Propane as an Alternative Fuel for School Buses | Case Study

Propane is one of the safest and most environmentally-friendly fuels out on the market today. It is non-carcinogenic, non-corrosive and non-toxic. Propane is also environmentally safe and emits the lowest amount of greenhouse gases compared to other alternative fuels. If a fuel leak were ever to occur, the fuel would dissipate into the air and not cause an environmental hazard by pooling on the ground (like other fuels such as gasoline and diesel). Propane gas has been used as a motor fuel for more than 80 years, is the third-most-widely used fuel in the world and uses a low-pressure system. Proving that it has been true, tried, tested and is safe for its users. Here in this article, I will explain some of the safety factors of these propane buses, as well as, dispelling myths about the fear of propane tanks and explosions.

 

The Explosions Myth

When thinking about propane and safety, two words usually come to mind: explosions and danger. Hollywood has played a major role in our thoughts on this matter. With movies like Shooter and Casino Royale, they have us believing that shooting even a rock with a slingshot at a propane tank would make it explode. However, this is not the case. In fact, it is nearly impossible to blow up a propane tank.

Mythbusters has proven that on one of their episodes and the only way they could get a propane tank to explode was to use incendiary rounds (explosives).

 

Propane tanks are actually 20 times more puncture resistant than diesel or gasoline tanks because they are made from dense carbon steel. State police from Alabama have also proved that it is pretty much impossible to actually blow up a propane tank, and even just trying to puncture the tank itself involves a fair bit of effort and firepower. Even when police shot the container with multiple rounds from several different handguns, they were unable to puncture the tank; only when they shot at it with a rifle were they able to penetrate the tank. On the other hand, shooting at a gasoline tank only took one round from a handgun to puncture the tank.

 

They also proved that, even if the tank has sprung a leak or been punctured, it is hard to even start a fire. Due to propane being such a unique gas, it needs the perfect amount of oxygen mixed with propane to produce fire. First of all, liquid propane itself will not ignite. It has to be in its vapor state. Second, it has to have the perfect air to vapor ratio. Too little propane and the mixture is too lean. Too much propane and the mixture is too rich, thus the propane vapor will not ignite. The ratio must be no more than 9.6% of oxygen and no less than 2.2% oxygen.

Propane bottles are affixed with pressure relief valves. Their job is to bleed off the excess gas from the tank if the propane inside exceeds 80% of the tank volume. Propane gas expands when heated. If a tank were in the middle of a fire, the pressure relief valve would bleed off the excess propane as it expands from the heat. Assuming the tank was not expired, meaning the valve is in correct working order, the pressure in the tank would be relieved, hence, keeping the tank from exploding. For even more precaution, tanks are never overfilled. The tank itself only allows for it to be filled up to 80% of its capacity. This allows for some room for expansion if the temperature were to rise.

As you can see, it is more likely that the propane gas coming from the tank will start a fire first instead of exploding. These tanks are very strong and are always being vigorously tested, constantly improved and regulated to ensure the safety of its users.

 

Shut-off Valves

Not only are the tanks themselves strong, but the tanks are designed with shut-off valves in case a fuel line has been compromised. These valves are positioned strategically away from the potential damage that could occur in an accident or from rough roads. There are two manual shut-off valves on each propane tank, as well as, an automatic shut-off valve within the fuel system itself. This ensures that when the bus is turned off, the automatic valves will close off the fuel lines and bleed off the excess propane fuel back into the tanks.

 

Steel Cage

As an extra precaution, a steel cage/shield has been added to the bus to protect the tanks and valves themselves, to deflect any materials that may try to come in contact with the tank. It is mounted overtop of the tanks in-between the frame rails of the bus for maximum protection.

 

Safety Requirements By Law/ Inspections

Each bus has to meet National Fire Protection Association requirements as well as Federal and State certification requirements before they are allowed to be sold on the market. Which most companies even exceed these requirements. After the buses are sold, they undergo three more inspections before they can even carry students on the road. Therefore, ensuring the safety of the children riding the bus as well as the drivers.

If you want more info on using propane as an alternative fuel in vehicle fleets, check out this little slideshow found on the web!

 

Stay tuned for more!

 

Shelby - The Mechanic Doctor

 

 

 

 

 

Article researched and written by Shelby Livetsky

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