Methods of Explosion Protection in Hazardous Areas ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Methods of Explosion Protection in Hazardous Areas

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Several methods are used to protect process and control equipment used in hazardous areas in many industries. The main goal of these protection methods is to eliminate one of the components in the fire triangle to prevent an explosion. The most common hazardous area protection methods in the process control industry are:
    1. Containment
    2. Energy limitation
    3. Avoidance
      Other methods of equipment explosion protection include exclusion and dilution.
       Hazardous Area Classification

      Containment Methods
      The containment method is used to contain an explosion within the equipment enclosure. Thus, sparks or flames will not leak into the hazardous atmosphere and cause another fire or explosion. Containment is often used with spark-producing parts, such as switch gears, control boards, or transformers. An explosion-proof enclosure has the
      symbol (Ex d).

      Explosion-proof Enclosures (Ex d)
      Explosion-proof (or flameproof) enclosures are used to surround equipment parts that could ignite an explosive atmosphere (e.g., by sparking). Explosion-proof enclosures must meet the following conditions:
      1.  All enclosure joints leading to the outside environment must be flameproof
      2. The enclosure must have sufficient strength to withstand an internal explosion  without rupture or   
           permanent deformation
      3. The enclosure’s surface temperature must never exceed the ignition temperature of the ambient gas-air  
          mixture.
      4. When selecting an explosion-proof material, users should consider the material’s thickness, corrosion
          resistance, impact strength, and porosity

      Energy limitation
      Explosions can also be prevented by removing sufficient energy from escaping gases so that energy levels are below the minimum ignition energy levels of any flammable gases and combustible dusts in the ambient atmosphere. If energy levels are maintained below these levels, an explosion will not occur. The most common method of energy limitation is to design an equipment to be intrinsically safe or to use intrinsically safe barriers as common in process transmitter installations. Intrinsically safe equipment has the marking or symbol (Ex i)

      Intrinsic Safety (Ex i)
      Intrinsically safe (I.S.) equipment and wiring prevents explosion by limiting the release of sufficient electrical energy to ignite explosive gases in the atmosphere under normal or defined fault conditions. The energy allowed into the hazardous location is limited by an externally mounted I.S. barrier (usually provided by the installer). Advantages of I.S. approaches include:
      1.  Less operator action required to maintain a safe system
      2.  Easier to maintain and repair the equipment.
      I.S. devices are assigned maximum voltage, current, capacitance, inductance and power supply limits. The magnitude of these parameters determines the level of energy storage allowed in the I.S. circuit.

                                                 Intrinsic Safety and Safety Barriers
      Avoidance
      The avoidance method protects against explosions by using equipment or parts of equipment that do not arc or spark in normal service, thus preventing the ignition source from ever occurring. Equipment can be designed to have increased safety designated by (Ex e) or designed to be non-sparking (Ex n).

      Increased Safety (Ex e)
      Increased safety is perhaps the most widely used method of protection. The design and manufacture of increased safety equipment excludes normally sparking components. Manufacturers design other components to reduce substantially the likelihood of the occurrence of fault conditions that could cause ignition by:
      1. Reducing and controlling working temperatures
      2. Ensuring reliable electrical connections
      3. Increasing insulation effectiveness
      4. Reducing the probability of contamination by dirt and moisture ingress (entry)
      Common increased safety applications include terminal and connection boxes, control boxes, and light fittings.

      Non-Sparking Equipment (Ex n)
      Non-sparking equipment is equipment with which special precautions are taken with connections and wiring to increase reliability. The equipment does not produce arcs, sparks, or hot surfaces in normal operation. Non-sparking equipment is commonly used with three-phase induction motors in hazardous areas.

      Typical Equipment Protection Schemes Used in Process Control Industry
      Listed here under are the common types of explosion protection schemes used in the process control industry and the symbols used on various equipment to represent the type of  protection:

      Type of Protection Symbol (Ex or EEx)
      Explosion-proof enclosures Ex d
      Intrinsic safety in Zone 0 Ex ia
      Intrinsic safety in Zone 1 Ex ib
      Increased safety Ex e
      Non-sparking equipment Ex n

                           Equipment Degrees of Protection - IEC Ingress  Protection Ratings