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1. Introduction There are many types of automation, broadly defined. industrial automation, and to some extent its related discipline of building automation, carries some specific principles. The most important skills and principles are discussed in Section 1 of this guide. It is critical to recognize that industrial automation differs from other automation strategies, especially in the enterprise or office automation disciplines. industrial automation generally deals with the automation of complex processes, in costly infrastructure programs, and with design life cycles in excess of 30 years. Automation systems installed at auto motive assembly plants in the late 1970s were still being used in 2008. Similarly, automation systems installed in continuous and batch process plants in the 1970s and 1980s continued to be used in 2008. Essentially, This means that it’s not possible to easily perform rip-and-replace upgrades to automation systems in industrial controls, whereas it’s simpler in many cases to do such rip-and-replace in enterprise automation systems, such as sales force automation or even enterprise requirements planning (ERP) systems when a new generation of computers is released or when Microsoft releases a new version of Windows. in the industrial automation environment, these kinds of upgrades are simply not practical. 2. Standards Over the past three decades, there has been a strong movement toward standards-based design, both of field instruments and controls themselves and the systems to which they belong. The use of and the insistence on recognized standards for sensor design, control system operation, and system design and integration have reduced costs, improved reliability, and enhanced productivity in industrial automation. There are several standards-making bodies that create standards for industrial automation. They include the international Electrotechnical Commission (IEC) and the international Standards organization (ISO). other standards bodies include CENELEC, EEMUA, and the various national standards bodies, such as NIST, ANSI, the HART Communication Foundation, and NEC in the United States; BSI in the United Kingdom; CSA in Canada; DIN, VDE, and DKE in Germany; JIS in Japan; and several standards organizations belonging to the governments of China and India, among others. For process automation, one of the significant standards organizations is ISA, the international Society of Automation. ISA's standards are in use globally for a variety of automation operations in the process industries, from process and instrumentation diagram symbology (ISA5 and ISA20) to alarm management (ISA18) to control valve design (ISA75), fieldbus (ISA50), industrial wireless (ISA100), and cyber security for industrial automation (ISA99). Three of the most important global standards developed by ISA are the ISA84 standard on safety instrumented systems, the ISA88 standard on batch manufacturing, and the ISA95 standard on manufacturing operations language. Other organizations that are similar to standards bodies but don’t make actual standards include NAMUR, OMAC, WBF (formerly World Batch Forum), WIB (the instrument Users' Association), and others. In addition, with the interpenetration of COTS computing devices in industrial automation, IEEE standards, as well as standards for the design and manufacture of personal computers, have become of interest and importance to the automation professional. There are also de facto standards such as the Microsoft Windows operating system, OPC (originally a Microsoft "standard" called object Linking and Embedding for Process Control, or OLE for Process Control, and now called simply OPC), and OPC UA (Universal Architecture). It is important for the process automation professional to keep current with standards that impinge on the automation system purview. 3 Sensor and System Design, Installation, and Commissioning It’s not generally in the purview of the automation professional to actually design sensors. This is most commonly done by automation and instrumentation vendors. What are in the purview of the automation professional are system design, installation, and commissioning. Failure to correctly install a sensor or final control element can lead to serious consequences, including damage to the sensor, the control element, or the process and infrastructure themselves. 3.1 The Basics The basics of sensor and system design are: identification of the application Selection of the appropriate sensor/transmitter Selection of the final control element Selection of the controller and control methodology design of the installation installing, commissioning, and calibrating the system 3.2 Identification of the Application Most maintenance problems in automation appear to result from improper identification of the application parameters. This leads to incorrect selection of sensors and controls and improper design of the installation. For example, it’s impossible to produce an operational flow control loop if the flow meter is being made both inaccurate and nonlinear by having been installed in a location immediately downstream of a major flow perturbation producer such as a butterfly valve or two 90-degree elbows in series. The most common mistake automation professionals make is to start with their favorite sensors and controls and try to make them fit the application. 3.3 Selection of the appropriate sensor/transmitter The selection of the most appropriate sensor and transmitter combination is another common error point. once the application parameters are known, it’s important to select the most correct sensor and transmitter for those parameters. There are 11 basic types of flow measurement devices and a similar number of level measurement principles being used in modern automation systems. This is because it’s often necessary to use a "niche" instrument in a particular application. There are very few locations where a gamma nuclear-level gauge is the most correct device to measure level, but there are a number where the gamma nuclear principle is the only practical way to achieve the measurement. Part of the automation professional's skill set is the applications knowledge and expertise to be able to make the proper selection of sensors and transmitters. 3.4 Selection of the final control element Selection of the final control element is just as important as selection of the transmitter and sensor and is equally based on the application parameters. The final control element can be a control valve, an on/off valve, a temperature control device such as a heater, or a pump in a process automation application. it can be a relay, a PLC ladder circuit, or a stepper motor or other motion control device in a discrete automation application. Whatever the application, the selection of the final control element is critical to the success of the installation. Sometimes, too, the selection of the final control element is predicated on factors outside the strict control loop. For example, the use of a modulating control valve versus the use of a variable-speed drive-controlled pump can make the difference between high energy usage in that control loop and less energy usage. Sometimes this difference can represent a significant cost saving. 3.5 Selection of the controller and control methodology Many automation professionals forget that the selection of the control methodology is as important as the selection of the rest of the control loop. Using an advanced process control system over the top of a PID control loop when a simple on/off deadband control will work is an example of the need to evaluate the controller and the control methodology based on the application parameters. 3.6 Design of the Installation As important as any other factor, properly designing the installation is critical to the implementation of a successful control loop. Proper design includes physical design within the process. Not locating the sensor at an appropriate point in the process is a common error point. Locating a pH sensor on the opposite side of a 1,000-gallon tank from the chemical injection point is an example. The pH sensor will have to wait until the chemical injection has changed the pH in the entire vessel as well as the inlet and outlet piping before it sees the change. This could take hours. A loop lag time that long will cause the loop to be dysfunctional. Another example of improper location is to locate the transmitter or final control element in a place where it’s difficult or impossible for operations and maintenance personnel to reach it after startup. installations must be designed with an eye to ease of maintenance and calibration. A sensor mounted 40 feet off the ground that requires a cherry-picker crane to reach isn't a good installation. Another example of improper installation is to place a device, such as a flowmeter, where the process flow must be stopped to remove the flowmeter for repair. Bypass lines should be installed around most sensors and final control elements. 3.7 Installing, commissioning, and calibrating the system Installation of the system needs to be done in accordance with both the manufacturers' instructions and good trade craft practices, and any codes that are applicable. in hazardous areas, applicable national electrical codes as well as any plant specific codes must be followed. Calibration should be done during commissioning and at regularly scheduled intervals over the lifecycle of the installation. 4 Maintenance and Operation 4.1 Introduction Automation professionals in the 21st century may find them selves working in maintenance or operations rather than in engineering, design, or instrumentation and controls. it’s important for automation professionals to understand the issues and principles of maintenance of automation systems, in both continuous and batch process and discrete factory automation. These principles are similar to equipment maintenance principles and have changed from maintenance practices of 20 years ago. Then maintenance was done on a reactive basis-that is, if it broke down, it was fixed. in some cases, a proactive maintenance scheme was used. in this practice, critical automation assets would be replaced at specific intervals, regardless of whether they were working or not. This led to additional expense as systems and components were pulled out when they were still operational. Recent practice has become that of predictive maintenance. Predictive maintenance uses the recorded trends of physical measurements compared to defined engineering limits to determine how to analyze and correct a problem before failure occurs. This practice, where asset management software is used along with sensor and system diagnostics to determine the point at which the automation asset must be replaced, is called life-cycle maintenance or life-cycle optimization. 4.2 Life-cycle Optimization In any automation system, there is a recognized pattern to the life cycles of all the components. This pattern forms the well-known "bathtub curve." There are significant numbers of "infant mortality" failures at the start of the curve; then, as each component ages, there are relatively few failures. Close to the end of the product's life span, the curve rises, forming the other side of the "bathtub." Using predictive maintenance techniques, it’s possible to improve the operational efficiency and availability of the entire system by monitoring physical parameters of selected components. For example, it’s clear that the mean time between failures (MTBF) of most electronics is significantly longer than the design life of the automation system as a whole, after infant mortality. This means that it’s possible to essentially eliminate the controller as a failure-prone point in the system and concentrate on what components have much shorter MTBF ratings, such as rotating machinery, control valves, and the like. 4.3 Reliability engineering For the automation professional, reliability is defined as the probability that an automation device will perform its intended function over a specified time period under conditions that are clearly understood. Reliability engineering is the branch of engineering that designs to meet a specified probability of performance, with an expressed statistical confidence level. Reliability engineering is central to the maintenance of automation systems. 4.4 Asset management, asset optimization, and plant optimization Asset management systems have grown into detailed, layered software systems that are fully integrated into the sensor networks, measure parameters such as vibration and software diagnostics from sensors and final control elements, and are even integrated into the maintenance systems of plants. A modern asset management system can start with a reading on a flow sensor that is out of range, be traced to a faulty control valve, and initiate a work order to have the control valve repaired, all without human intervention. This has made it possible to perform workable asset optimization on systems as large and complex as the automation and control system for a major refinery or chemical process plant. Using the techniques of reliability engineering and predictive maintenance, it’s possible to maximize the amount of time that the automation system is working properly-the uptime of the system. Asset optimization is conjoined to another sub-discipline of the automation professional: plant optimization. Using the control system and the asset management system, it’s possible to operate the entire plant at its maximum practical level of performance. Related Articles -- Top of Page -- Home |
Updated: Sunday, November 9, 2014 21:37 PST