Difference Between Flash Point and Fire Point

The key difference between flash point and fire point is that the flash point describes the lowest temperature at which the ignition of a substance initiates whereas the fire point describes the lowest temperature at which the fuel continues to burn for a short time period after the initiation of the ignition.  

All flammable liquids have a vapour pressure that increases with its temperature. The concentration of evaporated liquid in the air increases with an increase in vapour pressure. Different flammable liquids require different concentrations in the air to sustain combustion. Here, the flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. However, the vapour stops to burn if we remove the source of ignition. Whereas, the fire point is the temperature at which vapours of the flammable liquid continue to burn after being ignited even after we remove the source of ignition. However, both the flash point and fire point have no relation to the temperature of the source of ignition.

CONTENTS

1. Overview and Key Difference
2. What is Flash Point
3. What is Fire Point
4. Side by Side Comparison – Flash Point vs Fire Point in Tabular Form
6. Summary

What is Flash Point?

Flash point is the lowest temperature at which the vapours of a volatile material ignites when we provide an ignition source. This temperature does not depend directly on the temperature of the ignition source. However, usually, the temperature of the ignition source is considerably higher than the flash point. Also, every flammable liquid has a vapour pressure that increases with an increase in temperature. When the vapour pressure increases, the concentration of vapour in air increases. A flammable liquid requires a certain concentration of vapour in the air to sustain combustion in air, which is specific for flammable liquid (or a combustible liquid). Hence, the flash point gives the lowest temperature at which there is enough vapour in the air to induce the ignition of that fuel.

We can measure the flash point using either “open cup apparatus” or “closed cup apparatus”. This parameter is very important in distinguishing flammable fuels such as petrol and diesel. Also, it is important to characterize the fire hazards of fuels. For instance, if the flash point of a fuel is less than 37.8 °C, then that fuel is flammable. If the flash point is above that temperature, then we name the substance as “combustible material”.

For example, gasoline (petrol) is important as a fuel in automobiles. Where, this fuel requires preheating above its flash point and mixing with air before a spark from spark plug causes it to burn. Therefore, petrol needs to have a low flash point and high auto-ignition temperature. On the other hand, there is no ignition source in the case of diesel. Hence it requires having a high flash point and a low auto-ignition point.

What is Fire Point?

Fire point of a fuel is the lowest temperature at which the vapour of fuel can continue to burn for at least five seconds after the ignition has started. It means; the term fire point describes that it is the lowest temperature for a substance to hold the combustion for a small time period after the ignition by an open flame.

Usually, the fire point of a substance is about 10 °C higher than the flash point of the same substance. We can measure the fire point of a substance using the “open cup apparatus”.

What is the Difference between Flash Point and Fire Point?

The flash point and fire point are two very important features of fuels. These terms describe the initiation and the continuity of the combustion of a fuel. Hence, the key difference between flash point and fire point is that the flash point describes the lowest temperature at which the ignition of a substance initiates whereas the fire point describes the lowest temperature at which the fuel continues to burn for a short time period after the initiation of the ignition. Furthermore, we can find a difference between flash point and fire point based on their values as well. That is; the fire point is always a higher value than the flash point. In general fire, point is about 10 degrees higher than flash point of flammable liquids.

Summary – Flash Point vs Fire Point

Flash point and fire point are important characteristic parameters for flammable or combustible liquids. The key difference between flash point and fire point is that the flash point describes the lowest temperature at which the ignition of a substance initiates whereas the fire point describes the lowest temperature at which the fuel continues to burn for a short time period after the initiation of the ignition.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “Fire Point Definition.” ThoughtCo, Jun. 22, 2018. Available here
2. “Flash Point.” Wikipedia, Wikimedia Foundation, 18 Sept. 2018. Available here 

Image Courtesy:

1.”Flaming cocktails”By Nik Frey (niksan) (CC BY 2.5) via Commons Wikimedia 
2.”Fire Point Sign” by George Hodan (CC0) via PublicDomainPictures.net