The 2007 edition of the National Fire Protection Association Standard 68 for Explosion Protection by Deflagration Venting has a new provision to account for the turbulence level in combustible dust or powder processing equipment. This paper explains the development of this new provision for increased deflagration vent area requirements in highly turbulent combustible dust/powder processing equipment. The development includes a review of initial turbulence level effects on vented explosion pressures, and a review of turbulence levels measured in ASTM E1226 and ISO 6184/1 explosion test procedures to determine Kt. A review of operating conditions in some representative spray dryer plant equipment suggests that most equipment of this type probably do not have high enough air flows to require increased explosion vent areas due to turbulence, but some types of equipment with high tangential entrance air flows may well need larger vent areas. (C) 2007 Elsevier Ltd. All rights reserved.