AbstractObstacle separation distance (pitch) has received little systematic study in the literature. Either too large or small spacing between obstacles would lead to lesser explosion severity. Therefore, an optimum value of the pitch that would produce the highest flame acceleration and hence overpressure is needed. It was the aim of this work to investigate the influence of obstacle blockage ratio on the obstacle spacing in gas explosions. The explosion tests were performed using methane-air (10% by vol.), in an elongated vented cylindrical vessel 162mm internal diameter with an overall length-to-diameter, L/D of 27.7. Double 20-40% blockage ratio, BR orifice plates were used as obstacles. The spacing between the obstacles was systematically varied from 0.5 m to 2.75 m. The 40% BR produced the highest explosion severity in terms of overpressure and flame speeds when compared to 30% and 20% BR. However, the worst case obstacle spacing was found to be shorter with increase in obstacle blockage. In general, similar profiles of overpressures and flame speeds were obtained for all the obstacle blockages. This trend was equally observed in the cold flow turbulence intensity profile generated behind a grid plate by other researchers. In planning the layout of new installations, the worst case separation distance needs to be avoided but incorporated when assessing the risk to existing set-ups. The results clearly demonstrated that high congestion in a given layout does not necessarily imply higher explosion severity as traditionally assumed. Less congested but optimally separated obstructions can lead to higher overpressures.