cause of the problem will enable plant engineers and technical personnel to arrange necessary changes to prevent similar problems from occurring in the future.
With the field-centered architecture, intelligent field devices do most data reduction, before the diagnostic information is passed to the rest of the automation architecture. This means that diagnostic summaries can be sent to the system with a frequency of minutes or hours, not seconds. It also means that device or process specific diagnostics can be done in the field device, thus limiting the types of raw values needing to be sent through the automation architecture while adding to the types of diagnostics that can be provided. Information available to the raw sensor, but not available to the system can be used to perform analysis that is not possible in any other way. Superior sensing technology allows better information capturing and analysis. Some sensors with sampling in the 10s or 100s of Hz range can provide noise spectrum information that can detect potential problems in motors, pumps, or other un-instrumented equipment. This type of information is not available at all in conventional automation architectures. Finally, by combining this information with process control actions such as mode, setpoint, load, and other changes, fundamental insights to the health and performance of the equipment and the process become available for the first time. In addition, since much diagnostic information is communicated using foundation fieldbus, device descriptions can be used to make the information understood by hosts.
Superior sensor performance for fast responses and ability to transfer high frequency information furthered the capabilities of field-centered architecture to improve availability and reduce variability in an operating process plant. However, there is still a need for a higher-level technology that interprets the information gathered from various parts of the plant to provide the right feedback to the operators at a timely manner.
Technology for information interpretation
A diagnostic system that accesses only process control information, or only equipment information can draw incorrect or incomplete conclusions. For example, a setpoint ramp can look exactly like sensor drift. A bias can look like a setpoint or load change. The only really effective way to know if a change is due to a normal, or abnormal condition is to have access to both process control and equipment health information. By correlating the two, a diagnostic system can effectively determine if a change reflects a problem or a normal condition, and determine the root cause of the problem.
In a traditional architecture, there is no mechanism to make this type of information available to the field devices, thus requiring a diagnostic implementation that is centralised. In addition, most centralised diagnostic systems do not know the health of the field equipment. This means that centralised systems must assume the equipment is operating correctly. This can lead to both false alarms, and missed problems. A diagnostic system needs to be able to automatically access conditions such as setpoint, load, mode, and other changes that could affect the statistical model. This allows the diagnostic system to rebuild or modify its definition of normal to match the changing process conditions. This way a diagnostic system can effectively distinguish between problems and normal operational changes.
The field-based architecture is uniquely able to accomplish this. Since the field-centered architecture supports control in the field and equipment performance, this information is available in the field devices. The field devices can automatically compensate for operational changes while still providing detection of both process and equipment related problems. And the field-centered architecture can effectively isolate the problem to a specific loop or piece of plant equipment. This is critical to the speedy identification of the root cause of the problem, and hence to a fast repair and efficient use of plant maintenance, operations, and engineering resources. If a high level diagnostic system that is, for example, monitoring performance of a heat exchanger, is to be effective, it must know the health of the sensors and control elements associated with the heat exchanger.
If, for example, a temperature sensor fails, or even drifts, or if an impulse leg on a pressure transmitter measuring flow is partly plugged, it will provide false readings that will cause the diagnostic system to reach incorrect conclusions. These incorrect conclusions could result in an unnecessary shutdown, or worse may lead to equipment failure that could have been prevented. Any diagnostic strategy must have as its base solid and reliable information on the health and performance of the measurement or actuation point.
New architecture for diagnostic and monitoring systems
A failure is any event or condition that reduces production, increases cost, or negatively effects quality, safety, or the environment. To be effective, a diagnostic system must address this broader definition of failure. Effective diagnostics must address two fundamental areas of need. The first is availability. Simply stated, the plant equipment, and by extension, the plant must be available to make product. The second is performance. Simply stated, plant equipment and the process itself must have both high and repeatable performance to yield the right quality at the best economic point.
The most economically advantageous operating point is at the top of the safety band. In this area the plant is operating at the highest safe levels at all times. Most plants operate at far lower levels. Many operate in a comfort zone where production and profits are acceptable but sub-optimal. Through normal wear and tear, performance drops to unacceptable levels and failures or unscheduled shutdown may occur. Scheduled shutdowns for maintenance are often used in order to reduce unscheduled shutdowns and return performance to acceptable levels.
The cost of reduced plant performance is frequently underestimated. Studies have shown that a significant percent of loops are under-performing, sometimes seriously. In addition, loops that are performing adequately often have room for improvement.
It is necessary to understand the cause of poor performance, to be able to successfully address performance issues. One area of poor performance is excess process variability.
Clearly a better understanding of equipment health will improve both availability and performance. But to address both availability and performance, equipment health must be combined with process control performance to determine overall performance and the potential for improvement.
Summary
Current worldwide competitive conditions mandate lower cost, higher efficiency, and more consistent quality. Optimizing process control will contribute to accomplishing these objectives. However, without knowledge on the performance of the assets in the plant, process control optimisation cannot return the improvements needed. The field-based architecture provides the opportunity to determine current levels of performance and theoretical performance. In addition, it provides the capability to continuously monitor performance, isolate problems, and determine root causes in a more cost effective way than conventional architectures can provide. This combination of capability means that the field-based architecture can provide sustainable performance and financial advantages over conventional architectures.
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| Posted : 8/24/2005 |
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