Dcasto is exact and correct. This question comes up all the time – and I detect an inability to recognize the most obvious and common-sense analysis that Dcasto has painted for us. Common sense tells us:
Mom always let us boil our own Easter Eggs on a gas range where the 1,000+ oF flames directly impinged on the pot holding the water where our eggs were being boiled at 212 oF. Why didn’t the aluminum pot simply melt (or collapse) when subjected to 1,000+ oF??? The answer is obvious: the water heat sink absorbed all the heat put to it through the pot’s walls – keeping the pot’s wall relatively cool compared to the flame temperature (and closer to the cooler 212 oF). As long as an inventory of water is maintained in the pot, the pot’s metal and mechanical integrity is protected by the heat sink. The moment all the water is boiled away …..goodbye pot.
This example of simple, direct-fired, heat transfer is illustrated every day in industrial boilers – especially the fire-tube (“scotch-marine”) variety. A flame is created and maintained 24-7 in a closed environment where the extremely hot flame and resultant gases are transferring heat to a pressure vessel filled with water and generating saturated steam as a result. There are very important liquid level controls maintained on such a steam generator for the simple reason spelled out above in the previous paragraph – should the liquid water disappear, the result would be a boiler rupture with a lot of pressure being released instantaneously – a boiler explosion. And this was the precise reason the US government got involved at the turn of the last century and mandated that something had to be done to prevent people getting killed in boiler explosions during the steam engine and locomotives era. The result was the ASME code that we all love so dearly.
Today, the rules are the same: the same exact thing happens in an industrial pressure vessel that is filled with liquid and is exposed to a pool fire. The vessel’s integrity is secure as long as the vessel has liquid inventory to serve as a heat sink. And it is this liquid inventory converted to saturated vapor that serves as the medium that furnishes the “over-pressure” that is relieved through a PSV. As long as the excess vapor is being relieved the vessel is protected – and so is everything around it. It is when the liquid is depleted that a definite hazard is created: as Dcasto explains, the collapse of the vessel is inevitable if the flames continues. It took the API a long time to accept this simple and straight-forward reality in their standards 520 and 521 - but at least they finally did. It hasn’t been until recently that people have now realistically accepted the fact that mere PSVs do not totally protect a liquid-filled vessel – and much less, a gas-filled vessel. That has now brought on the API procedures for depressurization – which is the smartest and most positive approach for personnel protection – especially combined with water sprays and heat-resistant insulation.
The point I want to add and make sure is taken into consideration in this thread is that an engineer – particularly a chemical engineer – should realize that you can’t “design” a pressure vessel pressure for the worst case relief scenario. For that purpose, you have safety devices, instruments, alarms, and procedures to implement. It is, however, vital that one know what is physically happening to the vessel during – for example – the pool fire case. Every engineer should know by now that the worse heat transfer film coefficient known is a gas coefficient. That is why gases – such as air and Freons – are used as insulators. In fact, Artic insulation works well not because of the solid material used, but because of the insulating air it contains. The point here is that a 100% gas filled pressure vessel exposed to a pool fire is destined to total failure – and simultaneous violent rupture – unless it is protected by a water spray or insulation. This is due to the insulating properties of the internal gas film that effectively retards efficient heat transfer from the external flame to the internal gas. There is NO HEAT SINK, and the vessel walls have to take the brunt of the heat accumulation because they can’t transfer it anywhere else. Depressurization is a mitigation but it will not ultimately save the vessel; however, it will protect the environment and personnel from the mechanical failure.
All of this is explained or inferred in API 520 and API 521 and these documents should be the basis for good, safe design when dealing with pressure vessel relief.
