Scope today Cylinders in fires Acetylene a special problem Why is UK different? What can be done about it? Research
Cylinders in Fires All gas cylinders may explode in fires Increasing gas pressure overcomes reducing steel cylinder shell strength at ~300 o C Cooling restores shell strength and reduces gas pressure But Acetylene might re-heat
Acetylene is fuel gas of choice − Flame temperature of 3150 o C and lighter than air Carbon-Carbon triple bond – reactive, not unstable 3 x Acetylene reactions of interest in fire Decomposition not spontaneous!- needs > 350 o C C 2 H 2 + HEAT → 2C + H 2 + more HEAT Severe shock to a HOT cylinder? Acetylene (C 2 H 2 )
Acetylene Cylinders Porous monolithic mass Acetylene dissolved in a solvent − small cells act as flame arrestor Designed to stop decomposition Steel heat treated Low fill pressure But mass is a heat insulator
Acetylene cylinders have a hard life! – the myth of cold impact effect
Acetylene cylinder testing Drop test Elevated temperature test Backfire test Impact resistance test − 90g Plastic explosive charge
Impact stability of Acetylene cylinders Impact resistance test Test protocol requires cylinder to dent by 25% of its diameter. Example shown about 50% Porous mass not damaged No damage other than dent itself − No cracks − No sharp-edged deformations − No indication of decomposition
Cold Shock Gas expert opinion − For F+RS, Police and Highways Agency “Mechanical shock alone to a cold Acetylene cylinder, which remains intact and has not been exposed to fire, cannot initiate decomposition.”
Acetylene vs Propane Propane Acetylene Cylinder content (Typical) 47Kg 8Kg Gas density 1.55 0.9 Flame temp with O2 2500 3200 Flammability limits in air 2.2 - 9.5% 2.5 - 81% Max fill pressure (Bar) 10 18 Cylinder burst pressure (Bar) 67 105 Relative BTU yield 1 4 Cu Metres Oxygen needed 4 1 Special risk BLEVE Decomposition Cutting? Yes Yes Brazing? Yes Yes Welding No Yes
UK protocol for DA in Fires is different! Since 2003/2004 Initial 200m hazard zone whilst facts established Zone reduction based on dynamic risk assessment 24 hour precautionary cooling − Acetylene cylinder may not be moved or vented BUT – often 200m EXCLUSION zone held for 24hours – causes massive disruption!
Why the UK protocol became so? Death of John Wixey 1987 HSL Experiments in 1994 Fire Service training content Myth and folklore!
The ‘London improvement’ In 6 years ‘04 to ‘09 London had:- 543 Cylinder incidents 102 incidents really involved Acetylene after all 4 DA cylinders exploded – ALL IN FIRE, NOT AFTER. NO cylinder re-heated after 1 hour cooling – 437 wet tests passed on 140 cylinders – all found at ambient temperature! From August 07 to Oct 09:- London F+RS reduced incident frequency from 14 to 25 days and average disruption time from >19 to 2.67 hours! Problem solved! – why not elsewhere?
BAM Research German Federal Institute of Materials Science World renowned acetylene experts Independent contract research − commissioned by BCGA, HSE, DfT, TfL What cooling period is required for an Acetylene cylinder to be sure any decomposition is over and cannot re-start?
Screening: Acetylene only Explosive decomposition occurs at about 350 °C − ruining the pressure transducer Conditions as before, but start pressure 10 bar
Research What conclusions to date? Model works and can be interrogated Mass type makes no difference No decomposition until >350c Polymerization occurs <300c but is pressure reducing We assume DA cylinders which explode in fire do so because of decomposition? More likely simple over pressure
research – Phase III work Full scale cylinder tests (at Horstwalde!)
Summary Acetylene cylinders present a special hazard UK procedures have been over-cautious Good research progressing on heat transfer Cold impact case proven Acetylene is needed and is safe if stored, transported and used correctly