18 Best Practices When Applying Process Modeling to Overpressure Protection and Relief Networks

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1 WHITE PAPER 18 Best Practices When Applying Process Modeling to Overpressure Protection and Ron Beck, Industry Marketing Director, Aspen Technology, Inc. Wilfried Mofor, Product Management, Aspen Technology, Inc.

2 Abstract Safety is one of the issues at the forefront of recent innovations in modeling technologies to help companies optimize their systems and processes for safer performance. A barrier to achieving optimal designs, from a safety point of view, has been the separate working and modeling environments of different engineering specialists involved in the process design and the safety design of these systems. New advancements have brought these environments together to enable companies to achieve optimization across process, safety, and cost objectives. This paper explains how the innovative integration between process simulation, dynamic analysis, and rigorous pressure safety valve (PSV) and rupture disk modeling has evolved to make processes inherently safer and describes the benefits of the new PSV sizing features in process simulation to improve safety and optimize plant operations. One can now perform relief valve design analysis, sizing, and documentation from within the process simulation, making the workflow easier and faster than ever before. A number of best practices to bear in mind as one proceeds with this analysis are included. 2

3 Overview Hydrocarbon and chemical processes operate under elevated pressure and temperature conditions. This makes overpressure protection one of the most important safety issues governing the design and operation of a process facility, whether a production field, a transportation system, or a process plant. There is surprising complexity involved in the correct and appropriate design of overpressure protection systems, mainly due to the challenges in ensuring that each item of equipment and pipeline is protected under each identified safety scenario. The regulatory context has continued to become more rigorous, due to the continued occurrence of such overprotection incidents. For all these reasons, the design of pressure relief devices and the overall design of overpressure protection systems is an extremely important item during conceptual design and FEED, and is often a critical path item, with the experts tasked with these designs in high demand and short supply in an organization. This paper addresses several topics, including: (1) the ability to complete rigorous pressure relief studies within a process model from performing relief valve design sizing and analysis to completing documentation making the process easier and faster than ever before; (2) the close integration of flare system design with simulation models; and (3) easier-to-use dynamic modeling, which enables engineers to avoid overdesign when considering safety, thereby conserving capital. Finally, (4) to that end, new tools to evaluate economics from within these models, enabling the engineer to optimize designs to achieve safety objectives, while simultaneously optimizing capital and operating costs. AspenTech has put considerable time and attention to the tools and workflow for relief valve sizing and rating and flare network design within the aspenone Engineering framework. In versions 8.3 (August 2013) and 8.4 (November 2013) of aspenone Engineering, substantial new capabilities have been introduced in Aspen HYSYS and in version 8.6 (May 2014) similar capabilities were added to Aspen Plus and further fire scenario capabilities and validation data were added in version 8.8 (May 2015). This enables: a) performing steady state and/or dynamic modeling, b) placing pressure relief valves in a process flow sheet, c) automatically deriving process parameters and relief loads from the associated equipment or process stream (s), d) developing safety scenarios, e) sizing the relief valve or rupture disk, f) transferring relief valves, loads and scenarios to Aspen Flare Analyzer, and g) sizing the relief network. These growing requirements are driving innovations in modeling software to help organizations optimize their systems and processes for safer performance and reduced regulatory risk. 3

4 Key Engineering Challenges Overpressure protection systems design is a gating step in achieving safe plant designs. Overdesign of these systems can also cause significant and unnecessary capital expense. Flare networks always constitute a large portion of the capital budgets, and, once designed, the modification or expansion of flare networks can be a substantial capital cost factor. Key design factors include: Ensuring adequate overpressure protection system capacity Avoiding overdesign to avoid extra capital expenditure Considering all pressure relief sources and pathways in safety scenarios Ensuring that design comprehensively covers all scenarios Dynamic analysis of the relief system load over time during a worst-case incident Documenting complete and accurate PSV design parameters Re-rating and re-certifying of overpressure protection system under MOC Regulatory Context The following are the main API standard practices and regulatory directives that apply to overpressure protection system design and verification: API Standard 520, Ninth Edition, Sizing, Selection and Installation of Pressure-Relieving Devices API Standard 521, Sixth Edition, Pressure-Reliving and Depressuring Systems API Standard 2000, Seventh Edition, Venting of Atmospheric and Low Pressure Storage Tanks Canadian Safety Advisory, NEB SA , Overpressure Protection ISO :2010 API 520, 521 and 2000 are built into the PSV sizing functionality in Aspen HYSYS V8.6 and V8.8. 4

5 Problems in Current Safety Analysis Workflow Overall, gathering and generating information about process equipment, identifying relief loads and sizing pressure relief devices, and designing and rating the discharge systems can be a simulation-intensive project. Up to 90% of the information for the proper design and optimization of safety networks is generated during this step. The resizing and procurement of pressure relief devices and the review and maintenance of documentation are also time consuming, with up to 50% of the time dedicated specifically to documentation. In addition to maintaining the documentation for future use, extensive documentation must be created so the engineer s work can be understood by a supervisor. Additionally, rigorous relief device sizing methods are performed on an as-needed basis. In many companies, complex spreadsheets which are difficult to use, maintain, and distribute company-wide are set up to handle relief device sizing. Multiple methods are used within the same organization depending on which spreadsheet was generated by whom and how it was shared. Employing a spreadsheet method requires that significant time be spent copying and pasting values such as fluid properties and flow rates from one tool to another to perform calculations. In addition, more time is devoted to entering the inputs into different tools, with many errors reported in the transfer of information. Spreadsheets also offer no visual analysis of overpressure contingencies. The link between relief valve sizing and flare network design is also important. The integration between the relief load, relief valve, and flare system design delivers a clear benefit. The safety scenarios are identified and analyzed during the relief valve design task. Typically, in the traditional approach, the same safety scenarios are separately identified and analyzed during the flare system design. The benefits of bringing together these two environments will be discussed in more detail later. Also, the models from which the loads and designs are based are usually steady state in nature. Dynamic modeling of each safety scenario provides a more realistic view of the actual peak load, which helps further optimize operations. Systematic Overpressure Protection Analysis Size relief valves based on safety scenarios 2 3 Design/rate flare system 1 Create process model and model relief loads Figure 1: The Overpressure Protection Safety Analysis Workflow 5

6 Best Practices Steps Gathering and generating information is a multistep process. It includes identifying all equipment or systems that could fail due to overpressure in a process, gathering information from the conceptual design phase, developing process models or streams describing system conditions and fluid properties, deciding what material will go to the flare system and what material will be disposed via other mechanisms (sewers, treatment facilities, etc.), and determining the type of relieving devices necessary. Finding the relief loads involves determining the excess flow produced by each overpressure scenario. Once this is completed, the orifice size for each overpressure scenario must be calculated, along with identifying and analyzing all relieving devices (i.e., multiple valve analysis). Preliminary line sizing should then be performed for the relief area calculations and preliminary pressure drop calculations. The next step involves designing and rating discharge systems. First, the discharge locations for each relief device must be determined. The outlet lines for the relieving devices, as well as the flare tip, must also be designed and sized. This must be done in the context of all of the enumerated safety scenarios. Consideration of the scenarios entails making sure that each scenario is effectively served by the overpressure protection system or, put another way, identifying and analyzing the worst case load for each relief valve as well as the entire system, collectively. To complete this task, information must be accrued about the outlet lines, and site verification must also be done. A flare analysis software package saves significant time during this step. Once the discharge systems have been fully designed and rated, pressure relief devices should be resized by obtaining backpressure values from the flare system model. The relief devices can then be procured by sending process data to vendors for more accurate information and recalculation of relief areas. Relief device documentation must be reviewed and maintained for each scenario to ensure that the information is sufficient. Also, the relief device documentation should be grouped along with simulation files for future use, should a need arise. To help make this process easier to follow and understand, please reference the following: 18 Best Practices to Achieve Overpressure Protection 1. Fully study and understand the applicable regulations, rules and standards that apply to the process you are designing for, both from the regulatory authorities and internal company standards, such as API Standards 520, 521, and Build a clean process flow diagram and simulation model, such that the equipment and streams that will contribute load to a given pressure safety valve (PSV) and/or rupture disk can be clearly seen, analyzed, and enumerated. 3. Define all needed emergency scenarios to fully protect the process, and those called for in the API standards as well as the owner-operator s and engineering organization s standards. 6

7 4. After arriving at the tuned process model in steady state, convert the model to dynamic mode and analyze the design dynamically to realistically and conservatively identify maximum relief loads. 5. Use an equipment-based approach: Start from each equipment item, understand each item s relief load, and add adequate pressure relief valves to relieve each piece of equipment. 6. Identify all equipment and systems that could fail due to overpressure in the process. 7. Transfer fluid properties, relief loads, and other key process parameters from the process model to the PSV sizing calculator. When using Aspen HYSYS or Aspen Plus, these parameters are automatically transferred from the stream to the associated relief valve. When using Aspen HYSYS or Aspen Plus, alternate and multiple streams can be assigned to each relief valve. 8. Always use the same physical properties and hydrocarbon properties between the process simulation model and the relief valve sizing calculation. 9. Select relief valve type and metallurgy and size PSVs. 10. Understand the governing scenario for each piece of equipment, when sizing PSVs. 11. Develop regulatory compliance documentation. When using aspen HYSYS or Aspen Plus, design parameters automatically populate documentation, such as PSV process and mechanical datasheets. 12. Transfer sized PSV parameters to Flare Network Analysis Design program. 13. Transfer scenarios to Flare Network Analysis Design program. 14. Consider the dynamics of the relieving load arrival at the flares, to avoid overdesign. 15. Design Flare Network and test network design against all relief scenarios. 16. Store all files for each analysis together, to comply with MOC. This includes tying specific process simulation case, relief valve sizing calculations that apply to that case, resulting flare network design that applies to that case, and the full set of relief valve documentation. 17. Deliver process model to owner operator, in tuned and as-built condition, to ensure that the over pressure protection system can be verified by SHE on an ongoing basis. 18. Re-run the entire PSV and flare network analysis each time that a significant change is made to the process, to ensure the currency of the overpressure protection strategy. 7

8 Improving the Safety Analysis Workflow In order to improve the current workflow, certain measures can be taken, including the merging of design and overpressure protection steps to create inherently safer designs; the elimination of any walls between instrumentation, process, and safety groups so they speak the same language; creating evergreen documentation; and providing easier access to dynamic modeling to achieve realistic views of each scenario. Merging the design and overpressure protection phases of a project allows for the process safety analysis to be performed earlier in the design phase and for the reuse of models created at conceptual design or equipment design phases. By doing this, engineers are able to evaluate overpressure contingencies for possible design reconfigurations to find the best and safest alternatives, if they exist, and identify areas for capital cost savings. By removing barriers between instrumentation, process, and safety groups, the manual transfer of information from one group to the next is reduced and corporate governance and adherence to standards is simplified. Use of the same tools between groups makes communication easier and more efficient, while the automation of information transfer avoids errors and frustration. Automatically maintained documentation keeps design notes within design tools for easy referencing and reuse, while also ensuring that any modifications to each process are captured and updated. By automating the repetitive portions of the process, engineers are able to focus their talents on process improvements. Innovative equipment and tools that better protect relief systems help to further technology improvements for safety, as well. New styles of relieving devices are being created by leading manufacturers and new, improved engineering software packages help to enhance productivity and reduce mistakes. It is important to consider the longevity, usability, reputation, and robustness of vendors and their solutions when evaluating products for implementation. 8

9 Overpressure Protection in Aspen HYSYS V8.3, 8.4, 8.6, and 8.8 and Aspen Plus V8.6 and 8.8 There have been significant enhancements to Aspen HYSYS that have enabled companies to achieve safer operations at the design stage. In the aspenone Engineering V8.3 and V8.4 software package, in particular, Aspen HYSYS, Aspen HYSYS Dynamics, and Aspen Flare System Analyzer now enable engineers to optimize conceptual design, equipment design, overpressure protection (such as PSV and rupture disk sizing), and flare design and rating all from one unified and compatible environment. User interface innovation, based on research into the workflow used by process engineers, first introduced in HYSYS V8.0, has provided a highly intuitive environment where both new and experienced users can take advantage of Aspen HYSYS. Coherent simulation environments are intuitive and efficient, supporting process engineers who must construct a conceptual design. In Version 8.3, a safety analysis environment was introduced, so that safety engineers can avoid the details of process modeling and instead access detailed diagrams to determine appropriate protection for overpressure contingencies. These same engineers can also rate and design flare and discharge systems within the same environment. Instrumentation engineers can also access the product to find documentation to keep up-to-date with sizing and design rationale for pressure safety valves in the system and construct appropriate datasheets and calculation reports. The documentation builder also auto-updates with each revision of the calculation or project notes, so that engineers only must update the non-repetitive sections of the analysis. Add PSVs to Process Model Specify PSV Restraints Figure 2: The Safety Environment within the Aspen HYSYS process modeling system provides an extremely efficient approach for the process engineer or safety specialist to design and rate pressure safety valves within the model. Once the relief valves are designed, additional integration has been provided that automatically transfers each of the safety scenarios, together with the associated relief valves, into the flare network design environment. Additionally, usability advances have made it much simpler and more straightforward for the process modelers to conduct dynamics studies of process designs. They can evaluate the potential impact of worst-case relief loads that can occur by understanding the actual time sequencing of loads under each safety scenario. 9

10 Improved Workflow for Ovepressure Protection Design 2 Size relief valves Flare System Analyzer models flare Header 2 Con 2 Tip Integrated within simulation model Poe 1 3 Stack Con 3 Con 1 Header 3 1 Simulation modeling of equipment/plant + Dynamic modeling of maximum loads Figure 3: Innovations support an optimized workflow for improved safety in gas processing. Examples of Benefits Achieved E&C organizations conduct overpressure protection analysis and, in particular, pressure safety valve (PSV) studies on a regular basis, both during FEED and for clients and their operating plants. This aspect of the work is customarily done by specialists and often presents a schedule bottleneck during early FEED. The methods described here, in which rigorous and verifiable PSV rating and sizing is incorporated within the process modeling environment, can save significant time and remove these bottlenecks. Techint reported 80% reduction in engineering manhours for PSV sizing, using an earlier version of this approach (1), while Petrofac (2) reports excellent early results in their testing of the latest version of this software innovation. Wintershall, working with Inprocess Consultants (3), was able to save 70% of anticipated capital investment in flares through more accurate safety analysis enabled by dynamic simulation of process loads Flow Rate Total molar flow (MMSCPO) Flare capacity (MMSCPO) 0 10,000 12,000 14,000 16,000 18,000 20,000 22,000 24,000 Time (minutes) Figure 4: Innovations support an optimized workflow for improved safety in gas processing. 10

11 Summary References In conclusion, the way organizations are structured and the way process safety analysis is currently conducted can lead to a slow and inefficient work process. One of the main causes of this is the manual transfer of data from process simulator into spreadsheets and documentation, and between departments. New and improved software tools help to improve the process of safety analysis in terms of efficiency and accuracy. 1. Llorens, Andres Emilio (2011), Techint, presentation at AspenTech OPTIMIZE Global Conference, May, Brodkorp, Michael (2011), Inprocess Consultants, presentation at AspenTech OPTIMIZE Global Conference, May, Venkatesh, Lakshmi (2013), Petrofac, AspenTech press release, August, Narayan, Raghu and Ron Beck (2014), Process Modeling Innovations Achieve Safer Operations and Reduced Compliance Risk in Gas Processing and flare networks. GPA, Dallas, Texas, April

12 AspenTech is a leading supplier of software that optimizes process manufacturing for energy, chemicals, engineering and construction, and other industries that manufacture and produce products from a chemical process. With integrated aspenone solutions, process manufacturers can implement best practices for optimizing their engineering, manufacturing, and supply chain operations. As a result, AspenTech customers are better able to increase capacity, improve margins, reduce costs, and become more energy efficient. To see how the world s leading process manufacturers rely on AspenTech to achieve their operational excellence goals, visit Worldwide Headquarters Aspen Technology, Inc. 20 Crosby Drive Bedford, MA United States phone: fax: info@aspentech.com Regional Headquarters Houston, TX United States phone: São Paulo Brazil phone: Reading United Kingdom phone: +44-(0) Singapore Republic of Singapore phone: Manama Bahrain phone: For a complete list of offices, please visit 12