Water in diesel fuel is one of the most common reasons for engine breakdown and its removal is vital for the efficient and safe operation of diesel engines, anywhere they are used.
Therefore, fuel filters are an essential part of a diesel engine’s configuration, helping to remove both solid debris and water droplets, safe-guarding the engine and other components from damage. As diesel engine design has progressed, so too has the formulation of diesel fuels.
Nowadays, the increased use of additives, mainly in low sulfur fuels, can result in fine, quite stable emulsions of water droplets that are a lot more challenging to remove than larger droplets. Filter design is progressively demanding and filter testing techniques must be robust and demonstrative of performance in the field.
International standard ISO16332 Diesel engines — Fuel filters — Method for evaluating fuel/water separation efficiency (the US equivalent being SAE J1488) is presently undergoing revision and its publication is imminent. This new version standardizes the testing of filters between manufacturers and focuses on the effectiveness of the test rig and how probable droplet recirculation could impact droplet size distribution (DSD). It outlines testing of filter efficiency and gives a model droplet size distribution for this. Laser diffraction is stated in the standard as the only technique for sizing this distribution.
This article looks at how laser diffraction particle sizing, already a recognized tool in fuel filter testing, is stepping up to the challenges posed by latest fuels and the newest testing requirements. Malvern Panalytical considers the method’s application within the context of the new techniques being written for ISO16332, focusing on its implementation for measurements at high pressure, high concentration and high flow rates.
Introducing Laser Diffraction
A brief review of the fundamentals of laser diffraction analysis is applicable here as a prelude to exploring the measurement challenges inherent in the new fuel filter testing regime and how these may be resolved. Laser diffraction is a well-established particle sizing method capable of accurately measuring materials, ranging from hundreds of nanometers up to several millimeters in size. Widely adopted across a variety of industrial sectors ranging from cement to pharmaceuticals, one of its common applications is droplet distribution analysis of oil/water emulsions.
As an ensemble particle sizing method, laser diffraction generates a result for a whole sample, as opposed to building up a size distribution from measurements of each particle. Measurements are made by sending a dispersed sample via a collimated laser beam. Particles (or droplets) in the sample scatter the laser light over a range of angles, dependent on their size. Large particles produce a high scattering intensity at moderately narrow angles to the incident beam, while smaller particles create a lower intensity signal but at much wider angles. A range of detectors is used to record the angular dependence of the intensity of light scattered by the sample, and common scattering patterns of samples at two very diverse sizes are illustrated in Figure 1.
Figure 1. Diffraction patterns from a laser diffraction analysis, showing more diffuse scattering for smaller particles. (right) (Source: Dr Kevin Powers, PERC, University of Florida)
Here, the diffraction pattern for a 5 µm particle (left) has a strong center and very sharp rings over a range of angles. The reason is that some light is scattered around the edges of the particles by diffraction and some passes through, all later recombining to form a classic interference pattern. At smaller particle sizes (right), the scattering becomes more diffuse. Using a suitable theory of light scattering behavior to the detected scattering data allows calculation of the particle size distribution of the sample. The most recent version of ISO13320 (the ISO standard for laser diffraction) proposes the use of Mie theory for all particles in the size range over which laser diffraction is used and, for particles below roughly 50 µm in size, it is considered crucial.
The Mie theory represents the interaction of light with matter. It assumes that the particles are spherical and in a two-phase system, assumptions that are truly applicable for water droplets in oil, and requires awareness of the refractive index of the materials involved, also a direct task for these systems. The theory is effective for all particles sizes and for all wavelengths of light and envisages the dependence of scattering intensity on size. It also estimates that secondary scattering is detected for small particles, an issue that becomes progressively important at high sample concentrations, as explored below.
How Mie theory is applied to the measured data to deliver the particle size distribution of the sample can be seen in Figure 2. Since scattering models compute scattering patterns from a known size distribution, and not the other way around, the models are basically used in reverse and in a cyclical manner. Scattering data are initially calculated from the ‘guessed’ size distribution and then compared to the measured scattering data. The process is repeated till the difference is reduced and the final size distribution can be conveyed.
Figure 2. The application of Mie Theory.
The new test techniques expressed in ISO16332 pose specific challenges to laser diffraction systems and most importantly to laboratory-based analyzers. They request the analysis of high concentration samples presented at high pressure and high flowrates (usually up to 1500 L/h) and also the ability to measure alterations in particle size over time. A main requirement is that the particle size distribution should not be changed by the measurement system, so there is a need to avoid any possible shearing of the sample as well as sample dilution. Standard laboratory systems usually test dilute samples at atmospheric pressure in flow cells whose design may change the nature of the sample. Preferably, droplet size analysis must be done under the same conditions that are present during a filter test. To realize this, more industrially-focused tools that measure online at high pressure and high concentration are more suitable for this type of testing.
Avoiding Multiple Scattering
The recommended operating concentration for test systems in the new fuel filter standard is anticipated to be regularly up to 1500 ppm, with an optional 20,000 ppm. Not only are such high concentrations outside the capabilities of laboratory tools, their direct measurement requires a laser diffraction system that applies a number of scattering corrections. Multiple scattering happens when light scattered by one particle hits other particles in the system, before traveling to the detector as shown in Figure 3. Its occurrence causes the measured scattering to shift to higher angles, which produces too fine a particle size and, without correction, results in a measurement error. Two test droplet specifications are suggested in the standard, a fine fraction with a D50 of 10 µm and D90 < 30 µm, and a coarse fraction where the D50 is 150 µm and D90 < 350 µm. Fine transparent droplets pose an even bigger challenge from a multiple scattering angle than coarse ones, making correction imperative.
Figure 3. Multiple scattering increases the overall angle of scattering detected.
Although unconnected to the problem of multiple scattering, the high concentration of the samples can also present issues in keeping measurement cells clean.
The Perils of Sampling and Dilution
The new standard highlights the significance of examining particle size distribution over time, particularly as parameters, such as flow rate, change. While offline laboratory analyzers could still be used within the scope of the new standard, the act of repeat sampling and the required dilution and circulation via a tank pose huge risks to sample integrity. Sample representation is also taken into consideration. Extracting a few milliliters of emulsion for offline analysis increases potential errors that may result from velocity bias encountered while sampling a moving stream, and increases the probability of droplets coalescing during transport to the analyzer. Online laser diffraction systems capable of direct measurement at high concentration not only evade these risks, but also provide results in real-time for instant analysis of changes and the ability to examine the speed with which filtration happens.
High Flow Rate Issues
Analysis of the high flow rates encountered under filter test conditions introduces the risk of sample shearing, particularly if there are any restrictions in the cell that form shear points, something that will decrease the droplet size and generate erroneous results. Any bends in the cell design has to be avoided.
New Measurement Solutions
Even for those industry experienced analyzers already proven in fuel filter testing, such as the Insitec and Spraytec laser diffraction systems (Malvern Panalytical, Malvern Panalytical, UK), the addition of the new standard has prompted further hardware developments in anticipation of its heightened demands. One output from this development is a high-flow cell precisely designed to support ISO16332 testing stipulations, which functions at flow rates from 2 to 23 L/min, and whose novel design allows measurement of water droplets in diesel at high concentration without shearing.
A significant factor was the requirement in the standard for the sample to be unaffected by the measurement device. The resulting cell has a narrow path length of 0.5 mm that guarantees linear flow without any shear points. Experimental validation data using a distinctive water/fuel emulsion are illustrated in Figure 4, demonstrating that there is no reduction in the particle size; even at a flow rate of 15 L/minute. Operating at such a high flow rate also has the extra advantage of reducing the probabilities of droplet deposition on the cell windows.
Linearity was confirmed using both latex and glass beads, and similarly, seeding coarse particles into samples created wholly anticipated and consistent results, largely demonstrating measurement reliability without any bias.
Figure 4. Shear test for high-flow rate measurement cell showing no change to particle size at different flow rates.
Heavy duty sapphire windows enable the cell to endure pressures of over 10 bar, where standard cell materials might fail. Pressure drop testing at the necessary flow rates, using an ISO test assembly with and without the flow cell installed, verify that there are no problems with pressure drop likely to impact the measurement, since the flow cell caused a pressure drop of less than 3.5 kPa at the maximum flow rate of 15 L/min as illustrated in Figure 5.
Figure 5. Pressure drop tests using ISO test assembly with and without flow cell installed show that the flow cell caused a pressure drop of less than 3.5 kPa at the maximum flow rate. 
The application of online laser diffraction systems offers the greatest possibility of enhancing confidence in fuel filter testing and the development of strong test techniques. New solutions are needed to totally meet the high flow and high concentration measurement requirements of the new ISO16332 Diesel engines — Fuel filters — Method for assessing fuel/water separation efficiency standard. Online laser diffraction systems featuring test cells engineered to meet the rigors of the new testing method will help support its application.
 Information from Malvern Panalytical webinar https://www.brainshark.com/Malvern Panalytical/vu?pi=700599336&b=1&tx=83391&c1=22&nodesktopflash=1
 Data courtesy of Bonavista Technologies
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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