September 30, 2008 by Richard Kooren
The recent catastrophic explosion and fire on Aug. 10, 2008 at the Sunrise Propane Industrial Gases facility in Toronto, Ont. has raised many questions about the storage, handling and use of propane. In Canada the installation and use of all propane fired equipment, including propane fuelled vehicles is regulated by B149.1-05: The Natural Gas and Propane Installation Code. Facilities such as the Sunrise plant are regulated by B149.2-00: Propane Storage and Handling Code. In Ontario, these codes are enforced by the Technical Standards and Safety Authority (TSSA). It is their responsibility to ensure that all bulk storage and distributors, refill facilities and employees who handle the refilling and any person who installs or repair propane fuelled equipment are properly licensed.
Propane exists in atmosphere as a gas. The cylinders used to fuel barbeques contain both liquid and gaseous propane; this is because the gas is compressed into a liquid to minimize the size of the tank needed to store the propane. This is the same as the butane disposable lighters. Butane is similar in that it is stored as a liquid and vapourizes when released from its container. The boiling point of propane is -42oC so it is easy to see how readily this will change from a liquid to a gas. Unlike natural gas, propane is heavier than air and the gas will settle on the ground and flow like water, seeking the lowest point. This becomes critical in a spill.
One pound of liquid propane produces 8.5 cubic feet of propane gas. The 20 pound propane cylinders used on the barbeques holds 170 cubic feet of propane gas, but because it is holding a liquid, the cylinder is only roughly two cubic feet in size.
The explosive (flammable) range of propane is 2.4-9.5 per cent gas in air, which means the gas/air mixture has to fall within this range in order for the propane to burn. That 170 cubic feet of propane stored on the back deck would expand to a cloud of 1,800-7,100 cubic feet flammable gas/air mixture. Safely stored inside the 20 pound cylinder or 2,000 gallon storage tank, the propane cannot be ignited nor will it burn. It becomes a hazard when it is released into the atmosphere and comes in contact with an ignition source.
Propane storage sites can range from a single vertical or horizontal tank at a local gas station or retail store, to large facilities such as the Sunrise plant. Regardless of the size of the tank, the propane stored inside these containers is safe. The tanks will not explode because of the previously discussed flammable properties. The risk occurs when there is a leak or if the tanks are exposed to an external fire.
Propane tanks, the 20 pound cylinders and even the small 16 ounce cylinders used for torches or camp stoves will BLEVE if the cylinder cannot vent the pressure build-up inside the container. A BLEVE is a Boiling Liquid Expanding Vapour Explosion and this occurs when the container is exposed to an external heat source, typically a fire. The liquid propane begins to expand and become a gas, increasing the pressure inside the container. All propane cylinders have some type of pressure relief valve, which opens at a preset pressure to relieve this build-up. Of course, sometimes this venting adds propane to the already burning fire, but as long as the container is venting pressure inside is controlled. When the pressure build-up increases at a rate that cannot be controlled by the relief valve, then internal pressure rises until the cylinder or tank fails with sometimes catastrophic results. The sudden release of the propane, which immediately changes to a gas and mixes with air, will ignite and burn or explode if in a confined area. A 20 pound cylinder can create localized damage and be projected some distance. A 200 gallon tank, which could be found at a farm or a rural home, can explode with such intensity that buildings can be destroyed and injure or kill anyone nearby. Several years ago firefighters in both the United States and Canada were killed by such an incidents while trying to control fires impinging on a propane tanks.
The regulations and procedures required of these storage facilities are an attempt to minimize the potential risks. There are safety features which provide remote shut-offs to isolate the flow of propane should a leak occur and of course there are the pressure relief devices. Mistakes do happen and at other times regulations are violated. There are reports of company’s performing truck-to-truck transfers, which is a prohibited practice and a violation of the code. This method increases the risk of a leak or, worse, a hose separation and because these are not permanent fixtures they do not have some of the required remotely controlled safety devices.
Propane use in a residential property
In most suburban areas, the most common use of propane is to fuel a barbeque or other small portable appliance. This can be anything from an outdoor mosquito repellent device or patio heater, which use the 20 pound cylinders, to something as small as a soldering torch, camp stove or weed burner, all of which can use the small 16 ounce non-refillable cylinders. All of these cylinders have the potential of exploding (as a BLEVE) if exposed to a fire. The larger the cylinder the greater the explosive force. While common sense dictates that the larger cylinders should be stored outside or in a ventilated shed, a number of homeowners store pool equipment (pumps and heaters) in the same shed as the lawnmower, gas can and barbeque. The small cylinders are typically stored in the house in the workshop or with the camping gear.
In the rural areas, many residences use propane as their primary heat source, where natural gas service is not available. Now, instead of 20 pound cylinders there are 200 gallon tanks. These are typically located away from the building, but there are situations where smaller tanks are utilized and are placed against the side of the house. On farms it is not unusual to find large tanks situated in close proximity to a building.
The lines between these tanks are buried and not always deep enough to avoid damage from a garden tool or fence post stuck in the ground. If a propane line is nicked or cut the result may not be readily apparent. The gas will follow the path of least resistance. This could be the grass above, in which case the mercaptan will begin to kill (poison) the grass or vegetation, or follow the line or any other pipe to the house. Many years ago there was a situation where the septic pipe was crushed and the effluent corroded the propane line. The leaking propane entered the septic line and followed it back into the house where it accumulated and eventually exploded literally blowing out almost all of the exterior walls while the occupants were asleep. Fortunately, none of the occupants were seriously injured.
Obviously, during a fire, one of the priorities of the fire department is to keep these tanks cool in order to prevent a BLEVE. Once the fire is out and the fire department has left, it is assumed that the propane has been shut off. This may not always be the case. Alternately, smaller cylinders that were in the house or garage may be leaking due to damage to the valves from fire exposure. If the odour of rotten eggs is detected, the area should be evacuated immediately.
Propane, like natural gas, has no natural odour and is colourless. An odourant has to be added so that it can be detected. Mercaptan is the chemical which is added to these products and, while it is also flammable, it has a pungent odour that is readily detected at low levels. The code requires that mercaptan be detected at propane concentrations as low as 20 per cent of the lower explosive level (2.4 per cent). This means that at concentrations as low as 0.5 per cent propane in air the smell of rotten eggs (mercaptan) should be detected. At this concentration, no fire or explosion should occur since the concentration is too lean to ignite.
However, since propane gas is heavier than air, it will accumulate at floor level. Thus, while the concentration detected at nose level, some five feet above the floor, may be a lower explosive concentration, the concentration at floor level could be significantly higher. The gas does not have to mix to a homogeneous mixture if left undisturbed. Rather, it can form a layer that is a high concentration at floor level and decreases with height above the floor. Thus, it is important to test or smell close to floor level. A person could enter an area and not smell anything initially and while walking through the area disturb the layer on the floor and now suddenly smell the mercaptan.
Following an incident, ensure the propane line has been disconnected from the propane storage tank, and the gas line shut off valve is locked off (this is usually done by the local utility). This at minimum isolates the propane supply from the building. If gas is detected inside a house, then the hydro service should be shut off from the outside by having the meter pulled. Then, and only then, should the building be ventilated. If the building is ventilated first, the gas concentration could be above the explosive range and by introducing air the gas concentration is now lowered into the explosive range. This was a hard lesson learned in Winnipeg a few years back when a large gas leak occurred inside a building and exploded while being ventilated by the fire department who had failed to isolate the power inside first.
In summary, propane can be stored and handled safely if reasonable precautions are taken in the handling of the fuel. Unsafe or unacceptable practices can result in fires and explosions which can result in significant damage to property and the loss of lives.
Richard Kooren, P. Eng., CFEI, CFII, CCFI-C, is the vice-president and manager of the field offices at Origin and Cause. He has been a licensed professional engineer for over 20 years and is a designated consulting engineer.
Propane can be stored and handled safely if reasonable precautions are taken in the handling of the fuel.