Existent gum

Existent gums NF EN 26246 ISO 6246 ASTM D 381 Weight of evaporated residue... 

A number of laboratory tests are used to predict chemical stabihty. The amount of existent gum in a gasoline is determined by ASTM D381. This method involves evaporating a sample by a jet of heated air. The residue is weighed, solubles are extracted with / -heptane, and the sample is reweighed. 

The total is called unwashed gum and the insoluble portion is called existent gum. ASTM D4814 specifies that gasoline should contain less than 5 mg/100 mL of existent gum. Most gasolines contain less than 3 mg/100 mL. 

Another ASTM test method. Potential Gum (D873), combines the existent gum and the oxidation stabihty tests to measure potential gum. A sample of gasoline is subjected to the oxidation stabihty test for 960 min, filtered to remove particulates, and then subjected to an existent gum test. The potential gum is expressed as the total (unwashed) gum in this test. 

Other tests to predict stabihty of gasoline have been developed and reported in the hterature. One, developed by the U.S. military, stores gasoline at elevated (43°C) temperatures for up to 12 weeks and measures existent gum at the end of that period (26). Another measures existent gum in the presence of copper. The copper catalyzes oxidation and may be a better estimator of the stabihty of gasoline at high temperature/low residence time conditions. 

Specifications for gas turbine fuels prescribe test limits that must be met by the refiner who manufactures fuel however, it is customary for fuel users to define quality control limits for fuel at the point of delivery or of custody transfer. These limits must be met by third parties who distribute and handle fuels on or near the airport. Tests on receipt at airport depots include appearance, distfllation, flash point (or vapor pressure), density, freezing point, smoke point, corrosion, existing gum, water reaction, and water separation. Tests on delivery to the aircraft include appearance, particulates, membrane color, free water, and electrical conductivity. 

Another area of importance is contamination. Jet fuels are tested for the presence of heavier fuel contamination by use of an existent gum test, which detects the presence of heavier hydrocarbons from other products. Testing also is carried out to detect the presence of excessive levels of undissolved water and solids, as well as for surfactants that can adversely affect the ability of filters and coalescers to remove dirt and water from the fuel. 

Oxygen-containing impurities such as phenols and naphthenic acids can adversely affect water separation properties and initiate gum formation. No limit presently exists to control the amount of oxidized organic compounds found in jet fuel. However, tests for existent gums, neutralization number, and water separation indirectly limit the presence of oxygenated materials in jet fuel. 

The ASTM D-4814 gasoline specification states that an existent gum weight be measured. Existent gum is measured by ASTM D-381. This method is summarized by ASTM as follows ... 

Although other compounds may be found in the gum fraction, the components listed constitute the major portion of the existent gum volume. 

The ASTM D-4814 gasoline specification requiring an existent gum rating of <5 mg/100 mL of fuel has been established for all gasoline sold in the United States. A similar existent gum rating is in effect worldwide. If gasoline does not meet this specification, the addition of antioxidants will not reduce the existent gum level. Typically, existent gum levels cannot be reduced by chemical additive treatment. 

Presence of inorganic compounds in fuel can increase existent gums... 

Inorganic salts of organic compounds can increase existent gum... 

High residue in ASTM distillation may indicate a possible existent gum problem... 

Removal of existent gums from gasoline cannot be achieved by adding an antioxidant to fuel... 

Washing with water or caustic can be used to reduce and remove existent gum in some fuels... 

Clay filtration can be used to remove existent gum from fuel... 

Existent gum is the name given to the nonvolatile residue present in the gasoline as received for test (ASTM D-381, IP 131). In this test, the sample... 

Because gasoline contains traces of nonvolatile oils and additives, the residue left in the beakers is washed with heptane before the gum residue is dried and weighed. The existent gum test (ASTM D-381, IP 131) is useful as a refinery control but is to some extent unrealistic as a criterion of performance, and therefore engine tests have been developed to determine the tendency toward inlet system deposits. 

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