Flash Point: Liquid Flash Points

How does a Liquid Flash Point work in practice?

In a liquid molecules can move freely. The speed of the molecules depends on the temperature of the liquid; the higher the temperature, the higher the speed. Very fast moving molecules can break-out and "escape" fromthe fluid; the liquid evaporates.

The flash point of a liquid is the lowest temperature at which the liquid can form enough vapor to form an explosive mixture. Therefore, the vapor of any flammable liquids present must be considered be very carefully!

Diesel and kerosene flash points

Diesel, kerosene and heating oil have relatively high flash points. This means that these substances can only produce sufficient vapor at relatively high temperatures to form an explosive mixture. This makes these liquids relatively safe. However, if a storage tank is outside in the sun, an explosive mixture can still form due to heating. The following table shows the flash points of some familiar liquids:

Flash points of well-known substances

Minimal ignition energy 

To ignite an explosive atmosphere, energy must be added. In principle, this can only be done in two ways, by a spark or by a hot surface. The minimum ignition energy or Minimum Ignition Current (MIC) is the minimum amount of energy required to ignite a specific gas or substance.

For gases, the small amount of energy of a spark (ignition energy) is often sufficient and the ignition temperature of a hot surface is far less critical. It is extremely difficult to ignite a dust cloud with a spark, it requires the greater power of a hot surface; so, for dust the ignition temperature is more important. 

Gases are classified according to their susceptibility to ignition by spark energy. A distinction is made between the following groups with their specific minimum ignition energies (the groups are often referred to as gas groups):

• IIA   200 μJ
• IIB   60 μJ
• IIC   20 μJ

Dust particle sizes

The sensitivity and the explosive power of a dust cloud strongly depend on particle size. In addition, electrical conductivity also plays a major role because it affects the risk of electrostatic charging. Non-conductive substances can become statically charged, causing spark ignition. This is a risk that shouldalways be taken into account.

Based on electrical conductivity and particle size, solids are classified into dust classes where the following three groups are distinguished:

• IIIA  non-conductive dust with particles / fibers > 0.5 mm
• IIIB  non-conductive dust with particles < 0.5 mm
• IIIC  conductive dust

Maximum experimental safe gap

If an exploding gas mixture makes its way out through a narrow gap, the burning gas mixture will cool down by contact with the metal contact surfaces. The energy level of the escaping gas decreases and the flame is extinguished. If the gas cools down sufficiently, it will no longer be able to ignite any explosive mixture outside. The degree of cooling (energy loss) largely depends on the length of the gap and the distance between the contact surfaces.

Measurement setup for determing the MESG

The Maximum Experimental Safe Gap (MESG) specifies the dimensions of an opening in which emitted gas loses sufficient energy, cools down, and is extinguished. The MESG value can only be determined by experiment. There is a very strong relationship between the Minimum Ignition Current and the Maximum Experimental Safe Gap.

Bulkhead fitting for Ex d enclosures 

The principle of cooling a burning gas mixture through the use of narrow gaps is applied in flame arrestors and flameproof housings. Read more about protection methods.

Ignition and smouldering temperature

A surface must have a certain minimum temperature in order to be able to ignite a gas, the so-called ignition temperature. This ignition temperature varies widely for the different types of gases:

Ignition temperature of the gasses

Temperature classes

To make things more manageable, a system has been devised whereby these ignition temperatures are classified into temperature classes. A temperature class is assigned to the gases or vapors and to the equipment that can be used safely with those gases.

Temperature classes and maximum surface temperatures

The ignition temperature (flash point) is a property of the gas present which leads to its classification into one of the temperature classes. To be able to work safely, a device must be suitable for use with the gas present and must therefore never be warmer than the temperature class of the gas indicates.

Example

An explosion-safe electric motor can reach a maximum temperature of 145 ºC under full load and therefore receives a T3 classification. This motor can be safely used in an environment with a T1, T2 or T3 gas because these gases have an ignition temperature that is above 200 ºC. The T3 engine will not get hot enough to ignite the gas.

Ignition and smoldering temperature for dust

For environments where flammable dust is present, two temperatures are important:

• The minimum ignition temperature of a dust cloud;
• The smoldering temperature of a dust layer.

The ignition temperature of a dust cloud is the lowest temperature of a hot surface at which the most explosive mixture of that dust can be ignited. 

The smoldering temperature of a dust layer is the minimum temperature at which a hot surface with a 5 mm dust layer on it will ignite. The smoldering temperature is also called the glow temperature.

The following rules apply to the choice of equipment in a dust environment with regard to temperature control:

• The maximum surface temperature T of the equipment may be no more than 2/3 of the minimum ignition temperature of the dust cloud;
• The maximum surface temperature T of the equipment must be at least 75 ºC below the smoldering temperature.

Properties of a number of flammable substances and fibers

Molecular weight 

The molecular weight of a substance indicates the weight of 1 mol (6.023x10²³ particles) of this substance. For example, 1 mol of nitrogen weighs 28 grams and 1 mol of oxygen 32 grams. Air is a mixture of different gases and has an average molecular weight of 29 grams/mol. 

With a molecular weight of 2 grams/mol, hydrogen is much lighter than air and will therefore disappear into the atmosphere almost immediately. A small hydrogen leakage is therefore relatively harmless. With a molecular weight of 16 grams/mol, methane (CH₄, natural gas) is also considerably lighter than air. However, most gases and vapors are 2 to 5 times heavier than air.