Modeling Pressure Relief in Aspen Dynamics – the DIERS Benchmark Examples
Pressure Relief in Aspen Dynamics The pressure relief capability in Aspen Dynamics is provided by a collection of specialized pressure relief models together with a number of unit operation models with pressure relief support. All models use Aspen Properties for accurate and reliable estimation of physical properties. Pressure relief models can be added to a dynamic flowsheet and configured as necessary to define a relief system. This approach allows you to model and simulate pressure relief from entire flowsheets, including the control system, rather than just individual unit operations. The combination of equation-based solution strategy, rigorous physical property estimation and an extensive library of unit operation models gives Aspen Dynamics a unique and flexible capability for solving a wide variety of pressure relief problems, from steady state (rating) calculations through to dynamic simulation of relief events such as blocked-in operation or runaway reactions. The DIERS Consortium
Formed in 1976 under the auspices of AIChE, the Design Institute for Emergency Relief Systems (DIERS) was originally a consortium of 29 companies to develop methods for the design of emergency relief systems to handle runaway reactions. DIERS became a user group in 1985. Presently, over 120 companies have formed the DIERS Users Group to cooperatively assimilate, implement, maintain and upgrade the DIERS methodology. The purpose of the group is: • to reduce the frequency, severity and consequences of pressure producing accidents, and; • to develop new techniques which will improve the design of emergency relief systems.
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Of particular interest to DIERS is the prediction of two-phase flow venting and the applicability of various sizing methods for two-phase vapor-liquid flashing flow. DIERS has spent $1.6 million to investigate the two-phase vapor-liquid onset/disengagement dynamics and the hydrodynamics of emergency relief systems. Aspen Technology is a member of the DIERS Users Group. DIERS Methodology
The Aspen Dynamics pressure relief models include methods and principles developed by DIERS for calculating two-phase vessel onset/disengagement dynamics and relief system hydrodynamics (Fisher et. al., 1992; CCPS, 1998). The accuracy and validity of these models for rating pressure relief systems in Aspen Dynamics has been demonstrated through solution of the DIERS benchmark examples. These examples include various cases of compressible and incompressible single phase flows and flashing, hybrid and frozen two-phase flows through (i) a safety relief valve system and (ii) combinations of nozzles and horizontal and vertical piping arrangements. Solutions obtained with Aspen Dynamics give good agreement with consensus results.
DIERS Benchmark Examples The DIERS benchmark examples are a set of case studies that serve to validate computer software for rating pressure relief systems. Successful solution of the benchmark examples is demonstrated by attainment of consensus results with those reported by other members of the users group (Adair and Fisher, 1999).
There are two categories of benchmark examples: • safety relief valve case studies • pipe benchmark studies Full details of the DIERS benchmark examples and results are available in the open literature (Adair and Fisher, 1999). Safety Relief Valve Case Studies
The safety valve problem was formulated to illustrate the effect of viscosity, the presence of non-condensable gas and inlet quality (disengagement) assumptions on safety relief valve (nozzle) mass flow rate, inlet pipe irreversible pressure loss and discharge pipe back pressure calculations, for a relief system venting to atmosphere. The relief system comprises an inlet line, a safety relief valve and a discharge line.
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A summary of the safety relief valve case studies is given in Table 2.1. Case
Flow
1 Vapor 2 Flashing Liquid 3 Hybrid 4 Flashing Liquid 5 Hybrid 6 Flashing Liquid 7 Flashing Liquid 8 Liquid (water) 9 Liquid (water) 10 Frozen twophase 11 Frozen twophase 12 Flashing liquid 13 Flashing liquid 14 Hybrid 15 Hybrid 16a Flashing liquid 16b Flashing liquid 16c Flashing liquid 17 Frozen twophase
Viscosity
Inlet Quality
Low Low
1.0 0.01
Low Moderate
0.01 0.01
Moderate Low
Low
0.01 Churn-Turbulent onset/disengagement Homogenous onset/disengagement 0.0
High
0.0
Low
0.01
High
0.01
High
0.01
Low
Very high High Very high Very high High
Comments
0.01 0.01 0.01 0.01
Moderate
0.01
Very high
0.01
Constant viscosity nozzle flow Constant viscosity nozzle flow Constant viscosity nozzle flow
Table 2.1: Safety Valve Case Studies
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Running the Safety Relief Valve Case Studies To run one of the safety relief valve case studies: 1. Copy the relevant Aspen Dynamics input file (.dynf) and the appropriate Aspen Plus backup file (.bkp) to a convenient working folder. There is one Aspen Dynamics input file for each case study (named accordingly) and two Aspen Plus backup files; SafetyReliefValve.bkp is used for Cases 1-9 and 12-16, SafetyReliefValveFrozenFlow.bkp is used for Cases 10, 11 and 17. 2. Load the .bkp file into Aspen Plus and run it. Note that the .bkp file is used to create a properties input file, so you will not see a flowsheet within Aspen Plus. 3. To create an Aspen Plus Problem Definition file (.appdf), save the simulation as an Aspen Plus Document (.apw file) and exit Aspen Plus. The .appdf file contains all of the physical properties data required for the dynamic simulation. The .apw file is not required, and can be deleted. 4. Open the Aspen Dynamics simulation. 5. From the Tools menu, click Snapshots. 6. Select the converged snapshot that is marked as kept, then click Copy Values. Ensure the Run Mode is set to Initialization and Run the simulation. Once you have run the simulation, you can use the forms and plots for the streams and blocks to see the results. Results Results of the safety relief valve examples are summarized in the attached spreadsheet (click on icon to open):
AD121SafetyValveR esults.xls
The key results reported for each case are: • the relief mass flow rate, • the irreversible inlet pressure drop (the difference between the inlet stagnation pressure and the stagnation pressure at the valve inlet, less any pressure drop due to inlet pipe elevation), • the discharge pipe back pressure (the difference between the exit pressure of the valve and the relief system discharge pressure).
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Other results, including the choke conditions at the nozzle throat, are also reported. Pipe Benchmark Cases
The pipe benchmark cases were formulated to illustrate the effect of viscosity, the presence of non-condensable gas and inlet quality, and the pipe orientation on the mass flow rate through piping systems.
A summary of the pipe benchmark cases is given in table 2.2. Case
Flow
Viscosity
Inlet Quality
Comments
1 Liquid
Low
0.0
2 Two-phase flashing 3 Two-phase flashing
Low
0.001
Low
0.001
4 Hybrid
Low
0.001
4a Hybrid
Low
0.01
5 Initially subcooled/twophase flashing 6 Initially subcooled/twophase flashing
Low
0.0
Horizontal and down flow Horizontal and down flow Horizontal/vertical and vertical/horizontal Horizontal and vertical and inclined down flow Horizontal and vertical and inclined down flow Nozzle flow
Low
0.0
Pipe flow
7 Two-phase flashing 8 Two-phase flashing 9 Two-phase flashing 10 Cold Liquid flow and hot initially subcooled/twophase flashing flow
V. high
0.001
High
0.001
Moderate
0.001
Low
0.0
Horizontal and down flow Horizontal and down flow Horizontal and down flow Horizontal and down flow
Table 2.2: Pipe Benchmark Cases
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Running the Pipe Benchmark Examples To run one of the pipe benchmark examples: 1. Copy the Aspen Plus backup file pipebench1.bkp and the relevant Aspen Dynamics input file (.dynf) to a convenient working folder. There is one Aspen Dynamics input file for each benchmark example (named accordingly). 2. Load the .bkp file into Aspen Plus and run it. Note that the .bkp file is used to create a properties input file, so you will not see a flowsheet within Aspen Plus. 3. To create an Aspen Plus Problem Definition file (.appdf), save the simulation as an Aspen Plus Document (.apw file) and exit Aspen Plus. The .appdf file contains all of the physical properties data required for the dynamic simulation. The .apw file is not required, and can be deleted. 4. Open the Aspen Dynamics simulation. 5. From the Tools menu, click Snapshots. 6. Select the converged snapshot that is marked as kept, then click Copy Values. Note that many of the pipe benchmark examples include several sub-cases. Table 2.3 identifies the kept snapshots for each case. 7. Ensure the Run Mode is set to Initialization and Run the simulation.
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Once you have run the simulation, you can use the forms and plots for the streams and blocks to see the results. Case
Snapshot Name
Description
1
Pipe2x16_4 Pipe2x160_4 Pipe2x1600_4 Pipe8x16_4 Pipe8x160_4 Pipe8x1600_4 H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 HV16_4 HV160_4 HV1600_4 VH16_4 VH160_4 VH1600_4 H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 DEG45_16_4 DEG45_160_4 DEG45_1600_4
2” ID, 16.4’ pipe 2” ID, 160.4’ pipe 2” ID, 1600.4’ pipe 8” ID, 16.4’ pipe 8” ID, 160.4’ pipe 8” ID, 1600.4’ pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 160.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 160.4’ down pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe 45 degree 16.4’ down pipe 45 degree 160.4’ down pipe 45 degree 1600.4’ down pipe
2
3
4
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Case
Snapshot Name
Description
4a
H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 DEG45_16_4 DEG45_160_4 DEG45_1600_4 N2_0psi N2_1psi N2_2psi ID0_417 ID1_291 ID3_146 H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 H16_4 H160_4 H1600_4 V16_4 V160_4 V1600_4 H_cold H_hot V_cold V_hot
Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe 45 degree 16.4’ down pipe 45 degree 160.4’ down pipe 45 degree 1600.4’ down pipe 0 psi of N2 subcooling 1 psi of N2 subcooling 2 psi of N2 subcooling 0.417” ID pipe 1.291” ID pipe 3.146” ID pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe Horizontal 16.4’ pipe Horizontal 160.4’ pipe Horizontal 1600.4’ pipe Vertical 16.4’ down pipe Vertical 160.4’ down pipe Vertical 1600.4’ down pipe Horizontal pipe, cold water Horizontal pipe, hot water Vertical down pipe, cold water Vertical down pipe, hot water
5
6
7
8
9
10
Table 2.3: Snapshots for Pipe benchmark examples.
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Results Results of the pipe benchmark examples are summarized in the attached spreadsheet (click on icon to open):
AD121PipeBenchmar kResults.xls
The key result reported for each case is the relief mass flow rate.
References Fisher, H.G., Forrest, H. S., Grossel, S. S., Huff, J. E., Muller, A. R., Noronha, J. A., Shaw, D. A. and Tilley, B. J. (1992). Emergency Relief Systems Design Using DIERS Technology: the Design Institute for Emergency Relief Systems (DIERS) project manual. AIChE.
Center for Chemical Process Safety (CCPS). (1998). Pressure Relief and Effluent Handling Guidelines. AIChE. S. P. Adair and H. G. Fisher (1999). Benchmarking of twophase flow through safety relief valves and pipes. Journal of Loss Prevention in the Process Industries. 12, pp.269-297.
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