A Potentially Deadly Spread
When apple jelly appears, the fuel delivery system is toast
To most people, apple jelly is a sweet, lightly colored spread that is applied to breakfast toast. For many U.S. Air Force maintenance technicians, though, another kind of "apple jelly" is a nightmare, requiring extra work to clean fuel storage tanks and pipelines and to replace fuel filters. The military uses that term to refer to a contaminant, ranging from a thin, watery material to a thick, gooey brown gelatin that is frequently found in ground-based aviation fuel delivery systems.
A team of scientists at Southwest Research Institute (SwRI) spent a year solving the long-standing mystery of how and why this vexing substance finds its way into the fuel system.
The unusual appellation may be rooted in a mid-1980s presentation to a subcommittee of the American Society for Testing and Materials, referring to a contaminant found in the Alberta Products Pipeline (APPL) as "APPL jelly." However, the contaminant may be even older than its nickname.
The military, particularly the Air Force, has noted a similar contaminant in fuel storage and handling systems since the 1950s, when it was found in fuel controllers during flight tests of a new anti-icing additive. In those days, however, it occurred only occasionally and was considered more of an oddity than a maintenance problem.
Even today, thanks to careful maintenance and cleaning of ground-based fuel storage and distribution equipment, apple jelly seldom makes its way into an aircraft's fuel tanks.
Reports of apple jelly increased with the Air Force's conversion in the early 1990s from JP-4 to JP-8 as its primary aviation fuel. For safety and environmental reasons, JP-8 was given a new fuel system icing inhibitor, diethylene glycol methyl ether (DiEGME). Although the change made DiEGME a potential suspect, the new additive's known properties revealed no outward reason for suspicion and the apple jelly culprit remained a mystery.
In modern jet aircraft, icing inhibitors are added to prevent free water in the fuel system from freezing at high altitudes, potentially plugging the fuel lines and starving the engines of fuel. On the other hand, apple jelly can deactivate or disarm water separation filters and allow water to enter the ground-based fuel distribution infrastructure and, potentially, an aircraft's fuel tanks.
While apple jelly has never been known to cause an engine failure in an aircraft or ground vehicle that burns JP-8, it has increased maintenance costs by disrupting filtration and maintenance at some military fuel storage facilities. Specifically, it has caused more frequent maintenance procedures such as replacement of filters, cleaning of fuel lines and storage tanks, and removal of rust and other corrosion byproducts from the fuel distribution systems. Each report of an abnormality in the system requires a thorough investigation by maintenance specialists, further increasing maintenance costs and disrupting standard maintenance practices. To compound the problem, well-meaning technicians often erroneously identified as apple jelly such normal occurrences as contaminated water bottoms that are routinely removed from the system.
In late 2000, discovery of apple jelly in Air Force C-5 and C-17 cargo aircraft fuel tanks heightened official concern. In those cases, the apple jelly had prevented sensor probes from correctly measuring the quantity of fuel in the tanks.
Several Air Force studies failed to reveal the contaminant's chemical composition or the mechanism by which apple jelly is formed, although military scientists still suspected that the anti-icing additive may have been involved in some way. They struggled with the questions of why there were so many varieties of apple jelly with so many variations in color and consistency. (Significantly, the Army and Navy, which use different fuels and different fuel-additive treatment procedures than the Air Force, had not reported the same degree of apple jelly contamination.)
In 2001, the Defense Energy Support Center (DESC), the federal agency responsible for procuring and distributing fuel for the Department of Defense, asked SwRI to determine what apple jelly is, how it forms, and how it can be prevented or reduced. SwRI headed a team that also included Consulting for Energy Efficiency and Environmental Excellence (C4e) and Martin & Associates. SwRI provided project leadership, laboratory analyses and chemistry expertise, while C4e offered expertise on Air Force fuel-handling procedures, relevant past experience and contacts at Air Force installations. Martin & Associates provided knowledge of DESC procedures and assisted in gathering information from DESC databases.
SwRI team members from the Engine and Vehicle Research Division read everything they could about the problem and then requested samples of apple jelly from the field. In response, the team received 139 specimens and analyzed each for a number of compositional, physical and chemical properties. Test results were correlated to various properties of apple jelly. This approach had not been attempted previously; earlier investigations had involved only a very limited number of samples.
After a year-long study, the SwRI team confirmed that apple jelly is created when DiEGME interacts with water and other fuel components and forms a complex mixture that can have varying viscosity, color and other properties. Based on these analyses, prototypes of both thin and thick apple jelly were synthesized in the laboratory. The synthetic apple jellies exhibited physical and chemical properties consistent with samples submitted from the field.
One key discovery was an unexpected property of DiEGME: When combined with water, it forms a very aggressive solvent that can extract other components from the jet fuel and from the fuel filters themselves. The resulting contaminant thus can vary in color and consistency according to what combination of dissolved substances the fuel contains.
SwRI test results also showed that a chemical polymer component must be present to produce the thick, gooey variety of apple jelly. Although the identity of the actual polymer was not yet known, the results of the large testing program had identified many of the chemical and structural properties that this thickening polymer must have. Team members believed that the DiEGME/water blend might be extracting these polymers from the water-adsorbing fuel filters in some military fuel-handling systems.
This theory was further supported by an interesting observation that the SwRI team was the first to recognize: The thick variety of apply jelly occurred only in fuel systems where water-adsorbing filters were used.
SwRI researchers tested the polymer hypothesis by obtaining an actual fuel filter used by the Air Force and shredding it in the laboratory. After blending the filter's water-adsorbing material with DiEGME and water, the researchers saw a resulting product that was similar to the Air Force's most troublesome variety of apple jelly. The water adsorbing filter's manufacturer later confirmed that polyacrylate polymers were integral components of their products. Polyacrylate polymers were a perfect match for all of the chemical and structural properties predicted by the SwRI test data.
SwRI recommended improving fuel-handling procedures, installing new facilities to remove water from the fuel-distribution system, and ensuring proper mixing of additives in fuel. The Air Force has formed a technical group to study the Institute's findings and to recommend specific changes to its fuel-handling procedures and structures.
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Published in the Spring 2003 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.