Flammable Gas, Vapour & Liquid Testing

ASTM D 93 / ISO 2719 / ASTM D 3828

The flashpoint test is conducted to determine the lowest temperature at which a liquid will liberate sufficient vapour that will briefly ignite (flash).

Flashpoint measures the response of the test specimen to heat and ignite under controlled laboratory conditions. It is only one of a number of properties that must be considered in assessing the overall flammability hazard of a material. Flashpoint is used in shipping and safety regulations to define flammable and combustible materials and classify them. Flashpoint will also indicate the possible presence of highly volatile and flammable materials in a relatively non-volatile or non-flammable substance.

There are several closed cup methods used at Sigma-HSE to determine the flashpoint of a gas or vapour.

A test specimen (75 ml) is introduced into the test cup of the apparatus and the lip assembly is attached. A sample is heated at a rate of 1.0 to 1.6°C per minute with continual stirring at 250 rpm. A small flame with a diameter of 3.2 to 4.8 mm is directed into the cup containing the sample, at regular intervals, with simultaneous interruption of stirring.

Testing is started at 23 ± 5 °C below the expected flashpoint (or at ambient if unknown) with the source of ignition applied at 1 °C intervals, for temperatures up to 110 °C, and 2 °C for temperatures above 110 °C. Further trials on fresh materials are conducted starting 23 ± 5 °C, below that of the lowest ignition temperature observed in the initial trial. The flashpoint is the lowest temperature at which the application of a flame results in the ignition of vapour above the sample.

A test specimen (2ml for temperatures up to and including 100 °C, 4 ml for temperatures above 100 °C) is introduced into the test cup of the apparatus that is maintained at the expected flashpoint (or ambient temperature if the expected flashpoint is not known). After a specified time (1 minute for temperatures up to and including 100 °C, 2 minutes for temperatures above 100 °C) a source of ignition is applied, and a determination is made whether or not a flash occurred.

The test specimen is removed from the test cup; the test cup and cover are cleaned, and the test temperature is adjusted to 5°C lower or higher depending on whether or not a flash occurred. A fresh test specimen is introduced and tested. This procedure is repeated until the flashpoint is established within 5°C. Finally, the procedure is then repeated at 1°C intervals until the flash point is determined to the nearest 1°C.

Many products containing small quantities of materials with a low flashpoint can still dominate the flammability characteristics, especially flashpoint. Even in an aqueous solution, materials of high volatility will be driven off at low temperatures. Equally, a mixture of materials with similar volatility characteristics and differing individual flashpoints can produce an averaging scenario.

Therefore, assumptions are far from ideal when assessing liquid mixtures for flammability. Testing is the only way to confidently ensure a flammable atmosphere is either avoided or understood.

ASTM E681 / ASHRAE 34

This test is conducted to determine the lower and upper flammable concentration limits of chemicals that have sufficient vapour pressure to form flammable mixtures at atmospheric pressures at specific temperatures.

This test can be conducted on vaporised liquids and gaseous materials. Refrigerant gases are tested to ASHRAE 34, using the method detailed within ASTM E681, with specific ignition criteria and air moisture content.

The Lower Explosive Limit (LEL) or lower flammable limit (LFL) is the minimum concentration of a combustible substance that can propagate a flame in a homogeneous mixture of the combustible material and the air, under the specified conditions of the test. Generally, 1% v/v of the material is added to a glass vessel under a vacuum. Air is allowed back into the system and the material and air are mixed before an electrical source of ignition is activated, and observations for flame propagation are observed. The material is then incrementally increased in 1% steps until the ignition is observed and the LEL is defined.

Upper explosive limit (UEL) or upper flammable limit (UFL) is the maximum concentration of a combustible substance that can propagate a flame in a homogeneous mixture of the combustible material and air, under the specified conditions of the test. Once the Upper Explosive Limit has been established, a theoretical Upper Explosive Limit is calculated, and testing is started at a concentration above.

The material is added to a glass vessel under a vacuum, air is allowed back into the system and the material and air are mixed before an electrical source of ignition is activated, and observations for flame propagation are observed. The material is then incrementally decreased in 1% steps until ignition is observed and the upper explosive limit is defined.

To ensure that avoiding a flammable atmosphere as a ‘basis of safety’ is accurately conducted and monitored and to make sure that flammable limit alarms and detection systems are correctly set. A mixture of materials cannot simply be based upon worst-case constituent values, therefore testing of the product is essential.

ASTM E659

The auto-ignition temperature (AIT) test is conducted to determine the lowest temperature at which a substance will produce hot-flame ignition in the air at a specific atmospheric pressure, without the aid of an external energy source such as a spark or flame. It is the lowest temperature at which a combustible mixture should be raised so that the rate of heat evolved by exothermic oxidation reaction will overbalance the rate at which heat is lost to the surroundings and cause ignition.

The test is predominantly conducted on liquids, however gases and solids are capable of full vaporisation should be tested using the method below:

Working through a range of material concentrations, the material is injected into a heated and thermally stabilised, 500 ml round bottom flask. Once an ignition temperature is observed, the flask temperature is reduced, and the material concentration is varied until an auto-ignition temperature is established.

The auto-ignition temperature is required to define the maximum permitted surface temperature of electrical and non-electrical equipment used in areas where potential testing material is present. This allows for the correct and appropriate designation of equipment based on their temperature or “T” Class.

Many products are incorrectly given an auto-ignition temperature based either on untrustworthy literature or on its worst-case component (component with the lowest auto-ignition temperature). Other inert or less sensitive components within the product can greatly affect the overall auto-ignition temperature of the product. These products will usually produce a higher auto-ignition temperature.