User Manual

30
Siemens Switzerland Ltd
Building Technologies Division
4. Extinguishing
4.1. Introduction
For a fire to exist, four elements need to be present:
• Oxygen to sustain combustion.
• Fuel to support the combustion.
• Heat to raise the material to its ignition temperature.
• A chemical reaction between the above three elements.
Extinguishing a fire implies interfering with the combustion process in such a way that the preconditions to
maintain combustion are no longer present. This can be achieved by removing any one of the first three elements
or by interfering directly with the combustion process. This results in the four basic extinguishing mechanisms:
• Cutting off the supply of oxygen (smothering).
• Physically separating the inflammable materials from the flames (starvation).
• Reducing the temperature of the inflammable materials or the flames (cooling).
• Use of chemicals to inhibit the combustion process.
4.2. Extinguishing agents
A variety of extinguishing agents are used, each of which utilizes one (or more) of the above mentioned
mechanisms.
4.2.1. Water
The most commonly used extinguishing agent is water. Water is not only an effective extinguishing agent; in most
cases it is readily available, ecologically acceptable and inexpensive.
The extinguishing effect of water relies not only on the cooling of the burning material but also on the
suppression of oxygen by the steam that is generated. Water has a very high latent heat of vaporization, which is
four times that of all other non-combustible liquids. Water is absolutely non-toxic and can be stored at room
temperature and pressure.
The boiling point of water (100 °C) is considerably lower than the temperature range where the pyrolysis
(thermochemical decomposition) of solid combustible material can be expected to occur (250 - 400°C). It
therefore permits the surface temperature of the fuel to be cooled to well below these temperatures.
However, water also has a number of disadvantages and can itself cause considerable damage to property. As it
boils at 100 °C and freezes at 0 °C, it can also lead to damage of any water-carrying pipes. Water is a conductor of
electricity, making its unsuitable for electrical fires. It is ineffective in extinguishing certain combustible liquids,
particularly those that are not water soluble or float on water (such as oil, petrol or diesel). Water is also
unsuitable for some hot metals (e.g. magnesium) or for chemicals. For such materials other extinguishing agents
must be used, for example foam, gas or powder.