Application Note PA-E
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Simply connecting a sensor to an explosion proof head does not make a temperature assembly explosion proof. To ensure safety, all components must be properly rated.


nstrument engineers are responsible for providing safe
and appropriate instrumentation. To declare a system
safe, simply because a single component is approved,
is dangerous in more ways than one.
  Two important factors should be considered whenever a job involves electrical equipment in a hazardous location:

Safety. Is the equipment specified truly safe to use in the proposed environment? If the answer is yes, then on what premise is that answer based?
   
Liability. If you have determined that the equipment specified is safe, are you prepared to defend that determination in a court room?

  No one wants to be responsible for injuring others, especially when easily obtainable safeguards exist. In the event of an unfortunate incident, the consequences can be punitive if the injured party can prove that the designer did not take all reasonable and prudent measures to protect against predicable hazards.
  In the event of an accident involving an explosion in a hazardous environment, the attorneys for the injured party will attempt to prove negligence on the designer's part. To do this, they usually will retain some individual or organization recognized as an expert in the field. The designer might be asked what measures were taken to ensure that the equipment was safe. Then, their expert would attempt to point out items missed or not listed under direct examination.
  In the United States, Factory Mutual Research Corp. (FMRC) is recognized as one of the experts in the field of industrial safety. For certain installations, many companies, local governments and regulatory agencies require that all electrical equipment must be approved by FM and Underwriters Laboratories (UL). In general, these rules and regulations spell out the location's category by class, division and group. (See Sidebar)
  When an installation requires that electrical-measuring instruments be placed in locations that are considered hazardous, it is incumbent upon the engineer to see that the installation is safe. The user generally will specify the degree of safety required. For example, a petrochemical company could specify that supplied equipment must be explosion proof for Class I, Division 1, Group B. The user is being very specific as to the level of security expected, referring to Article 500 of the National Electrical Code. Factory Mutual subscribes to this code and references it in their approval guide.
  When an installation requires that electrical-measuring instruments be placed in locations that are considered hazardous, it is incumbent upon the engineer to see that the installation is safe. The user generally will specify the degree of

 


safety required. For example, a petrochemical company could specify that supplied equipment must be explosion proof for Class I, Division 1, Group B. The user is being very specific as to the level of security expected, referring to Article 500 of the National Electrical Code. Factory Mutual subscribes to this code and references it in their approval guide.
  In general, the classification defines the extent of the hazard, taking into account the energy required to ignite certain gas mixtures and the potential forces released as a result. When an assembly is called explosion proof, the sensor assembly manufacturer is certifying to the user that specific steps have been taken to ensure the safety of all Plant personnel.
  But, no chain is stronger than its weakest link; likewise, no system is explosion proof if it contains non-explosion proof components. Systems are rendered explosion proof by two means:
• Design the system so that it cannot release sufficient electrical or thermal energy to ignite the specified gas mixture. Such systems are classified as intrinsically safe.
• Design the system such that it is capable of containing an internal explosion of a specified flammable vapor air mixture and may contain sufficiently long thread paths such that any escaping gases will be cooled below the ignition temperature of the surrounding atmosphere.

CASE IN POINT:
  Installing a temperature sensor in a hazardous location requires more than an approved explosion proof connection head. To meet the safety requirements of FM, you must consider the complete system: including the sensor, the thermowell (if required and used) and the connection head.
  Each component in the system must meet the code. For example, many thermowells are supplied with 1/2-14 NPSM threads for the sensor connection. FM dictates that to meet the requirements for explosion proof, a tapered thread is required. Most sensors are supplied with a 1/2-14 NPT thread; however, FM also specifies a meinimum thread engagement between the sensor and the connection head. The thread engagement requirement makes it necessary to overcut the thermowell thread beyond the ASME definition of 1/2-14 NPT.
  Those manufacturers wishing to offer explosion proof assemblies must meet FM's requirements. Once a design is constructed with the proper thread size and engagement between the head cover, head body, wire conduit and seal, sensor, lag fittings and thermowell, the manufacturer submits the design to FM for testing to ensure that the specified gas mixtures are properly contained. Once approved by FM, the manufacturer is not permitted to alter any of the approved configurations without FM approval. If changes are

     
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necessary, FM may require additional testing before approving any changes. In addition, FM performs periodic unannounced inspections of the manufacturer's plant to ensure that the assemblies are constructed in accordance with the approved drawings.
   With explosion proof assemblies, it is truly a case of ‘you get what you pay for’. Using unapproved sensors or thermowells with an approved, explosion proof connection head, for example, will not provide the protection you need for your plant and personel. Failure to consider the entire system, or failure to utilize the knowledge and resources of recognized experts in this critical area, is at best risky, and at worst dangerous and irresponsible.

 

 

 

 
Explosion proof probe assemblies incorporate stainless steel fittings, stainless stell or aluminum lag unions, tig welded construction and cast aluminum heads.


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hazardous locations:
Understanding Class, Division and Group Definitions

While nothing can replace the published codes defining Class, Division and Group ratings for explosion proof applications, these guidelines provide a general overview of the terms and how they are used.

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Class: The term "Class" is used to categorize the nature of the hazard. For example, Class I comprises flammable gases and vapors; Class II comprises combustible dust; and Class III comprises ignitable fibers.

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_ Division: The term "Division" categorizes the area in which the equipment is to be installed. Division 1 covers all electrical devices including sensors and describes an environment that is usually or likely to be hazardous. Division 2 is applied alone to lighting to limit operating temperature and defines an environment that is usually safe but may become hazardous in the event of some kind of accident or failure.
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_ Groups: Within Class I and Class II, there are material groups representing the degree of hazard. For instance, Class I refers to gases and vapors; Group A belongs to Class I and is defined as atmospheres containing acetylene. (Acetylene is explosive when mixed with air over a wide — 3 to 80% — range.)
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Groups A, B, C and D apply to Class I environments (containing combustible gases):

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Group A: acetylene

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_ Group B: hydrogen, fuel and combustible gases containing more than 30% hydrogen by volume or equivalent hazard such as butadiene, ethylene oxide, propylene oxide and acrolein.
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_ Group C: ethyl, either ethylene or gases, or vapors of equivalent hazard
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Group D: acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, propane or gases or vapors of equivalent hazard

 
Groups E, F and G apply to Class II environments (containing combustible dusts):
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Group E: metal dusts: aluminum, magnesium and their chemical alloys, or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards to the use of electrical equipment, characterized by resistivity < 100 ohm – centimeter.

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_ Group F: carbonaceous dusts: carbon black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles, or dusts that have been sensitized by other materials so that they may present
an explosion hazard, characterized by resisitivity between 100 and 108 ohm ® centimeter (> 105 ohm –centimeter in Division 2).
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_ Group G: combustible dusts not in Groups E or F, including flour, grain, wood, plastic and chemicals and characterized by resistivity
>108 ohm®centimeter.
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Published 2003, RdF Corp., expanded. Original publishing: PROCESS HEATING, July 1998