Intrinsically Safe or Explosion Proof?
The distinctive ring from the radio phone told the control room operator that the person on the other end would be the tank level technician. He also knew that if a call was coming in, something was amiss at the remote site tank farm. “I can’t get on the last tank” said the voice on the other end.... Here are considerations for deciding between intrinsically safe and explosion-proof.
Top: Wireless temperature transmitters are among the intrinsically safe equipment used by chemical plants to reduce the hazards of explosive environments. Such devices are simple to install and maintain. (Source: Emerson Process Management)
The distinctive ring from the radio phone told the control room operator that the person on the other end would be the tank level technician. He also knew that if a call was coming in, something was amiss at the remote site tank farm.
“I can’t get on the last tank” said the voice on the other end. “There’s a cat on the steps.” Well, just shoo it away, the control room operator started to say, but stopped abruptly as the technician curtly interrupted. “It’s a mountain lion and I’m not going near it,” he snapped. “I’m coming in.” One tank level would remain unread.
The situation is unusual, and admittedly rare in today’s world of automated measurement systems, but this case is true. Before automating tank gauging at a distant location, this company actually had to deal with a mountain lion on the tank steps.
Using to automated tank gauging systems, which many have done, eliminates personnel hazards such as tank climbing, exposure to product, and inclement weather, but raises other issues— among them, the type of protection to select for such hazardous applications.
The dangers involved in the production and processing of fuels, such as explosive atmospheres, may require the use of intrinsically safe (IS) devices under some circumstances, or explosion-proof enclosures in others. Which option is best for your application? When should it be wireless? What benefits does one technology hold over the other? The decision is not always simple or easy.
What type of hazard is it?
Deciding which type of protection to apply is rarely cut-and-dried for a variety of reasons, from equipment involved and type of hazard to company policy or preference. Intrinsically safe and explosion-proof enclosures, devices and practices have more relevance in certain industries than in others. Here, examples focus on the oil and gas, refining, and chemical industries, but similar safety risks exist elsewhere. In water and wastewater operations, for example, digesters release hazardous methane gas. Agriculture and mining industries face potentially explosive dusts. But in oil and gas production and refining or in chemical operations, hazardous environments are common, and a given. In other circumstances, “hazardous” may indicate poisonous.
Choosing explosion-proof or intrinsically safe devices is really more site- than application-specific. In practice, such devices are under consideration in only a small percentage of cases (less than 5%). Therefore, among the first steps any facility would take in its hazard analysis would be to ascertain if intrinsically safe or explosion proof practices and devices are indeed required.
In the analysis, safety compliance personnel survey an application while it is operating using sniffers or other sensing devices to detect the presence of hazardous gases and determine the degree of the hazard [by NEC class and division]. If hazardous gases are present normally, a Class 1, Div. 1 situation exists. If they are present only occasionally or under unusual circumstances, the area is designated Class 1, Div. 2. With Class 1, Div. 2 situations, non-incendive devices typically are sufficient. An overwhelming majority of applications fall into this category. It is for the remaining cases—those Class 1, Div. 1 situations—that IS or explosion-proof devices (EXD) and practices are intended.
Briefly, explosion-proof technology places a device within an enclosure sturdy enough to withstand an explosion of a specified level. Should a fire or explosion occur in the equipment, the event will be contained within the enclosure and any gases will cool below ignition level before they can escape to the atmosphere.
Explosion-proof enclosures are rated and tested by independent laboratories (such as Factory Mutual) for specified pressures and flame lengths. Typical applications include light fixtures, engine switches, pumps, and other equipment such as motors that must be placed in hazardous areas. Explosion-proof technology should be the sole option only when an IS-rated device is unavailable. A pump or agitator with a large motor, for example, is simply of a size that can’t be made to be intrinsically safe. Explosion proof equipment and practices then would be mandatory.
Intrinsically safe devices and associated wiring limit the amount of power (voltage, current, inductance, capacitance) to and in a device, so that a spark cannot be generated. They are powered from beyond a Zener-diode or galvanic-isolation barrier in a safe area away from the hazard, typically a control room. Intrinsically safe devices can be opened, maintained, or otherwise handled while powered. Such an instrument might be used to measure the temperature inside a tank or a pipeline, for example, where an enclosure would be impractical. IS options are often less expensive than explosion-proof techniques, but tend to be more complicated to apply.
If a risk assessment reveals an area or a process to be a hazardous atmosphere in need of one of these protective technologies, management must thoroughly familiarize itself with all appropriate codes and standards (NEC, Atex, IEC)—at the corporate, local, national, and international levels—and follow them scrupulously, understanding that standards are dynamic and that, although progress is being made toward global harmonization, variations are still significant. Further, codes are subject to interpretation, and the local authority having jurisdiction typically has the final say in what standards are applicable and how.
A company with multiple sites should consider that identical situations in plants in separate geographical locations may be interpreted differently. As with any safety technology, a safety professional with expertise in the technologies and all applicable codes and standards should be consulted before moving ahead with any hazardous-area project.
Tank farm example
With this background in mind, let’s look at how these technologies play out in real-life refinery operations. Mark Menezes, PEng., is measurement business manager—Canada for Emerson Process Management. He offers one reason why refineries face greater-than-normal hazards: “In the last 10 years or so, many refineries have upgraded their installations to operate on heavier crude oils, such as those found in the Canadian tar sands or in Venezuela. So, if you want to operate a refinery using some of these feedstocks, you need to crack this heavy oil with a lot of steam under high pressure.
“A lot of hydrogen is in use here, used to make these heavy oils lighter. Hydrogen is one of the most highly flammable gases there are. Any hydrogen leak at high pressure is very dangerous. In addition, sulfurs are commonly stripped out of these sour heavy oils. Any plant of this nature is going to pay a lot of attention to hazardous environments.”
One area of special concern is the tank farm. Operations needs to know how much fuel, chemical, or other liquid resides in each tank. The job, explains Frank Van Bekkum, senior product marketing manager, Honeywell Process Solutions, can be done manually by sending someone to the tank (as described above). But this approach is difficult, dangerous, and undesirable. In addition, he says, “Tank farms are often situated in desolate areas. Oil, gas, and chemical products may be hazardous to health; tanks can be difficult to reach through ice, snow, or, in another actual case, an eagle’s nest.”
Modern refineries use a variety of equipment for measuring tank level. Options, says Van Bekkum, include radar gauge systems that are typically powered by the main service, equipped with communications, and packaged in an explosion-proof box. “In addition,” he adds, “an intrinsically safe temperature gauging device on the tank goes into the product. So what we have is a separate device supplied in an explosion-proof box with a connection and temperature probes that go into the tank that are intrinsically safe. Even if the probe—which is basically a stainless steel hose—should leak or fail, the installation is still safe.”
The approach is straightforward, but the complexity of many operations make such safety systems difficult to apply correctly, warns Van Bekkum: “Intrinsically safe devices aren’t always an option in a tank farm, because you have pumps and control valves that have power and communication requirements that go beyond the definition of intrinsically safe. If you are using IS instrumentation in the field, it must be kept separate from a powered device to avoid any confusion as to the type of device it is.”
Intrinsically safe devices and practices are crticial at loading racks. Here, a load computer (bottom), and tang grounding equipment help achieve safe and efficient hydrocarbon loading. Grounding equipment for the road and rail tankers helps prevent accidents by slowly discharging static electricity without creating dangerous sparks. Once the discharge is complete, a permissive signal is transmitted to the system controlling the load process. Source: Honeywell Process Solutions.
Historical views, future developments
Regardless of the technology, adds Van Bekkum, most problems result from human error, and most occur not when new equipment is installed, but when modifications are made without adequate planning and communication.
“For example, an existing transmitter is replaced with a new technology, perhaps from a different manufacturer, and the maintenance department is aware of the change but the production department is not,” he explains. “Care must be taken to ensure specifications match and that all technicians are informed of any changes.”
One concern is that the use of both technologies in the same area could lead to confusion, error, and, therefore, danger. Workers must be trained specifically for the area in which they will work; indeed many today are being cross-trained to handle both technologies as more companies find themselves using both approaches at the same site. Menezes and Van Bekkum agree that, in most cases, the use of one technology or the other is not necessarily application driven. Rather, it is a site-by-site decision, influenced by, among other things, habit and tradition rather than technology and hazard.
In the opinion of Emerson’s Menezes, explosion-proof practices have historically been more common in North America vs. intrinsically safe practices in Europe. “Intrinsically safe is likely to have a slightly lower installed cost, but is more complex to implement. You have to think about a lot more things with IS. Explosion-proof technology is fairly simple to apply. You buy robust devices that will contain the potential explosion and install them using robust conduit. Such practices ensure if an explosion occurs within a device, it is contained, and by the time the gases have escaped they have cooled to the point that they are no longer dangerous. Products used in oil and gas operations and refineries are robust to start with because of the nature of the industry. Most North America operations are so used to installing robust devices that they assume all products are explosion proof.”
Influence of bus networks, wireless
Both Menezes and Van Bekkum agree that the world is moving toward intrinsically safe practices and away from explosion-proof techniques. Intrinsically safe practices are gaining popularity in North America for two reasons: increasing use of bus technologies and proliferation of wireless systems. Notes Menezes, “If I have, for example, a Foundation fieldbus installation, I can put one barrier in the safe area and connect five transmitters on the same bus. My cost is divided by 5. Many users who have historically used explosion-proof wiring practices turn to intrinsically safe when they adopt Foundation fieldbus. Bus technologies are also catching on with motors—not the power to the motor, but the signal. Eight motors, for instance, connected together on an IS low-power Profibus or DeviceNet bus, can be started at once.”
Wireless also is promoting the adoption of intrinsically safe practices. Many wireless devices are battery operated and, in one sense, inherently intrinsically safe. If a plant needs to monitor the temperature in a reactor, it is relatively inexpensive to install a wireless temperature transmitter on it. “The power level is just not sufficient to generate a hazard,” says Menezes. “If there’s a problem, for example, with a battery inside a transmitter, the technician goes into the field, unscrews the cap, removes the battery, replaces it with a fresh one, and screws the cap back on right in the hazardous environment. In many respects, it is an IS design.”
Menezes also offers a more detailed example: The failure of a primary seal. A pressure transmitter may be correctly installed using either intrinsically safe or explosion-proof practices, he explains, “but if a hazardous fluid bursts through the primary seal (the transmitter diaphragm that connects the device to the process) and the process fluid finds its way into the transmitter, it’s not a leap to see that the process fluid may burst through into everything else and follow the conduit into the control room. You could conceivably have 2,000-psi hydrocarbon squirting onto the floor of what is supposed to be a safe area. A wireless device would mean no connections from the transmitter in the hazardous area to the safe area.” He admits there are other ways to solve this problem using dual sealing devices and conduit seals, but they add cost and complexity.
“With legacy systems, the explosion proof approach has been simpler, more comfortable,” says Menezes. “But once you have multiple devices on a bus or installations using wireless technology, intrinsically safe becomes more compelling, particularly with instrumentation.”
Van Bekkum agrees, his views on wireless are shared by colleague Jeff Becker, Honeywell Process Solutions’ global wireless director. “Customers are applying wireless technology to measure potentially critical process variables in hazardous locations that were previously inaccessible,” says Becker. “Applications are related to, among others, butane sphere pressure monitoring, inventory management, safety relief valve monitoring, safety shower monitoring, and rupture disc monitoring—all critical points in a refinery or chemical plant.” Wireless has empowered and enabled these applications and met regulatory needs related to environment, explosive classified areas, safety regulatory, and bottom line company profitability objectives, he adds.
“Although there are still some emotional issues associated with wireless, the industry is moving faster and faster toward wireless and there are a lot of benefits,” adds Van Bekkum. “When making wireless decisions, it’s critical to develop an infrastructure for the entire plant that meets current and future needs. Wireless is more than a one-to-one replacement of copper wire. If you build a good system according to current standards, it will be safe and secure. Nothing is 100% foolproof, but you make things as safe as possible. You reduce the risk to an acceptable level.”
Jeanine Katzel is a contributing editor to Control Engineering. Reach her at email@example.com .
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