Uncertainty in the analysis of the risk of BLEVE Fireball in process plants and in transportation Joaquim Casal Centre for Studies on Technological Risk (CERTEC) EEBE, Universitat Politècnica de Catalunya CERN Workshop An angineering perspective on risk assessment: from theory to practice November 2018
Outline - Introduction - Uncertainties, BLEVE: - frequency - time to failure - vessel pressure - amount of material - energy released - peak overpressure - ejected fragments - Uncertainties, fireball: - frequency - amount of material - Conclusions
Introduction BLEVE: an explosion resulting from the failure of a vessel containing a liquid (+ vapor) at a temperature significantly above its boiling point at atmospheric pressure. BLEVEs are just mechanical explosions. Their effects: - overpressure wave - ejection of fragments. Often, substances involved in a BLEVE are flammable. In such cases, BLEVE is usually followed by a fireball; its effect: - thermal flux So, many people consider BLEVE and fireball practically synonymous. But this is not correct (example: steam boiler).
Introduction Survey: 202 accidents with BLEVE, 1960 2018 (Hemmatian et al, 2018): Origin Nº accidents % --------------------------------------------------------------------- Transport 87 43 (rail/road: approx. 5/4) Storage 42 21 Process plant 33 16 Transfer 25 12.5 Other 15 7.5 202 100 --------------------------------------------------------------------- Substances involved: LPG: 42% water: 8% (?)
Uncertainty: frequency Frequency of instantaneous release of the complete inventory (not necessarily BLEVE) --- Stationary vessels: Atmospheric storage tank 5 10-6 year -1 Pressure storage vessel 5 10-7 year -1 Process vessel 5 10-6 year -1 --- Road and rail tankers in an establishment: Atmospheric tank 5 10-5 year -1 Pressurized tank 5 10-7 year -1 [Rupture of loading/unloading hose: 4 10-6 h -1 ] In a QRA: the frequency of the BLEVE of a vessel is obtained from the estimation of the frequency of the scenario (pool fire, jet fire ) that will cause it. To take into account domino effect (for ex., if there are several vessel at short distance) that frequency can be multiplied by 2.
A situation that can lead to a BLEVE If there is flame impingement on a vessel, failure is a combination of high pressure (heating of liquid wetted wall) + wall weakening (heating of wall above liquid level). Worst case: jet fires Approximate heat fluxes (impingement): natural gas, sonic: 200 kw m -2 propane gas, sonic: 300 kw m -2 propane 2-phase, sonic: 180 kw m -2 propane 2-phase, not sonic: 150 kw m -2
A situation that can lead to a BLEVE Impingement below or above liquid level (vessel 80% fill with water; Birk, 2006):
Uncertainty: frequency Frequency in transportation 73.5% of accidents in the transportation of hazmat are originated by collisions (Ronza et al., 2003; Linkuté, 2011). A common sequence: collision release fire BLEVE. It is not possible to predict the frequency.
Uncertainty: time to failure Time to failure depends on: fire heat flux fireproofing existence/condition vessel volume/surface ratio wall thickness fire exposure area vessel fill level. Chiba (Japan), 2011: 1 h Failure can occur: in 1 min in half an hour or more or can never occur. Zarzalico (Spain), 2011: 73 min Tivissa (Spain), 2002: 20 min Mexico City: 67 s, 1 h, 1.5 h, never
Uncertainty: vessel pressure Possibilities: - Operating pressure Example: storage pressure if there is an impact. - An intermediate pressure between the operating one and the PRV set value. Example: a vessel being heated fails before PRV opens. - PRV set pressure Example: a tank heated by a fire, after a certain time PRV opens. - A pressure higher than the PRV pressure. Example: the device is blocked or heat flux exceeds the design conditions. General hypotheses: - PRV set pressure for vessels heated by a fire. - Operating/storage pressure for impact (no fire).
Uncertainty: vessel pressure / time to failure Moodie-1/4t-40% Moodie-1t-80% 60 s 180 s 60 s 180 s 10 cm 20 cm Scarponi, G.E., Landucci, G., Birk, A.M., Cozzani, V., 2018. LPG vessels exposed to fire: Scale effects on pressure build-up. J. Loss Prev. Process Ind. 56, 342 358.
Uncertainty: vessel pressure / time to failure The temperature stratification can originate a quick pressure built-up, depending on tank size and shape. Scarponi, G.E., Landucci, G., Birk, A.M., Cozzani, V., 2018. LPG vessels exposed to fire: Scale effects on pressure build-up. J. Loss Prev. Process Ind. 56, 342 358.
Uncertainty: amount of material - Worst case: the maximum amount legally possible. - If the explosion occurs a certain time after the valve opening, some material will have been released. So, some authors suggest 80% or 90% of the maximum amount legally possible. PRV reduction of pressure + reduction of content
Uncertainty: contributions to energy released Two contributions: 1. Expansion of the pre-existing vapor. 2. Instantaneous vaporization of the superheated liquid (volume increase: water, 1700 times; propane, 250 times). Experimental DP measurements show two consecutive peaks corresponding to these two phenomena: Johnsons, Pritchard, Wickens, 1990. 5.7 m 3 vessel, propane, filled 80%. Birk et al., 2007 1.89 m 3 vessel, propane, filled 51%.
Uncertainty: contributions to energy released -- Traditionally it had been assumed that the essential contribution is the one due to liquid vaporization. -- Birk (2007, 2018) has suggested that flash vaporization is too slow to produce a shock wave, blast being essentially created by the expansion of the previously existing vapor. The relative magnitude of both peaks will depend on the mass fractions of liquid and vapor in the vessel and on the material (LPG, water ) properties. A conservative assumption seems reasonable: combining the energy released by both phenomena.
Uncertainty: estimation of the peak overpressure Not all the energy released is devoted to create overpressure, some is lost in breaking the vessel and ejecting the fragments. Again some uncertainty: It is usually assumed that 50% (or 40%, less conservative) remains for overpressure.
Uncertainty: estimation of the peak overpressure D. Laboureur et al., 2014: Model with irreversible expansion (Planas et al. 2004, Casal et al., 2007) (left) and with isentropic expansion (Prugh, 1988) (right) compared with all scales experimental data.
Uncertainty: estimation of the peak overpressure For cylindrical tanks: DP varies significantly with the direction (axial or perpendicular) in the near field. A common method for DP: TNT equivalency The main features of TNT explosions are well known. If the energy involved in overpressure is converted into the equivalent mass of TNT, the explosion peak overpressure can be predicted from the scaled distance (better in the far field).
Uncertainty: number and direction of fragments Cylindrical tanks Typical number of fragments: 2 or 3, depending on the heating circumstances. An initial crack progresses along a weld (end cap); the most common possibilities: - the tank open and flattened: no ejection, or end caps may be ejected - tank broken in two pieces, one cap and the remainder of the vessel - tank divided in three pieces; diverse possibilities The most frequent one: 60% of cases (Gubinelli and Cozzani, 2009) Sector 1 probability: 0.62 Sector 2 probability: 0.38 Reach: can be estimated with Baum equations (maximum: 1100 m for large tanks).
Uncertainty: number and direction of fragments Tivissa, Spain, 2004: the initial position and the two major fragments on a straight line.
Uncertainty: number and direction of fragments Zarzalico, Spain, 2011. The three major fragments remained near the explosion point.
Uncertainty: number and direction of fragments Spherical tanks The number, direction and range is practically impossible to be predicted. Number: 2 to 15 (usually less than 5) Range: maximum distance reached: 600 700 m México City accident 600 m
Fireball If the BLEVE is followed by a fireball, often the thermal effects are worse than those due to blast. BLEVE involving a flammable material: probability of ignition (fireball) = 1 Amount of fuel: the same as for BLEVE. Prediction of diameter and height: 23 expressions have been proposed. When applied to 20,000 kg of propane, D ranged between 103 m and 170 m. I propose (Martinsen and Marx, 1999; Roberts, 2000): D (m) = 5.8 M 0.333, t (s) = 0.9 M 0.25, H = 0.75 D
Very bright flames, high E Surface emissive power depends on the radiant heat fraction (0.2 0.4). Approximate value: LPG: 250-350 kw m -2
Very strong radiation, which often is much more hazardous than blast: Chiba, Japan, 2011: BLEVE-fireball of propane, 2000 m 3 sphere, fill level 15%, P = 20 bar, failure time: 1 h, D fireball = 350 m, t fireball = 26 s. Fill distance to 0.3 bar*, m fireball D, m t, s d** 100% lethality, m 0.05 72 308 23 300 0.1 75 334 25 320 0.2 82 377 28 360 0.4 92 440 33 460 0.6 100 490 37 595 0.8 107 531 40 650 Birk et al, 2013 * Total energy (l + v) ; P = 0.3 bar => lethality 100% usually accepted in risk analysis (all effects) ** This communication
Conclusions The prediction of the risk associated to a BLEVE is subjected to diverse inevitable uncertainties: - Frequency of occurrence: for fixed vessels can be estimated by analyzing the frequency of the possible domino effect; for transportation can not be predicted. - Time to failure if there is flames contact: could be predicted for pool or jet fire flames impingement. Can not be predicted if a previous impact could have damaged the vessel or the thermal insulation. Emergency management: assume it can be quite short. - Amount of material involved: unknown in most cases; usually a conservative approach (maximum amount legally possible) is applied. - Vessel pressure: assume operating pressure if impact, or PRV set pressure if heating. - Energy invested in overpressure wave / peak overpressure: can be estimated with models; considering the contribution of vapor + liquid is a good approach. - Number and direction of ejected fragments (often not considered in QRA): impossible to be predicted with spherical vessels; can be estimated with cylindrical vessels. Reach: maximum distances can be considered.
And for fireball (only for flammable materials): - Frequency: that of BLEVE (assume P ignition = 1). - amount of material involved: unknown in most cases; usually a conservative approach (maximum amount legally possible) is applied. Further research can reduce some of these uncertainties (for ex., energy invested in DP), but other will remain (for ex., amount of material involved).
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