01 March 2004
Blocking the big boom
Preventing explosions in hazardous locations.
By Joe Kaulfersch
Preventing explosions ignited by electrical circuits can be a challenge, but with containment, segregation, and prevention, you can eliminate one or more of the ignition triangle components to reduce the risk of explosion. Of course exceptions to the rule always exist, and some materials can explode spontaneously without supplied energy.
Hazardous locations-areas typically related to petrochemicals and their derivatives-are industrial process applications areas ripe for explosion. In the past, we used pneumatic controls in those locations due to their inherent safety. Even though other parts of the world still use pneumatic equipment, those manufacturers are quickly displacing it with newer electronics with increased capabilities. In the U.S., explosionproof protection has had the lion's share of installations using electronics.
Studies forecast the North American intrinsically safe barrier market will grow from $190.6 million in 2001 to $261.2 million in 2006, at a compound annual growth rate of 6.5%. Analysts expect more than business expansion to drive this relatively high growth rate. North America's preferred approach has been to provide explosionproof hazardous environment protection. Several applications see movement away from this approach toward intrinsically safe solutions, which some believe are safer, and that generally offer lower operating costs.
Large European users adding facilities in North America prefer to use the same hazardous environment protection methods as they do in Europe, where intrinsic safety (IS) is favored. Larger North American firms with facilities in Europe are more likely to adapt this protection method into their North American operations.
The main reasons for the growing popularity of intrinsic safety are:
- Semiconductor advances allow you to carry out increasingly complex electrical operations in hazardous areas at permitted power levels-typically in the order of 1 watt.
- You can calibrate simple, light, and inexpensive hazardous-area equipment and service it live.
- You can use ordinary instrument wiring in hazardous areas instead of armored cable.
- It provides inherent safety for personnel, because it employs low voltages.
- Progressively harmonizing standards governing the design of IS equipment allows the sale and use of the same product in many countries without any variation.
- Advances in certification standards allow a modular approach to IS systems. With a certified IS interface, safe-area equipment needs no certification, and the end user can choose or change the hazardous-area equipment within wide limits. Simple, non-energy-storing sensors require no certification.
The intrinsic safety method of explosion prevention precludes the ignition of the dangerous mixture, while simplifying the installation and use of the required apparatus that is connected to the electrical circuits directly located in a hazardous location.
According to Article 504 of the National Electrical Code, NFPA 70, an intrinsically safe electrical circuit is defined as one in which no spark or thermal effect generated during normal functioning and/or during specific fault conditions is able to ignite a given explosive atmosphere.
An electrical circuit typically consists of a connected voltage (V), resistance (R), inductance (L), capacitance (C), and switch (S). For an electrical circuit to be intrinsically safe, you need to consider the parts of the circuit able to store energy-the inductor and the capacitor. When the switch in the hazardous location is open, the capacitor accumulates energy that discharges when the switch closes. This causes an electrical spark. In the same way, when the contact is closed, the inductor stores energy that releases in the form of an electrical arc when the switch opens. The energy the circuit can release must be lower than the minimum ignition energy (MIE) of the air/gas mixture in the hazardous location. You can then apply safety factors to ensure the values allowed are well below those required for ignition.
A theoretical estimation of energy inherent to an electrical circuit is not always possible, especially when the power source's energy is higher than the energy the reactive components store. Therefore, you can apply data normally used in considering intrinsic safety to correlate between electrical parameters of the circuit, voltage and current, and the minimum ignition energy level of the hazardous atmosphere.
To reduce the risk of explosion in hazardous locations, you need to eliminate at least one of the components in the ignition triangle. From a chemical point of view, oxidation, combustion, and explosion are all exothermic reactions with different reaction speeds. For such a reaction to take place, each of the following three components must generally be present simultaneously in due proportions:
- Fuel: flammable vapors, liquids or gases, or combustible dusts or fibers
- Oxidizer: generally, air or oxygen
- Ignition energy: electrical or thermal
The general consensus is that in a properly designed safety system, two or more independent faults must occur, each one of low probability, before a potential explosion can occur.
It is possible to draw an ignition characteristic for each type of fuel, and an MIE exists for every fuel that represents the ideal ratio of fuel to air. At this ratio, the mixture is most easily ignited. Below the MIE, ignition is impossible for any concentration.
For a concentration lower than the one corresponding to the MIE, the quantity of energy required to ignite the mixture increases until you reach a concentration value. Below this value, you cannot ignite the mixture due to the low quantity of fuel. This value is called the lower explosive limit (LEL). In the same way, when increasing the concentration, the energy requirement increases, identifying a concentration value-called the upper explosive limit (UEL). Ignition cannot occur above this value due to the low quantity of an oxidizer. The following table lists the explosive characteristics of hydrogen and propane.
From a practical point of view, LEL is more important and significant than UEL, because it establishes, percentage-wise, the minimum quantity of gas needed to create an explosive mixture. This data is important when classifying hazardous locations.
The MIE (minimum energy required to ignite an air/gas mixture in the most favorable concentration) is the factor upon which the intrinsic safety technique is based. With this technique, the energy released by an electrical circuit, even under fault conditions, is limited to a value lower than the MIE.
The minimum ignition temperature of an air/gas mixture is the temperature at which the dangerous mixture ignites without a supply of electrical energy. This parameter is important because it establishes the maximum surface temperature allowed for devices located in a hazardous location, under both normal and fault conditions. This value must always be lower than the ignition temperature of the gas present.
The flash-point temperature is a characteristic of a volatile liquid-defined as the lowest temperature at which the liquid releases sufficient vapors that can ignite from an energy source. Because a liquid above its flash point constitutes a source of danger, you should consider this parameter when classifying locations.
Behind the byline
Joe Kaulfersch is an applications engineer at Pepperl+Fuchs in Twinsburg, Ohio.
Harmonizing intrinsic safety standards around the world
By Ellen Fussell
Timeliness and cost of certification are two issues driving the harmonization of U.S. standards for intrinsic safety with other international standards, according to Ted Schnaare, engineering manager at Emerson Process Management, Rosemount Division, in Chanhassen, Minn. From a U.S. manufacturing perspective, a couple of significant things are going on. "One is the harmonization of international requirements into the U.S. requirements," he said. "The ISA SP12 committee has been in the process for some time of adapting International Electrotechnical Commission (IEC) requirements for use in the U.S. ISA has published several of these IEC standards as U.S. national standards-for example the ISA 12.02.01 (IEC 60079-11 Mod) standards."
The committee is continuing with the harmonization process "and even accelerating to some degree," Schnaare said. "A lot of this is due to not only harmonization of equipment standards but also harmonization of conformity assessment through the IEC Ex Scheme, which is an effort to internationally harmonize the assessment and certification of equipment for use in hazardous areas," he said.
U.S. manufacturers are interested in harmonization of equipment standards and conformity assessment processes to shorten certification lead times and lower costs. "The end goal of harmonization is to allow equipment manufacturers to design to one standard and to get equipment examined by a single conformity assessment authority, resulting in a product that is accepted around the world," Schnaare said. "Focusing on intrinsic safety (IS), the major activity there is an effort to publish the next edition of IEC 60079-11, which is the IEC IS standard. The IEC has received and is sifting through many comments and suggestions for improvements and changes to the standard-from different sources: manufacturers like myself, test houses and authorities, and end users."
The comments and suggestions for the new edition now run the gamut, Schnaare said. A number of comments address segregation requirements. "One of the tools the standard provides for achieving intrinsic safety is the physical separation of circuit conductors. There's a move to revisit these separation requirements and in some cases replace them with test criteria. There's also a move to improve the way the standard addresses the mounting of surface-mount components, especially surface-mounted shunts zener diodes. The IEC is finding that with the broader application of the standard have come more and more divergent interpretations of the requirements."
This situation has driven an effort to minimize the interpretation space within the standard and to write more detailed and inclusive requirements, Schnaare said. "In the world of harmonized standards and conformity assessment, you have to get the whole world agreeing on how to do something, and that can be very inefficient. Currently, conformity assessment authorities have the latitude to make more or less independent interpretations of the standard." However, in the future, they will be offering certification to an internationally harmonized standard, the interpretation of which will have to be agreed upon by all other conformity assessment authorities. "In the end, harmonization doesn't work unless everyone is interpreting the requirements in the same way," he said. "You don't want an authority in the U.S. to have a different interpretation of the standard from one in Europe."
FISCO and FNICO
Two fieldbus concepts for hazardous locations are making a splash on the scene as well. The fieldbus for intrinsic safety concept (FISCO) and fieldbus nonincendive concept (FNICO) are explosion-protection concepts that simplify the use of fieldbus in hazardous areas, said Schnaare. (See related story on page 59.)
"FNICO is an emerging concept that is similar to FISCO, but limited to use in Zone 2 areas. Both of these concepts seek to make the use of fieldbus in hazardous areas attractive to people by simplifying it and making it cheaper. FISCO and FNICO preserve the flexibility of the entity concept, where each component is assigned parameters that allow them to be used with other components with compatible parameters, while offering the simplicity of the system concept, which defines each component explicitly. The entity approach is flexible from the standpoint of being able to use a wide variety of equipment together," he said. "But it's limiting from the standpoint that you pay for that flexibility by a reduction in the overall energy available in the field and the number of faults that have to be applied to the systems to ensure its safety."
What has been done with FISCO and FNICO is to maintain that flexibility. "So people can use different manufacturers' equipment together but also maximize the energy available to field equipment by making some simplifying assumptions about the type of field wiring, the amount of stored energy available in a field device, and the amount of energy contributed back onto the fieldbus by field devices," Schnaare said.
The FISCO and FNICO concepts apply fairly tight restrictions to these parameters, but because you can optimize these limitations for fieldbus systems, they do not need to get in the way. "The additional energy available to field devices allowed by these concepts translates into more devices running on a single bus, which translates into increased cost savings," he said.
Intrinsic safety defined
Hazardous locations are present in industries such as munitions, petrochemical, auto (paint spray booths), grain, wastewater, printing, distilling, pharmaceutical, brewing, cosmetics, mining, plastics, and utilities.
ISA-RP12.6 defines intrinsically safe equipment as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration." You can achieve this by limiting the power available to and generated by electrical equipment in the hazardous area to a level below that which will ignite the hazardous atmosphere.
The European standards define the general specifications and the detailed guidelines for methods of protection against explosion. The national requirements primarily contain installation requirements.
In the past, the U.S. and Canada have classified hazardous areas by classes, divisions, and groups. Although this system is still in use, North America is gradually beginning to adopt a classification system based on zones as standardized in many countries of the world.
Common instruments in hazardous areas
Switches-Include push buttons, selector switches, float switches, flow switches, proximity switches, and limit switches.
Thermocouples-Inexpensive temperature sensors constructed of two dissimilar metals that generate a millivolt signal varying with temperature.
I/P converters-Convert a direct current milliamp signal to a proportional pneumatic output signal, which usually positions a control valve.
Transmitters-In control systems, they convert a process variable to a proportional electrical signal. The electrical output is a 0/4-20 mA, 0/1-5 volt (V), or 0/2-10 V signal.
RTDs-Resistance temperature detectors (RTDs) convert temperature into resistance. A resistance change could be 0.385 ohms/°C for a 100-ohm platinum RTD.
Light-emitting diodes (LEDs)-Don't use standard incandescent bulbs in explosive areas because of radiant heat, current requirements, and the susceptibility of the bulb to breakage.
Solenoids-Electrically actuated valves allow full flow or no flow of gases or liquids. Don't use standard 24 volts direct current solenoids in the hazardous area due to the coil's energy storing capacity.
IS solenoids-To design for IS certification, one common procedure is to embed two diodes connected in parallel to the coil. These diodes eliminate the potential arcing if a wire were to break. They suppress the arc and provide the solenoid with a low inductance rating.
Strain gauges-Measure stress, force, weight, and pressure in load cells, scales, and transducers.
Potentiometers-Adjustable resistors with resistance value (ohms) that changes with mechanical wiper movement.
Audible alarms-Horns or buzzers signal a hazardous event has occurred. Typically, barrier choice would be the same for audible alarms as it is for solenoids.
Serial communications-Transferring data in a sequence of bits, generally in the form of a low voltage signal (0-15 V), the most common serial communications protocol is RS-232.
Fire detectors-Detect flames in a hazardous environment. In the normal state, a low current (4-6 mA) passes through the detector circuit.
Source: Scientific Technologies Inc., Fremont, Calif.