The filters remove any solid components (rust, paint chips) that may cause wear on the engine as well as undissolved water (that will be present in the diesel).
Large water droplets are reasonably easy to remove, but increasingly the water droplets are present in the form of a fine emulsion. The recent addition of various fuel additives (especially in low sulphur diesels) act as surfactants and have the unwanted side effect of reducing the emulsion droplet size. The droplets would have been larger in the past and easier to separate, but now have a higher zeta potential (near-surface charge) in the new formulation so are more stable at small sizes than in the past, so a new problem has been inadvertently created.
As part of filter design, filters must be tested to examine how they will behave in the field. Currently, new test methods are being written to standardize testing conditions between laboratories and filter manufacturers. A new version of the ISO16332 standard will address this.
The principal use is to validate the effectiveness of the test rig, and how potential droplet recirculation during testing could influence the particle size distribution.
Laser diffraction has long been used as a testing mechanism for these filters. This has ranged from traditional laser diffraction measurements (such as a Mastersizer) to high concentration laser diffraction measurements (Spraytec or Insitec). Traditional laser diffraction will require dilution, and as it is known that droplet size depends on the surfactant concentration reducing this (by diluting the sample) can have the unwitting effect of changing what is being studied, so a high concentration measurement makes more sense. The high concentration laser diffraction measurements are also real time, so how rapidly filtration occurs can also be studied and the system can even be automated and have its results fed back into the control system.
Creating a new cell
The new standard outlines testing of filter efficiency and specifies a model particle size distribution for this testing.
Laser diffraction is specified in the standard as the only method for sizing this particle size distribution, other methods could be used but need to be validated. On-line particle sizing is also mentioned as an option in the standard, the only stipulation being that the device should not change the particle size distribution (though that is far more likely should conventional laser diffraction with dilution be used). If dilution is a risk, then designing a cell with the highest concentration range you can is a must.
Besides minimizing or eliminating dilution the other factor which could change the PSD is the cell design, any bends or restrictions run the risk of shearing the emulsion reducing the particle size, so as well as the existing high concentration behavior. An ideal cell for this application should be able to operate at a wide range of flow rates, the higher ones at which would certainly cause shear in a traditional cell. The new cell has been designed to operate at flow ranges varying from 2 -23 liters per minute.
For further information on the cell design and results please attend the forthcoming webinar on the 16th September titled “In-line PSA streamlines fuel filter testing (ISO16332)“.