Protection methods, containment, segregation, and prevention

Transcription

1 .7 Intrinsic safety 101 Protection methods, containment, segregation, and prevention By Robert Schosker ' This article answers the question, "Why worry about hazardous locations?" The area classifica- tions. for the U.S. and the European (the International Electrotechnical Commission [IEC] stan- dard) approaches are described and contrasted, including protection methods, containment, segregation, and prevention What is intrinsic safety? Intrinsic safety (IS) has been around a long time. It is the concept of limiting ignition-capable energy to Below that of the hazardous material a process may be working with. IS, as most people callit, was born out of the ashes of a horrible roining accident in the U.K. in The explosion was caused by Substitute l! Operat r 1 the dangerous Residual training/ substances protection risk qualification Minimize the Risk reducing by tertiary explosion protection 54 INTECH SEPTEMBER/OCTOBER

2 a low-voltage signaling system that was used to advise the surface crew that coal cars were ready to be brought to the surface. The ensuing research revealed that the most important factor in determining the safety of, an electrical circuit is the energy stored in the circuit. The concept of intrinsic safety was born. So why do we worry about hazardous locations? Many explosions are the result of limited knowledge by employees, which can be directly related to inadequate training, lack of documentation, and questionable safety measures. These disastrous results may have been prevented with proper training, new instrumentation, inspection, maintenance, or revised safety procedures. Of course, even if the plant takes these steps, an explosion could still happen; however, every effort to reduce "residual risk" is a benefit. Residual risk is the risk left Over after all other forms or mechanical means of risk reductions have been taken. A hazardous area is an area containing (or likely to contain) an ignitable concentration of flammable gas, vapor, or dust, where an electrical spark of sufficient power will cause an explosion. However, an explosion can only occur when an oxidizer, the hazardous substance (gas, vapor, or dust), and energy (thermal or electrical) are present at the same time. These three groups comprise what is known as the hazardous location ignition triangle. Energy (thermal or electrical) who defines a hazardous location? What is the difference between zones and divisions? These are all very good questions, and ones that are often asked. We already know that a hazardous location is an area that contains of is likely to contain an ignitable concentration of flammable gas, vapor, or dust. Once we have determined that a hazardous area exists, then we classify that area using either zones or divisions. Although the physical principles of explosion protection are the same worldwide and are not differentiated, the procedures determined by national legislation in the approximately 100-year history of explosion protection have resulted in various solutions. Each solution has a unique perspective on how to classify a hazardous area. Division.. I Class ; Division; Groups; T Class (t-lit) -Gas groups (x--y) Temp codes (T1-T6),ÿ (1-2)ÿe Divisions, mainly a North American-approved method of classifying an area, use four categories: class, division, group, and temperature. They are commonly seen like this: Hazardous due to the presence of flammable substances such as gases or vapors Hazardous due to the presence of flammable substances such as dusts or powders. - Hazardous due to the presence of flammable substances in a fiber.or flying state. Oxidizer Fuel (air) (gas, vapor or dust) If any one of these groups is removed, an explosion cannot happen. As we will discuss throughout this article, intrinsic safety is a method of protection that removes one of these groups from the ignition triangle. The article will first answer some basic questions about hazardous locations. We already know why we worrÿ but what are we worrying about? What is a hazardous location? What or The categorization of these areas is carried out in North America in accordance with the National Electrical Code NFPA 70, article 500. Class I, II, or III represent the form of the hazardous material you must deal with. Danger can be present during normal functioning, during repair or maintenance, or where a fault may cause the simultaneous failure of electrical equipment. Combustible material is present but confined to a closed container or system, is normally vented or in an area adjacent to a'division I location. Division is a categorization by the probability that these materials will be present in a potentially hazardous quantity (Division 1 and Division 2). The division is commonly the perimeter or designated area of the hazardous material. INTECH SEPTEMBER/OCTOBER

3 Class I i GroupA Group B Iÿ Fÿrouÿ C i: Group D \ Class Ii I Group E i Group F : i GroupG Atmospheres containing acetylene Atmospheres containing hydrogen and flammable process gasses with more than 30 percent hydrogen by volume, or gases or vapors posing a similar risk level such as butadiene and ethylene oxide Atmospheres such as ether, ethylene, or gases or vapors posing a similar risk level Atmospheres such as acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane methanol, methane, natural gas, naphtha, propane, or gases or vapors posing a similar risk level Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electronic equipment Atmospheres containing combustible carbonaceous dusts including carbon black, charcoal, coal, or coke dusts that have more than 8 percent total entrapped volatiles, or dusts that have been sensitized by other materials so that they present an explosion hazard Atmospheres contai.ning combustible dusts not included in Group E or Group F, including flour, grain, wood, plastic, and chemicals The graphical representation below illustrates the concept of division-based areas. Hazardous locations Key Division I Grade Division 2 Sump or trenchÿ In Europe, the zone method of area classification is used. The European zone practice is described in IEC/EN In accordance with this standard, any area where there is a probability that flammable gas or dispersed dust may exist must be classified into one of the following areas: Furthermore, classes, of hazardous areas are divided into subgroups depending i3n the type of flammable gas or vapor present.. Finallyÿ apparatus installed directly in a hazardous area must be' classified for themaximum surface temperature that the device will produce.under normal operation or in the event of a fault, The maximum surface temperature must be below the minimum ignition temperature of the gas present, Maximum temperature ( C) ( F) 450r 30O i65 160' , Temperature class in North America TI T2 T2A T2B T2C T2D T3 T3A T3B T3C T4 T4A T5 T6 Zone 0 Zone 1 Zone 2 Zone 20 Zone 21 Zone 22 An area!n which an explosive airlgas mixture is continuously present or present for long periods. An area in which an explosive airlgas mixture is likely to occur in normal operation. An area in which an explosive airlgas mixture is unlikely to occur, but, if it does, only for short periods of time. An area in which a combustible dust cloud is part of the air permanently, over long periods of time, or frequently. An area inwhich a combustible dust cloud in the air is likely to occur In normal operatlon. An area in which a combustible dust cloud in the air may occur briefly or during abnormal operation. The European zones then require that the apparatus be subdivided into two groups, according to IEC/EN These two groups in general terms indicate whether the apparatus is located,above or below ground. Group I Group II Apparatus to be used in mines where the danger is represented by methane gas and coal dust. Apparatus to be used in surface industries where the danger is represented by gas and ÿapor that has been subdivided into three groups: A, B, and C. These subdivisions are based on the maximum experimental safe gap (ME SG) for an explosion-proof enclosure or the rai!!mum ignition current (MIC) for intrinsically safe electrical apparatus. And as with divisions and gas groups, there are subgroups of Group I and II within the zones. Each is associated with a certain number of gases having an ignition 56 INTECH SEPTEMBER/OCTOBER 2013 WWW, ISA,ORG

4 energy in the value reported and is represented by the gas referenced in the table below. Included in this comparison are the Class I and II division-based classifications. Methane Material Apparatus classification Europe (* IEC) North America Group I (mining) Class I, Group D Acetylene Group IIC Class I, GroupA > 20 IJJ Hydrogen Group IIC Class t, Group B > 20 IJi Ethylene Group lib Class I, Group C > 60 IJJ Propane Group IIA Class I, Group D > 180 pj Conductive dust(metal) Group IIIC* Nonconductive dust Group IIIB* (carbon) Class 11, Group E Class II, Group F Cereal/flour Group IIIB* Class II, Group G Fibers/suspended Group IliA* Class III particles Ignition energy A temperature code is also required just like divisions; however,' the zone method is simplified. With both divisions and zones, the maximum surface temperature must be below the minimum ignition temperature of the gas present. Maximum surface temperature ' ( c) 45O 3OO OO T3 The graphical representation in the upper right illustrates the concept of zone-based areas. The article has explained hazardous areas and how those areas are classified, but what methods of protection exist for your hazardous areas? As previously mentioned, to reduce the risk of explosion, you must eliminate one or more of the components of the ignitiontriangle. There are three basic methods of protection: explosion containment, segregation, and prevention. Temperature class in Europe TI T2 T4 T5 T6 Boundaries. Zone method - Boundaries (Designated areas)... ; Zone 0 Zone 1 Iÿ Zone 2 Explosion containment: the only method that allows the explosion to occur, but confines it to a well-defined area, thus avoiding the propagation to the surrounding atmosphere. Flameproof and explosion-proof enclosures are based on this method. o Segregation: a method that attempts to physically separate or isolate the electrical parts or hot surfaces from the explosive mixture. This method includes varimis techniques, such as pressurization and encapsulation. e Prevention: a method that limits the energy, both electrical and thermal, to safe levels under both normal operation and fault conditions. Intrinsic safety is the most representative technique of this method. The protection methods, based on the containment and segregation concepts, are methods that contain the explosion in order to prevent the energy source--electrical or thermal-- from coming in contact with the potentially explosive mixture. In both cases, the use of appropriate enclosures and specific wiring and installation systems is required. The intrinsic safety method prevents the ignition of the exp!osive atmosphere, while simplifying the installation and use of the required apparatus that is connected to the electrical circuits directly located in a hazardous location. The choice of a specific protection method depends on the degree of Safety needed for the type of hazardous area. The idea is to determine the best protection method based on the lowest probable degree of the simultaneous presence of an adequate energy source and a dangerous concentration level of an air/gas mixture. Of course, other important factors to consider include the size of the apparatus to be protected, the flexibility of the system, the possibility of needing to perform maintenance, and the installation cost. Respective of these factors, intrinsic safety has many, advantages. So what is intrinsic safety? Intrinsic safety is based on the.principle of limiting energy to a level below that required to ignite the hazardous gas or dust. The energy limitation als0 pertains to thermal energy as well. Therefore, in normal operation or in the event of a fault, no sparks or thermal effects may occur that could lead to the ignition of a potentially explosive atmosphere. INTECH SEPTEMBER/OCTOBER

5 o Intrinsic safety "i," example Ex ia - Divisions 1, 2/Zones 0, 1, 2 Barrier Isc/Io Cable Hazardous area Associated apparatus It is one thing to understand intrinsic safety as a concept, but it has to be applied correctly to w6rk. As shown in the illustration, there are three components to an intrinsically safe circuit: The field device--intrinsically safe equipment located in the hazardous location o Intrinsically safe barrier orassociated apparatus located in nonhazardous location Interconnecting wire between the two apparatus The.example here is simple, because the fault combinations are few, and the:knowledge of the apparatus safety Setting the Standard for Automattmw Birthdays, holidays, star.ring college... what great occasions to give a gift that has a lasting impact. Show someone how much you respect and value his or her career aspirations, with a gift of membership in ISA. parameters and cable characteristics are sufficient to verify the safety of the system, A more complex system (combinations of barriers or using multiple cables) requires a more detailed analysis because there are many fault combinations to veriÿ and they are not ap ways evident. As mentioned above, more complex analysis may be required with intrinsic safety, but with this in mind, intrinsic safety has several advantages over the other methods of protection. They include allowing trained personnel to connect and disconnect circuits under live operation. They are guaranteed to be safe during short circuits or lead breakage. Intrinsic safety is the only ignition protection class that allows connect'ors to be opened and intrinsically safe apparatus to be removed and replaced by an equivalent device in a hazardous area. Intrinsic safety is also the only method that allows general-purpose wiring methods to be used in the hazardous area. These simple advantages also generate cost savings through installation and maintenance, because this method allows live maintenance with no need for plant shutdown. Intrinsic safety is also more reliable due to the use of infallible and derated components as prescribed by the standards. Intrinsic safety was born from the ashes of a horrific accident to become the most widely accepted method of protection. Not only does' it limit voltage and current, but it provides a 50 percent safety margin even during fault conditions; no other method of tÿrotection can claim that. In the end, it is the end users who must determine the area of classification, the required process equipment, and the method of protection they want toj use. If they decide to use intrinsic safety, I think they will find that it provides the best mix of an affordable system and the safety required. ABOUT THE &UTHOR To enroll someone, call +I (919) or visit: Robert Schosker is the product manager for intrinsic safety, HART, signal conditioners, and surge protection for Pepperl+Fuchs. Since joining Pepperl+Fuchs in 1995, he has been focused on technology and product-related support, and is involved in, a wide range of activities and roles including certifications, sales, and marketing. He has been the key lead in many IS and HART projects resulting in the developmenlÿ of new products for intrinsic safety and HART infrastructure. Schosker holds a BSEE from the University of Akron. Direct questions or comments to pa-info@us.pepperl-fuchs.com. 58 INTECH SEPTEMBER/OCTOBER