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Boosting performance in the process plant are several new innovations triggered by the digital spark of fieldbus. Jonas Berge reports.
Quality, throughput, and availability are critical in 21st century process plants. Ensuring these are maintained at high levels requires that control room and field operators make timely, informed and appropriate decisions concerning production assets – vessels, pumps, valves, heat exchangers, motors, etc.Enter digital fieldbus technologies.
Innovative, embedded capabilities of today’s fieldbus technology include Electronic Device Description Language (EDDL), function blocks, control-in-the-field (CIF), device/process diagnostics, and device alerts. These provide control room and field operators with the necessary information to ensure the all important optimal quality, throughput,and availability.
It was the Space Program that sparked electronics research and miniaturization, which in turn enabled modern consumer and industrial electronics. The CD (compact disk) provides good, high-fidelity sound, but switching to digital had secondary benefits such as innovations like MP3 players and purchasing and transferring tunes across the Internet. Digital was introduced for photography and telephony also resulted in new innovations. Thanks to digital fieldbus, that same innovation is now prevalent inprocess control.
Digital game changer
FOUNDATION fieldbus (FF, IEC 61784 profile 1/1 of IEC 61158) technology is the digital game changer that connects devices in a seamless web and bringing with it developments in the process control market for both devices and applications; it is a core technology of digital plant architecture for process manufacturing. Other protocols that provide acyclic data in addition to cyclic I/O (input/output), such as Profibus DPv1 and DeviceNet, have resulted insimilar innovations for factory automation.
In the first generation of fieldbus devices, a temperature transmitter provided similar functionality as an analog temperature transmitter. However, fieldbus is bidirectional and can transfer large amounts of information. Moreover, the control system in a digital plant architecture is permanently networked to the devices and continuously communicating in a tightly knit web. This enables device diagnostics to becommunicated in real time.
Hence, the first generation process diagnostics made a quantum leap in capability, including the ability to detect plugged impulse line in pressure transmitters, sense increasing amounts of friction in control valves, and provide statistical process monitoring (SPM) for abnormal situation prevention(ASP).
EDDL is the language used to describe device information available in products using HART, FOUNDATION fieldbus, PROFIBUS, and OPCUA. Born from the same file structure as is used by HTML web browsers – EDDL files are host platform independent thereby supporting mixing and matchingfield and host control system products.
Digital plant architecture uses digital device managers as part of asset management tools for device management. Combined with intelligent device management software, field device processors are able to “mine” predictive diagnostic data, place that data in the EDDL (IEC 61804-3) format confident that it will be displayed in the host system in a consistent, easy to use manner that permitstechnicians to take corrective action.
With fieldbus users and manufacturers openly sharing experiences and ideas, second generation fieldbus capabilities are appearing, including the distribution of advanced calculations and control in to field devices. These second generation fieldbus innovations have introduced several new categories of devices, especially in the area of process measurements. At the same time, the presence of device resident function blocks is being used by end-users to develop innovative applications.
Measurement innovation
Remote seals are devices used with differential pressure transmitters to measure the level in pressurized tanks or the density of liquids. Despite their many benefits, the use of remote seals in applications with large temperature swings can introduce significant measurement errors, especially in remote seal installations using small diaphragms or long capillaries.
However, using the peer-to-peer communication capability available only in FF devices, the value of one pressure transmitter can be communicated to a second transmitter where the differential pressure is computed using an arithmetic function block thus eliminating the need for remote seals in many applications. The overall advantage is that the total probable error (TPE) of the application is reduced thereby allowing the setpoint to be moved closer to optimum for increased plant throughput.
An increasing number of fieldbus temperature transmitters are capable of accepting two independent temperature sensor inputs, communicating value and status and making it possible to reduce the total number of temperature transmitters required for monitoring and control, with subsequent reduction in project cost.
Multiple temperature input cards in the control system I/O racks have long been used for monitoring applications with a large number of temperature points such as temperature profiles of columns, reactors and tanks. Although this solution can be cost-effective, fieldbus now enables an even better option for bulk monitoring of temperature.
A multi-channel fieldbus temperature transmitter is capable of simultaneously measuring up to eight temperature points and transmitting the individual values and status to host devices as well as other field devices. Mounting the transmitter in the field near the sensors, including in hazardous areas, eliminates the need for running three to four wires for each RTD (resistance temperature detector) or expensive thermocouple extension wires to each thermocouple.
Not only does this reduce cable cost and preserve valuable cable tray space, it also reduces cable installation labor costs. More importantly, when the transmitter, with its A/D converter, is located in field junction boxes near the sensors or within their own enclosure, the shorter wire runs increase accuracy and at the same time minimize the possibility of induced electrical noise interference.
Apart from project cost reduction, the improved accuracy and reduced electrical noise interference equates to reduced temperature reading variability resulting in greater quality and yield (a more uniform product results in less waste and rework), which far exceeds the initial project installation savings.
Device management innovation
Beyond better measurements, being aware when process conditions and device performance begin to degrade is highly valuable information when exploring opportunities to extend the time between plant turnarounds. Capturing failures that have already occurred or which are imminent is fundamental to device management, particularly for a predictive (proactive) maintenance scheme.
In spite of digital fieldbuses being able to communicate diagnostics, and software being able to interpret and display the results, intelligent device management did not become a reality until devices were continuously monitored and failures intelligently and automatically classified and prioritized.
Regular fieldbus alerts for device failure such as input fault, output fault, memory fault, and configuration error share a common priority with all other faults in a device. However, in a process plant, certain devices are more critical than others. A flow transmitter on a non-essential measurement point is less critical than a critical control valve. Likewise, some conditions detected by the device diagnostics are only anomalies while others can indicate a pending fault.
An innovative alert scheme, supported by some devices using FOUNDATION fieldbus, allows all device conditions to be classified, prioritized, and reported to the appropriate personnel including management, supervision, operators and maintenance technicians. Each alert can be classified as failure, maintenance, or advisory and be accompanied by supporting information so that informed decisions regarding the appropriate corrective action(s) can be determined.
Diagnostics innovation
Before device management can capture failures of machinery and other plant assets, supporting devices must be in place to monitor each machine’s health. The diagnostic information available from a fieldbus machinery health transmitter provides valuable predictive insight to help avoid untimely shutdowns by enabling a health check at any time and issuing an alert when device performance degrades belowpredetermined levels.
For example, in the case of a centrifugal pump, for which an untimely shutdown can be costly far beyond just the cost of repairing the pump, a diagnostic transmitter is mounted near the pump and includes multiple sensors for measurements such as vibration, temperature, coil flux, speed, etc. as well as computing an easy to understand performance index for the AC induction motor, coupling, and pump as well as an overall performance index for the entireassembly.
Detecting degrading performance is fundamental to avoid surprise shutdowns in the plant and to minimize unscheduled downtime by allowing technicians to take action before it is too late. At the sametime, production is not halted to service a pump which is stillhealthy. The result is greater availabilityand therefore greater output, all the whilereducing maintenance costs.
The overall impact on the plant can be quantified in terms of increased revenue, reduced expenses, and greater margins. The more critical the devices being monitored, the greater the return on investment of using intelligent fieldbus devices to improve process reliability and availability.
Innovative accessories
With the use of fieldbus technology potentially reducing the number of proximity switches, solenoids, on/off valves, and other simple discrete devices, and the benefits of using digital fieldbus being so compelling, many plants have begun efforts to move all their sensors and actuators to fieldbus technology.
However, some discrete points do remain and a fieldbus device that can be field mounted in hazardous areas, in field junction boxes, or completely within its own enclosure can be used to interface digital signals to the fieldbus. One such device provides eight discrete inputs, four discrete outputs, and user programmable logic capabilities. Similar to the wiring and installation labor savings described for process measurements, the use of digital FF logic devices reduces discrete I/O installation costs.
FF’s peer-to-peer communication capability permits a remote indicator to receive the measured value from a transmitter or the actual position feedback from a valve positioner. The remote indicator displays information from up to eight other devices while embedded function blocks permit computations such as pressure and temperature compensation, minimum and maximum selection, and characterization etc. to be performed andthe result displayed.
Beside the convenience of locating the display at eyelevel, fieldbus technology can improve operation. Rather than using impulse lines or remote seals to mount pressure transmitters at eyelevel, direct mount transmitters can be placed above or below the tank, while the remote indicator provides eye-level readings.
It should be noted that since remote diagnostics made possible with fieldbus reduces the need for routine inspection, the need to mount the transmitter in an accessible location is reduced, and therefore transmitters can be installed where it is optimal for accurate measurement and without compromising readability and accessibility. Because the measurement improvements are beneficial to plant output, installing transmitters in this way is rapidly gaining in popularity.
System integration innovation
While the benefits of using fieldbus are now well apparent and accepted, it is understandable that plants may want to “test drive” the technology on a small scale before committing an entire plant.
It is possible to attend to such applications by using a field-mounted fieldbus interface device housed in a metal enclosure providing power and communications coupling for up to four H1 fieldbuses having up to 16 devices each. Users have created applications where several interface devices are networked using Ethernet or RS485 and connected to the control system and intelligent device management software running on a separate workstation.
The intelligent device management software application is a critical component of an effective asset management optimization solution. Connected via FF to all the field instruments via the high-speed Ethernet, the solution provides technicians on-line, without interruption, access to all data and functions in the fieldbus devices, including identification and tag, general information, diagnostics, performance analysis, operational statistics, parameterization and ranging, simulation and override, calibration trim, and monitoring.
An HTTP Web server embedded in the interface enables configuration of its communication ports and underlying devices from most Web browsers connected to the same local network, but not across firewalls. The user loads EDDL files into the interface making it possible to configure any fieldbus device through the interface.
Because the interface device supports Modbus/RTU and Modbus/TCP, and has an optional embedded OPC server, it can also be connected to most legacy control systems while providing process variables from fieldbus devices to these legacy systems despite the lack of native fieldbus support.
Intelligent device management software can also be deployed separately from the control system with all device related information efficiently passing between the field instruments and the device management software application, thus legacy systems can evaluate the benefits of intelligent device management software and advanced FF diagnostic capabilities.
It is possible to build control or inferential measurement strategies such as compensated flow or level using the function blocks in the underlying devices. Simple PID (proportional, integral, derivative) loops can be executed in the field, often with better result than in a central controller although good engineering practices must be applied to each control strategy. Control in the field takes a little more planning, but can often create benefit throughout the life of the system.
A field-mounted interface device is an ideal solution for control and integration of small package units or skids. The interface device provides a single point of connection for 64 fieldbus devices and connects to the main control system providing access to key process parameters and, optionally, device management data.
Future innovation
Fieldbus is a great innovation that has triggered several new innovations, and it shows no sign of stopping. Based on the innovation in the consumer market that the development of digital audio, digital telephony, and digital photo has produced, who knows what the future will bring? The same digital spark that drove innovation such as CD, DVD, and the MP3 player is driving innovation around fieldbus.
The added value of the fieldbus technology goes well beyond reduced installation costs and faster startup. The real value is in increased process effectiveness and reduced cost of operation and maintenance benefiting plant owners, shareholders, and operators. This is enabled by a digital plant architecture that uses the power of field intelligence to improve plant performance, by delivering accurate, actionable information to the right person at the right time to make a difference.
With a digital plant architecture the combined improvements of these innovative devices are pulled together into a high performance fieldbus solution. World-class manufacturers are already adopting these solutions globally. CEA Jonas Berge is Senior Manager, PlantWeb Consulting at Emerson Process Management Asia Pacific, and the author of the books ‘Fieldbuses for Process Control’ and ‘Software for Automation’.
Fieldbus brings in diagnostics presented using Enhanced EDDL in device management software
Accessories: a fieldbus logic transmitter
