LNG Tank Top Fires

11th Topical Conference on Gas Utilization
2011 AIChE Spring Meeting
AIChE Spring Meeting and Global Congress on Process Safety
March 15, 2011 - 8:00pm

The regulatory requirements of 49 CFR 193, Liquefied Natural Gas Facilities: Federal Safety Standards, require thorough engineering reviews before approval of any proposed LNG facility. Such a process must consider potential and credible hazardous events. The proposed paper will specifically note, and make comment on the necessary safe fire radiation exclusion zones and the appropriateness, verification and sensitivity of the model used to define these as well as the lack of consideration for ‘knock-on’ events; in particular, the escalation of the tank-top fire that could be considered to result in a catastrophic tank failure and thus a significant increase in radiant exposure to the public and its infrastructure beyond the site boundary. To do this a comparison between the results of LNGFIRE and a NIST FDS cfd model will be used in the determination of a safe fire radiation exclusion zone for a typical LNG facility. A safe fire radiation zone to the general public has been defined by the FERC in their EIS’s as a distance beyond which radiation exposure is 5kW/m2 or less (NFPA-59). Melham et al[1] states that “(s)ome experts argue that a 20-second 5 kW/m2 limiting thermal radiation exposure criterion is sufficient to establish safe separation distances for the general public. … An opposite view argues that these criteria cannot be applied to sensitive population or critical areas/infrastructures. Thus the literature concludes there exists no definitive and absolute value of ‘safe’ exposure. The tank top fire used by the FERC to establish whether or not the radiation exclusion zone extends beyond the site perimeter is one where the tanks outer diameter is about 80 to 90m and at an elevation of 40 to 50m. LNGFIRE has only been verified for LNG fires to 35 m in diameter and not for tank-top fires. Actual tank top fires have &'(t)he wind flowing round the sides of the sides of the tank tends to drag the flame down over the edge of the tank towards the ground.&'[2],[3]. This also results in significant flame drag[4] and an increase in the apparent diameter and length of the emitter. Additionally LNGFIRE &'..can only be applied to the specific type of fire examined in the experiments which form the basis of the model.&' (italic emphasis added). What these reservations mean is that LNGFIRE, though an approved regulation, via NFPA59A, and relied on by the FERC to provide protection to the public is NOT an appropriate model to establish zones of impact from the effects of tank-top fires.

[1] Melham et al. Understand LNG Fire Hazards, ioMosaic (2007).

[2] Cowley, L T , and A D Johnson, Oil and Gas Fires: Characteristics and Impact, Work Package No. FL1, Blast and Fire Engineering Project for top-side Structures (1991).

[3] Cleaver, P, Johnson, M, and Ho, B, “A summary of some experimental data on LNG safety”, 2007, Journal of Hazardous Materials 140, 429–438.

[4] LNGFIRE presumes that there is no flame drag for a tank-top fire and that the wind velocity at the tank-top is that estimated at the elevation of 10m: both obvious non physical errors.


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