Two-phase blowdown and depressurisation calculator

Emergency depressurisation of a vessel holding a saturated liquid is not a simple gas blowdown. As the pressure falls the liquid flashes, the vapour cools sharply, and the wetted and dry portions of the wall follow very different temperature paths. This module tracks the vessel pressure, the phase split and the wall temperatures so that an orifice size can be checked against both the depressurisation target and the low-temperature limit of the steel.

Open the Two-Phase Blowdown module

What this calculator returns

Pressure, temperature and inventory versus time
Vapour and liquid mass split during the blowdown
Wet-wall and dry-wall metal temperature histories
Minimum wall temperature and time to reach the target pressure

Required inputs

Initial pressure, temperature and vessel geometry
Fluid or pure component selection
Restriction orifice diameter and discharge coefficient
Wall thickness and material properties

Calculation method

The vessel contents are treated as a saturated pure component; the vapour pressure curve fixes the temperature at each pressure step.

Mass leaves through the orifice using the discharge relations of the gas-discharge module, evaluated on the vapour phase at the current saturation state.

The wall is split into a wetted zone, where boiling heat transfer keeps the metal close to the fluid temperature, and a dry zone, where only vapour convection acts and the metal cools much more slowly.

Governing equations

dP/dt = ( m_out - m_flash ) / ( V ( d rho / d P ) )

Pressure decay from the net vapour mass balance in the vessel.

M_w Cp_w dT_w/dt = h A ( T_f - T_w )

Wall energy balance used separately for the wet and dry zones.

T = Tsat( P )

Saturation constraint linking fluid temperature to vessel pressure.

Nomenclature

P: vessel pressure, Pa
m_out: mass flow through the orifice, kg/s
m_flash: vapour generated by flashing liquid, kg/s
V: vessel volume, m3
T_w, T_f: wall and fluid temperature, K
h: heat transfer coefficient, W/(m2 K)
M_w Cp_w: wall thermal mass, J/K

Assumptions and limitations

Pure-component saturated behaviour; multi-component flashing is approximated by the dominant component.
Vapour space and liquid space are each treated as well mixed.
External heat input from fire is excluded unless explicitly added.

Reference practice

Aligned with the depressurisation guidance of API 521 for emergency blowdown system design.

Worked example

A saturated propane separator depressurised through a restriction orifice against the API 521 guideline of reaching 6.9 barg, or half the design pressure, within 15 minutes.

Step	Value	Basis
Initial state	Saturated liquid and vapour at the operating pressure	The liquid inventory flashes as pressure falls, so the vapour mass grows during the blowdown
Orifice sizing	Iterate the orifice diameter	Increase the diameter until the 15-minute target pressure is met
Wall temperatures	Wet and dry wall tracked separately	The dry wall follows the cold vapour and is normally the governing metal temperature
Acceptance check	Minimum wall temperature against the MDMT	A larger orifice depressurises faster but drives the metal colder, so the two checks are traded off

Blowdown sizing is a two-sided problem: the orifice must be large enough for the depressurisation target and small enough to keep the dry wall above the minimum design metal temperature.

Illustrative numbers only — rerun the module with the project basis of design before using any result.

Common questions

Why is a two-phase blowdown colder than a gas blowdown?

Flashing liquid absorbs latent heat from the remaining fluid, so the vapour and the wall in contact with it cool far below what an ideal gas expansion would predict.

Which wall governs the low-temperature check?

Usually the dry wall above the liquid level, because it exchanges heat only with cold vapour and has no wetted-wall heat sink.

What target does API 521 use?

Depressurising to 6.9 barg, or to 50 percent of design pressure, within 15 minutes is the common basis for vessels exposed to fire.

Related calculators

Gas Discharge — Gas discharge rate calculator for pressurised releases
Chemical Properties — Chemical and mixture property data for safety calculations