High-Pressure Side (Tube)
Low-Pressure Side (Shell)
Tube Geometry
Rupture area uses tube bore (ID) per the selected failure model × tube count.
Advanced Kinetic & Thermodynamic Factors
(refine the steady-state relief load)Friction “On” replaces pure-nozzle discharge with a loss model that reduces the predicted relief load (prevents over-sizing). Gas uses a compressible Fanno/isothermal-duct mass-flux equation with f·L/D; liquid uses a velocity-head loss coefficient.
API 521 Tube-Rupture Exemption Screening
Screening of the “10/13 rule” / pressure-ratio criteria. A pass here is a candidate for exemption only — full API 521 §5.19 review and PE judgment are mandatory.
Decision Logic
- Candidate exempt: HP design ≤ hydrotest pressure of LP side, i.e. HP design ≤ (13/10) × LP design. The LP side can momentarily contain the surge to its test envelope.
- Conditional review: ratio near the 1.3 threshold, gas-filled LP side, large surge volume, or brittle-fracture / fatigue concerns.
- Relief required: HP design > LP hydrotest. A dedicated relief path (PSV / rupture disk / open vent) sized for the rupture load is needed.
10/13 ≈ 0.769. Equivalent: relief unnecessary when LP design ≥ 10/13 of HP design.
Governing Equations
API 526 Orifice Selection
Step-by-Step Calculation (expert trace)
Transient Model Inputs
1-D Method of Characteristics (distributed acoustic) surge solver, with 0-D lumped fallback for comparison.
MOC network: rupture → shell → junctions → relief device. Each segment carries its own length, bore & wave speed. Used when solver = network.
Pressure vs. Time (relief node)
Spatial Profile P(x) at peak
Dimensionless Risk Dashboard
⚠ The 1-D MOC models the shell as a single acoustic conduit (Joukowsky surge + reflections) — a legitimate distributed idealization, but not a multi-segment network simulator; gas-filled cases run as linear acoustics. Confirm severe surges with dedicated dynamic software (e.g. BOSfluids).
Omega-Method Inputs
DIERS / HEM screening (Leung). Requires the ω parameter from fluid properties.
Critical / Flashing Flow Result
⚠ Screening only. The Omega method is sensitive to inlet quality, non-condensables and property evaluation. Confirm two-phase relief loads with rigorous DIERS analysis and validated software.
Direct-Integration HEM
Homogeneous Equilibrium Model with piecewise flashing-fraction tracking yₜ(P). Marches pressure from stagnation to throat.
Mass flux G vs. throat P
Vapor fraction yₜ(P)
⚠ Flashing fraction now uses an entropy-conserving (isentropic) flash with Clausius–Clapeyron and user properties — improved over a simple energy balance but still screening-grade (constant Cp, h_lg). Rigorous HEM requires a validated equation of state. Verify against DIERS / process-simulator data.
Burst / Rupture Disk Sizing — Critical Two-Phase Flow
Sizes the disk for the two-phase critical mass flux from the HEM (run HEM first) or a manual G. A = W / (Kd·Kc·G).
Cubic EOS Property Engine
Self-contained Peng–Robinson / SRK for a pure component. Solves the cubic for Z, departure H/S, JT coefficient, isenthalpic flash & saturation pressure.
Isenthalpic (JT) Expansion Result
⚠ Pure-component cubic EOS with ideal-gas Cp. Not a multi-component VLE flash and not a substitute for CoolProp/REFPROP/NIST property packages. For mixtures (LNG, refinery cuts) use a validated property method.
Transient Stress Advisor™
Hoop / membrane stress from the transient peak vs. material allowable, plus failure-before-relief logic.
Multi-Scenario Governing Case
Screens common overpressure cases and ranks the governing relief load. Run the main Analysis first.