Aviation Fuel Particle Detection (Continued)

Parker Hannifin (UK) Ltd

Please click here to see the begining of this article on Aviation Fuel Particle Detection.

Furthermore, microbial contamination will generally occur wherever moisture is present in hydrocarbon fuels, and where the liquid remains relatively undisturbed for periods of time.  Typically therefore algae will grow in fuel farm storage tanks and even in areas of pipework and distribution systems where the structure of the system causes eddies or slow moving zones of liquid.

There are currently no operational limits on the content of microbial contamination and even the experts in the industry hesitate to put numbers to such a limit. Each case has been cause for individual investigation and if necessary, treatment. Ongoing actions include ASTM D-6469 Standard Guide to Microbial Contamination in Fuels and Fuel Systems, (2003) IP Guidance document update, and IATA Microbiology task Force.

Finally, dispersed solids in fuels can be caused by the ingress of dust, pollens and debris during the process of fuel transfer or by poorly maintained storage tanks, while particles of corroded metal can easily find their way into the fuel at any stage of the fuel handling process.  In each case, the results can include blocked or partially blocked fuel ways, additive depletion, deposition in storage, equipment failure from wear, and premature filter blocking.

Clear and Bright / Gravimetric Tests

To date, all jet fuel specifications have required an assessment of fuel appearance, such as the Clear and Bright Visual test method (IP216 or ASTM D2276) and the Gravimetric test method (IP423 or ASTM D5452). The earlier requirement may be summarised in the words of a previous Joint Checklist: Fuel should be clear and bright and visually free from solid matter and undissolved water at normal ambient temperature.

All current testing methods carry some risk. For example, Clear and Bright test is subjective and not quantitative, in that many contaminants are undetectable by the human eye, and normally detects only gross contamination above 30 ppm free water. Clear and Bright is also subjective as the eye can only detect particles above 40 microns unless present in very large amounts; aviation tolerances are of course much lower.

Although filtration systems have been developed and installed in order to remove solids, water and other contaminates, with fuels on average being filtered up to 14 times before being burnt, it is essential that contamination be measured accurately to ensure the safety of the engine systems in which it is being used. The science of dirt and water removal from jet fuel includes filtration processes and nominal/absolute filter ratings, dispersed water coalescence such as coalescence mechanisms and coalescer disarming, and water removal by absorbent media. Practical devices include filter/water separators, filter monitors and microfilters.

Coalescence is the process of joining together two or more droplets of the same fluid, separated by another immiscible fluid. One of the fluids must be a liquid; the other can be a liquid or a gas. Auto-coalescence entails droplet to droplet interactions, while fibre-bed coalescence entails droplet to fibre interactions. Good coalescence produces small droplets, Clear and Bright. If a coalescer is disarmed by surfactants, the released droplets are micronic in size and produce a haze.

Particulate methods comprise gravimetric or filtration time testing, both of which are laboratory methods and typically use 0.8 micron filters. In the former case, filters are then dried in an oven and weighed, and in the latter case, time to filter under gravity is measured, or the volume of fuel that passes in a given time. Filter efficiency decreases with use and filters need maintenance to avoid allowing particulate matter through.

The Gravimetric test method requires an operator to draw a sample of fuel through a filter cell containing two 0.8 micron filter membranes. The first is the working membrane which captures the dirt and the second is a control membrane which is used as a datum when measuring the solids loading. This filter cell is then taken to a laboratory where it is dried and the two membranes separated.

Again, there are important limitations with this type of testing. The time taken to analyse the sample can be between 24 and 48 hours, making it impossible to prevent an aircraft taking off before problems are found. Furthermore, manual error is also a problem with this type of testing. Results can be misinterpreted if the sample is not the correct volume, the sample membranes are not matched weight, or if there is a leaky path within the membrane cell.

Gravimetric is not available in real time, can sometimes be erratic and is non-informative in terms of condition monitoring. Filtration time is a more realistic analysis for filter performance prediction but requires a laboratory environment.  There are also water detectors where a 5ml sample is taken through a detector attached to the end of a suction syringe, with a change of colour from yellow to green indicating the presence of free water. Other proprietary water testing methods include Aquaglo and Karl Fischer.

Filter monitors are designed to sample the whole flow of fuel, with the presence of particulate and/or water triggering a rise in operational differential pressure.  These are often referred to as 'fuses', although the analogy with electrical system fuses is more correctly 'slow-blow' fuses, as they have some solids and water holding capacity before shutting off flow.

Quantitative Fuel Contamination Assay

There is clearly a need for new quantitative fuel contamination assay methods and there are several additional available technologies to measure fuel contamination. While electrical probes that measure conductivity can show sensitivity to water droplets, they are compromised by the variability in fuel composition and additive types and levels.

The alternative is light scattering and light obscuration detection methods. Light scattering is useful as a go/no-go method but is non-quantitative and non-referenced, whereas light obscuration is both quantitative and referenced. However, there remains the issue of differentiation between water and particulate.

Particle counting based on the principle of light obscuration is otherwise known as light blockage or light extinction. These terms relate to an object passing in front of a light source creating a shadow of a particle suspended in a fluid and is measured by way of a voltage drop across a light sensitive diode.

The signal generated as a result of the shadow is dependent on the size of the particle and the speed at which it passes through the light. There are other types of particle counting but the light obscuration technique is a more common and accurate method and is well regulated through ISO standards and practices.

Particle counting is ideally suited for use on fuels, specifically those used in the aviation industry, as it provides accurate results immediately. This enable more frequent, real time testing of fuels to be undertaken, ensuring the safety of aircraft and minimising costs as no laboratory consumables are required. Just as importantly, particle sizes and numbers can be measured extremely accurately and consistently, eliminating the problems of subjectivity that have been found with previous test methods.

The technology is also well governed and regulated by ISO standards (ISO11171), with the latest generation of particle counters using technology that has been proven time and again to deliver fast, reliable, repeatable and reproducible data, previously only available in the laboratory. Now for the first time, fuel suppliers from the refineries through distribution hubs, to the final delivery of fuel into the aircraft can be sure that the fuel being delivered is clean and dry.

Particle counting used in laboratory fuel quality measurements can provide a pervasive cleanliness parameter across industry, from refinery production, through distribution to into-plane activities. Sometimes the analysis may be best accomplished online so the provision of an instrument that has both laboratory and field applicability will be desirable. For example, the Parker ACM20 is the only instrument with this capability and includes a Z2 version for use in hazardous zone 2 areas.

In 2007, before the Defstan guidelines came into force a study team comprising QinetiQ, ExxonMobil and Parker had already developed an instrument and method specification that can be adopted by the aviation industry for contaminant monitoring. The aim was to develop one test protocol in particle counting. The new ACM20 will replace current subjective methods of fluid monitoring still in use today for checking and maintaining the quality of today's modern aviation fuels, from the point of manufacture through the distribution system to the point of uplift into the wing.

Following the DEFSTAN accreditation, the ACM20 can now be used in place of traditional and often subjective methods of measuring jet fuel contamination levels. The unit can discern moisture and contaminant particles as low as four microns in size, which can contaminate fuel during the distribution process as well as through installation and maintenance procedures, or through degradation of components, and can be used effectively with the minimum training.

Complementing the ACM20 is the Icount PD online particle detector, featuring independent monitoring of system contamination trends, early warning LED or digital display indicators for low, medium and high contamination levels, and optional moisture % RH LED indicator. The laser based technology helps prolong fluid life and reduce machine downtime. Icount PD has visual indicators with power and alarm output warnings, providing continuous performance for dependable analysis with self diagnostic software, and full PC/PLC integration technology using digital interfaces, such as RS232, and analogue interfaces, such as 0-5 Volt or 4-20mA.

Conclusion

There are a number of reasons why airport operators should use particle counting over current methods. Particle counting data is available in real time, and is regulated by ISO11171. It provides both on-line and off-line testing, connects to existing sample points, and has in-built alarms. Furthermore, it is portable and suitable for high pressure systems, so laboratory cost is reduced.

In addition, with the advent of Defstan 9191-Issue 6 and its incorporation into the JIG Check List issue 23, particle counting will now be recognised as the de-facto method of monitoring the quality of aviation fuels on a global scale.

JIG Bulletin No. 17 Issue 23 has the agreement of: BP, Chevron, ENI, Exxon Mobil, Kuwait Petroleum, Shell, Statoil, and Total. It defines the fuel quality requirements for supply into Jointly Operated Fuelling Systems. The Aviation Fuel Quality Requirements for Jointly Operated Systems (AFQRJOS) for JetA-1 embodies the most stringent requirements of the following two specifications:

British Ministry of Defence Standard DEFSTAN 91-91/Issue 6 of 8 April, 2008 for Turbine Fuel, Aviation Kerosene Type, Jet A-1, NATO Code F-35, Joint Service Designation AVTUR.
ASTM Standard Specification D 1655-08 for Aviation Turbine Fuels "JetA-1".
What this means in simple terms is that in order to provide a level playing field, the Checklist combines the most stringent requirements of the two specifications into one document, which is agreed to by all the major oil companies for use by Joint Operating Systems.

In the US, this means that those refineries dealing internationally will be required to provide ISO code reports on the condition of the fuel they supply. In addition, whereby 9191 only applies to the point of manufacture, Check list is used and recognised all the way along the distribution chain up to and including airport fuel farms.

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