Taking safety to the next level: part two

Read part one of this article here.

Dynamic simulation and HAZOP analysis

Background

Steady state process templates (flowsheets) are used extensively in the engineering design workflow for all UOP process technologies. UOP engineers utilise the Honeywell UNISIM® Design Suite of process modelling tools^{1 - 3} in the preparation of a process design package to users called Schedule A. This suite of tools allows for the development of steady state and dynamic process models.

In the process phase of the Schedule A workflow, there was development of a large database of validated steady state process templates for all UOP process technologies. Modifications to these templates are made based on customer specific requirements. All the equipment and piping associated with a project are sized based on flow rates, calculated from steady state heat and mass balances, which interface all of the engineering sizing tools. Simulation results are downloaded to the standalone hydraulic tools to determine line sizes and generate tabulation reports. If needed, various parts of the steady state template can be checked using the equivalent dynamic simulation template.

Dynamic simulation templates

Transitioning the converged steady template to a dynamic simulation template requires a different approach towards specifying unit operations. In the implementation of dynamic templates a sequence of important actions are required to insure a successful high fidelity dynamic analysis:

• Converge a realistic steady state simulation of the problem to be studied.
• Add valves and pipes to ensure pressure-driven flow between unit operations.
• Set terminal feed and product pressures.
• Determine approximate, or include exact, dimensions of all process equipment.
• Specify sizing data for all pipe runs (lengths, diameters, etc). Size the CVs.
• Include nozzle elevations for all valves, piping and process equipment.
• Add all necessary controllers (FICs, TICs, etc.) to all control loops.
• Run the dynamic simulation: tune the controllers, check pressure drops, and line out the model to match the original steady state performance.
• Add events to the event scheduler to model upsets and other conditions.

The above steps need to be completed only once and serve as the starting point for dynamically evaluating all normal and upset operating scenarios.

Each of the dynamic models is preconfigured to include upset conditions, strip charts and tabulate historical data. In UNISIM^®, the event scheduler is used to model the preconfigured dynamic upset scenarios. Additional upsets are also manually activated as a dynamic run is proceeding during time integration.

Over the years, a strategy and timetable have been established to implement dynamic simulation methods into the current UOP process workflow. The use of dynamic process templates has been introduced to supplement the work as needed during the Schedule A projects. Various sections of these dynamic templates are used to identify specific design issues for each project (e.g., a project HAZOP review, emergency shutdown events, customer inquiries, etc.) that normally would not be recognised through the use of steady state simulators.

Sections of the dynamic templates are utilised to explore transient behaviour around individual equipment, e.g., reactors, columns, etc. There are many different levels of fidelity possible when creating dynamic flow sheets. In UOP engineering dynamic templates are used that implement the basic regulatory control schemes shown on all the process flow diagrams for all UOP process units.

From a full dynamic template of a UOP process unit, the process engineer is able to break down the process flows and create smaller process snapshots for analysis. To consider and evaluate time dependent behaviour in the Schedule A work process, a separate group of flowsheeting skill specialists are utilised on Schedule A projects to assist in preparing and designing these dynamic flowsheets.

The recently included flowsheeting specialists support the internal HAZOP team reviews. This support includes the preparation and evaluation of dynamic models as needed from the working HAZOP worksheets. Supporting data (e.g., strip charts, tabular historical data of key events/new hazards/safety issues) from these dynamic runs are included in the reports. Additional dynamic runs are made to address any user’s concerns after the reports are issued.

With the use of dynamic templates from the steady state design templates, quantative testing can now evaluate the effectiveness of proposed strategies for emergency situations. Furthermore, dynamic analysis has helped to identify new unknown hazards and safety issues. The consequences of any deviations can now be accurately determined from normal design. Dynamic analysis has also been included in the HAZOP reviews of new technologies, as proposed by the R&D process design and development groups.

Part two of this article will be available soon.

References

1. Process Design Simulation: Fundamentals – Dynamic Modeling Using UNISIM® Design Suite Implementation, PDS-4528 R380 Student Guide, Rev 01.0, 07/2010.
2. UNISIM^® Design Dynamic Modeling-Reference Guide, May 2012 R410 Release.
3. UNISIM^® Operations Guide, May 2012 R410 Release.

Written by Scott M. Wozniak and Bill Weide, UOP, a Honeywell Company, USA. This is an abridged article taken from Hydrocarbon Engineering’s January 2016 issue.

This article is based on a paper that was first presented at the American Institute of Chemical Engineers' 2015 Spring Meeting and 11^th Global Congress on Process Safety, held from 27 - 29 April 2015 in Austin, Texas.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/21012016/taking-safety-to-the-next-level-part-two-2181/