Equations for Flammability Parameters, Pmax and KG

The maximum pressure of gas combustion, P_max, and the deflagration index, K_G,are very important flammability parameters that are used for risk assessment and relief vent design. It is difficult to predict the P_max and K_G theoretically because of the complicated mechanism of combustion.

A very large experimental data base for hydrogen / oxygen / nitrogen mixtures in a 20L closed vessel (Jo and Crowl, 2006) was used to investigate the relationship between the gas concentration and P_max, K_G.

Preliminary work showed that the best way to correlate these parameters was along the constant nitrogen line in a triangle diagram.

An on-line chemical equilibrium calculator was then used to calculate the maximum pressure of combustion assuming constant volume and internal energy.  The equilibrium pressure was then compared to the experimental pressure. This only agreed with the experimental data near the stoichiometric combustion concentration.  Since this is typically the concentration with the maximum pressure, this is a very useful result.

A commercial computer program, TableCurve, was used to fit the experimental data. A set of simple equations was identified to predict P_max and K_G.

We believe these empirical equations will be valuable for estimating these important combustion parameters.