Technical Report

There are various literatures on dust explosions. Most of explosion suppression systems and explosion emission systems are based on a guideline by NEPA (National Fire Protection Association), “Guide for Explosion Venting” (NEPA, No. 68-2013) or ATEX directive (ATEX2014/34/EU). For example, it is general that the area of the emission holes is determined from the capacity of the container.
NEPA is a compilation result from various experimental data, which is very easy for us to use. It is also highly evaluated by factory safety managers in U.S., and there are many practical examples adopted in real facilities. It is said in the column of “vent opening pressure” of this guideline that (quote) “the pressure rise is alleviated when the vent is opened during explosion. The smaller the pressure to open the vent, the smaller the rising pressure. The opening pressure should not be reduced to the extent that the vent opens due to the change in pressure during the normal operation in the container, but the vent opening pressure should be as low as possible. (skipping over the explanation in between) Vent device must not harm a person or equipment when it opens. For example, the vent panel should be the appropriate hinge type. At the same time, a large mass of flame that forms during venting must not harm a person or equipment.” (unquote) And in another columns, the guideline states “The purpose of providing the emission holes is to dissipate the combustible dust before it develops into a pressure that could damage the container. The maximum combustion pressure in most air-containing dust-sealed vessels reaches 100 psig (7 barg, 0.7 Mpag) under atmospheric pressure. An emission hole which has a low mass per area, opens by low pressure and has sufficient area can reduce the maximum developing pressure to a smaller value. (skipping over the explanation in between) The required vent area in the emission hole design becomes the function of (1) container capacity, (2) dust level, (3) vent opening pressure, (4) ignition energy, and (5) maximum pressure.” (*)
Figure 1 shows the explosion vent panel for Fluidized Bed Dryer for ABS, designed in accordance with this NEPA guideline. In addition to this, a rupture disc, a suppression system, and a nitrogen closed circuit have also been constructed.