1 July 2007
Intrinsic safety time saver, cost cruncher for chemical industry
By Ellen Fussell Policastro
You have heard the saying, “If you can smell it, it might be hazardous.” That is why one fragrance chemical manufacturer makes sure it is on board with the latest in intrinsic safety methods. Inter-national Flavors and Fragrances manufactures chemicals for ingredients that go into the production of perfumes, laundry detergents, beverages, and confectionary and other food products.
Manufacturers at the Hazlet, N.J., plant initially housed their input/output (I/O) systems in a safe area, using barriers to make the area intrinsically safe. (See related article on protection methods on page 42.) With a new intrinsically safe remote I/O system for placement in hazardous areas, they can now wire field instruments from field device to the I/O system and run a set of cable from the I/O system back to the safe areas.
They used to install the I/O system into the motor control system and wire the individual systems back to the motor control center, which could take up to 500 ft of cable installation. But with the new remote I/O system, they can wire directly from field device to the I/O module, so there is nothing in between. “We save money on installation and time on commissioning the system and overall project management costs,” said Dennis Pakla, the company’s manager of corporate engineering technology. “Since the I/O system is right in the hazardous area, we can wire our field instruments from field device to I/O system. And we can run a set of cable from the I/O system back to the safe areas.”
Now with shorter wire runs from field devices to the I/O system, the overall project costs are less because of decreased installation cost for the I/O system. “You can build a building that houses all the motor controls and everything,” Pakla said.
Intrinsic safety barriers are a core component of an intrinsically safe solution because they limit current, voltage, and total energy delivered to a sensor or other instrument located in a hazardous area. Limiting the energy prevents fires and explosions from occurring in areas with hazardous chemicals, gases, or other ignitable atmospheres.
“Intrinsically safe equipment and wiring means the products are not capable of releasing sufficient energy in a circuit to ignite a flammable atmosphere in a hazardous area,” said Brian Traczyk, an application engineer at Pepperl+Fuchs in Twinsburg, Ohio.
The new system saves time with wiring techniques because, even using intrinsically safe barriers before, there was still a lot of wiring involved, Pekla said; “we had to go from field device to intrinsic safety (I.S.) barrier, and from I.S. barrier to I/O block,” he said.
By using a DIN-rail mounted, component-based plug-and-play system alongside intrinsically safe inputs and outputs, “a remote intrinsically safe system can also interface to non-intrinsically safe field devices installed in Div 2 and non-intrinsically safe solenoids installed in Div 1,” said Catherine J. Smith, an automation manager at R. Stahl in Houston.
“All of our processing areas in the aroma chemical division are Class 1, Div 2 or Div 1,” Pakla said. “In the I.S. system, it’s rated for Class 1, Div 1. So even if we have Class 1, Div 2 areas, we still use it as if it’s a Class 1, Div 1 area.”
“Until about four or five years ago, you couldn’t put your I/O into the hazardous area unless it was inside an explosion-proof enclosure or purged enclosure,” said Pat Moyer, a manager of distributed I/O at Rockwell Automation in Mayfield Heights, Ohio. A reason to distribute I/O instead of having it in the control room is to save money on wiring costs from terminations. “You have to put your wiring inside these hardened conduit that will go long distances,” she said. Using distributed I/O in a hazardous area lets the user bring out only one network cable instead of hundreds of point-to-point wiring. “Instead you just bring your network cable out to the hazardous areas, you locate your I/O close to the field devices it’s picking the signals up from, then it just puts it out to the network,” she said. You could put non-intrinsically safe equipment out into the hazardous areas, but you would have to put it inside these bulky explosion-proof or purging enclosures. If you have intrinsic safety distributed I/O, you can put them in standard enclosures, and it saves you money. It reduces your wiring costs and makes it easier to implement and maintain your system.”
Crop science leads I.S. in U.S.
Bayer, a European-based company involved in crop science, material science, and health care, was one of the forerunners of I.S. in the U.S. But I.S. is still proving itself to U.S. manufacturers. “Overall, there’s a perception that intrinsically safe equipment is very expensive. But it’s not quite as expensive as one would think,” said Steve Susko, a process control system engineer at Bayer CropScience in Kansas City, Mo. There are special provisions in terms of separating wiring to make sure the low energy or I.S. circuits could not possibly be compromised by other higher energy wiring. “You have to take special precautions, but it’s worth it.”
People who have not applied the technique think it is too expensive or too much of a problem to do the installation, Susko said. “Here in Kansas City we’ve seen the benefits of it primarily in terms of safety aspects.” Because of the greater acceptance of intrinsic safety in Europe, Bayer’s headquarters, “our engineering organization has embraced the use of intrinsic safety,” he said. “In terms of what you can do in the plant, the overall installation cost using I.S. is now comparable to other methods.”
Years ago, the I.S. barrier and associated equipment drove up the cost of intrinsically safe installations. Today most control equipment manufacturers offer integrated safety barriers in their I/O while also providing the added advantage of an isolated input. “While the installation cost may end up slightly higher, you have the added benefit of knowing you have installed the safest hazardous method protection available,” Susko said.
Since most of the areas in the Bayer plant are Class 1 and 2, Div 2, and some Div 1 areas, there’s a potential for some flammable vapors and in some cases dust to be present. “Our preferred method of installation is two-wire intrinsically safe equipment for which the majority of sensor types are available,” Susko said.
Safety techniques revisited
One of the most widely applied techniques in the U.S is explosion-proof equipment, which allows the ignition to occur but attempts to contain it in the equipment enclosure. “Unfortunately in a number of cases over the life of the equipment, the protection method becomes compromised, and if something did occur, the enclosure wouldn’t provide protection,” Susko said. With this method of protection, an explosion still can occur in the equipment enclosure, but since the enclosure is designed to contain the explosion and prevent the ignition of the flammable atmosphere in the plant area, “this will most likely result in damage to the equipment in the enclosure,” he said.
A major problem with this method of protection is the fact that with age the protection capability of most enclosures becomes compromised either through missing fasteners or damage to the enclosure surface.
“This is not a problem with I.S. since you’re not sending enough energy out to the hazardous area to even cause an explosion,” Susko said. The intrinsically safe technique is inherently safe, which means “you make provisions up front so the amount of electrical energy sufficient to cause an ignition cannot be transmitted out to the field,” Susko said. “With this method of protection, you could take an explosive mixture of gas and short two wires together generating a spark, and it wouldn’t cause an explosion.”
Another advantage of intrinsic safety is maintenance people do not have to de-energize equipment in hazardous areas prior to working on a piece of field equipment. “In one case, you’re trying to mediate or contain the problem, and with I.S., you’re eliminating it.”
ABOUT THE AUTHOR
Ellen Fussell Policastro is the associate editor of InTech. Her e-mail is firstname.lastname@example.org.
Protection methods primer
Three most popular methods of protection in North America are explosion-proof, pressurization, and intrinsic safety.
When explosive gases or dusts enter an enclosure with electrical equipment inside, an explosion could occur. With the enclosure cover properly installed and tight tolerances maintained, the flame will be unable to propagate to the outside atmosphere.
Pressurization (purged enclosures)
The concept keeps the enclosure filled with a positive pressure of air or inert gas. Therefore, as long as the pressure inside is higher than the outside pressure, the explosive gases cannot enter the enclosure.
You need three things to start a fire: oxygen, a flammable material, and an ignition source. Oxygen is always present, and releasing a flammable material in the air gives you two out of three. But if you can take away the ignition source, you have prevented the fire. That is what intrinsic safety does for manufacturers in the chemical industry. Intrinsic safety limits the energy to the hazardous areas through safety barriers mounted in safe areas. Due to the energy limitation of the barrier, regardless of the fault in the hazardous areas, sufficient energy cannot be released to ignite the explosive atmosphere.