Petrell - Bespoke blowdown analysis

As a leading provider of cutting-edge technology within multiphysics process simulation, Petrell offers VessFire, a system for blowdown analysis of process segments that offers increased safety, documented fire integrity and reduced costs.

Petrell has analysed more than 1,000 systems of almost 100 process plants onshore and offshore, in order to determine whether the plants would survive a dimensioning fire case. The analyses are performed using VessFire, a tailor-made system for blowdown analysis. The best results are obtained when such analyses are performed in the concept and FEED stages of a project.

The pressure relief and blowdown systems are the most important safety systems of a process plant. If the pressure, for some reason, increases, a relief is necessary to protect the plant from over-pressurisation and a rupture that could potentially escalate into a disaster. Such systems are commonly designed following the industry standard API 521 'Pressure-relieving and Depressuring Systems' (ISO 23251). Despite the standard's good intentions, its application is often unsatisfactory, and it has weaknesses; for example, regarding heat loads.

The objective of a blowdown system is to handle both cold blowdown (for example, in case of a compressor trip) and exposure to a fire. Petrell has performed numerous depressurisation studies as well as blowdown, survivability and fire integrity analyses. Observations from these analyses include:

  • the capacity of the blowdown valves (BDV line) and orifices is too low
  • pressure is too high and/or there is too much remaining mass inside the segment at the time of rupture
  • pipework ruptures before the BDV opens (in case of manual activation)
  • passive fire protection is applied where it is not required, and vice versa
  • pressure safety valves (PSVs) do not protect against fire.

Quality as standard

We see that the fire loads defined in 'Guidelines for the Protection of Pressurised Systems exposed to fire', which API 521 makes reference to in the section on "more rigorous calculations", may be too conservative, particularly when it comes to application on onshore facilities. The peak heat load is often set to 350kW/m2, but the global heat load can vary significantly with the size and geometry of the fire and exposed system.

The global heat load causes heating of the inventory inside process vessels, pipework and flowlines. In order to predict the correct heat transfer, it is important to include all exposed pipe and vessel surfaces of a segment with its inventory.

There are many possible mitigating measures available in the concept and FEED phases of a project. In detailed design, the only available risk-reducing measure is passive fire protection. In the early project phases, one can modify pipework routing, equipment layout, material specifications, wall thickness, process segment sizes, BDV arrangement (single and multiple) and flare capacity. We often see that the need for passive fire protection can be removed or reduced by increasing wall thickness, or by applying a different specification.

The investment in a blowdown analysis (using VessFire) can give a very high payback. The output from VessFire includes performance data for all pipework and vessels for the defined scenarios: process data, release rates (BDVs and PSVs), temperature profiles, calculated stress vs ultimate tensile strength, and time to rupture (should rupture occur). The project will thus have complete documentation of the fire integrity of the plant (for the parts that have been analysed).

VessFire is developed by and is the property of Petrell. VessFire is built on Brilliant, a general multi-purpose CFD system which includes a complete thermodynamic library and a materials database.

Company profile

Petrell offers a wide range of specialist services related design and engineering of onshore and offshore oil & gas installations, typically within process safety and technical safety. Our services are based on VessFire and Brilliant, our own CFD (Computational Fluid Dynamics) platform. We perform analysis and simulations for clients world-wide, including oil companies, engineering companies, consultancies, manufacturers and authorities. We also perform consequence analysis, safety studies, HAZOPs, and verifications.

Brilliant is a CFD-code based on object technology for analysis of transient and stationary phenomena. Brilliant's unique features include fully coupled analysis of all relevant physical phenomena within flow, heat transfer and stress analysis. Other features include a complete thermodynamic database, database for material properties, and a very flexible and unique grid system. Brilliant constitutes a superb basis for developing tailor-made simulation systems and numerical laboratories.

VessFire is an application using Brilliant as the engine. VessFire is tailor-made to analyse the thermo-mechanical response of process systems subject to cold blow-down and depressurization during fire exposure. VessFire is probably the only tool available in the market that fully complies with the simulation approach addressed in ISO 23251:2006. The tool has been subject to extensive verification and validation. VessFire is also available to clients on a license basis. Examples on the use of VessFire include:

  • Identification of the most unfavourable leakage rate from a fire and blow-down point of view.
  • Design of blow-down systems (orifice sizing, multiple BDV arrangements).
  • Fire integrity of process vessels and piping.
  • Identification of the need for, and the extent of passive fire protection.
  • Process system sectionalisation.
  • Verification of PSV capacity.

Major on-going R&D activities include development of a Near Well-bore Simulator for ConocoPhillips and BP with support from Innovation Norway, and SolidCO2Sim, a JIP with Total, Statoil, Gassco and the Research Council of Norway as partners.

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Illustration showing the concept of peak and global heat load on a vessel exposed to jet fire.
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