Incident Overview


A driver of an automobile lost control of his vehicle at high speed, and drove under the right side of an oncoming MC-331 cargo tank truck, which was making a righthand turn onto a bridge.

Upon arrival at the scene, the Incident Commander (IC) sizes up the situation.

Under the protection of hoselines and in full protective clothing and self-contained breathing apparatus (SCBA), firefighters approach the accident site and find the automobile wedged under the propane truck, with the car's roof crushed level with the dashboard. A quick survey of the driver's injuries confirms that he has received fatal injuries. The propane truck driver has received minor injuries which will require medical treatment at the hospital.

Inspection of the damaged tank truck reveals the truck is placarded "1075" (the correct placard for propane). The truck is painted with the name of the propane transport company.

Under the protection of hoselines, a fire department officer observes that the truck's piping is severely damaged and has cracked open. The valve stem appears to be bent and is producing a gas cloud. A liquid propane leak is observed on the bottom, lower section of the tank.

 

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Summary of Tank Truck Construction Features


MC-331 cargo tanks have a maximum design pressure of 500 psig [49 CFR 178.337-1(b)]. They are constructed from steel metal sheets that are formed into a cylindrical shape, and 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:

  • Relief valves
  • Temperature, pressure and liquid level gauges
  • Liquid and vapor transfer lines and valves
  • 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 the 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.

 

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Incident Action Plan


Tactical Objectives

  1. Control or stop the propane liquid from escaping the bottom of the tank truck.

  2. Maintain site safety until the propane in the damaged tank truck can be transferred to another tank truck.

Methods of Leak Control

The first initial action by the IC 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 the propane transport company should be made as soon as possible. The transport owner 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 and off of the bridge. Two 1-1/2 or 1-3/4 inch hoselines flowing 100 gallons per minute (gpm) or higher are recommended; however larger hoselines or master streams may be necessary. A combustible gas indicator (CGI) should be used to 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.

Given that the tank truck's piping and valves are severely damaged and there is a visible propane liquid leak on the bottom half of the tank, the cargo tank will have to be unloaded.

Considerable time may pass until a compatible MC-331 is available on scene for product transfer. In the meantime, some attempt should be made to slow down or stop the leaking propane on the damaged tank.

 

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Additional Factors


Additional Factors to Consider for this Operation Include:

  1. When the ambient temperature is above 90ºF, hose water fog played on the top of the tank can be effective in keeping the tank's shell cool. The average temperature of hydrant water is 52ºF. When ambient temperature is below 40ºF water will warm the tank.

  2. CGI readings should be taken throughout the incident to ensure that hoselines are effective in dispersing flammable vapors.

The Incident Commander Has Three Tactical Options for Leak Control:

Fiberglass Wrap
A vapor leak can be stopped with the use of a commercial fiberglass wrap material known as "Stop It". Field experience has shown that fiberglass wrap kits work successfully on most propane vapor leaks. The roll of Fiberglass Wrap is wetted and wound around the damaged pipe or valve. When the wrap sets up it will stop a vapor leak up to 200 psig. However, if there is a liquid leak producing a vapor cloud that can be seen, these kits will not work.

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, and then wrapped around the leaking valve or pipe. The wet cloth will freeze due to the propane liquid's very low temperature. CAUTION: Firefighters must wear heavy neoprene gloves suitable for working with low temperatures to avoid frostbite. Cloth or leather gloves can absorb liquid propane and result in injury. Freeze wraps should be monitored for reapplication.

Water Injection
Propane has a specific gravity of .504 (the specific gravity of water is 1.0), so water will sink to the bottom of the tank. Looking at the problem from a weight perspective, propane liquid is in the bottom of the tank and weighs 4.20 pounds per gallon. Water weighs 8.34 pounds per gallon. If water can be injected into the tank, it will sink to the bottom of the tank and form a layer of water which will leak out of the damaged valve or pipe. If the flow of water into the tank can be maintained, flammable vapors will not be produced and the transfer of product can be conducted under safer conditions. In some cases the leaking water will come into contact with the cold metal on a valve or pipe and form an ice patch. Fires on the lower half of a propane tank have been extinguished in the same manner.

 

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WARNING


WARNING!

When using the water injection method, special consideration must be given to the possibility of creating a hydrostatic condition which will pressurize the propane tank and activate the relief valve.

It is vital that the person operating the water injection nozzle be prepared at all times to shut down the line. This technique should only be attempted by or under the supervision of an experienced propane or container specialist.

If the support of qualified personnel is not available, the tank should be protected by unstaffed monitors and emergency responders should switch to a defensive posture.

 

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 Combustible Gas Meter


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.
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 Incident Map


This is a top-perspective view of the incident area. Its purpose is only to get the general idea of the incident and surroundings.
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 Incident Video


In this section, we present an animated version of the incident.