Building intrinsic safety systems
There’s an old story that tells of a stage coach company that was advertising to hire a new stage coach driver. Three applicants had been selected for interviews. Each was asked the same question: "How close to the edge of the cliff can you drive and still be safe?” The first man said: "I am such a good driver that I can drive within one inch of the edge and still be safe.”
Not to be outdone, the second man answered: "I am such a good driver that I can drive with half the wheel over the edge and the other half of the wheel on the ground and still be safe.”
The third man simply said: "I don’t know how close I can safely drive near the edge of the cliff. I always stay as far away from the edge as possible.” The third man got the job.
Methods of protection
Perhaps the philosophy of the stage coach company should be applied to the design of modern-day process automation plants. How safe do you make the plant where your employees work? How close should the design engineer come to the minimum ignition curves? Should I use intrinsic safety technology or go with explosion-proof technology?
In Europe, the factories are relatively close to where the workers live and plant management rightly does not want to chance even a possibility of an accident. So they typically use intrinsic safety technology as the primary method to achieve plant safety. In North America, large petrochemical plants are built as far away as possible from residential areas, and the explosion-proof protection method has normally been the technology of choice for safety. Many believe that explosion-proof technology is too entrenched to embrace intrinsic safety technology. Also, most plants have been around for many years and retooling just cannot be justified.
Perhaps hazardous area protection is influenced by habit, but data is beginning to reveal that North American engineers are beginning to break those habits. Less than a decade ago, many engineers were not able to put their I/O into hazardous areas unless it was inside an explosion-proof or purged enclosure.
To be fair, explosion-proof housings still have their place. The best argument for their use is for high-voltage equipment, or where energy levels are impractical for intrinsic safety protection.
Signs of improvement
It would be inaccurate to say that intrinsic safety it is not popular in North America. That might have been true 15 years ago, but the intrinsic safety business has been growing with an impressive growth per year except for the last year during the economic downturn. Intrinsic safety barrier manufacturers have seen an increase in sales, and there has been an increase in the number of companies involved with intrinsic safety.
Actually, several industry segments have standardized on intrinsic safety for all of their plants in North America as well as their plants around the world. There are several reasons for this growing trend.
If North American companies want to do business in a world market, they must adapt to international standards. Today, CE and ATEX Zone 0 electrical specifications require intrinsic safety in most of the world outside of North America. It is interesting to note that intrinsic safety is the only method acceptable in Zone 0 areas. Many North American companies have been acquired or merged with other foreign organizations, so their practices and philosophies have filtered into their engineering cultures.
IS Remote I/O
A little more than ten years ago, the industrial automation controls industry came up with remote I/O with imbedded intrinsic safety and HART technology. This allows the customer to run a single or redundant pair of wires back from hundreds of field devices saving tens of thousand of dollars in wiring costs and enabling rudimentary diagnostics on the field hardware.
With Foundation Fieldbus and PROFIBUS, intrinsic safety is actually part of the specification and requires interoperability so any vendor can plug in and play. It’s a tall order, but it is working.
The industry is nurturing innovation to provide technologically-advanced diagnostic and configuration software for Fieldbus technology. It is requiring high availability, lower mean time between failures and lower mean time to repair. Additionally, SIL2 and SIL3 are requirements of the safety shutdown system and safety control market today.
Industry continues to search for ways to keep costs at a minimum, so wireless technology is quickly gaining ground over wired technology. But wireless still needs to be powered in some way, so intrinsic safety is required. Some have said that it will decrease point-to-point barriers. This may have been true a few years ago with the old technology, but that is the same thing experts have said about Fieldbus installations.
Large sums of money have been invested into R&D to solve those problems. Higher voltage, redundant conditioned power supplies allow more segments and instruments on the Fieldbus trunk. New advanced diagnostic modules are available that reduce commissioning time, predict failure, and troubleshoot any problem. New technology can even indicate which wire is coming loose on which module in which cabinet.
Size, weight, and corrosion
Space, weight, and corrosion reduction is always a concern, and it is especially critical on offshore platforms. Explosion-proof housings are big, bulky, and heavy. And because the housings are never completely airtight, the components inside are susceptible to corrosion.
Additionally, if an explosion occurs within an explosion-proof housing degrades the electronics or will result in significant damage of the electronics and components inside the housing. Explosion-proof conduit and appropriate seals must also be installed. Also, power must be disconnected before any covers may be removed. Improper opening and closing of an explosionproof enclosure may compromise its integrity.
Newer intrinsic safety barriers are significantly smaller and lighter than previous generations of barriers. The heat they produce has been reduced to increase the life of the electronics and reduce the stress they could induce in other electronics that may share cabinet space.
Using lower power has another hidden benefit. All safety systems require very expensive uninterruptible power supply backup systems and air conditioning. In larger SIS and DCS installations, experience has shown us that the user can select a smaller sized UPS and air conditioner systems by using these barriers. The cost savings is huge.
When a plant is making $20,000 an hour or more on a product or process and an instrument can be fixed “hot” or while running, it doesn’t pay to stop production unless it’s absolutely necessary.
With explosion-proof technology, users need a hot permit, which takes time to obtain. Also, it’s important to remember that when the enclosure is opened for repair, not only are the electronic components exposed to the environmental elements, it is no longer explosion-proof. The process must be shut down for repair; there is no option.
After the repair, it is possible that technicians did not close the systems properly. Enclosures have interlocks and bolts that securely hold the enclosure closed, due to time constraints there is a high probability that interlocks are not always secured and not every bolt is correctly torqued. Remember the story of safety and the stage coach? How far from the edge is safe enough?
With an intrinsically safe system, I/Os can be connected and disconnected while the unit is operating. An intrinsic safety barrier has a very quick return on investment. Plus, it is an insurance policy: It eliminates an explosion. Even if everything is done correctly explosion-proof technology only reduces the chance of an explosion propagating outside the explosion-proof enclosure. It does nothing to eliminate the explosion from occurring inside the enclosure.
Many facilities are identified as Class I Division 2 hazardous areas. This type of classification reduces insurance costs since the area is only considered hazardous during abnormal conditions as opposed to Division 1 areas that are considered hazardous in normal operation. But, the customers do should consider the case of their technicians not being familiar with a certain part of the plant, or if he/she is new to the job.
If a technician makes a mistake of any kind when using intrinsic safety barriers, the circuitry is incapable of releasing sufficient electrical or thermal energy, even under abnormal conditions, to cause ignition of a specific hazardous atmospheric mix. Therefore, many plants standardize on intrinsic safety for Class I, Division 1 and Division 2 classified areas.
In the last 10 years many of the leading process automation control companies have invested millions of dollars in new intrinsically safe control solutions. They would not have proceeded with the investment if it were just a passing trend. Also, many companies that never used intrinsic safety in the past are now using it not only for safety reasons but also for the return on investment that it provides.
If intrinsic safety is regarded as the safest method of protection shouldn’t explosion-proof categorically be replaced with an intrinsically safe system? If you were in that stage coach would you want to have the driver stay as far a way from the edge as possible? I would tend to say yes. Of course, you may need explosion-proof technology depending on your facility or operations. In fact, intrinsic safety, purge and explosion proof protection method may all be present in a modern processing plant.
All three methods have appropriate applications as well as benefits and drawbacks. Good engineering practices and training with the electrical codes in your country and area is the best protection from explosions in hazardous areas.