Wireless HART’s Price Point

July 29, 2008
I Would Do This About 100 Times, and all My Stranded Diagnostics Finally Will Show Up in My Asset Management Software
By John Rezabek

The HART Foundation recently approved a standard for wireless HART, and one of the benefits for end users is the ability to “access stranded diagnostics.” If you’re like me and like the majority of end users, per recent trade press surveys, you have a significant number of intelligent devices from which diagnostics are accessible only when a handheld device is attached, if at all. Now, by installing an antenna on a HART-capable transmitter, you can view and record these on your asset management system.

It’s 2009 budget time, so if you have an initiative to use diagnostics and do more predictive maintenance, now is the time to complete a budgetary estimate. While most of the plant where I work is integrated with Foundation fieldbus, I have a not-insignificant number of 4-20 mA HART-smart devices, the majority of which are connected directly to our Triconex ESD system. I have about 100 such instruments with stranded diagnostics, and I want to start monitoring them.

This solution needs to be cheap. While the data is interesting, we thankfully haven’t been plagued by a lot of spurious instrument trips or any failures to trip due to a sick device, for which timely diagnostics might have saved the day. The only spurious trips have been due to lightning strikes, where the HART-smart temperature transmitters went dutifully to their NAMUR-standard upscale limit, while the rest of the electronics recovered from the jolt. Maybe diagnostics would have helped with the forensics in these situations to figure out how the process died, but perhaps only if the asset management system was polling the device at the time of the fault. Our HART devices have been impeccable otherwise. Most are of the same vintage as the plant, eight years and counting, so deriving a payoff will be tough.

If, on the other hand, you have a lot of control valves with HART-smart positioners, you probably can generate some better payback numbers. Users calculate this benefit by claiming that the number of control valves pulled for repair will be greatly reduced, spare parts will be reduced and plant productivity will improve. Only valves flagged for repair by diagnostics will be pulled, early warning allows for just-in-time part orders, and faulty valves are detected sooner.

The installation effort consists of installing a radio-plus-antenna package in the unused conduit connection of the HART device. After obtaining the necessary permits, the installer would coordinate with operations to take the device out of service—place loop in manual or bypass the associated shutdown, open the transmitter enclosure and connect the radio/antenna wires in parallel with the signal wiring, just as one would hook up a HART-capable communicator. The protocol is designed to be plug-and-play, so the instrument will in theory start sending data to a nearby gathering point.

I’d do this about 100 times, and all my stranded diagnostics will show up in my asset management software. What, then, is the cost per point?

The radio and antenna combos look like they will be available from multiple vendors. I would caution those whose instruments are in hazardous areas to consult with the instrument OEM about adding the antenna. Some are going to show up with integral seals that preserve, for example, a NEMA 7 rating.

The other end of the pair that’s landed in the transmitter ends up in the control house. What about the old-school method for extracting HART diagnostics, the HART mux? My asset management solution provider sells a 32-point version for roughly $1,500 each. For 100 transmitters, I’d have about $6,000 invested in hardware. The installation effort would be similar: lift each pair once while co-terminating the wires leading to the mux. The risks arguably could be greater. What happens if the tech gets on the wrong wire? What if something gets shorted out? If one is nervous about such activity, it might be deferred to a shutdown.

If I add a premium of $40 per pair for wire and incidentals, will a wireless solution be able to compete at $100 per device? Having a wireless infrastructure in place to support future “pure wireless” devices can be claimed as another benefit that won’t be there for the old-school practitioners. But if you budget for a wireless solution, it appears you will have plenty of money to get it done the old way.

John Rezabek is a process control specialist at ISP in Lima, Ohio.

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