CONSTRUCTION INDUSTRY INFORMATION PORTAL
Homepage For Construction Industry Professionals
Information resources for construction industry employment and operations
Explosive Chemicals - How Dangerous are They?by Thomas Yoon
Enter at your own risk! The flammable hazard signs with their bright red flame graphic and danger wordings say it all. You are now entering a restricted area...
Many people have a natural fear of chemicals that catch fire easily. In fact, I know of some people who became so nervous whenever they have to come into the chemical areas that they lost their concentration and fumble about with their activities, visibly shaken. It is as if the whole place is going to explode in their faces anytime.
However, if this natural fear is allowed to paralyze us, there will not be any oil refinery, oil wells or even petrol stations anywhere. Industrial processes will literarily grind to a halt. People will be so afraid to handle these chemicals in their work.
This article tries to eliminate this natural fear by explaining the properties of flammable chemicals so that people will treat flammable chemicals with respect but not with fear.
Chemicals that are flammable will usually have a low flash point. What is this low flash point? It's the temperature at which the chemical will give out fumes sufficiently enough to catch fire when a lighted flame is brought near to it.
This means that a chemical having a lower flash point than room temperature will give out fumes capable of catching fire even though it is stored at normal room temperatures.
Thus, gasoline with flash point of -20 degree Centigrade will already be able to catch fire at normal room temperature if a light flame is present, while kerosene with flash point of 38 degree Centigrade will not burn when it is kept at a room temperature of 30 degree Centigrade.
Well, that's not totally correct either. In order to burn, three things must be present at the same time: fuel, oxygen and heat. When we talk about flash point, we are talking about the heat to generate sufficient gaseous fumes that can burn, but the chemical will not burn until a higher temperature is reached. That temperature is the ignition point.
The ignition point can be reached if a lighted flame is brought near to the combustible fumes, or it can be from a sparking electrical contact or even from sparks produced from mechanical impact. Very often, it can even come from sparks generated by static electricity.
Even when all these conditions have been reached, fire will not start if there is not sufficient oxygen to support the combustion. This is a very important factor to consider especially when storing flammable chemicals.
Have you heard of the expression, too lean mixture or too rich mixture? If you have been messing around with diesel or petrol engines, you will certainly know about this. Garage mechanics know that if the fuel in the engine is either too lean or too rich, the engine will not start. What does this mean?
Diesel and petrol engines burn fuel very rapidly. The combustion of the fuel is so rapid that explosions occur inside the engine cylinders rather that slow burning like in a gas stove.
The explosion of the fuel-air mixture is what gives it the power to move pistons that will turn the crankshafts of engines.
Back to the issue of getting the right combustible mixture...
There is a range in the fuel-air mixture ratio that is just right for explosions. If the ratio is out of this range, no explosion can occur. Too lean mixture is when the amount of fuel is not sufficient to burn. Too much air, too little fuel. When the fuel percentage increases further, the Lower Explosion Limit or LEL is reached. This is an explosive mixture.
If the fuel percentage increases further, it will reach a fuel-air mixture ratio that is too rich to support combustion, i.e. too much fuel, too little oxygen. That point is the Upper Explosion Limit or UEL. Both the LEL and UEL are expressed in percentage by volume.
So any percentage between the LEL and the UEL is within the explosive range. This is the explosive range that we have to control. In this range, fuel is present, and air is present. We have to be very careful not to have a spark or hot spot.
Let's have a hypothetical scenario. When a flammable chemical with low flash point is spilled in a room, the fumes will evaporate and fill the room. When the mixture has reached the Lower Explosive Limit, the whole mixture in the room is capable of exploding when a source of heat is brought in. This can be in the form of an electric spark, as when someone switches on the light.
If the chemical is left for a sufficiently long time, it will fill the whole room and saturates the atmosphere. The mixture then becomes too rich for combustion. It has reached the Upper Explosion Limit and the mixture will not explode even when a spark occurs. But this situation can turn dangerous when a person opens the door and switches on the light. The oxygen coming from the opened door may be just sufficient to bring down the too rich mixture to become an explosive mixture.
With this understanding, which do you think will be more dangerous: a tank full of flammable chemicals or a tank with only 1/4 full? If you are not sure, the answer is the 1/4 full tank. With both tanks having the same chance of being heated, the 1/4 full tank has more empty space that can contain oxygen whereas the full tank almost inevitably will be too saturated with the fuel and don't have sufficient oxygen to burn.
But what happens when a chemical in a full tank has to be pumped out?
Inert Gas Systems
On tanker ships, whenever crude oil or other flammable oil is pumped out, the space occupied by the oil must be replaced, otherwise, there will be a vacuum formed in the tank. This makes it impossible to pump the oil out further. To avoid atmospheric air from being sucked into the tank and creating an explosive mixture, inert gas is led into the tank at a slightly higher pressure than atmospheric.
This inert gas, containing mostly carbon dioxide and nitrogen, is generated from the burning of fuel in the steam boilers. This inert gas is pumped into the tank by means of blowers. The oxygen content in the exhaust gas must always be monitored. Usually it is around 5% and does not support combustion. To prevent corrosion and contamination of the oil, the exhaust gas is cleaned by passing them through a scrubber system. In this case, eventhough the tank may be nearly empty, the atmosphere above the chemical does not contain oxygen and there is no explosive mixture.
Explosion Proof Fittings
In these types of flammable chemical storage areas, care must be taken to avoid sources of heat that can trigger an explosion. To avoid sparking, flammable chemical storage facilities will have to follow certain safety standards with regards to installation of explosion-proof and intrinsically safe electrical fittings, lightings, wiring and earthing.
Handling of flammable chemicals need not be dangerous if we take the necessary precautions to avoid conditions that can cause fires to occur. So far, we have talked about engineering control only. The other aspect of control is people. They must be competent.
Prior knowledge, training and proper engineering design is necessary in order to avoid problems. Some data on the properties of combustible chemicals can be obtained at http://www.free-engineering.com/searchdata.htm
About the Author