When it comes to a high pressure vapor release, the scary visuals, i.e., the billowing apparition rapidly enveloping the facility is not as dangerous as what might already be much closer. An invisible killer surrounds the escaping fuel, a combustible wrapper that makes ignition and death possible.
Visible clouds of vapor represent a region of relative safety. The richness of the fuel itself drives away the oxygen it needs to burn. You might suffocate but you will not incinerate. However, just beyond the vapor the leading edge of the release is mixing with the surrounding atmosphere, turning clear and flammable.
Granted, with a multitude of ignition sources readily available, chances are the escaping vapor will touch off long before the ERT can do anything. The plant’s own fire pumps, automatically activated in such an emergency, could easily provide the devastating spark. But what if the gas continues to vent uninterrupted for minutes, maybe even an hour. Is it worth the risk to try and mitigate the hazard? Are there that many acceptable options that can be employed?
Procedures for dealing with these events are straightforward. Move upwind. Turn on any fixed monitors or nozzles available to dissipate the vapors. If you have a nozzle on an aerial ladder, use it. Once you dissipate the vapors, gather your hand lines and charge straight at the leak. Close the magic valve and go home a winner.
It might be time to reevaluate that strategy. First of all, a vapor cloud can originate from many different sources. Not every vapor release shares the same characteristics. Compare liquid natural gas to liquid propane. LNG, with a relative density of 0.63, is lighter than air. Release it to the atmosphere and it rises.
However, LPG has a relative density of 1.56, making it heavier than air. Instead of rising, it sinks to the lowest point available. If your facility has storm drains, that can mean big trouble.
But first you have to find the gas. Otherwise, where do you spray the water? What happens if the wind shifts? At the current rate of release what will the expanding parameters of the vapor cloud be in a half-hour?
How do you find what you cannot see, feel or smell? Remember, pungent-smelling methyl mercaptan is added to give natural gas any odor at all. Without knowing you could be standing waist deep in flammable vapor.
Technologically, the answer is to equip the responders with pocket detectors to monitor for flammable gases. However, this brings us to another question. Do emergency responders have any business walking into a hazardous situation like this, personal detectors or not? If the product involved is not immediately known, neither is the flammable range.
There are two types of ignition to worry about, both bad news. If the vapor cloud ignites as a deflagration, an ignition wave moves through the flammable material at a leisurely subsonic rate. You might have enough time to dive for cover, if any is handy. Trying to run is futility itself.
Worse, if the ignition is a sharp, supersonic detonation traveling at 3,500 feet per second, there is not even enough time to think about moving. Either way, full PPE complete with breathing apparatus, is only marginally better than being stripped to your BVDs. Do we want personnel operating inside an area that can flash into that kind of fury in a heartbeat?
No, we do not. The risk is unacceptable. I have watched a 38,000 gallon railroad tank car launched like a Fourth of July bottle rocket. Given that it misses you when it lands, the experience leaves you with a healthy respect for the power that a vapor explosion can generate.
This is not going to be a popular pronouncement. But doing nothing while expensive product vents away only carries the stench of failure. If it ignites with emergency responders in its midst, the overwhelming smell afterward could be the stench of death.
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