Advancements in the prediction of blast damage due to vapour cloud explosions. Hans Boot

Transcription

1 Advancements in the prediction of blast damage due to vapour cloud explosions

2 2 Contents Predicting VCE Damage zones? = the WHY Explosion modeling = the WHAT Using the GAME correlations = the HOW 1 Explosion regions : connecting dispersion to explosion models = the HOW 2 Calculation procedure = RESULT Conclusions

3 3 Technology evolves! New models have been published after the last version of Yellow book (1984,1995,2005) Government does not want changes: other risk distances!! Yet, new, improved insight models are being added to TNO consequence modelling tool e.g: 1. Dynamic BLEVE (Martinson & Marx) 2. Two Zone pool fire (Rew & Hulbert) 3. Statistical spray release (Dam) 4. GAME correlations (van de Berg & Mercx)

4 4 Vapour Cloud Explosions (VCE s) One of the most destructive accident scenarios on (petro)chemical sites Much effort in prevention, but.. what if it happens? Avoid domino effects! Use site layout guidelines/requirements for plant buildings Conscious choice for location operator rooms Constructions designed to withstand certain overpressure Evaluation requires knowledge of potential blast strength

5 5 VCE damage modelling Knowledge on potential damage requires: - Prediction of overpressure / impulse (blast strength) - Information about construction strength (vulnerability) Explosion models: 1. Numerical simulations 2. TNT equivalency 3. Blast curve methods Can a VCE affect the operator room?

6 6 Typical explosion models Numerical simulations (CFD based) Requires detailed 3D geometry Total site: multiple sources/receivers/ wind angle s: laborious & expensive TNT equivalency method Requires equivalency factor Primary aimed at solids: short duration high amplitude Blast Curve methods TNO Multi Energy : developed for VCE s Requires estimation Blast curve # + % confined explosive mass

7 7 Multi-Energy method Determination of Blast class (curve #) based on: Ignition strength: High, Low Obstruction: High, Low, None Parallel plane confinement: Yes, No / only ground surface Estimation of % confined mass of the cloud Blast strength category Ignition strength (High / Low) Obstruction (High / Low / None) Parallel plane (Confined / Unconfined) Class 1 H H C H H U 7 10 LFL footprint 3 L H C H L C H L U H N C L H U H N U L L C L L U L N C L N U 1 Only small part of cloud covers installations: is this 10%?? We need to estimate overlap LFL cloud AND congested area

8 8 GAME correlations Guidance on the Application of Multi Energy: a HSE-TNO project aimed at providing a quantitative relation to provide overpressure of VCE s Describes maximum overpressure as function of quantifiable parameters: P max = 0.84 (VBR * L p / D avg ) S L 2.7. D avg 0.7 (open 3D) P max = 3.38 (VBR * L p / D avg ) S L 2.7. D avg 0.7 With: P max = (Dimensionless) Maximum Overpressure VBR = Volume Blockage Ratio L p = Flame Path Length D avg = Average Diameter Obstacles S L = Laminar Burning Speed fuel (2D confinement)

9 9 GAME correlations Instead of estimating the Blast strength category (based on expert judgement), the maximum overpressure is calculated The GAME correlations were extensively validated!! TNO-ME method also provides dynamic pressure & pressure impulse curves The initial P max relates to starting point of curves: P max can be translated into a Representative curve #

10 10 Determining VCE damage zones Practical usage of GAME correlations P max => Blast Curve number representation Log interpolation: Peak overpressure, Dynamic pressure and Pressure Impulse vs. distance But what about the flame path length and confined explosive mass fraction? Overpressure [bar] Blast Curve 9 (Very strong deflagration) Blast Curve 10 (Detonation) GAME calculated; Distance from center mass of explosive cloud [m] Flame path L p and % Confined mass requires projection of flammable cloud on congestion area!

11 Area [m2] 11 Dispersion model connected to GAME A dispersion model is required to calculate Explosive mass in the cloud and Footprint of the LFL Cloud Dispersion models can report Mass in LFL cloud and Area of LFL cloud as F(time) Available dispersion models have been extended with reporting dynamic cloud = location, size and mass Explosive mass [kg] 9,000 8,500 8,000 7,500 7,000 6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1, Instantaneous release - Explosive mass vs. Time Instantaneous release - Area of the LFL cloud vs. Time Time [s] ,500 7,000 6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1,

12 12 Interaction of LFL cloud & congestion areas The GAME method requires to define congestion areas (with properties VBR and D avg ) The (moving) LFL cloud is projected upon congestion areas The overlap of LFL cloud and congestion area determines explosion strength: Overlap region = Explosion region LFL cloud as function (time) Max flame path length Cloud size determines explosive mass involved VBR and D taken from area: overpressure contours Overpressure contours resulting explosion in centre congested area Release point Congested area size determines mass involved VBR and D taken from area: overpressure contours

13 13 Calculation procedure 1. Define release location, Chemical, and Loss of Containment event (Instantaneous, Leak, Rupture) 2. Define environmental conditions (temperatures, wind speed, stability class, surface roughness) 3. Define potential congestion areas (polygon on the map with typical VBR and D properties) 4. Calculate source term/rate and conditions 5. Perform dispersion calculation (LFL mass, area and location as function time) 6. Calculate maximum expl. mass in LFL cloud overlap with congestion areas: (Confined explosive mass, Flame path length) 7. Calculate overpressure contours (corresponding to worst case confined mass, congestion area properties)

14 14 Example procedure Project LFL cloud on congestion area Explosion region (red contour) determines %confined and flame path length First assumption: explosion centre in centre of gravity explosion region

15 15 Remaining choice: explosion centre The explosion centre determines the damage range! The explosion centre: anywhere in the explosion region The explosion region: (almost) anywhere in congestion area Construct the overpressure contour (example 0.1 bar) around the full congestion area!

16 16 Limits for explosion centre Part of congestion areas can be out of reach of LFL concentrations The explosion centre location is limited to max LFL distance Construct the overpressure contour around the congestion area within LFL!

17 17 Conclusions OK: A practical, quick and thus highly feasible method has been developed to create overpressure contours for total site analysis OK: The usage of the GAME correlations avoids the need of expert judgment in selecting blast curves, providing a more transparent and traceable blast pressure calculation OK: The method described can now also be extended with damage calculations on receiving objects (using vulnerability criteria for buildings or constructions)

18 18 Conclusions To Do: The explosion region method can be extended with the concept of Critical Separation Distance Note: The GAME method requires detailed knowledge of site congestion areas with parameters VBR and D avg Note: not a real 3D modelling; does not involve height : LFL cloud and congestion areas are 2D projections. (Height area is used as max volume limit) Be aware: The empirical dispersion models don t account for cloud displacement (concentration deviations) due to large obstacles (requires real 3D modelling of surroundings: a CFD calculation for multiple wind directions) Method remains an estimation of reality we simply gained improved realism

19 19 Finally Thank you for your attention Any questions??