Physical Effects Screening Calculator

Table 2 Gas orifice (Isentropic choked / unchoked)

Compressed-gas outflow. Gas stored above ambient pressure expands through a hole. For an ideal gas with constant heat-capacity ratio γ, isentropic nozzle theory gives two regimes:

If the downstream/upstream pressure ratio is low enough, the throat reaches sonic speed (choked flow): further lowering of P₂ does not increase mass flow. Above the critical ratio the flow is unchoked (subsonic) and depends on both P₁ and P₂. C_d again corrects for real orifice behaviour.

Critical (choked) when P₂/P₁ ≤ [2/(γ+1)]^(γ/(γ−1)).

Table 3 Flash balance (Enthalpy–KE)

Flashing / two-phase energy balance. When a pressurised liquid flashes to vapour (or a two-phase mixture accelerates), specific enthalpy and kinetic energy trade off. Neglecting heat loss, shaft work and slip between phases, a steady-flow balance equates inlet enthalpy + KE to exit enthalpy + KE.

Use this panel to check consistency of He, ue, Hf and uf, or to solve for one unknown. It does not compute flash fraction from thermodynamic flash calculations by itself.

Table 4 Pool evaporation (Mass-transfer screening rate)

Pool Evaporation describes a model for estimating evaporation from liquid pools on land or water. The model can be applied to spreading or contained pools and to both boiling and non-boiling liquids. It was developed to address several numerical difficulties associated with earlier calculation methods, although the resulting procedure is relatively complex.

Principle (this calculator): for a non-boiling pool, vapour leaves the surface by mass transfer into the wind. Rate scales with pool area, a wind-driven transfer coefficient k, and the saturated vapour concentration at the surface (from vapour pressure and ideal-gas law). Boiling pools, spreading dynamics, and heat balance with the substrate are outside this screen.

Table 5 Vapour cloud dispersion (Heavy-gas / PG screening)

Vapour Cloud Dispersion. Once vapour (or aerosol) is airborne, it mixes with the atmosphere. Buoyancy and atmospheric stability control how far a hazardous concentration travels.

Heavy gas: if the cloud is denser than air (cold release, high molecular weight, or liquid aerosol), gravity drives lateral spreading and the cloud stays low. A simple “box” model dilutes mass over a growing width and a characteristic height.

Passive gas: if density ≈ air, Gaussian plume / Pasquill–Gifford methods apply: concentration falls with downwind distance as the plume grows in σ_y and σ_z, which depend on atmospheric stability (A–F) and wind speed.

This calculator: continuous ground-level release. If cloud density > air → dense-gas box; else Briggs open-country σy, σz centreline form.

Table 6 VCE (TNT equivalent + Sachs ΔP screen)

Vapour Cloud Explosion (VCE). A flammable cloud that finds ignition can deflagrate or, in strong confinement/congestion, produce damaging blast. The classical TNT-equivalent method converts a fraction η of the cloud’s combustion energy into an equivalent mass of TNT, then reads side-on overpressure from a scaled-distance (Sachs) curve.

η is typically a few percent for outdoor unconfined clouds and higher where congestion accelerates flame. For congested plant layouts, the Multi-Energy method (blast strength linked to confined volume and congestion) is preferred over TNT; this panel remains a first teaching screen.

Table 7 Fire heat flux (Point-source radiation)

Heat flux from fires. Pool fires and jet fires harm people and equipment mainly by thermal radiation. Combustion power is burn rate × heat of combustion. A fraction χ_r of that power is radiated; the rest is convection and incomplete combustion.

Point-source idealisation: treat the flame as a point at the flame centre and spread radiated power over a sphere of radius R. This is often conservative in the far field. Near the flame, solid-flame / view-factor models (flame shape, emissive power, orientation) are needed.

Table 8 Vessel burst (Brode energy → TNT / ΔP)

Rupture of pressurised vessels. When a gas-filled vessel fails catastrophically, stored internal energy expands into the atmosphere and can generate a blast wave (and fragments/projectiles). The Brode estimate uses the excess pressure–volume energy of an ideal gas as the available expansion energy.

That energy can be expressed as an equivalent TNT mass and mapped to overpressure vs distance with the same scaled-distance idea as for VCE. Fragment flight and BLEVE fireballs are separate phenomena not covered in this panel.

Table 9 Model chain (Couple module outputs)

Interfacing of models. Between a release and the impact on people or property, several physical effects often occur in sequence. Each model’s predictions (outputs) must be adapted and transferred as inputs to the next model — matching units, phase, and meaning of the quantity.

Example: orifice mass flow may feed a pool; pool evaporation rate feeds dispersion; flammable mass in the cloud feeds VCE; burn rate feeds fire radiation. Vessel burst energy is a parallel blast pathway, not an input to a plume model.