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elcome to this newest addition to the Digital Fieldbus Network world. It is my intention that every month we will discuss at least one aspect of what is happening in the world of industrial fieldbus. Since there are in excess of more than 30 different fieldbuses in use in the industrial marketplace this should not be too great a challenge, though to make this column successful it will require input from you the readers on topics that you would like to learn more about.
Being from the wet process industries, my natural tendency will be to focus on the fieldbus networks used in this realm Foundation Fieldbus, Profibus, DeviceNet, AS-i, HART, and Modbus.
So what makes a fieldbus? This question has been debated for some time with the purists saying a fieldbus must be all digital, while others profess that it need only be able to communicate digital information. It appears that the consensus is leaning towards the latter definition and is why the HART communications protocol that uses a Frequency Shift Key (FSK) signal superimposed on an analog signal is normally included on the fieldbus list.
Of course, incorporating fieldbus technology in a device and control system is not done simply to please the engineers so they have something new to play with, it must have an economic incentive and payback. This payback must appeal to both the people approving the project initially, project managers and such who are concerned with Capital savings but also more importantly the operations people who need to operate and maintain the equipment of 15+ years after the project has been turned over to them.
The first fieldbus installations were justified on wiring savings. Since that time, experience has shown that in many cases the cabling costs or more precisely the physical layer costs are about the same, partly because fieldbus systems require additional components not found in a traditional analog loop. Where the savings are being found, however, is in the field labor and installation. Fieldbus systems, being multiplexed signals, have far fewer terminations per signal, as much as 50% less than a traditional loop. At todays labor rates and shortage of skilled tradespeople, this can quickly add up to a significant savings.
Another opportunity for fieldbus is its use as a standard multiplexer-demultiplexer. It always seems that the location that runs out of spare cables first is the one furthest from the control center, for example the far end of the tank farm. Fieldbus technology offers an easy way to at least triple the number of cables in the field. By converting the field instruments and I/O card in the host to fieldbus technology, the multiplexing capability of fieldbus will allow for installation of more than one device on a single wire. The down side is that the control system is now a mixed signal and a way must be found to differentiate one signal and cable type from the other. Maintenance, engineering and operations personnel also will have to learn about multiple technologies and, of course, the host system will have to be able to support the fieldbus technology as well. In the case of some older control systems, this may require installation of newer technology being grafted on to the legacy system, perhaps through a gateway.
The more significant economic component of any installation is the Operational Expense phase, since that is where more than 80% of a project's life cycle costs are incurred. Day-to-day operation of a facility over 10+ years also has the advantage that any savings realized here are captured every day throughout that period, versus the typical one time only for each phase of the Capital portion of the program. Despite the fact that there is little documentation in the literature about the life cycle savings attributable to fieldbus technologies, there are references to savings equivalent to between 1-3% of plant capacity. This extra capacity is being captured without any physical changes to the process, but instead is being realized from the diagnostic capabilities of the field devices that enable a facility to better predict the true health of equipment and thus operate a facility longer, reduce the number and impact outages and effectively result in true prognostic maintenance working only on those devices that need maintenance when they are closest to failure and not before.
The above gives you an idea of the breadth of topics we can cover in this column, so please contact me with ideas on what aspects of fieldbus technology you would like to explore from the nuts and bolts mechanics, through operational and maintenance concerns to economic analysis and justifications. As control folks, all of you know that it is through feedback that control improves, so please provide me the feedback I need to help you better.
My next column will cover the thorny issue of why many Engineering Procurement Construction (EPC) companies are not adopting fieldbus technologies as part of their offerings.
