Source Models Discharge Models

Most of the incidents and hazards associated with escaping hydrogen such as fire, explosion, and even formation of a flammable vapour cloud usually involve the escape of liquid or gaseous hydrogen from the containment owing to either the failure of the containment or an abnormal discharge from an engineered outlet such as a relief valve, followed by vaporization and dispersion. In particular, a large quantity of liquid hydrogen and/or vapour may be released by failure of the tank, line or valve.

For hydrogen gas at low pressure, the flow through an orifice is governed initially by the equation of isentropic (constant entropy) flow, by solving the conservation of energy and mass Eq. E-1 (Appendix E). Meanwhile for liquid hydrogen, the release may be gas, liquid, or a two-phase mixture, depending on certain conditions. For example, if the release is from a container holding liquid under pressure, it will normally be liquid if the aperture is below the liquid level, and vapour or vapour-liquid mixture if it is above the liquid level. For a given pressure difference, the mass rate of release is much greater for a liquid or vapour-liquid mixture than for a gas. To calculate the rate of release given the size of the aperture using thermodynamic and physical properties of escaping liquid hydrogen the Bernoulli Eq.E-4 is used (Appendix E). Pool Spreading and Evaporation

Immediately after release, the liquid hydrogen spreads out on the ground. It will spread until it meets an artificial boundary such as a dyke, until it reaches a minimum depth at which it no longer spreads, or until the evaporation rate is equal to the release rate so that the amount of liquid in the pool is no longer changing. Also immediately after release, the liquid hydrogen starts to boil off as it absorbs heat from the air, the ground and the sun. Mass is also lost from the pool when wind removes the evaporated material from the surface of the pool so that hydrogen evaporates in order to restore the partial vapour pressure. If hydrogen is released from containment as a liquid, vaporization must occur before a vapour cloud is formed. Immediately after release the heat for boil-off is taken from the ground. Equations used to calculate the pool radius, and evaporation rate are given in section E.1.2 (Appendix E).

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