Fieldbus - The first engineering solution for a new instrumentation technology
By Meir Stein
Contents
Changing Standards Pave the Way for New Technology
Fieldbus Design – Considerations and Restrictions
DCS Configuration and Data
Successfully Combining Old and New Technologies
Fieldbus, a new technology, is an all-digital, serial, two-way communications system that connects measurement and control equipment. There are three main reasons for the move toward fieldbus technology. These include: 1) the benefits of using digital transmission, such as receiving more information and increasing information accuracy, 2) the growing trend toward distributed control, and, primarily, 3) reasons of interoperability. All of these will result in lower installation and commissioning costs. The uniformity and standardization of fieldbus technology has been an issue for over a decade, but consistency in standards has recently begun to emerge. The ultimate goal is a single international fieldbus standard.
More and more owner/operators are incorporating fieldbus technology in their new projects – including small-scale projects, facility revamps, or pilots – and some are even using it in their large-scale, grassroots projects. For engineering, procurement, and construction firms (EPCs), this technology means changing design philosophies and adapting to new documents and terminology, which could lead to more competitive, less costly designs.
Fieldbus technology will bring more accurate and detailed information to management and maintenance, making instrumentation better manageable, and a more valuable asset. Process and plant decisions that once had to be based on assumptions and estimations will become more precise.
– Ian Verhappen
Engineering Associate
Syncrude Canada Ltd.
Intergraph Process & Building Solutions is taking a leading role in providing fieldbus engineering and design solutions that will benefit both plant owner/operators and EPCs. These solutions will result in lower cost of instrument maintenance, and better, more accurate information.
Changing Standards Pave the Way for New Technology (Return to Contents)
The emergence of fieldbus technology has brought about a new standard in instrumentation. Fieldbus technology introduces three major changes:
Each of these changes has, in turn, introduced changes in design and the presentation and documentation of engineering data.
Fieldbus instrumentation uses digital transmission and can measure and transmit more than one signal, not necessarily only as implied by the tag name (i.e., FT-100 may measure the flow, temperature, and static pressure of the fluid in the process), as whoever is reading the document should be aware.
Networking affects how instruments are connected and routed to the control system. The concept of a single pair of wires and a tag having a one-to-one relationship is no longer true. The relationship is now one-to-many, affecting the way wiring and loop drawings should be portrayed (see Figure 1).
The segment concept introduces new types of documents that need to be considered and engineered. New terms, including "home run," "spurs," "topology," and "chicken foot," will be integrated into instrumentation terminology and everyday use. Off-the-shelf termination blocks (Figure 2) will provide a standard, reliable way to implement system topology.
The control strategy assigned to the field instrument, which could be distributed between the DCS and the instrument, affects Field Instrument Data Sheets and documentation and presentation of the types of controls to be used.
Function blocks are software control algorithm blocks that require documentation and management, and were previously handled only by control engineers.
Fieldbus technology has introduced new terminology and new types of systems. Raising the issue of preparedness, this requires:
The following should be considered in the design process.
New Symbols for Piping and Instrumentation Diagrams (P&IDs)
Currently, there is no standard set of symbols to represent the fieldbus-connected instrumentation and illustrate that control may be implemented in one or more instruments within a loop. Clearly, new symbols are needed to support the new technology.
Instrument Index
Fieldbus instruments have a larger set of properties and corresponding values than conventional instruments. The set limitations and restrictions of power consumption, attenuation, and bandwidth on a single network require these properties to be documented, validated, and stored. Function blocks must also be specified and stored.
Data Sheets
The new set of properties should be individually specified for each fieldbus instrument, and the instrument vendor should comply with this new set of specifications.
Segment Assignment
Fieldbus has segments to which a finite number of select instruments are connected. Engineering tools must be capable of connecting more than one instrument on a single paired cable, using various topologies and commercially available termination accessories.
Fieldbus Design – Considerations and Restrictions (Return to Contents)
Fieldbus systems require special attention to correct design, particularly from a wiring and loading standpoint. Rules must be followed to ensure adherence to design parameters, avoiding signal attenuation across segments, and remaining within bandwidth limitations. If the rules are not followed, the validation tools provided by INtools will notify the designer.
Input/Output (I/O) Assignment Design
Conventional DCS I/O cards have channels. Fieldbus cards have segments connected to physical segment cables, each carrying the networked instruments.
In fieldbus, segment allocation is referred to rather than individual tag assignment. Once a segment is allocated to a position in the card, all signals connected to the segment are automatically in the DCS. INtools allows for association of segments to the fieldbus DCS I/O cards.
DCS Configuration and Data (Return to Contents)
Instrumentation engineering data common to the data within the DCS needs to be shared. Moving to fieldbus and smart digital instrumentation makes this common set of data even more important, because the DCS configuration tools can now read (and write to) the instrument directly. This makes the data in the DCS the "as built" data.
Plant Commissioning
After installation is complete, loop drawings are used as deliverables for plant commissioning. During startup, the plant is usually divided into process loops, which can be started as soon as all the instrument loops in the process units have been validated. Because instrument loops are a key element in the plant commissioning phase, conventional loop drawings as well as segment wiring drawings need to be considered. For standard maintenance practices, however, the ability to generate individual loop drawings may still be desirable.
Successfully Combining Old and New Technologies (Return to Contents)
Process involves regulatory control, and safety and shutdown instrumentation. Over time, plant owner/operators and EPCs should grow increasingly more confident in using fieldbus instrument systems for critical measurements or emergency shutdown systems.
As the changeover from conventional instrumentation to fieldbus technology occurs, it may foster a hybrid situation – mixed systems of both conventional and fieldbus technologies. Not only may they be mixed on a system level, but also possibly on a loop level – one instrument being a conventional 4-20 mA, and the other a fieldbus instrument! Our engineering tools will need to be capable of handling both kinds of designs, and provide tools for these diverse technologies during the interim period.
Intergraph Process & Building Solutions has identified the needs associated with mixing the old and new technologies. The new version of INtools (5.1) will provide the necessary tools to integrate and complement the emerging fieldbus technology.