BLOWDOWN

Blowdown predicts the depressurisation of a process vessel through a throttled valve or a fixed orifice: time to target pressure, fluid and vessel-wall temperature through the event, and the peak load at the flare header. It handles a vapour-only vessel and one holding a flashing liquid with the same rigour.

Capabilities

• Time to a target pressure through a throttled valve held at a fixed percent-open, or through a fixed orifice.
• Fluid and vessel-wall temperature through the event, including the minimum reached.
• Peak discharge rate to the flare header.
• Automatic detection of a liquid heel from composition, pressure and temperature, switching between a vapour-only and a two-zone (liquid pool plus vapour) model.
• Draw from either the vapour space or the liquid pool.
• Liquid-pool composition tracked as lighter components boil off, with the component driving the change reported.
• Ambient heat input through the vessel wall and any insulation.
• A vessel at its bubble point specified by pressure alone or temperature alone.
• Vessel geometry from saved presets shared with Tank Gauging.

Why the results hold up

At each time step, Blowdown re-solves the vessel contents by UV-flash on the suite’s Peng-Robinson engine: phase equilibrium at fixed volume and fixed internal energy, the condition a rigid vessel losing mass imposes. Discharge follows the Homogeneous Equilibrium Model, which integrates the real isentropic expansion path through a PS-flash instead of applying the ideal-gas choked-flow equation. A flashing-liquid draw is therefore resolved on the same basis as a vapour one. The throttled-valve path follows ISA-75.01.01 compressible flow; the orifice path follows API 521 choked and subsonic flow.

The two-zone model is built on conservation of energy and mass. Liquid and vapour share one pressure but carry separate temperatures. Ambient heat reaches the contents through a series wall-and-insulation resistance, with natural convection from the Churchill-Chu correlation and nucleate pool boiling from the Cooper correlation. The app’s test suite checks the engine’s integration against conservation of energy and entropy.

Verification

ALKANE’s V&V Diagnostics includes a depressuring suite, which checks the engine against a closed-form ideal-gas reference. Details are on Validation & Verification.

Scope

Fire-case heat input is not modelled. Blowdown is built as an operations planning aid — it must not be used for relief valve or orifice sizing, flare header loading studies, or MDMT/brittle-fracture fitness-for-service decisions. These are safety-critical design questions, and no ALKANE module is intended for them; see the Software & Engineering Disclaimer for the suite-wide statement.