While driving down a major interstate highway in a metropolitan area, an MC-331 cargo tank truck was cut off by an automobile making a quick lane change causing the tank truck driver to swerve sharply and lose control of the vehicle. The cargo tank truck overturned onto its left side, sliding along the concrete highway until it came to rest against a guardrail.
Upon arrival at the scene, the Incident Commander (IC) sizes up the situation. The driver has already extricated himself from the tractor, and an EMS unit has arrived on-scene and has begun treatment of the injured driver.
There are no visible leaks or spills from the cargo tank. Visual inspection of the damaged cargo tank truck reveals the truck is placarded "1075" (the correct placard for propane). The truck is also painted with the name of the propane transport company.
EMS personnel inform the IC that the driver has reported that the product is propane, that the cargo tank is approximately 80% full, and that he was enroute to make several deliveries.
Under the protection of hoselines, the IC sends an entry team in to assess the damage to the container. Using a combustible gas indicator (CGI) the entry team determines that there is a small leak on the bottom liquid unloading valve (i.e., "belly valve"). They also report that there is a "street burn" that extends along the entire length of the cargo tank caused by the container sliding along the highway. There are no other obvious dents or gouges visible on the tank underside.
MC-331 cargo tanks have a maximum design pressure of 500 psig [49 CFR 178.337-1(b)]. MC-331’s may be constructed to a design pressure not less than the vapor pressure of the commodity contained at 115º F. They are constructed from steel metal sheets that are formed into a cylindrical shape, then welded along the seams. Hemispherical or elliptical tank heads are welded to the barrel to form the tank.
All MC-331 cargo tanks have pressure relief valves; temperature, pressure and liquid level gauges; liquid and vapor transfer lines and valves; and remote emergency valve controls. Openings for transferring propane liquid and vapor are located in the bottom of the tank, either in the underside of the shell and/or in bottom half of the rear head. Openings for the pressure relief valves are located in the top of the tank. Most propane cargo tanks are uninsulated and are required to have the upper two-thirds of the tank painted white, aluminum or a similar heat reflective color.
The first initial action by the Incident Commander should be to implement site management procedures, (e.g., isolate the area and deny entry, remove bystanders to a safe area, and establish Hazard Control Zones).
A call for technical assistance from both the regional Hazardous Materials Response Team (HMRT) and the propane transport company should be made as soon as possible. The transport company representative should be briefed on the nature of the problem so that the proper personnel and resources are dispatched to the scene.
Firefighters in full protective clothing and SCBA should deploy hoselines to disperse and dilute the flammable gas away from the container. Two 1-1/2 or 1-3/4 inch hoselines line flowing 100 gpm or higher are recommended; however, depending upon the rate of release larger hoselines or master streams may be necessary. A combustible gas indicator (CGI) should be used to monitor the hazard area and determine the effectiveness of hoselines in dispersing the gas. If temperatures are expected to rise, water for container cooling as well as vapor suppression should be provided.
With the cargo tank lying on its left side, all of the container valves and piping will be in the liquid product. Although the leak appears to be minor, some attempt should be made to slow down or stop the leaking propane on the damaged tank.
The Incident Commander has several tactical options for leak control:
Freeze Wrap — A liquid leak can sometimes be sealed off by the use of a freeze wrap which forms an ice patch over the leak area. A towel or fabric wrap is wetted, then wrapped around the leaking valve or pipe. The wet cloth will freeze due to the propane liquids very low temperature.CAUTION: Firefighters must wear heavy neoprene gloves suitable for working with low temperatures to avoid frost-bite. Cloth or leather gloves can absorb liquid propane and result in injury.
The nature of both the leak and the mechanical stress applied by the rollover will determine the path forward. Conducting a damage assessment will assist the IC in determining if the cargo tank must be off-loaded in a timely manner, or if the container can first be uprighted and then off-loaded. While there are general guidelines on uprighting versus off-loading, technical assistance should be obtained during the hazard and risk evaluation process.
Much of the available literature on damage assessment of pressurized containers is based upon testing and experience with railroad tank cars. It should be noted that design and construction standards for tank cars are significantly greater than those for MC-331 cargo tank trucks.
Damage assessment should evaluate the following factors:
Examine the surface of the container, paying attention to the types of damage and the radius (i.e., sharpness) of all dents.In this scenario, the street burn is actually a long dent that is inherently flat. It is generally caused by a container overturning and sliding some distance along a cement or asphalt road. If there are no accompanying sharply curved dents or gouges, the cargo tank can often be safely uprighted providing a sufficient lifting capability is present.
This is an interactive combustible gas meter. Move your mouse or finger (touch device) over the leak area to view the Gas-in-Air percentage change.
This is a top-perspective view of the incident area. Its purpose is only to get the general idea of the incident and surroundings.
In this section, we present an animated version of the incident.