Intrinsic Safety

Reader Feedback: July 2008

Covering Intrinsically Safe Systems and Asking, How to Preserve Instruments/Valves In A Closing Plant?

Exaggerated Obituary?

I read with interest the recent column that raised the question “Instrinsic Safety Obsolete Yet?” (John Rezabek’s “On the Bus,” March 2008, p. 19 [The temptation to reproduce the spelling mistake was irresistible.] The column may reflect the current attitude of the North American refining industry, which has never been the most enthusiastic user of intrinsically safe systems, but possibly fails to take into account some other factors.

For example, many plant designs aim to be such that they can be reproduced in any part of the world with the minimum of modification to suit local requirements. These plants frequently use intrinsically safe instrumentation because it’s the only explosion-proof technique that’s universally acceptable. The increasing recognition of IEC Ex certification ensures this preference will grow.

Area classification is changing and Zone 1 installations are becoming smaller, but Zone 0s continue to exist, and intrinsic safety is the preferred solution in that situation. The effect of the risk analysis approach to plant safety implicit in the European ATEX directive and being introduced in the IEC standards may have an impact on the choice of instrument systems. The replacement of “nL” systems by the “ic” concept (intrinsically safe in normal operation) in IEC standards extends the use of intrinsic safety to all three zones. The consequent reduction in spares holding and the need for education in different techniques may prove significant. How far this change will penetrate the North American non-incendive scene is open to debate. The probability is that it will be patchy and will take some years.

The increasing use of intelligent instruments makes fault diagnosis from the safe area much easier, and reduces the need for complex test equipment in the hazardous area. However, the more complex field instruments are potentially less reliable, and there are still many simple devices, such as switches and thermocouples, that are much easier to check when not isolated. Working on a fieldbus network that has to be isolated to permit replacement or checking isn’t desirable. There remains a significant advantage in being able to do “live maintenance” without gas clearance certificates, and because the probability of making mistakes is reduced, it’s safer.

There are other applications of intrinsic safety, such as portable personal communicators and gas detectors, where intrinsic safety is the only practical solution. These applications probably aren’t relevant to the matter of the original note, but are a considerable factor in keeping intrinsic safety alive. Similarly, there’s potentially a significant use of intrinsic safety where the explosion hazard is created by dust.
In short, to paraphrase a well-known saying, “Reports of the obsolescence of intrinsic safety are possibly a little premature.”

Chris Towle
Founder, MTL Instruments

Help Wanted

I’m a technician at Vale’s Hydromettalurgical Plant in Argentia, NL, Canada. We’re getting ready to close down our plant for a couple of years. It’s a small-scale plant used as a test facility to determine if hydrometalurgy was the most economical and effective process to recover nickel, copper and cobalt from ore. We have about 2,500 instruments and valves in our plant. The majority of our instruments are Coriolis meters, magmeters, temperature, pressure, radar level and pH txs and small (1/2 to 1 in.) Bauman & Fisher valves. We run Emerson’s Delta V control system. The plant will start up again in two years. How do we preserve all the instruments/valves in our plant for the next couple of years? Thanks!

Cory Kelly
Instrument Technician, Vail Inco

Send suggestions to wboyes@putman.net, and we’ll forward them to Mr. Kelly.

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