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Severe jet fires and vapor explosions

05.01.2012  |  Bradley, I.,  International Paint, Felling, UK

Operators should consider alternative test procedures, capable of reproducing conditions found within jet fires.

Keywords: [severe jet fires] [vapor explosions] [safety] [storage] [terminals] [precautions] [API] [guidelines] [injury]

Jet fires are pressurized releases of hydrocarbons that result in impinging flames with significant momentum. The potential for jet fire exists wherever storage, process equipment or pipe work contains flammable gas or liquid/gas (two-phase) mixtures at pressures approximately 2 bar or greater. At such pressures, choked flow occurs and the gas, flashing liquid or two-phase flow reaches sonic velocities. Jet fires can, therefore, have a significant erosive force, unlike hydrocarbon pool fires.

Jet fire characteristics vary significantly depending on a number of factors, such as the type of jet fire previously described and the mass release rate (which itself depends on the pressure and hole shape/size). The potential flame lengths involved mean that the heat flux to steel can be significant within a large fire scenario envelope. Fabig technical note 111 gives the following benchmark flame lengths from test data on various size releases:
• 3 kg/s gives a 22-meter (m) flame
• 10 kg/s gives a 34-m flame
• 50 kg/s gives a 50-m flame.

Table 1 is adapted from the Fabig technical note and gives an estimate of typical heat fluxes to objects engulfed by a jet fire. These are considered conservative estimates of the initial heat flux, but it can clearly be seen that the values are significantly higher than those expected in hydrocarbon pool fires.

 


Gas jet fires generally have higher exit velocities than flashing liquid jet fires and so produce less buoyant flames with greater horizontal reach. The fraction of heat radiated from gas jet fires is lower than that of liquid jet fires, as well-ventilated gas flames typically burn cleaner, producing less soot, than liquid jet flames. The majority of radiation emitted by gas flames is from water and carbon dioxide.

Flashing liquid jet fires generally have higher release rates for a given aperture size, but the lower exit velocities result in less air entrainment. This reduces the combustion efficiency and, combined with the higher carbon content in liquids, produces sooty flames that radiate more heat to their surroundings. Two-phase jet fires are generally worst case scenarios, as they can combine the worst aspects of both gas and flashing-liquid jet fires, which are high exit velocities and highly radiant, sooty flames.2

API 2218 and UL1709 limitations

API 2218 (second edition) clearly states within its scope that it does not address jet fires. Jet fires are commented on in Appendix C of the document, but at the time of its writing, there was no standard UL, ASTM or ISO test procedure. The Appendix C comments are now outdated, due to publication in 2007 of ISO 22899-1: Determination of the resistance to jet fires of passive fire protection materials. This standard provides, for the first time, an internationally recognized test procedure suitable as a basis for assessment and certification. A third edition of API 2218 is in draft, and within the latest version it does recognize ISO 22899-1; it actually goes so far as stating it is accepted as the preferred method of fire test.

The recommended test procedure for pool fires within API 2218’s second edition (and upcoming third edition) is UL 1709. It is important not to assume that a UL 1709 listing provides information on the ability of a material to withstand jet fire. Furnace-based pool fire testing cannot replicate the severe heat flux and erosion conditions found in jet fires. The key differences between the two fire test procedures are compared in simplistic terms within Table 2.

 


Such differences make it necessary to consider alternative test procedures, capable of reproducing the conditions found within jet fires.

Jet fire testing

Jet fire testing can fall into three categories, generally characterized by the relative size of the jet flame: large, medium or small. All tests use a directional jet flame impacting on a test specimen to simulate the erosive and rapid heat-rise conditions within a jet fire, with each test having its advantages and disadvantages.

The large-scale tests (Fig. 1) replicate realistic release rates by using the types of fuels that may be expected in real release scenarios, such as crude oil or natural gas. A significant number of large-scale jet fire tests have been conducted by industry and academia, and they together form the basis for the majority of current knowledge on jet fire behavior. Despite the obvious benefit of being the most accurate type of test, the ability for fire protection manufacturers to perform such testing was limited due to the extreme cost. An economic, yet consistent and representative, test was required for industry to use for fire protection evaluation.

  Fig. 1.  Large-scale jet fire test.


The small-scale tests, commonly known as torch tests, do involve an impinging flame, but the release rates, temperatures, erosive forces and heat fluxes are significantly less than those experienced in large-scale realistic jet fire testing. Examples of small-scale torch tests include the test procedures in NFPA 58 and NFPA 290. Materials tested and certified to these standards demonstrate they perform under the specific conditions described within the test procedure; however, the materials should not be considered as having been characterized for resistance against realistic jet fires on the basis of such tests.

Medium-scale tests are the recommended method of assessing the resistance to jet fire of passive fire protection (PFP) materials. Extensive work has been undertaken comparing both the erosive forces and the heat fluxes obtained in the medium-scale tests with those experienced in large-scale tests. The results have shown the tests to be representative of the conditions found in the majority of potential jet fire releases, attesting to the suitability of the test procedure.3

ISO 22899-1 is the preferred medium-scale test, with suitable alternatives being OTI 95-634 or the Sintef high-heat-flux jet fire test procedure.

Figs. 2 and 3 show the ISO22899-1 test setup. It consists of a 1.5 m × 1.5 m × 0.5 m box with a central web or tube as required. Propane is the fuel used, released from the nozzle visible in Fig. 2. Eighteen thermocouples are used to measure the temperature of the steel, as visible in Fig. 3 from the rear of the specimen.

  Fig. 2.  The front of the ISO test specimen.


  Fig. 3.  The rear of the ISO test specimen.


Specifying ISO 22899-1

In order to ensure protection against jet fires, it is recommended that the hydrocarbon extraction and processing industries make certification to ISO 22899-1 a requirement when specifying jet fire protection. The benefits of specifying the ISO standard are:
• An internationally recognized and accepted test procedure
• Standardized method of assessment to ensure a level playing field between materials
• Takes limiting steel temperature into account
• Allows type approval certification from all classification societies, not reliant on technical reports
• Type approval ensures that products are part of ongoing surveillance and factory audit schemes.

To ensure the correct sizing of PFP against jet fires, operators should ensure that they consider and provide to the PFP manufacturer the following:
• Fire scenario (pool, jet or combination)
• Fire durations (including separate pool/jet durations, if applicable)
• Heat flux
• Limiting steel temperature
• Steel dimensions.

Vessels and BLEVEs

Boiling liquid expanding vapor explosions (BLEVEs) are potentially catastrophic incidents that can occur when fire engulfs storage or process vessels containing volatile liquids. Simultaneous heating of the contents and weakening of the shell cause the internal pressure to increase to the point where the stress on the steel is greater than the yield stress. The consequent rupture of the shell can result in an explosion with far-reaching consequences, as the full energy content of the vessel inventory is released almost instantaneously (as opposed to pool or jet fires where the energy release is often over a substantial period of time).

If the pressure release valve or blowdown system is not capable of reducing the pressure to a point where rupture cannot occur, the resulting risk to life and property can be extremely high. It should be noted that pressure relieving valves alone are generally insufficient to prevent a BLEVE of a vessel exposed to jet fire. Some form of active or passive protection is usually necessary to delay the temperature rise and give the blowdown system or PRV time to work.

The occurrence of a jet fire is particularly dangerous around process and storage vessels, as the possibility for escalation of the incident and the domino effect is high. A study of 84 historical jet fire events undertaken by Catalonia University showed that 56% of these caused a subsequent explosion or BLEVE.4 One such example was from 1984 in Mexico City, when the escalation of an incident led to multiple BLEVEs and an overall loss of more than 500 lives and the evacuation of 200,000 people. During this incident, one of the BLEVEs occurred within 69 seconds of an impinging jet fire commencing.4

API fire protection guidance

Fireproofing of liquefied petroleum gas (LPG) vessels is covered by the API guidance documents 2510 and 2510A. The recommended practice is to fireproof the supports in accordance with UL 1709 testing and use an active (water deluge) system on the sphere body for protection against pool fires. For jet fires, fixed water monitors capable of delivering sufficient water to the impingement point of the jet have been shown to be effective, so long as the water application rates listed in Table 3 are achieved.5

 


Water deluge systems have been shown to be effective against vessels engulfed in hydrocarbon pool fires, limiting the shell temperature to no more than 120°C by maintaining a water film on the surface of the vessel.

In instances of jet fire, research has shown that water application rates of a typical deluge system (10–12 l/m2/min) cannot be relied upon to form a continuous water film on the surface of the vessel. At the point of impact, the force of the jet can result in a localized dry spot, leading to failure of the vessel and a BLEVE. Fixed water monitors delivering significantly higher water rates have been shown to be effective against jet fires; however, both water deluge systems and fixed water monitors have a number of disadvantages. The primary one is the potential for the system to be inactive during a fire due to corrosion or maintenance or damage from a vapor cloud explosion possibly preceding a fire. Other potential problems include delays in operation, if the fire is shielded from the detector, risk of danger to an operator if they are manually controlled (in the case of water monitors) or the potential for corrosion of the vessel and the legs/saddles due to regular testing of the deluge system.

For these reasons, operators routinely consider installing PFPs to vessel bodies as well as supports in areas at risk of jet fire.

Specifying vessel fire protection

Having decided to specify passive fire protection to a vessel shell, operators quickly encounter a problem: there is currently no internationally recognized test standard for PFP to vessels. Although a number of guidance documents (such as those published by API) offer advice on what types or what duration of protection to apply, there is limited information available on how tests should be performed to prove PFP fire resistance. ISO TC92 SC2 is currently working on a test standard that should solve this problem; however, this will not provide a solution over the next couple of years.

Having no internationally recognized test standard makes it currently impossible to get type approval certification of fireproofing materials for vessels, meaning the sizing of protection requirements is usually left to manufacturers themselves, done on the basis of their own testing and assessment.

The lack of a standardized test makes it very difficult for operators to compare fireproofing materials, and leaves them in the position of having to decide for themselves, which company’s recommendations are correct and suitable.

As a minimum, operators should insist on a product that has undergone testing on an actual pressure vessel containing LPG or other suitable flammable inventory. Testing conducted solely on plates, bulkheads or structural sections should be avoided, although such data may be used to assist in characterizing the performance of the material. Examples of bad practice are:
• Treating the vessel as a structural section and taking the thickness directly from existing type approvals without further validation
• Using a bulkhead or a deck certificate of similar rating (e.g. H120)
• Treating the vessel as plate and using thicknesses derived from plate testing (without further validation).

To ensure the correct specification of any PFP, the operator should consider and provide to the manufacturer the following:
• Fire scenario (pool, jet or combination)
• Failure temperature of the vessel
• Steel shell material, thickness and vessel diameter
• Vessel inventory, typical fill level and expected minimum fill level during service
• Pressure-relieving valve type and settings.

Additional considerations

In addition to ensuring that the product is correctly tested and the thickness correctly calculated, operators should take into consideration a number of factors that affect the ability of a PFP material to perform its function throughout the life cycle of the vessel or structural steel. Such factors include, but are not limited to:

• The ability of the material to withstand mechanical stresses and deformations (e.g., due to filling of a vessel, transportation of a steel section or stresses induced due to thermal expansion as a result of non-uniform heating)
• The ability of the material to withstand the environmental conditions (including those during testing of deluge systems). Norsok M501-rev 5 and UL 1709 exterior listing test procedures are recommended methods of testing for fire protection materials
• The blast resistance of the PFP material.

Be aware

Despite the consequences of jet fires being potentially far more severe than hydrocarbon pool fires, API 2218 second edition does not provide sufficient guidance on how to specify passive fire protection to mitigate against these hazards. Although there are a number of test procedures available to test against torch or jet fires, past research has shown that the medium-scale jet fire test methods are the most suitable for characterizing the ability of PFP to resist jet fire. The standard ISO 22899-1 is the preferred method of test (as is likely to be stated within API 2218 third edition), as it brings a number of benefits including standardized methods of assessment, the consideration of steel failure temperature and type approval certification.

When applying PFP to vessels, operators should be aware of the lack of test standards and the current impossibility of referring to type approved certification for sizing of the PFP. They should seek guidance from reputable manufacturers that have conducted tests on actual pressure vessels, avoiding thicknesses provided on the basis of unrepresentative structural section or bulkhead tests. In instances of jet fire exposure operators should consider the need to apply PFP to a vessel shell and not necessarily rely solely on a water-deluge system, insisting that jet fire tests have been conducted to ISO 22899-1.

Whether specifying PFP for structural steelwork or vessels it is important that operators know what information is necessary to ensure correct sizing of the PFP, and that they take every effort to provide this information to the PFP manufacturer. HP

LITERATURE CITED

1 Fabig, TN11: Fire Loading and Structural Response, 2009.
2 Lowesmith B. G., G. Hankinson, M. R. Acton and G. Chamberlain, “An overview of the nature of hydrocarbon jet fire hazards in the oil and gas industry and a simplified approach to assessing the hazards,” Process Safety and Environmental Protection, Vol. 85, 2007.
3 International Standardization Organization, ISO TR22899-2, 2011.
4 Gomez-Mares, M., L. Zarate, and J. Casal, “Jet fires and the domino effect,” Fire Safety Journal, Vol. 43, 2008.
5 American Petroleum Institute, “Fire-protection considerations for the design and operation of liquefied petroleum gas storage facilities,” API publication 2510A, 1996.

The author

Ian Bradley is the standards and certification manager of worldwide fire protection for International Paint Ltd. His current responsibilities include testing, assessment and certification of fire protection products and industry representation to a number of trade associations and standards committees. He is a UK delegate to TC92 SC2 WG8: Jet Fires. He holds an MEng degree in materials engineering from the University of Sheffield, UK. 




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