Chemical Processing - May 2008 - (Page 22)

harder to find. Self-powered or loop-powered, the wireless network provides an information pathway without other major changes to wiring or control system hardware. Short-term monitoring. The low cost nature of wireless technology allows you to consider temporary measurements to help diagnose process problems. This could be as simple as using a strap-on temperature transmitter or replacing a local pressure gauge with a new pressure transmitter. To get a sense of how wireless can change the way you troubleshoot, consider the following issues raised at a refinery operations morning meeting: Operator A: “We have a problem in the PP splitter column (Figure 3). I just don’t have enough information to isolate and diagnose the problem. What I really need is a new pressure measurement at the top of the column.” Traditionally, the engineer responsible for the unit might reply: “I’ll call a meeting with Instruments and Electrical. We might have some spare cable pairs and trays up there, but we probably don’t. We’ll have to call in the construction subcontractors. I’ll look into it and get back to you later in the week.” Instead, with WirelessHART, the engineer might say: “No problem. We have a wireless pressure transmitter. We’ll stick the transmitter on the piping vent up there and have it running by lunch time. Just give me the tag and how often you want the data updated.” Operator B: “We seem to have more fouling in the pre-heat exchangers again and I’m not sure which ones to clean. We’re measuring a few of the temperatures, and there’re even some spare thermowells, but there aren’t any real data to help me with this problem.” Here, the conventional response might be: “I’ll call a meeting. We’re going to need some new cable trays in there as we used the last of the spare cable pairs during the ‘little learning event’ we had last year. We can probably find a few spare slots in the input modules but they’ll be spread out a bit. I don’t like these spaghetti solutions, though, because they make for trouble later on.” With WirelessHART, the engineer might instead say: “I’ve got seven wireless temperature instruments and thermocouples in stock. I could even borrow some more from another unit for a few days. They’re all WirelessHART, so there’s no problem with compatibility. We just need the mobile platform for access and a list of which ones to fit first. If you want, I can leave some up there permanently. Let me know when I can have the ones you don’t need back.” Closed loop control? One of the clear messages from customers is that they won’t consider closed loop control using wireless for at least five years — they want to get experience with the technology first. While 22 • May 2008 it’s possible to transmit process data from a measuring device over the mesh to a positioner/valve, this requires care in setting up the network topology to reduce time delays. Plus, the positioner would be constantly modulating the valve and would need significant power, thus perhaps ruling out a self/battery-power option. Given that the measuring instrument typically isn’t far away, a local controller using wired connection to the instrument and positioner would be a good solution when coupled with instrument WirelessHART connectivity back to the host. Meshing efforts Early adopters of wireless instrument networks have been using either proprietary solutions (e.g., for tank level or safety shower operation) or running early pilots of WirelessHART. The proprietary networks often relied on a star configuration with single line-of-sight connection to a wireless hub/gateway. Process plants have steel vessels, exchangers, piping, structural steel work, reinforced concrete, etc. So, it’s tough to set up a star wireless network with clear lines of sight from the gateway to each instrument. Then, what happens in the future as the plant is modified and maintained with scaffolding being erected near the wireless network? A self-healing mesh network with redundant pathways that can adapt to changing plant environments avoids such issues (Figure 4). ABB and other vendors have tested such mesh networks within plants and observed the mesh adapting to the changing environment and effectively coping with other interference sources. As often is the case with emerging technology, other groups — here, ISA (via the SP100 Committee) and Foundation Fieldbus and Profibus — have similar ef- >> Troublesome splitter Figure 3. Wireless pressure transmitter offers a quick and simple way to get necessary data. www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - May 2008

From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor WirelessHART signals a change at plants Avoid costly fabrication mistakes Watch out with variable speed pumping Hot cutover boosts control system migration Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - May 2008

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