JT EXPANSION ANALYSER

JT Expansion Analyser predicts the downstream temperature and phase across a point restriction — a choke, control valve, orifice or leak — and places that temperature against the stream’s hydrate dissociation temperature and a supplied minimum design metal temperature (MDMT).

Capabilities

• Downstream temperature and phase for an isenthalpic pressure drop.
• Classification of the expansion path: single-phase vapour, retrograde condensation, partial or complete condensation, partial or complete vaporisation, flashing liquid, near-incompressible liquid, or Joule-Thomson heating.
• Sensible and latent components of the temperature drop, where the path enters the two-phase region.
• Density and compressibility factor upstream and downstream — vapour, liquid and bulk.
• Joule-Thomson coefficient at the operating condition.
• Hydrate dissociation temperature, Structure I and Structure II, including the effect of methanol, MEG and dissolved salts.
• Freezing point of the aqueous phase at the stream’s actual salinity and inhibitor content.
• Margin to a user-supplied MDMT.
• Phase envelope with the hydrate boundary and the expansion path overlaid.

Why the results hold up

The expansion is followed along its full isenthalpic path on the Peng-Robinson equation of state, so a path that enters the two-phase region is resolved through it rather than solved as a single endpoint. Phase stability is established by Michelsen tangent-plane-distance analysis before a flash is attempted. The Joule-Thomson coefficient is derived directly from the equation of state rather than from a tabulated approximation, and the derivation is verified numerically against an independent method.

Hydrate equilibrium uses van der Waals-Platteeuw statistical solid-solution theory, with Structure I and Structure II lattices evaluated independently and the stable structure reported. Langmuir constants follow the Parrish-Prausnitz/Munck correlation, with Munck et al. (1988) reference-state properties; gas-phase water content follows the Bukacek correlation. Inhibitor and salt effects are applied through a water-activity model that reproduces the Hammerschmidt correlation to within about 3% up to 20 wt%. Inhibited results show how inhibitor type and dose move the hydrate boundary; they are indicative, not a dosing calculation.

Verification

ALKANE’s V&V Diagnostics includes a hydrate suite covering the dissociation-temperature calculation against published reference data. Details are on Validation & Verification.

Scope

The isenthalpic basis applies to a point restriction, where transit time is short enough for heat exchange to be negligible; heat transfer along a length of pipe is outside its scope. Hydrate and MDMT results are reported as margins, not pass/fail verdicts. The module is a proximity check, not a hydrate-management or brittle-fracture fitness-for-service assessment; see the Software & Engineering Disclaimer for the suite-wide statement on safety-critical use.