Flare system design challenges - Onshore LNG and Floating LNG projects

Mitsuyoshi Ninomiya's picture
Mitsuyoshi Ninomiya, Head of Engineering HSE Section, Integrity Management Unit, Chiyoda Corporation
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The LNG industry faces numerous challenges. Safety and Environment play a more pivotal role than before, whilst cost pressure increases due to the current economic situation. Furthermore, regulatory permitting processes take longer and require more transparency. Under these situations, the plant design, particularly the flare, should be concluded as early as possible, prior to starting a major permitting process. Ideally during Pre-FEED to avoid late changes.

Needless to say flare system is one of the key elements for onshore LNG and FLNG projects in terms of safety, environment and cost. As the flare occupies a large area, flare design affects siting of onshore LNG plants. For FLNG, it affects hull size and layout including location of process plant, turret and living quarters.

There are several challenges to the flare system design which Chiyoda has extensive experience in resolving. These include the following:

  • LNG plants contain higher inventories of light components.  This introduces the potential for BLEVE (Boiling Liquid Expanding Vapour Explosion) when vessels are exposed to fire.  API 521 depressurisation requirements result in huge flare load.
  • LNG plants have huge amounts of lighter components such as refrigerant and gas, hence flare loads other than depressurizing are also higher.
  • Higher flare loads lead to higher possibilities of flow induced vibration (FIV) and acoustic induced vibration (AIV) causing damage to flare system.
  • When LNG projects are expanded, the new train is often connected to the existing flare, and the chance of overload exceeding flare capacity becomes higher due to additional relief sources from the new train. Multi-train relief scenarios such as total power failure exist and can incur extraordinary flaring rates.
  • Flare is located close to the process deck due to space limitation on FLNG and heat radiation from flare is one of key challenges, especially for Epoxy Intumescent Coating, which is widely used offshore as Passive Fire Protection (PFP) material. This coating degrades from approx. 100 deg. C. resulting in loss of PFP function when required. Other items affected by the heat radiation includes instruments, ESD valves, cold insulation materials and cables.

In normal engineering practice, flare load is calculated after vessel sizing and hydraulic design. This means flare system design usually concludes at a relatively late stage of process design. Chiyoda employs the combination of the following to cope with the above challenges during Pre-FEED:

  • Identify vessels to be protected from BLEVE, and determine protection measures for each vessel including depressurization and application of PFP,
  • Assessment of major sources of flare loads with sufficient accuracy to determine flare sizing,
  • Identify possibility of FIV/AIV and establish protection philosophy for implementation during FEED/EPC,
  • Calculate overload frequency exceeding the flare capacity especially during multi-train relief scenarios and confirm the frequency is low enough from a safety point of view. If not, implement measures such as Safety Instrumented Function enhancement to prevent excessive relief.
  • Heat radiation under credible flaring scenario is calculated using Computational Fluid Dynamics and high heat radiation areas are identified. Protection philosophy is established for implementation during FEED/EPC.

As shown above, Chiyoda is an experienced LNG contractor is able to contribute to flare system design in the early phase and minimize design changes during FEED/EPC, thus preventing BLEVE, FIV/AIV, flare overload and degradation of PFP materials.

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