Beyond the Rule of Thumb
Survival Tip 16
By Steven De Lisi
First responders likely will have to deal with compressed gas cylinders on many calls they respond to. They can be affected by not only the cylinders’ contents, some of which are flammable or poisonous, but also by the extreme pressure under which the materials are stored, which has the potential for extreme danger.
For example, if the valve on a compressed gas cylinder is damaged, gas or vapor may be rapidly released. If this occurs on unsecured portable cylinders, the sudden release of gas could propel the cylinder at high speed, endangering people or property in its path. The direction of travel is often uncertain and the consequences are potentially fatal.
First responders must also remember that compressed gas cylinders store volumes of liquefied or non-liquefied gases that are far greater than the physical size of the container. In particular, when released, liquefied gases will expand in large proportions that can quickly engulf first responders. The rapid expansion of gases and vapors absorb heat energy from surrounding surfaces, which can then lead to frostbite injuries to first responders who may be near the source of the release. In addition, first responders engulfed in flammable gases and vapors may suddenly find themselves trapped in a deadly fireball.
Many compressed gas cylinders include a pressure relief device that is designed to automatically prevent a catastrophic failure of the container resulting from excessive internal pressure, usually caused by external heating from sources such as exposure to direct sunlight or flame impingement. Any activities conducted in proximity to cylinders that are exposed to thermal stress must be performed with extreme caution, since pressure relief devices will open without warning, possibly exposing first responders and the public to a hazardous material. Once open, some of these devices may not close, either because the device is not designed to close (such as a frangible disk) or because it malfunctions, as could be the case with a spring-loaded device. Pressure relief devices used on cylinders that store liquefied gases may not operate properly if the cylinder is not in an upright position in which the pressure relief device is in contact with the vapor space above the liquid. Remember, too, that cylinders used to store certain materials, such as those that are poisonous, may be designed without pressure relief devices to prevent a release resulting from an accidental activation of the device.
It is important for first responders to understand that pressure relief devices can protect a compressed gas cylinder only as long as the device is the weak link of the container. For example, a cylinder built to withstand an internal pressure of 400 pounds per square inch (psi) may have a pressure relief device designed to activate at only 350 psi, thereby opening up before the internal pressure could reach the maximum allowed.
However, if this same cylinder has been damaged from a physical impact suffered during an accident, the container’s ability to withstand internal pressure at the site of damage may have been reduced to less than the 350 psi setting of the pressure relief device. The damaged portion of the cylinder may now become a weak link, allowing the cylinder to rupture at the site of the damage even before the pressure relief device would operate.
Physical damage to a cylinder may appear as a crease, dent, or gouge, and the extent of the damage will determine the extent to which the integrity of the cylinder has been compromised. Damage is especially troublesome when it occurs at “weld lines,” where pieces of metal are joined when building some cylinders; these areas are weaker due to the effects of the welding process on most metals.
If a damaged (and therefore weakened) cylinder cannot withstand its internal pressure immediately after an accident, the cylinder will likely explode prior to the arrival of first responders. However, if the cylinder has been weakened but is still able to contain the internal pressure immediately after an accident, first responders may underestimate the potential for serious consequences later on–the internal pressure of cylinders can increase as ambient temperatures on scene rise throughout the course of an incident or if there is heating from flame impingement.
For example, an incident that occurs outdoors at 3:00 p.m. during a hot summer afternoon will likely result in far more external heating of the contents of a container than if the same incident occurred at 3:00 a.m. As a result, if the internal pressure increases sufficiently to a point just above that which the weakened cylinder can resist, the container may fail without warning, even before the operation of a pressure relief device.
First responders should consider this fact when assessing containers that have not yet released their contents, since long-term incidents have a potential for the internal pressure of these containers to fluctuate because of changes in weather conditions. Those who fail to recognize this potential may be in for an unwelcome surprise several hours or perhaps even days into an incident that involves damaged compressed gas cylinders.
Cylinders exposed to fire present an even greater danger because excessive heat will not only rapidly increase internal pressure exerted by the contents, but may also weaken the cylinder at the point of flame contact. When considering compressed gas cylinders that store liquefied gases, the weakening of a cylinder from flame impingement usually occurs where the flame contacts the vapor space above the liquid level, since most of the heat applied at this location is absorbed by the cylinder itself. Heat applied at the liquid level may not weaken the cylinder (since this heat is absorbed by the liquid), yet heating of the liquid will increase the internal pressure exerted on the cylinder. For containers of nonliquefied compressed gases that are stored in un-insulated containers at normal atmospheric temperatures (such as compressed gas cylinders containing oxygen or nitrogen as well as SCBA cylinders), there is no liquid component so that flame impingement anywhere on the container can quickly cause damage.
Whenever compressed gas cylinders are exposed to fire with a subsequent increase of internal pressure, first responders must remember that the operation of a pressure relief valve is no guarantee that the cylinder will not explode. Remember that a cylinder built to withstand an internal pressure of 400 psi may have a pressure relief device designed to activate at 350 psi, thereby opening up before the internal pressure reaches the maximum allowed. If this cylinder is exposed to fire, internal pressure will increase and at the point when this pressure exceeds the setting of the relief device, the device will open.
However, if flame impingement is directed to the vapor space of a cylinder, the cylinder may rapidly weaken, and as a result, its ability to withstand increasing internal pressure may soon be reduced to less than the 350 pounds per square inch setting of the pressure relief device. As with situations involving physical damage discussed earlier, the container will now become the weak link with the likelihood for the cylinder to explode even with the pressure relief device open.
The difference between compressed gas cylinders weakened by fire exposure and those weakened by physical impact is that the effects of fire rapidly weaken the cylinder while also rapidly increasing the internal pressure because of the large volumes of heat energy. This rapid pace of events may provide first responders will very little time to evacuate nearby areas or make attempts to safely cool the affected areas of the cylinder by setting up hose streams.
First responders should also remember that the sound of gas and vapor escaping from an operating pressure relief device will increase in both volume and pitch as internal pressure continues to rise. This change in sound should serve as a warning to first responders that they may need to immediately evacuate the area.
The failure of any compressed gas cylinder can have devastating consequences that include flying debris. This problem is only magnified when dealing with flammable materials where the ignition of rapidly expanding gases and vapors create huge fireballs that can extend for hundreds of feet in all directions.
When dealing with compressed gas cylinders, be smart, be safe, and remember, everyone goes home!
Discussion Points
- Although first responders naturally expect to find compressed gas cylinders at many industrial sites, consider that these types of containers may also be found at buildings that house many retail businesses, auto repair shops, and medical offices. Review the inspection reports for your response area to determine those occupancies where compressed gas cylinders are stored.
- When reviewing the inspection reports, determine the frequency and types of violations associated with the storage and use of compressed gas cylinders. Common violations include portable cylinders that are not secured when stored in an upright position, portable cylinders with incompatible contents that are stored in proximity to each other, and inadequate markings on cylinders that can make it difficult for first responders to readily identify the contents.
Click here for more info on Steven De Lisi’s book, “Hazardous Materials Incidents: Surviving the Initial Response.”
Steven M. De Lisi is a 27-year veteran of the fire service and is currently Deputy Chief for the Virginia Air National Guard Fire Rescue located at the Richmond International Airport. De Lisi is a Hazardous Materials Specialist and a former chairman of the Virginia Fire Chiefs Association’s Hazardous Materials Committee. He is also an adjunct instructor for the Virginia Department of Fire Programs and a former member of the NFPA committee on hazardous materials protective clothing. De Lisi began his career in hazardous materials response in 1982 as a member of the HAZMAT team with the Newport News (VA) Fire Department. Since then, he has also served as a Hazardous Materials Officer for the Virginia Department of Emergency Management and in that capacity provided on-scene assistance to first responders involved with hazardous materials incidents in an area that included more than 20 local jurisdictions.
