Best Practices in Overpressure Protection

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2014 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved.11-5746-0614

18 Best Practices in Applying Process Modelingto Overpressure Protection and Relief Networks

An Industry White Paper

By Ron Beck and Wilfried Mofor Aspen Technology, Inc.


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18 Best Practices in Applying Process Modeling toOverpressure Protection and Relief Networks

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Key Engineering ChallengesOverpressure 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 ContextThe 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 2012-01, Overpressure Protection

ISO 4126-10:2010

API 520, 521 and 2000 are built into the PSV sizing functionality in Aspen HYSYS V8.6


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Problems in Current Safety Analysis WorkflowOverall, 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 engineers work can be understood by a

supervisor.

Additionally, rigorous relief device sizing methods are performed on an as-needed basis. In many companies, complex

spreadsheetswhich are difficult to use, maintain, and distribute company-wideare 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.

Figure 1: The Overpressure Protection Safety Analysis Workflow

Create process model andmodel relief loads

Design/rate flare system

Size relief valves basedon safety scenarios

Systematic Overpressure Protection Analysis

Size relief valves basedon safety scenarios

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Best Practices StepsGathering 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 analyzingthe 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 2000.

2. 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) 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-operators and engineering organizations standards.

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 items 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.


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7. Transfer fluid properties, relief loads, and other key process parameters from the process model to the PSV sizing

calculator. When using Aspen HYSYS, these parameters are automatically transferred from the stream to the

associated relief valve. When using Aspen HYSYS, 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 andthe 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, 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.

Improving the Safety Analysis WorkflowIn 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.


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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.

Overpressure Protection in Aspen HYSYS V8.3 and 8.4There 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 ratingall 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.

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.

Add PSVs to Process Model Specify PSV Restraints


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Figure 3: Innovations support an optimized workflow for improved safety in gas processing.

Examples of Benefits AchievedE&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.

Figure 3: Innovations support an optimized workflow for improved safety in gas processing.


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FlowR

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Total molar flow (MMSCPO)

Flare capacity (MMSCPO)

+ Dynamic modeling of maximum loads

Size relief valves

Improved Workflow for Ovepressure Protection Design

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Integrated withinsimulation model

Simulation modeling of equipment/plant

Flare System Analyzermodels flare

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SummaryIn 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 datafrom 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.

References1. Llorens, Andres Emilio (2011), Techint, presentation at AspenTech OPTIMIZE Global Conference, May, 2011.

2. Brodkorp, Michael (2011), Inprocess Consultants, presentation at AspenTech OPTIMIZE Global Conference, May, 2011.

3. Venkatesh, Lakshmi (2013), Petrofac, AspenTech press release, August, 2011.

4. 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 2014


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Best Practices in Overpressure Protection

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