NFPA 68 – New Gas Venting Equations

The 2013 edition of NFPA 68 Standard on Explosion Protection by Deflagration Venting will replace vent area sizing equations containing the gas Deflagration Index, K_G, for high-pressure applications with equations containing the laminar burning velocity, S_u.  Modified Swift-Epstein equations replace both gas venting equations in previous editions of NFPA 68.  A scale-dependent flame enhancement factor, λ, has been included in both the low-strength and high-strength methods to account for turbulence and flame instabilities prevalent in many large enclosure deflagrations.  The new vent sizing method predicts vent areas that are equal to or larger than the vent areas actually used in the overwhelming majority of tests in unobstructed large enclosures, and in many tests with large-scale obstacles simulating large process equipment.  While further work needs to be done to address corrections for enclosures with congested piping arrays, the framework has been set to facilitate this activity going forward.  This paper addresses the gas equations implemented in the 2013 edition, modifications to prior vent area corrections and examples of vent sizing.