Suppression of hydrogen–oxygen–nitrogen explosions by fine water mist: Part 2. Mitigation of vented deflagrations

The simultaneous use of water mist and dilution by nitrogen has not been previously considered for the mitigation of hydrogen containing gas explosions and there is little guidance currently available to plant and safety engineers in the nuclear industry. This gap in knowledge is addressed and data reported for the reduction of rates of pressure rise experienced in a vented apparatus. Such information will also be of use in subsequent, separate modelling studies. loding wire igniter was used, powered from a capacitor charged to 3 kV to provide energy of about 9 J. Using this ignition source it was found that the flammable region of hydrogen–air–nitrogen mixtures was reduced in the presence of fine water mist. The combined effect of nitrogen and water mist was found to be greater than either when used alone. For water mist densities of about 200 mg/l or more, it was not possible to ignite mixtures containing less than about 0.1 fuel-free (v/v) oxygen fraction. For mixtures that did ignite, initial rates of pressure rise were reduced in the presence of water mist. The effect of water mist in reducing the rate of pressure rise was consistent with its effect on burning velocity (reported in part 1 of this study).