What is the gum content?
The quality of different fuels, e.g. aircraft fuels and motor gasoline, and other volatile distillates can be verified by the amount of contaminations. The gum content is the nonvolatile residue that is left after the evaporation of the sample under controlled conditions. This residue can be a result of, e.g., higher boiling oils that were not removed during the downstream process, or particulate contaminations, which are impurities resulting from poor handling of the fuel. Gum can also be caused by chemical reactions of some fuel components with each other or with oxygen. This is the reason why stabilizing agents, such as oxidation inhibitors, are added in order to increase the fuel stability.
Why is measuring the gum content important?
The gum content can be used to judge the quality of various products. The amount of gum indicates the condition of the sample at the time of the measurement. Large quantities of gum are an indicator for the contamination of fuel caused by higher boiling oils or particles. Since the gum content can change according to the age of the fuel and the exposure to oxygen, the reported content is only true at the time of measurement.
Generally, the gum content reflects inappropriate production processes and poor fuel handling in the refinery or the storage facility. The amount of gum should be as low as possible since the use of fuels with high gum contents can lead to deposits in induction systems or cause intake valves and fuel injectors to stick.
How can I measure the gum content?
The general procedure for the determination of the gum content in fuels is described in the following standards: ASTM D381 (“Standard Test Method for Gum Content in Fuels by Jet Evaporation”), ISO 6246 (“Petroleum products – Gum content of fuels – Jet evaporation method”), and IP 540 (“Determination of the existent gum content of aviation turbine fuel – Jet evaporation method”).
During the test procedure, a measured quantity of fuel (50 mL) is evaporated under controlled temperature conditions by a constant flow of either hot air or steam, depending on the sample. The test methods ASTM D381 and ISO 6246 require the use of steam as the evaporating medium for aviation turbine fuels. The use of air, on the other hand, is used for testing motor gasoline. In contrast to the ASTM and ISO method, the IP 540 method allows both air or steam as the evaporating medium for examining aviation turbine fuels but requires air for testing motor gasoline.
The beaker containing the sample is weighed before and after the evaporation to determine the weight of the gum content, which is then reported as milligrams per 100 mL. For motor gasoline an additional treatment is necessary: The residue is washed with heptane after the evaporation to remove any additives the gasoline was initially blended with. In this case, the beaker containing the residue is weighed before and after the washing procedure.
Limiting values for gum content
According to the fuel specifications ASTM D1655 (“Standard Specification for Aviation Turbine Fuels”) and ASTM D4814 (“Standard Specification for Automotive Spark-Ignition Engine Fuel”), the limitations for the gum content are 7 milligrams per 100 mL¹ for aviation turbine fuel and 5 milligrams per 100 mL² for motor gasoline.