Mastering the Hazard: Your Guide to Controlling Solvent Ignition Conditions

Understanding the conditions for combustion is fundamental to industrial safety. Ignition of a combustible substance only occurs when it is mixed with an oxidizer in the right proportions and has sufficient energy. Therefore, the ignition hazard of a solvent is determined by key factors, including its combustion limits and flash point. This article details the science behind these ignition hazards, explaining how a solvent's vapor concentration, combined with heat, determines its potential for fire or explosion. By providing a clear breakdown of combustion ranges and a comprehensive table of data, this guide aims to help professionals understand and manage the risks associated with various flammable solvents.

Combustion requires a combustible substance to be mixed with an oxidizer in appropriate proportions and to obtain sufficient energy to occur. If these three conditions are not met simultaneously, ignition cannot occur. Therefore, the ignition hazard of a solvent is determined by the following factors:

Combustion Limits

Flammable solvents, under certain temperatures and pressures, form combustible mixtures (explosive mixtures) with air or oxygen when their vapors mix with these gases. If the composition of the mixture is outside a certain range, even if a large amount of energy is supplied, ignition will not occur. The range of possible compositions (concentrations) for ignition is called the combustion range or explosion range, and the composition limits are called combustion limits or explosion limits. The lowest concentration at which solvent vapor mixed with air reaches a certain concentration range and ignites or explodes when exposed to a heat source is called the lower limit; the highest concentration is called the upper limit. Concentration is expressed as the volume percentage (vol%) of solvent vapor in the mixture. If the concentration is below or above this range, no explosion will occur.

Flammable solvents all have a certain explosion range, and the wider the explosion range, the greater the danger. For example, the lower explosion limit of acetylene is 2.5%, and the upper limit is 80%; the lower explosion limit of ethane is 3.22%, and the upper limit is 12.45%. The following table lists the explosion limits (combustion limits) of various gases and vapors in air.

Explosion limits (combustion limits) of various gases and vapors in air

Flash point

The flash point indicates the minimum temperature at which the vapor-air mixture on the surface of a flammable liquid ignites upon contact with a flame. There are two methods for determining the flash point: the open cup method and the closed cup method. The former is generally used for liquids with high flash points, while the latter is used for liquids with low flash points.

Ignition point

The ignition point, also known as the flash point, is the lowest temperature at which a flammable liquid, when heated, causes the vapor-air mixture at its surface to ignite immediately upon contact with a flame and continue burning.

The flammability and explosiveness of solvents

Some solvents are highly flammable, while others are prone to explosion or explosive decomposition at normal temperature and pressure. Still others require a strong heat source to explode. The flammability and explosiveness of solvents typically depend on the following conditions:

1. Low boiling point and high volatility, making them prone to evaporation at normal temperature and pressure;

2.  Low flash point;

3. The solvent vapor can form an explosive mixture with air;

4. The density of the solvent vapor is greater than that of air. A low flash point indicates a high risk of ignition, but the flash point does not refer to the temperature at which the solvent continues to burn; it merely indicates that the vapor at the liquid surface is flammable. For continuous combustion to occur, vapor must be continuously produced. The temperature at which combustion continues is typically about 10°C higher than the flash point. In general, solvents with a low flash point are flammable solvents.

When using flash point, ignition point, and explosion limits to indicate the fire hazard of a solvent, the following points should also be noted:

1. Ethers can form explosive peroxides;

2. Non-flammable solvents or carbon tetrachloride can react explosively when in contact with metals such as sodium, potassium, calcium, magnesium, and barium;

3. Trichloroethylene reacts with sodium hydroxide or potassium hydroxide to form dichloroacetylene, which can explode due to self-oxidation;

4. Nitro compounds (even mononitro compounds) are explosive.

Precautions for using flammable solvents

Flammable solvents pose risks of ignition, combustion, and explosion, so precautions must be taken when using them.

1.  Solvents and solvent vapors must be stored in sealed containers;

2. Solvents and solvent vapors must be kept away from heat sources. Since solvent vapors are heavier than air, they can easily reach explosive limits in low-lying areas, so extra care must be taken to keep them away from heat sources.

3. Ensure adequate ventilation in the workplace. Since solvent movement can lead to static electricity buildup, equipment should be grounded.

4. Containers should be protected from direct sunlight and should not be stored in high places.