There are a number of important concepts that drive facility design requirements and therefore need to be well understood, filtration is one of those: Perhaps the simplest type of filter is the mesh strainer. This equipment is a simple dirt capturing filter. The degree of filtration efficiency is determined by the mesh size.
Although they do perform a useful function in the fuel supply system, these types of filters are not adequate contamination control for modern diesel power generation or transport equipment. Their primary use is in protection of transfer pumps and metering systems from gross contamination.
For diesel fuel filtration, the performance needs to be well specified and defined. There are a multitude of industry specifications for different applications and equipment. It is always recommended to contact the OEM to determine the level of fuel quality specified for their individual equipment.
The main challenge with the various fuel specifications is the alignment between lab test and field experience. The test methods use defined test dusts and challenge the filters in a defined way. This may not match with the real world filtration challenge. Further, there are many variations of fuel additives (i.e. cold weather, lubricity, antistatic) that can also alter the filtration systems capability to remove water.
It is useful to understand the sources of potential contamination and the impact on filtration systems. The illustration above gives an indication of particle sizes of various materials in microns.
The primary issue that needs to be understood is that visible particulate is not the primary concern, as dirt contaminants that are large enough to see will most likely settle out and are easily filtered. It is the sub 40 micron materials that are of particular concern and especially those in the range below 15 microns that tend not to settle during storage as both time periods are insufficient and thermal convection prevents it. Particulate in this size range will cause fuel system wear and disruption in diesel power generation and transport equipment. To provide some sense of fuel cleanliness, the pictures below show a diesel fuel sample with a particulate content defined by an ISO Code of 22/20/18. The fuel is clear and bright and free of visible particulate, but this fuel contains high levels of sub-40 micron particulate.
If fuel is generally manufactured by distillation of crude oil, how does fuel get dirty? This issue is largely defined by the distribution system from the refinery storage tanks through to delivery to the end-user’s power generation or transport. Contaminants build up in:
- Pipelines, Ships, trucks and Rail Cars
- Storage tanks
- Delivery vehicles
Real world maintenance practices may also accentuate the problems, where transfer nozzles may be contaminated as no dust caps are fitted, schedules are missed and replacement of expired or damaged equipment is not performed. Sources of contamination may occur from many sources:
- Simple rust, airborne dirt and water,
- Temperature variation (precipitation of dissolved water)
- Additive impact and compatibility issues (note most low sulphur diesel will contain a lubricity improver which may stabilize water haze) as well as possible low temperature and combustion improver (cetane) additives.
- Biodiesel compatibility, ageing and purity issues
- Microbiological spoilage
Coalescers are particularly impacted by these types of contaminates since contamination is always present in the system, it is imperative to manage the system to ensure clean fuel is delivered to the final point of use. In this regard, in addition to proper storage tank design features, a minimum inlet filtration to the supply tank should be used,especially power generation or transport equipment. In many cases, it may also be appropriate to include outlet filtration, depending on the criticality of the equipment function.
Contamination can occur from many different sources, so care is needed on the appropriate selection of filtration. There are simple dirt filters, sometimes referred to as microfilters, coalescing filters to remove free water and dirt, and water absorbing filters to provide a higher level of water removal. Before choosing a filter for the intended facility it is useful to have an understanding of the type and extent of contaminant that is routine. Using a particle counter to conduct a survey of inlet fuel quality and outlet fuel quality from the storage will provide insight into the volume of particulate. These particle counters can also now eliminate water effects with appropriate test condition management and allowing assessment of the dirt and water content in the fuel stream to be determined.
These may be conventional pleated cellulose blends, polymeric fiber, or glass fiber systems with replaceable cartridges. In terms of particle capture efficiency, the replaceable cartridge filters are available in a range of particle sizes (0.5 to 100µm) and in high dirt systems, these provide the most cost effective approach. Selecting the right type of media for application should be done with care. In some high flow systems, use of cyclone type filters may also be appropriate to remove gross contamination. Cyclone system users should be aware of the full cost of these systems, including power consumption and maintenance. The turbulence in a cyclone can chop up agglomerations creating many more smaller particles. The cyclone type filter is generally most effective with larger particle sizes (>30µm). Pleated products offer the best solution for fine particle removal.
Be aware that there are many laboratory methods to test filters and systems. The optimal way to assure the user will attain their filtration requirement is for the user to have the filter manufacturer test the filter at the desired flow rate at a reasonable challenge level (dirt injected at a given ISO4406 level) injection, and measure the actual downstream retention results in an ISO code using a calibrated system.
Coalescing filtration is primarily designed to allow the removal of dirt and free water from flowing fuel streams. Stable hazes contain many small water droplets and due to the small size, the water may take days to settle to the bottle of a tank or vessel. The coalescing filter contains a media that coalesces the small droplets into increasingly larger and larger droplets that due to their specific gravity easily settle to the bottom of the housing to be removed by daily flushing of the sump.
In high dirt and water environments, using the coalescing media to control both the dirt and water contaminant may not be the optimal economic approach. In order to deploy the most cost effective solution, a properly sized Microfilter should always be installed upstream of filter water separator.
The coalescing/separating necessarily contains hydrophilic (water attracting) and hydrophobic (water repelling) elements. The surface properties of the media can be susceptible to deactivation by certain types of aggressive surfactants (fuel additives) and care is needed in monitoring the filter to ensure that this is identified and appropriate action taken. Poor quality or high concentration (>B10) biofuel can also act to disarm coalescing elements over time. Since biodiesel quality varies significantly throughout the globe, care should be taken by consulting Parker Velcon when implementing coalescing technology when using a diesel-biofuel blend.
Water Absorbing Filters
Water absorbing filters contain a polymeric media that is designed to adsorb water permanently from flowing fuel streams. Unlike the coalescing media, this type of filter is not susceptible to surfactant deactivation, but they are a relatively costly filter type to remove bulk water and should only be used for final polishing of the fuel to remove any small quantities of dirt and water immediately prior to the delivery to the power generation or diesel transport system. The water volume needed to be removed should be the primary consideration for implementing these types of water and solids removing filters.
Once the type of filtration system has been chosen the next question that must be answered is where in the system the filter or filters will be installed. In theory, the fuel should be filtered each and every time it is moved from one storage location to another. This would include filtration to assure quality upon delivery throughout the system to maintain the fuel quality during storage, and especially at dispenser to assure proper cleanliness level for end user.
Where there is a concern about the integrity or quality of supply, it might be useful to configure the outlet pipework to allow tank recirculation using a system filter. This type of system (shown diagrammatically below) will allow remediation in tank of any contamination that is present in the tank.
Depending on the criticality of the system, inlet filtration may be sufficient to provide the integrity of a clean fuel supply. Where higher integrity is needed, there is no option but to install both inlet and outlet filtration.
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