THE DEVELOPMENT OF FLASH POINT TESTING: A HIGHLY NUANCED AND EXACT SCIENCE
Safety is one of the most important considerations when handling various petroleum products due to their chemical nature and tendency towards flammability. The need for assessing the safety of liquid fuels, liquid lubricants, and their mixtures has led to the development of flash point testing. The flash point of a liquid is defined as the minimum temperature, corrected to a barometric pressure of 101.3 kPa, at which the vapors of the liquid will briefly ignite given a nearby ignition source. The term “flashing” of a liquid has been defined as when a flame appears and spreads itself across the vapor formed by the liquid [1]. In some asphalt samples, flame may not spread itself but be localized at the occurrence of flash point. There is a similar characteristic to flash point called the “fire point,” where the flame that is generated after the ignition of the flammable vapors is sustained for at least five seconds. At the flash point temperature, the vapor is flammable enough to ignite briefly but the flame is not usually sustained for more than five seconds. Although the flame is not usually sustained at the flash point, it is still a very important temperature to recognize because even the very brief ignition of vapors can lead to catastrophic results with regard to safety in transporting and storing the liquid petroleum products. In this article, we will go over the primary methods that have been developed to test flash point and highlight what makes them different from each other. We will mainly attempt to address the question of why there are so many different flash point testers and methods and why one method cannot be used to cover every different type of flammable product.
Abel Closed-Cup Test (ISO 13736)
In the UK during the 19th century, Parliament passed the Petroleum Act in 1862, which declared that liquids with a flash point temperature below 37.7°C were flammable. This was developed due to a number of fires resulting from kerosine blending with lighter hydrocarbons. Recognizing the necessity of accurate testing for flash point of various liquids to comply with the Petroleum Act, Sir Frederick Abel designed his own apparatus, which was established by Parliament in 1879. The apparatus is a closed-cup apparatus with a brass test cup of 14 mm thickness, 55-57 mm depth, and a sample size of around 78 mL. The close-fitting cover and stirrer are also made of brass and the heating vessel is made of copper. The heating device can be anything, such as a gas flame or electric heater, that is suitable to heat the vessel at the prescribed rate. The procedure describes a heating rate of 1°C/min and a stirring rate of around 30 rpm [2,3]. The manual version of this tester is shown in Figure 1.
Tag Closed-Cup Test (ASTM D56) The first method to be standardized by ASTM International
was the Tag Test, issued in 1918 by ASTM Committee D02. The apparatus was designed by an American scientist named Charles J. Tagliabue. The apparatus is a closed-cup apparatus with a brass test cup of 0.9 mm thickness and 54.5 mm depth, a lid made of any nonrusting metal, a heater of any type, and a liquid bath made of brass or copper. The test procedure calls for a heating rate of 1°C/min for lower flash points or 3°C/min for higher flash points. A sample size of at least 50 mL is required. It is important to note that there is no stirring in this method [2,4].
Cleveland Open-Cup Test (ASTM D92)
The Cleveland open-cup test was approved by ASTM in 1921. This apparatus consists of a test cup usually made of brass, a heating plate (as opposed to liquid bath), and either a gas or electric heater. The test cup does not have a uniform thickness, but rather the sides are 2.25-2.5 mm thick and the base is 2.8-3.5 mm thick. The procedure is to keep the heating rate at 5 to 17°C/min and then decrease the heating rate to 5 to 6°C/ min when the test specimen is around 56°C below the expected flash point. This method does not involve any stirring and the sample size is around 70 mL [2,5].
Figure 1. Manual Abel Closed Cup Tester (Koehler Instrument Company).
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