Existent gum testing

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. 

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. 

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. 

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. 

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. 

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. 

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... 

Thus formation of this undesirable gum is strictly limited and is assessed by the existent and accelerated (or potential) gum tests. 

The existent gum value (ASTM D-381, IP 131) is the gum actually present in the fuel at the time of test and is measured as the weight of residue obtained after controlled evaporation of a standard volume of fuel. The accelerated gum test (ASTM D-873, IP 138) is a safeguard of storage stability and predicts the possibility of gum forming during protracted storage and decomposition of the antiknock additive. In this test, the fuel is heated for 16 h with oxygen under pressure in a bomb at lOO C (212 F) and then both the gum content and amount of precipitate are measured. 

Although the conventional (storage) stability of aviation fuel has long been defined and controlled by the existent and accelerated gum tests, another... 

D381 Test Method for Existent Gum in Fuels by Jet Evaporation ... 

Results of this test are reported as unwashed gum values or as washed gum values. The gums which are shown to exist in fuel can be either organic or inorganic in nature. 

Within this group, the linear polymers have been most intensively studied and researched. The main area of interest lies in water-soluble polymers. However, there have also been many studies into organic solutions, although there exist problems with odor and toxicity in the case of analytical studies. Among the water-soluble additives, tests were made especially on polyethyleneoxide (PEO), polyacrylamide and the coacrylates (PAAm, PAAm/AAcNa), polyacrylic add (PAA), guar gum (GG), carboxymethylcellulose (CMC) and sodium salts, as well as hydroxyethylcellulose (HEC). From these tests, PEO proved to be the most effective flow improver, followed closely by PAAm, which is somewhat more stable than PEO in turbulent flow. 

Other ways of expressing the uncertainty exist [43] as recalled by the GUM ( 6, page 22) e.g. replacing the above by a factor of 2 or 3, lead to the famous two or three. v tolerance limits. A factor of 2 is in fact equivalent to when the number of measurements n becomes high and corresponds to an approx. 95 M> level of confidence. A factor of 3 (approx. 99% level of confidence) is very cautious and may increase arbitrarily the final certified uncertainty of the material thus harming its interest for accuracy testing. 

There is however some evidence not in accord with this hypothesis. Experiments by Lai (1970), which included treatment of existing natural rubber gum-stock vulcanizates to both, separately, insert and remove poly sulphide links, did not show any dependence of strength on the nature of the cross-link. Mention may also be made of observations on carbon black loaded natural rubber vulcanizates cured by conventional accelerated sulphur and by EV systems (Des-ramault and Brydson, 1970). Plots of tensile strength against 300% modulus (the force required to extend the sample 300% in a conventional tensile test (not an equilibrium modulus) showed no dependence on the type of cross-linking. 

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