THE MANEUVER INVOLVES USING A LARGE, DURABLE TARP TO COVER A COMPRESSED LIQUEFIED GAS TANK THAT IS RELEASING ITS CONTENTS.
BY DAVID F. PETERSON, CHMM
Incidents involving gases or matter in the gaseous state cause the most problems for responders. This is because gas can travel where it wants and presents an inhalation exposure problem (especially if it is toxic or irritating) and because a gas and air mixture can ignite if the gas is flammable. These inherent problems are compounded if the gas is also compressed into a liquid and stored in a pressurized tank. The gas will exit the tank under pressure in a release, and the escaping gas will present a cold temperature or thermal hazard.
Responders may use numerous methods to handle compressed liquefied gas (CLG) releases (e.g., cold patches, hot tapping, off-loading to another container); the Triple-T Maneuver is one of them.
GAS BEHAVIOR
First, however, responders must understand the behavior of gases.
Boyle’s Law. Boyle’s Law states that as pressure of a gas increases, the gas’s volume decreases. The exact opposite is also true; there is an inverse mathematical relationship in which as one quantity increases, the other decreases. A balloon illustrates Boyle’s Law well: When you squeeze a balloon, the pressure of the gas inside (air) increases and the volume of the air inside the balloon decreases.
Charles’ Law. According to Charles’ Law, as volume of a gas increases, the temperature of that gas increases.
Combined or Ideal Gas Law. The first two laws have been combined to truly indicate the behavior of gases in relation to pressure, volume, and temperature, because, in the real world, these are not constants. The Combined or Ideal Gas Law explains how these components are related, both directly and indirectly. Essentially, the relationships are as follows:
-As volume decreases, temperature increases.
-As volume increases, temperature decreases.
-As pressure increases, temperature increases.
-As pressure decreases, temperature decreases.
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The Refrigeration Cycle. The continuous cycle that a common refrigerator uses to cool its interior is a good illustration of the Combined Gas Law. The refrigerator’s compressor compresses the gas in a closed system that pressurizes the gas and causes it to increase in temperature. The coils in the back of the refrigerator dissipate the gas’s heat, reducing its temperature. As the gas cools, it condenses into liquid at high pressure. The high-pressure liquid flows through an expansion valve (a small orifice) and changes to a low-pressure gas. The liquid immediately boils and vaporizes at the expansion valve, thereby dropping the temperature of the gas to near the boiling point of the gas’s liquid form. This makes the inside of the refrigerator cold, and the compressor then sucks up the cold gas, and the cycle repeats.
Another good example of gas behavior pertains to a self-contained breathing apparatus (SCBA) air cylinder when it is filled with air. Compressed air under pressure is allowed to travel into the lower pressure air cylinder when the appropriate valves are opened. This action causes the smaller air cylinder to heat up. When the contents of the small cylinder are allowed to exit, the escaping gas cools the cylinder. Frost, or condensation, can occasionally be observed when the air exits quickly, not only from air cylinders but also from other containers such as propane tanks.
Autorefrigeration. The properties and characteristics of CLGs allow them to cool themselves or autorefrigerate in certain situations. Most CLGs have very low boiling points, which means that when they are released from their containers they will rapidly boil or evaporate into a gaseous state. Because of the Combined or Ideal Gas Law, their expanding gas will create a cooling effect.
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If a CLG is released from its container through normal means (i.e., through unloading valves), the escaping gas takes with it energy in the form of heat. Again, this has a cooling effect on the liquid that remains in the container. The whole process described here is called autocooling, and this state of autorefrigeration causes the temperature of the CLG to cool to the boiling point of the specific material.
TRIPLE-T MANEUVER
Triple-T stands for tarping the tank. This maneuver uses a large durable tarp to cover a CLG tank that is releasing its contents, thereby initiating autocooling of the tank and its contents. The tarp confines escaping vapors at or near the tank, which induces cooling of the tank. As the tank and contents cool to the boiling point of the substance, the vapor pressure gets lower and lower, and the vapor production will nearly stop.
Tools and equipment. Assemble the following equipment:
-Tarp. It should be large enough to cover the container and of appropriate durable construction. Canvas or other durable materials are preferable to plastic because they are more cold resistant.
-Rope and webbing. They should be long enough to secure each corner of the tarp. The length depends on your estimation of what is needed.
-Positive-pressure ventilation (PPV) blowers. PPV is used to direct the release of product away from responders and for responder decontamination.
-Pike poles. Poles should be long enough to raise the tarp over the object to be covered.
-Personal protective equipment (PPE). At a minimum, responders should wear structural fire protective clothing (SFPC) and SCBA. Consider Level A or Level B protective apparel if appropriate.
The maneuver as described below involves a truck-mounted propane gas container with a leak at the rear (photo 1).
- Before deployment, attach webbing to all four corners so personnel can maintain control of the tarp during operations (photo 2).
- Using 12-foot pike poles, the lead firefighters at each corner of the front end of the tarp hook the tarp through the eyelets and raise the tarp over the leaking container, moving toward the leak. Firefighters at each rear corner of the tarp hold the webbing to maintain control (photo 3).
- Bring the tarp down over the source of the leak, and wrap it under the rear of the vehicle (photo 4).
- Secure the webbing at each corner of the tarp in the final step (photo 5). Operations cease when the tarp is pulled over the end of the tank or pulled underneath the tank and all tarp ends are secured. Monitor the situation, and adjust accordingly.
- The fully tarped and secured tank is in the process of autorefrigeration (photo 6).
- Decontaminate personnel, if necessary, using PPV (photo 7). PPV may also be used to direct the flow of escaping material away from responders (photo 8).
Safety Precautions
As with any procedure or technique for handling CLGs, all responders must be aware of safety precautions. The Triple-T Maneuver is not risk-free, and it depends on adequate training for safe and effective deployment. Practice the maneuver repeatedly, andnever use it for the first time at an emergency. Personnel must be competent and confident with the Triple-T Maneuver before field application.
-Tactical leader. The tactical leader is in command of the operation and should brief all personnel concerning incident objectives. At the leader’s signal, operations commence and should proceed at an even pace. A “freeze!” command immediately stops all personnel movement.
-Protective hoselines and safety personnel. Protective hoselines and safety personnel should be in place at all times for fire protection and, more importantly, to protect personnel from fire and chemical exposure.
-Positioning. It is important to position responders upwind of the release at all times and away from vapor or liquid exposure hazards.
-Apply the tarp to the container or tank from the upwind direction. Use Mother Nature to your advantage. Consider augmenting the natural air currents with one or more positive-pressure blowers. It is also possible to create a microclimate by using enough PPV blowers to provide a wind direction at the scene.
-Tarp should be large enough for the job. Use tarp that is large enough to complete your objective of covering the tank. The tarp should also be very durable, rip-resistant, and cold-resistant. Consider canvas tarps or another durable fabric.
Plastic and thin tarp materials have a high likelihood of failure from the thermal shock of the cold liquid and vapor.
-Use PPV blowers to decontaminate personnel. Low boiling-point materials will evaporate very quickly, and PPV blowers will accelerate the process. Check with personnel exiting the decontamination area with appropriate monitoring instruments to ensure safety. Personnel should not remove SCBA or SFPC until they have been decontaminated.
-Prepare for flammability hazard. Eliminate or reduce static electricity and other ignition sources at emergencies involving flammable gases. Bond and ground the tank/container/vehicle before employing the tarp.
-Liquid product may pool at the bottom of the tarp. It is a distinct possibility that a pool of liquid product may accumulate at the bottom of the tarp. This must be handled carefully and safely. Consult with product specialists on this task.
-Tarping the tank is not advisable in all situations. When to employ the tarping is a judgment call. Large releases may be too dangerous, or tarping may be less effective than other methods. If the release is substantial, the product may evaporate quickly, making the risk of employing the tarping exorbitant.
AMMONIA SAFETY AND TRAINING INSTITUTE
The Ammonia Safety and Training Institute (ASTI) has conducted training sessions for covering an ammonia tank that is releasing product. Its extensive testing with ammonia has shown the tarping concept to be a viable alternative for handling these types of emergencies. The ASTI provides comprehensive ammonia safety training across the country to ensure this chemical is handled safely.
ADVANTAGES AND APPLICATIONS
The Triple-T Maneuver has many potential applications.
Preventing large-scale evacuations. Tarping a tank can help prevent large evacuations, especially in highly populated areas. Instead of allowing a gas or vapor to drift downwind from a container that is releasing product, the vapors can be confined. This may be critical in locations where hospitals, schools, or office buildings exist and it may be dangerous to expose people to the vapors of a toxic or irritating material.
Buying time while awaiting off-loading specialists. The Triple-T Maneuver can also buy responders time while off-loading vehicles are summoned to the scene. Depending on the location and the available resources, it may be hours before professional assistance arrives. Rather than stand by idly, responders can actively employ this concept and better control the outcome.
Permitting shutoff of a leaking valve. Once the tarp is in place and secured and the product has been autocooled, a leaking valve on a container can be shut off. With adequate personal protection, a responder can turn a valve from the other side of the tarp and still be protected.
A key factor in obtaining a favorable outcome in an emergency response is knowing and understanding the potential of the situation. In a haz-mat response, this extends to knowing and understanding the product involved in the emergency. CLGs have multiple hazards and properties, and responders can use these properties to their advantage and mitigate the emergency safely. The Triple-T Maneuver, with proper safety precautions, can be employed to mitigate CLG releases efficiently and effectively.
Thanks to Madison (WI) Fire Department Firefighters Garza, Kuhl, and Shortreed for their assistance with the photographs and to Amerigos for the use of its tank truck.
References
- Hildebrand, Michael S. and Gregory G. Noll; Propane Emergencies. (Developed for the National Propane Gas Association.) Red Hat Publishing, 1999.
- Sax, N. Irving and Richard J. Lewis, Sr., eds.; Hawley’s Chemical Dictionary, 11th Edition. Van Nostrand Reinhold Publishing Company Inc., 1987.
- Snyder, Carl H.; The Extraordinary Chemistry of Ordinary Things. John Wiley & Sons, Inc., 1992.
- Ammonia Safety Training Institute (ASTI) Manteca, California, (209) 825-8567.
DAVID F. PETERSON, CHMM, a 21-year veteran of the fire service, is a lieutenant with the Madison (WI) Fire Department. Previously, he was a training coordinator for the regional Level A Haz-Mat response team. Peterson is the owner of Americhem Safety & Environmental, LLC, a haz-mat training and consulting firm in Janesville, Wisconsin. He is an IAFF Master Trainer, an adjunct instructor for the National Fire Academy and the Emergency Management Institute, and an FDIC presenter. He is a member of the NFPA Classification and Properties of Hazardous Chemical Data Committee. Peterson is the founder and past president of the Wisconsin Association of Hazardous Materials Responders, Inc.
