By John Rezabek,Contributing Editor
Why do we install an intrinsically safe instrument system? One reason is the ability to do live work, such as connecting and disconnecting an instrument for calibration or troubleshooting. The other reason is the significant likelihood that the area where an instrument is located routinely has a flammable or explosive mixture of fuel and air present. Were at a point today where, in many cases, both of these needs are going or have gone away already.
There was a day when a cloud of hydrocarbon vapor could unexpectedly emerge from a sewer in a refinery or petrochemical plant. Knock-out drums, separators and storage tanks all require periodic blow-down to remove accumulated water, and the condensate, basic sediment and water were usually accompanied by flammable liquids.
Furthermore, 20 years ago, interface detection was imprecise at best, and virtually no one applied any microwave or capacitance detectors to achieve a better cut. Every day, many times a day, slugs of flammable liquids headed down the drain on the way to in-plant treatment systems.
Then about 15 years ago, the National Emissions Standard for Hazardous Air Pollutants (NESHAPS) went into effect. Refineries and other process plants that had to be in compliance spent many millions installing new closed sewer systems, which serviced practically every hydrocarbon-containing vessel that previously drained to the sewer. And, since the separation and treatment facilities that treated the material were limited in size and scope, greater discipline was applied when any such vessel was drained.
Regulations dealing with fugitive emissions also have been in play for some time, and most of us are subject to Leak Detection and Repair (LDAR). In my plant, a specialty contractor who checks every valve stem, pump seal and flange in a Hazardous Air Pollutant (HAPS) service, comes in at least quarterly for an inspection. Sniffers, as we call them, seek out leaks with sensitive detection equipment, and were bound by regulation to fix anything found on a strict schedule.
One consequence of these relatively new practices, I would suggest, is that many plants no longer have any bonafide Zone 1 areas. Below-grade areas, where heavier-than-air flammables can accumulate can be minimized, and we can avoid installing instruments in the few genuinely Zone 0 or Division 1 areas we still have. Furthermore, such areas require an entry permit for any personnel to enter, essentially the same effort as a hot work.
Another factor that reduces or removes the need for IS systems is digitally integrated instruments. Newer plants are being built from the ground up to allow digital access to all instruments, whether through a standalone asset management system or the DCS/host system itself. If routine calibrations and diagnostics are checked from the control house using wireless Ethernet to the house or wireless HART to individual devices, even opening an enclosure to hook up a test device can be eliminated. With the exception of the hopefully rare instances where a field device needs to be replaced, I can think of very few situations when one would ever need to lift a wire.
The emergence of digitally integrated instruments, stricter environmental regulations and wireless access to diagnostics are converging to make live maintenance in normally hazardous atmospheres a rare occurrence. Shouldnt end users ponder whether the value once delivered by an onshore IS installation is still worth the added complexity, effort and expense?
