When our technician saw an alert, an instrument alarm of sorts that would typically show up in his AMS Device Manager alarm summary, he ignored it. It was a "travel accumulator” alert. Like an odometer for a valve, it expresses the totalization of valve travel. Like the one in your car, it’s not the best indicator of "service needed soon.” Typical automobile oil change guidance goes something like "8,000 miles unless your driving includes dusty, hilly, stop-and-go, etc. Then change every 3,000 miles.” We’re left confused about when to change the oil, and so it is with valves.
The right thing to do for our automobile might be to have our oil analyzed every 2,500 miles or so, and characterize its viscosity, contamination, oxidation, water, etc., to see how far along it really was from "like new” to "totally spent.” Maybe you can skip an oil change, or maybe it should have been changed at 1,000 miles. Like cars, valves vary greatly in the severity of their service, but the cost and complexity of maintenance, as well as the high cost of neglect, are orders of magnitude higher than a stop at the corner Quik Lube. Preemptive valve maintenance — the equivalent of the guy who changes his car’s full-synthetic oil every 2,500 miles — is difficult, dangerous and burdensome on operations and maintenance schedulers. Breakdown maintenance can have dire consequences for plant availability. Ideally, we want our smart positioners’ alerts to wake us up at the ideal time — long enough before a failure to plan and schedule a repair. Many times, this is well within the realm of possibility, but probably not with factory default alert settings.
Analyzing a setting or alert at any level of travel accumulation is a bit of a conundrum. Do we have any basis for determining at what "mileage” we want the "needs maintenance soon” lamp to light up? What does 1 million percent of travel represent?
Every fieldbus valve positioner I’ve seen and most of their HART brethren have some setting for a travel accumulation diagnostic, and if you don’t bother to change it or turn it off, it will come on at the default setting. All the Metso ND9000 positioners I’ve looked at have a default of 250,000 (percent, one would assume) and also have the same setpoint for actuator travel. So I get both at about the same time, assuming no slippage between actuator and valve stem. Does this mean the actuator needs service at the same time as the valve?
Probably not. It’s not Metso’s fault. End users need to decide whether 1) we enable a given alert, 2) where exactly we want to set the alert and, oh yeah. 3) what priority do we give it? It would be nice to determine specific actions we want technicians to take. And when an alert comes in, and I decide I don’t care (yet), how do I reset it or raise it to a new threshold?
A Fisher valve (www.emersonprocess.com) positioner’s travel accumulation alert is set at 1 million. I don’t believe the folks in Marshalltown are implying that their valve will go four times as long as one from Neles. It’s just a different arbitrary default. HART and fieldbus positioners and fieldbus "placeholders” in DCS engineering tools all ship with a similar set of defaults for scores of diagnostic settings. There are 10 tabs of settings for a Fisher DVC6200, including alerts for supply pressure (high and low) and temperature (high and low), drive signal, travel and maybe 20 others. The default settings could leave the impression that most of them are set so they’ll never bother you.
If users want to get value from their smart devices, they’ll have to start tinkering with these defaults. Turn off the vague ones like travel accumulation. Enable and tighten up settings like supply pressure, travel deviation or stiction that have a chance of alerting us to a potential breakdown. Most important, train and empower your people, so they can demonstrate how thoughtful, proactive use of intelligent devices can have a real impact on reliability. That won’t come "by default.”