Figure 5. Such networks rely on a direct line of sight between devices and the gateway.
Figure 6. Self-building and self-healing mesh adapts to plant changes, even temporary ones for maintenance.
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 (Figure 4). 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 from the engineer 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 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.
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 (Figure 5).
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?