FLAMMABLE LIQUID RELEASES FROM MC-306 TANKERS: AN OVERVIEW
Vehicles that transport bulk quantities of motor fuel and related products are commonly referred to as gasoline tankers. Tractor-trailer versions haul up to approximately 9,000 gallons, depending on product mix, gross vehicle weight, and highway weight limits.
Vehicle weight is reduced by using frameless tube-type aluminum trailers with a nominal 0.20-inch thickness designed to hold a maximum of 3.0 psi working pressure and no vacuum. The current U.S. Department of Transportation (DOT) specification for these vehicles is MC-306, which will be replaced next year by MC-406. While many safety areas are updated by the new specification, the basic tube design will not change substantially.
Compartments are numbered from front to back, with 1 and 5 having the largest capacities in most trailers.
TYPICAL CAUSES OF EMERGENCY RELEASES
Stresses to MC-306 trailers that result in emergency releases come principally from rollover, impact with another object, failure or improper use of appliances, internal pressure from ignition, and external heat from an exposure fire.
Rollovers. In general, rollovers present the greatest potential for a major incident, since the impact points will be at the sides or head of the front compartment. Guardrails, other vehicles, and highway curbs stress the relatively thin aluminum shell. Rollovers typically are caused by excess speed, roadway failure, or collapse of the landing gear. Products are released through manway covers, stressed vapor recovery components, or damage to the piping and shell.
Recent DOT information indicates that one-third of reported incidents involving all types of tankers (LP, gasoline, and so on) are rollovers. If the rollover involves an MC-306, there will be a release in two-thirds of the cases.
Ignition of the released product at the time of rollover is not the norm if only the aluminum trailer contacts the ground. Analyses of case studies and news reports indicate that five to 10 minutes may pass before the liquid, with its ever-present vapor cloud, reaches an ignition source. Rollovers in rural settings often do not ignite if the accident scene is kept free of vehicles and smoking materials. If foam can be applied in a timely manner, even releases in more populated areas can be controlled before ignition.
When fire does start, the breached compartment melts quickly and causes the top of the other compartments to melt out. In the meantime, product is forced out of the relief devices, increasing the ground fire. When not cooled, a rollover can burn from 45 minutes to an hour and a half, as the product boils over the sides of the remaining compartments. As the contents boil over and burn, the low melting temperature of the aluminum causes the sides to fold in as the liquid level falls.
Control of ignition sources, application of foams, and damming the spill are very effective in suburban and rural areas. Rollover releases in densely populated areas usually find an ignition source.
- Impact with or by another object. Upright breaches occur when the vessel is impacted by another moving object, such as a car in traffic, or when the trailer strikes a fixed object. One recurring cause is low centering on
- grade crossings that are under construction.
Most upright releases are at low rates from weeping at cracked seams to running leaks in the range of one to five gpm. A one-gpm leak in a 2,000gallon compartment will last 33 hours. Rapid action to control the runoff, shut off ignition sources, and control vapor with foam will allow time to obtain appropriate sealing materials and set up to pump off the products in the damaged compartment. A bar of hand soap rubbed across a narrow crack will work until more sophisticated materials arrive.
(Photo at left courtesy of the Normandy (MO) Fire Protection District; photo at right by Jerry Patton, Robertson (MO) Fire Protection District.)
The highest risk to firefighters occurs during the leak-control phase. If firefighters must move to the leak, a foam cover on all spilled material must he maintained. To reduce the disturbance of the foam blanket, pallets, cribbing, or bags of sand can be used to provide a stepping path. If at all possible, firefighters should not enter the foam blanket. However, if firefighters must move through the foam, they should slide their feet to reduce product pickup and locate potholes and ditches.
Since foam cannot stop a vapor release from the flowing spill or extinguish a flowing fire, the safety team should be equipped with both a safety foam line on spray pattern and dry chemical extinguishers. Once the foam blanket is down, all water lines should be removed from the work /one. If ignition occurs, keep a foam blanket under the entry team as they leave the leak area and extinguish the
fire with dry’ chemical on the running product. Use caution with come-alongs, straps, air bags, and other systems to put pressure on a leak. Remember, these aluminum tankers are not rated for any vacuum.
- Failure or improper use of appliances ancl overfilling. Ruptured hoses, valve failures, and procedure errors while loading and unloading are the causes of many small releases. These releases generally are not reported and are absorbed or flushed by employees unless the incident involves a massive overfill or unloading with a plugged vacuum relief that will collapse a compartment.
Releases frequently can be stopped by proper use of emergency bottom valves on the leaking trailer or by shutting off the source of the overfill.
The safety release located behind the driver on the front left of the trailer will set the internal emergency valves on all compartments, stopping a release from a broken hose, defective delivery valve, or ruptured piping in the trailer-discharge system.
- Internal pressure from an ignition. This rarely occurs, due to the high vapor pressure of most motor fuels, control of ignition sources, and bottom loading.
Ignition of vapors inside a compartment requires oxygen and an ignition source in addition to the normally available vapors. This usually occurs when a compartment has been bottom-unloaded with no vapor recovery, which allows air to enter the vapor space from the top. If a highresistance product such as diesel fuel then is allowed to free-fall into the tank or if a spark occurs due to a charge from lightning, ignition can happen.
- External beat from an exposure fire. Exposure stress to an MC-306 can happen when the tractor and its fuel supply are involved, as was the case in the Highway-63 incident. Gasoline, with its high vapor pressure and low boiling temperature, requires very little outside heat to overpressure the relief devices.
As exposure stress comes from a radiant fire and direct flame contact, extinguishing the ground or exposure fire is extremely important, but so is cooling the top of the heated vessel. If cooling cannot be accomplished in time, the top will melt out to the liquid line, and boiling gasoline will expand the exposure fire.
In the Highway-65 incident, the exposure fire had almost died out from lack of fuel, but the contents of compartments 1 and 3 were so preheated that application of foam for the first 15 minutes had almost no noticeable effect on the flame plume. Foam had to flow over the top and sides of the burning compartments until the pressure and heat dropped enough to allow some buildup inside the compartments. Initially, any foam that dropped into a compartment was vaporized in the high-heat environment or broken down due to the boiling gasoline.
Lengthy application of agent before any change in flame appearance is normal for challenging, deep Class B fires. These can occur in melted vessels such as tankers, in storage tanks, and in industrial applications such as dip tanks. Key to the success of foam attacks on deep fires is this continued steady application of agent. This presents a particular challenge to structural firefighters, who normally fight Class A fires, when nozzle movement frequently increases extinguishing effectiveness.
(Photo courtesy of Western Taney Fire Protection District.)
A deep fire also severely stresses the foam agent. Here, we see the true value of a foam listed by a national testing agency and appropriate for the product involved. While wetting agents, high-expansion foam, and Class A foam may extinguish shallow spill fires in training environments, only agents specifically formulated for Class B fires will be effective in deep high-challenge incidents. Remember, each gallon of gasoline weighs about six pounds. At 20,000 Btus per pound of fuel, a 2,000-gallon compartment can release more than 240 million Btus. This is not the time for the cheapest foam.