Author(s): Jesse Roman. Published on March 1, 2016.

Cloud Hazard

New research takes a closer look at the explosive potential of vapor clouds formed by LNG


VAPOR CLOUD EXPLOSIONS have the potential to cause the most destructive LNG incidents. They can form if LNG is spilled or leaked, causing the liquid to vaporize as it mixes with ambient air temperatures above its boiling point of –260 degrees F. Because the cold vapors are initially heavier than air, they form a cloud close to the ground, which slowly rises and dissipates as it warms. If a viable ignition source is present when the cloud is large enough and at a 5–15 percent concentration in the air, it can ignite and possibly explode.

An LNG explosion is a very rare occurrence that requires a near-perfect alignment of events, according to Guy Colonna, NFPA's division manager for Industrial and Chemical Engineering, who spent part of the 1980s participating in research that informed many of the LNG vapor cloud computer models still used today. “The fact is, it is really hard to blow up LNG—I have tried many times under many different scenarios,” he said. “You have to have the right concentration, the right amount of evaporation, and a delayed and perfectly timed ignition.” A more likely scenario is an LNG pool fire, not a big boom, he said.

Though remote, the danger posed by vapor clouds isn’t taken lightly, especially when it comes to facility siting. Sophisticated computer models developed by government and industry attempt to show what the off-site consequences would be in a potential vapor release. The models take hundreds of inputs—causes of LNG leaks, locations of leaks, tank pressure, rate of release, prevailing winds, ambient temperatures, and much more—and map out what would happen in hundreds of different scenarios. How would the gas disperse? Where would it go? How many people might be in danger if it were to explode? A 2009 Fire Protection Research Foundation (FPRF) project collected all of the experimental data and created a validation database to enhance these dispersion models. The models have been referenced in NFPA codes for years. Even now, they are still being refined.

This year, the U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) is funding a project to learn more about what happens when vapor clouds explode, under numerous conditions and scenarios. Members of the technical committee for NFPA 59A, Production, Storage, and Handling of Liquefied Natural Gas, served as technical advisors for the first phase of the project, which was completed in February by the Health and Safety Laboratory in the United Kingdom. Phase II, which will begin this year, will likely involve FPRF participation. The project will look beyond gas dispersion and what happens during an explosion. It will assess the effectiveness of current vapor cloud explosion models, review regulations and current mitigation measures, and determine what research and development might be needed.

The end result, NFPA and other participants hope, will further inform NFPA 59A and help government regulators better assess siting requirements for the growing number of LNG facilities under consideration.

JESSE ROMAN is staff writer for NFPA Journal. He can be contacted at