In 1999 a study was conducted by Purdue University and Union Carbide Corporation to compile and expand upon previous benchmark studies with the goal to compare differences between various two-phase flow safety relief sizing methodologies and software programs. This effort focused on studies conducted within the Design Institute of Emergency Relief Systems (DIERS) Users Group in 1995 and 1996 that consisted of completing multiple two-phase flow safety relief sizing and pipe flow problems utilizing such methodologies. The thorough documentation of the inputs and results of the two-phase flow calculations provide a valuable reference that can be used to benchmark new and existing methodologies and software implementations of these methodologies. This article summarizes the results obtained using FERST powered by CHEMCAD for a number of these scenarios. FERST powered by CHEMCAD is an easy-to-use software tool to quickly apply adiabatic calorimetry data (e.g. VSP2 and ARSST) to full-scale vessels using built-in material properties of over 3,000 components and 40 different thermodynamic mixing models available to represent a wide range of mixtures. FERST performs static vent size evaluations and is currently beta testing dynamic functionality that allow for the transient behavior of vessels for reactive and non-reactive upset scenarios to be simulated. This article uses the FERST dynamic functionality to replicate a number of the Adair and Fisher test conditions. The Purdue and Union Carbide Corporation study (Adair and Fisher, 1999) primarily focused on TPHEM, CCFLOW, and RRERSP computing models. Within that study these methodologies are related to one another, as TPHEM and CCFLOW utilized input from RRERSP to have closer agreement. RRERSP was performed multiple times within each case to obtain successive iterations, TPHEM and CCFLOW were also run multiple times utilizing different material property sets, and each calculation was documented. In turn, the provided data comparisons are somewhat biased toward these methodologies. We have found that FERST’s outputs have relatively good agreement with the documented methodologies, especially within the larger more diverse data sets. A number of documented cases within those studies were omitted here because they assumed viscosity parameters within the problem statement, whereas FERST calculates viscosity and other fluid properties in conjunction with CHEMCAD.
Safety Relief Sizing Problem Statement
Flashing liquid (Water: vapor and liquid) flows through a 4P6 conventional/bellows safety relief valve which has a set pressure of 52.55 psig from a vertical 1595-gallon vessel. Given the specified inlet/outlet isometrics and vessel specifications, the initial relief conditions through the vent line were calculated. (See Adair and Fisher (1999) for all specified input parameters.) The results calculated within FERST are summarized in Table 1. These results are compared with the results in Adair and Fisher in Figure 1 through 4. All documented data from Adair and Fisher were included within the calculated mean unless otherwise specified.
Figure 1: Mass Flow Rate
Figure 2: Non-recoverable Inlet Losses
Figure 3: Choke Pressure
Figure 4: Discharge Backpressure
Particularly within cases 2, 8, and 10, TPHEM, CCFLOW, and RRERSP collectively make up over half of the documented data for these cases. Variance within these cases is due to differences in methodologies and can also be attributed to differences in material properties. FERST utilized CHEMCADs material property database to readily calculate material properties, which are slightly different than the material properties given within Adair and Fisher.
Pipe Flow Problem Statement
In combination with the relief scenarios described above, a secondary study was conducted to compare the effects of varying the relief line configuration between different methodologies. Water discharged from the specified vessels given the specified conditions and the resulting mass flow rate was calculated. See Adair and Fisher (1999) for all specified input parameters. The pipe flow benchmark cases in Adair and Fisher Pipe Flow have varying number of methodologies included in the datasets, which impact the variation in results. For instance, pipe flow benchmark case 4 had no other methodologies documented for benchmark 4 outside of multiple TPHEM, CCFLOW, and RRERSP, and for benchmark 1 only two other methodologies were documented. The results calculated within FERST are summarized in Table 2 and these results are compared with the results in Adair and Fisher in Figure 5. This figure shows good agreement between the results calculated by FERST and Adair and Fisher.
Figure 5: Pipe Benchmark Mass Flow
Conclusion
Comparison against previously documented vent sizing and pipe flow scenarios is an important step for benchmarking FERST powered by CHEMCAD. The results from FERST are in good agreement with all of the Adair and Fisher (1999) data sets and excellent agreement with the more diverse data sets. Two-phase flow is a complex phenomenon, and multiple methodologies exist to model complex behavior. Fauske and Associates offers a robust and easy to use option with FERST. The capabilities of FERST are growing with the latest dynamic simulation feature, allowing for transient behavior of vessels and relief lines to be modeled prior to and during reactive and non-reactive relief events. For further information on FERST contact info@fauske.com.
References
- FERST powered by CHEMDCAD Version 1.0.1.18090, Fauske & Associates, LLC, 2024.
- Adair S. P. and Fisher F. G., “Benchmarking of two-phase flow through safety relief valves and pipes,” Journal of Loss Prevention in the Process Industries 12, 1999.
- Leung, J. C., "Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels,” AIChE Journal, Vol. 32, No. 10, p. 1622-1634, 1986.
- Leung, J. C., “Flashing Two-Phase Flow Including the Effects of Noncondensable Gases,” Journal of Heat Transfer, pp. 269-272 (February 1991).
- Leung, J. C., “Vent Sizing for Gassy and Hybrid Systems,” Safety of Chemical Batch Reactors and Storage Tanks, A. Benuzzi and J. M. Zaldivar (eds.) p. 299-310
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