Role of Water Additives
The role of water in extinguishing a fire can be summarized as follows: The main effect is cooling down the fire by absorption of heat energy either at the fuel surface or in the gas phase. A contributing effect is diluting the atmosphere by adding vapor and thereby removing oxygen from the fire The main limits to the use of water are directly linked to the physical-chemical characteristics of water itself: - Water cannot be used on certain type of fires :
- Fires where live electricity is present- as water conducts electricity it presents an electrocution hazard
- Hydrocarbon fires - as it will only spread the fire because of the difference in density
- Metal fires - as these fires produce huge amounts of energy (up to 7.550 calories/kg for aluminum) and water can also create violent chemical reactions with burning metal (by oxidization)
- Fat fires - as vapour will carry and spread burning oil everywhere.
Since these reactions are well-understood, it has been possible to create specific water-additives which will allow:
- A better heat absorption with a higher density than water
- Carrying free radical catchers on the fire
- Carrying foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release
- Carrying specific reactives which will react and change the nature of the burning material
Water-additives are generally designed to be effective on several categories of fires (class A + class B or even class A + class B + class F), meaning a better global performance and usability of a single extinguisher on many different types of fires (or fires that involve several different classes of materials).
Read more about this topic: Fire Triangle
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