Emergency relief device sizing and evaluations are performed with a sophisticated in-house computer program developed by Rankin Chemical Technologies. This program has been used for evaluating and/or sizing hundreds of devices, and uses sophisticated VBA algorithms in an Excel environment. Validation of the two-phase DIERS portions of the program was made against numerous published test cases.

General Program Features

1. Standard scenarios handled:
   • Fire
   • Fire (unwetted)
   • Vapor Relief (blocked outlet)
   • Abnormal Process Heat Input
   • Chemical Reaction
   • Entrance of Volatile Material
   • Regulator Failure
   • Liquid Relief (blocked outlet)
   • Flashing Liquid Relief
   • Hydraulic Expansion
   • Two-phase Relief
   • Two-phase Dynamic Analysis
   • Inbreathing and Outbreathing (API-2000 low-pressure tanks)
2. Sizes and/or rates pressure safety valves and rupture disks for pressure vessel applications (API-520/521).
3. Sizes and/or rates pressure safety valves and combination pressure-vacuum safety valves for atmospheric and low pressure tank applications (API-2000).
4. Handles multi-component mixtures with up to ten liquid phase components.
5. Vapor-liquid equilibrium can be modeled as ideal, or liquid phase activity coefficients can be specified.
6. Full export capability of results to PDF documentation file with automatic bookmark creation.

Special Program Features

1. Different vessel head types handled, including ASME F&D, 2:1 elliptical, hemispherical and conical.
2. Liquid volume and wetted area calculated automatically based on specified mass inventory.
3. Vessel heat transfer in agitated vessels can be calculated using built-in modeling feature.
4. Inlet and tailpipe hydraulic evaluations performed based on user-specified pipe geometries.
   • Critical and sub-critical flows handled
   • Total backpressure at pressure relief valve outlet checked to ensure code compliance
   • Reaction forces calculated at pressure relief valve and/or tailpipe exit
5. Additional backpressure contribution from simultaneous relief into common blowdown header can be modeled for vapor and two-phase streams.
6. Automatic creation of datasheet specifications for pressure relief valves and rupture disks for pressure vessel applications (API-520/521) and pressure-vacuum safety valves for low-pressure tank applications (API-2000).

Two-phase Flow Modeling

1. DIERS methodology used to predict the onset of two-phase flow during relief events.
2. Churn-turbulent, bubbly and homogeneous disengagement flow models used.
3. PRD mass flux calculated using the two-point Omega method.
4. Hydraulic calculations utilize Joseph Leung’s method for pressure relief valve backpressure and choking pressure determination.
5. Dynamic modeling of two-phase relief event from onset through disengagement to all-vapor flow.
   • Ability to change relief valve size to quickly evaluate overpressure response
   • Time-dependent graphs generated to track key parameters, including pressure, liquid inventory, void fraction, PRD mass flux, and vapor quality at the PRD inlet
   • Results display the maximum expected overpressure as well as the total liquid and vapor vented at both the PRD and the vent outlet