# NFPA 68 formula 8.3.2 seems to have a fault

Question asked by aan on Aug 29, 2017
Latest reply on Sep 21, 2017 by aan

The formula uses Kst, Filter Volume & Pred to calculate a pressure developed. This is applied to the number of vents in a system. The idea being to find a weight in Kg/m2 that a deflagration panel may have before the size must be compensated.

Without detailed knowledge of the background of the equation I cannot confirm my hunch that there is a fault but intuitively there seems to be a disconnect in the way the number of panels is applied in the equation (represented by n).

In the equation the number of vents "n" is multiplied through which gives an increase in how much each panel may weigh (kg/m2) with each additional panel in the system. Assuming that the force of the deflagration is constant you would suspect that the forces on each panel would decrease as the number of panels increases and as a result the overall kg/m2 permissible would also decrease. I suspect that the number of panels should be divided into the equation. so "n to the power of 0.3" should in fact be "1/n to the power of 0.3).

I have created a simple excel sheet which demonstrates the use in both cases. The green field is the current formula 8.3.2, the red field is the suggested change. Start be putting n to 1 and increase. You will get the idea. Again this is only a guess at what is wrong, the true fault may lie elsewhere in the formula.

Please note that the formula number refers to the current second draft numbering. In the original 2013 version of the standard the formula was numbered as 8.2.6.2