NFPA 68: Explosion Protection using Deflagration Venting

NFPA 68: Standard on Explosion Protection by Deflagration Venting products offered by IEP Technologies.

Deflagration is acombustion that propagates through a gas or along the surface of an explosive at a rapid rate driven by the transfer of heat. IEP Technologies are designed to comply with NFPA 68 and to detect and contain explosions from spreading to interconnected equipment and other locations within the processing facility.

There are two approaches to handling deflagration from spreading down a pipeline damaging equipment to meet the NFPA 68 standard.

The first is to use a chemical suppression system. In this system, the pressure wave is detected by a sensitive press sensor that is measured by a control unit which instructs a chemical suppression agent to be discharged into the pipeline mitigating the passage of the flame and burning material further down the pipeline.

The second is to use a mechanical isolation system. In this system, the pressure wave is detected by a sensitive press sensor that is measured by a control unit which instructs a high speed knife valve to close forming a barrier, stopping the flame and burning material from traveling further down the pipeline.

Both of these approaches to eliminate a deflagration condition from causing damage to equipment and injury to employees has been proven highly effective. IEP Technologies used to eliminate deflagration problems are being applied around the world.

The NFPA 68 standard applies to the design, location, installation, maintenance, and use of devices and systems that vent the combustion gases and pressures resulting from a deflagration within an enclosure so that structural and mechanical damage is minimized.

NFPA 68, Venting of Deflagrations, applies to equipment or enclosures needing to withstand more than 1.5 psig pressure. Most dust collectors need additional reinforcement for that capability. The maximum pressure that will be reached during an explosion will always be greater than the pressure at which the vent device releases. NFPA 68 calls for a pressure differential of at least 50 lbs./ft2 or 0.35 psi between the vent release pressure and the resistive pressure of the dust collector (enclosure). This NFPA guide lists the following basic principles that are common to the venting of deflagrations. You should become familiar with these principles so that you can correctly specify the conditions the dust collector and explosion vent must satisfy.

  1. The vent design must be sufficient to prevent deflagration pressure inside the dust collector from exceeding two-thirds of the ultimate strength of the weakest part of the dust collector, which must not fail. This criterion does anticipate that the dust collector may deform. So do expect some downtime with the dust control system after an explosion.
  2. Dust vent explosion operation must not be affected by snow, ice, sticky materials or similar interferences.
  3. Dust explosion vent closures must have a low mass per unit area to reduce opening time. NFPA recommends a maximum total mass divided by the area of the vent opening of 2.5 lbs./ft2.
  4. Dust explosion vent closures should not become projectiles as a result of their operation. The closure should be properly restrained without affecting its function.
  5. Vent closures must not be affected by the process conditions which it protects or by conditions on the non-process side.
  6. Explosion vent closures must release at overpressures close to their design release pressures. Magnetic or spring-loaded closures will satisfy this criterion when properly designed.
  7. Explosion vent closures must reliably withstand fluctuating pressure differentials that are below the design release pressure.
  8. Dust explosion vent closures must be inspected and properly maintained in order to ensure dependable operation. In some cases, this may mean replacing the vent closure at suitable time intervals.
  9. The supporting structure for the dust collector must be strong enough to withstand any reaction forces developed as a result of operation of the dust explosion vent.
  10. Industrial exhaust system ductwork connected to the dust collector may also require explosion venting.

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