Fuel Stability

Today's commercially available diesel fuels and heating oils can deteriorate rapidly for a variety of reasons. Degradation and oxidation reactions that occur when fuel is thermally stressed or stored for long intervals are complex chemical changes. These changes lead to deposits or sediments from certain hydrocarbons and traces of naturally occurring nitrogen and sulfur containing compounds in the fuel. In addition to fuel composition, environmental factors directly influence the rate at which these processes proceed.

Hydrogen treating of fuel to meet low sulfur diesel fuel requirements generally improves stability, however, since sulfur is more easily removed than nitrogen, many low sulfur diesel fuels still have potential for instability under a variety of use conditions.

Diesel fuel is increasingly being used as a coolant for high-pressure fuel injection systems with high temperature fuel wetted walls, which can thermally stress the fuel. This thermal stress, along with an increase in recirculation fuel temperature, is often responsible for fuel degradation and the formation of sediments, which can cause fuel flow restriction through filters and injection systems.

Heating oil is subject to long periods of storage while exposed to oxygen and degradation catalysts. Complex reactions, occurring between oxygen and trace fuel components, can generate fuel particulate that eventually becomes the sludge found in fuel tanks, fuel lines and fuel filters.

In the field, these products can cause deterioration of fuel pumps and injector performance. Degradation products also lead to filter plugging, fuel line restriction, nozzle fouling and deposit formation.

The following graph typifies this type of problem nationwide. To obtain this data, fuel samples were gathered from around the country and tested in an industry accepted bench test to determine the thermal stability of each fuel. The graph also indicates the type of corrective action that is easily taken to correct the problem.

This Graph shows how dramatically Octel Starreon Performance Plus Additives, Fuel Oil Additives and Stabilizers boost fuel stability.
 

Thermal Stability Analysis

Accelerated Fuel Oil Stability Test (ASTM D-6468, Octel Starreon F21) is a test method that determines the relative instability of a fuel subjected to a thermal degradation process. The test is significant since the fuel is exposed to an environment similar to actual operating conditions where the fuel cools the injectors during engine operation. The fuel is forced to the injectors and a only a small portion is injected. The balance of the fuel cools the injector and returns "hot" to the fuel tank. This test is currently one of the test methods specified by the N.C.W.M., the T.M.C. and the E.M.A. for defining one criteria of a Premium Diesel Fuel.

During the test, prior to heating, the fuel is passed through a filter pad to trap any solid material that might already be present in the fuel. The fuel is then subjected to a heating process, which accelerates chemical reactions naturally occurring in unstable fuels.

The newly created fuel degradation by-products, in the form of insoluble gums and solid particulate matter, are then trapped as the fuel is passed through another clean filter pad. The pads are evaluated measuring the percent of reflected light using a specialized photometer. The lower the percent reflectance, the heavier and larger the deposits. For proper performance, a fuel should not have a percent reflectance of less than 80% after aging for three hours at a temperature of 302° F (150° C). 

Oxidative Stability Test

Accelerated Fuel Oil Stability Test, Octel Starreon F31-81 is a test method that determines long term storage stability of distillate fuels such as home heating oils, kerosene, or diesel oils under moderately accelerated conditions within 7 to 14 days. The test method is not suitable for highly volatile fuels with flash points under 38°C (100°F). The test method is particularly useful to determine relative storage stability; for example, with and without stabilizing additives. 

For most distillate fuels, the F31 test run for 7 days aging at 80°C (175°F) is equivalent to storage for 4-8 weeks at 43°C (110°F), or 4-8 months at typical ambient storage temperatures. The F31 aging time should be considered accordingly. Storage intervals of 7 or 14 days are usually adequate.

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