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CALCULATION OF BLEVE CONDITIONS FOR

CARBON DIOXIDE

BLEVEs are very unusual but extremely catastrophic events that can occur with carbon dioxide under pressure. The last really bad explosion with carbon dioxide occurred some time ago in Worms, Germany. Since then carbon dioxide had been stored cool at atmospheric pressure. BLEVEs also occur with other gases, notably with propane at higher temperatures, usually as a result of fire.

However, now that carbon dioxide and its mixtures are being used under pressure for gas and oil recovery, carbon capture and storage and supercritical fluid processes, BLEVE must again be considered. This involves calculating the range of conditions of pressure and temperature where BLEVEs can occur. Even under these conditions a BLEVE is unlikely but, because of the enormity of BLEVE explosions, it is prudent to avoid these conditions. In fact failure to avoid BLEVE conditions is quite inconsistent with the current safety culture in oil and gas companies.

The explosion at Worms

There have been a number of reported BLEVEs with carbon dioxide, mostly involving fire extinguishers. However, one of the most catastrophic examples was the explosion of a tank of CO₂ at a plant in Worms, Germany in 1988. A tank of 30 tonne capacity was shattered into a number of pieces and only 20% of the tank was present in the original premises after the explosion. Most of the tank was propelled 300 metres into the Rhine. There were three fatalities and a further eight casualties. The premises containing the tank originally were devastated. The event is reported in: W.E. Clayton and M.L. Griffin, ‘Catastrophic failure of a liquid carbon dioxide storage vessel’ Process Safety Progress, 1994, 13, 202-209. (Their Figure 5 is above)

Explanation of BLEVE

Why do BLEVEs occur? A theory developed by Professor Reid and colleagues at MIT (M.E. Kim and R.C. Reid, ‘ The rapid depressurization of hot, high pressure liquids or supercritical fluids’ in Chemical engineering at supercritical fluid conditions, edited by M.E. Paulaitis, Ann Arbor Science, 1983) shows that a very spectacular physical event must occur under certain circumstances, and this is likely to be the explanation of BLEVEs. A simplified explanation of this theory can be given with the aid of the figure below, which is a diagram of the relationship between the pressure in a substance and the volume it occupies as a liquid, gas and fluid.

The continuous thick black line ABCD shows the behaviour of the substance at a constant temperature and at thermodynamic equilibrium. In the section AB, the substance is a liquid and as the volume it occupies is expanded the pressure falls dramatically. Eventually the pressure falls to the vapour pressure of the liquid at the particular temperature at B. The liquid then starts to evaporate to become a liquid-gas mixture, and the pressure stays constant at the vapour pressure. Eventually it reaches C, where the liquid has been completely converted to gas. The pressure then drops as it is expanded further.

However, if the pressure falls suddenly, due to a failure in the container, the substance can become an unstable liquid along the path BS, coloured red. Along the path BS the substance is metastable and can at any time boil to return to the equilibrium horizontal line BC. Although such an event can be very violent is not thought to be a BLEVE. Typically violent boiling will occur before the point S is reached and a BLEVE will not occur.

However, in the unlikely event that the point S is reached a special and catastrophic situation arises. S is known as a spinodal point and the slope of the line at this point is zero (i.e. ). The dotted red line connects these points at different temperatures and is known as the spinodal curve, which ends at the critical point. The special nature of situations represented by points along this curve are that large density fluctuations can occur because of the insensitivity of pressure to volume ().

Once the spinodal curve is reached separation into liquid and gas must occur. The density variations develop spontaneously into liquid and gas regions. This occurs homogeneously throughout the whole liquid. The rise in pressure on to the vapour pressure line BC is not large but it happens at great speed, homogeneously and at time scale of molecular motion. The shock to the containing vessel is huge and a disastrous BLEVE happens.

Calculation of conditions for BLEVE

For a BLEVE to occur, the substance has therefore to find itself on the spinodal curve between 1 bar and the critical point where the curve ends. When a catastrophic failure occurs there is not time for heat to pass into the system and so the path during failure is adiabatic. In thermodynamic terms the entropy of the system is constant during failure and this allows the conditions for BLEVE to be calculated. It can be shown that for CO₂ the temperature-pressure conditions must be within the red envelope shown in the figure. For other gases and gas mixtures the envelope will be different, but again can be calculated. These envelopes should be prudently avoided

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