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DIESEL FILTER DIRT HOLDING CAPACITY – The Misleading Factor

1. Abstract

The diesel fuel filter business is a multi-billion dollar industry with hundreds of manufacturers around the world producing a wide variety of products for contamination control in diesel engines and for bulk storage and dispensing applications. Competition to produce these filters and gain a slice of this lucrative market is extremely high and as such, manufacturers are continually striving to gain a competitive edge; be it through product innovation or sales and marketing strategies.

One of the more popular strategies of recent times is to advertise the dirt-holding capacity of the filter element, simply represented as total weight in grams or ounces. This paper is intended to provide an insight into this widely used reference factor and will explore the reasons why the average consumer should avoid using such information as a standalone determining factor for filter selection or as a predictor of actual filter service life.

2. Introduction

Various filter element manufacturers use and report dirt-holding capacity data as a method for advising consumers how well a filter performs its function. This is mainly because it is very easy for the everyday person to interpret the information. If two filters look the same and one holds more dirt than the other, it should be relatively easy to determine which is the better value for money. On the surface this would be a logical determination, however due to the variations in tests being used to obtain dirt-holding capacities, relying on such information as a standalone determinant is fraught with danger. When reviewing the dirt-capacity of a diesel fuel filter element, it is important to know which test has been used to obtain the information and how that test was conducted. Even then, the information is not always useful or relevant to the specific application. Despite uniformly more stringent filter test methods, there remains some inconsistencies in both the gathering and reporting of data.

3. The History of Testing

The reporting of filter element dirt-holding capacities has steadily increased since its rise to popularity during the 1980’s. Such an increase was primarily the result of an explosion in filter element competition following the release of advanced 3μm (β 3 ≥200) filter elements into the market during that period. These filter elements quickly gained in popularity as they targeted and significantly reduced the fine silt contaminants that were causing new electrohydraulic servo proportional hydraulic valves to fail. In order to gain a competitive edge, leading filter element manufacturers looked at new methods of marketing the benefits of their product verses the competition; dirt-holding capacity being one such option available to them.

The reporting of dirt-holding capacity information originally came from the ISO4572 Multi-pass test standard developed by International Standards Organization (ISO). Hydraulic filter elements tested under this method were evaluated on their ability to separate contaminants of certain sizes from a fluid and under specific operating conditions. Filtration performance was measured by analyzing three areas:

  1. Efficiency or absolute rating and percent efficiency
  2. Dirt holding capacity (DHC)
  3. The pressure drop across the element at a specific absolute efficiency.

The ISO4572 test required that a known and predefined test contaminant – AC Fine Test Dust (ACFTD), be added to the test system until the filter element terminal differential pressure (∆P) was reached. Following the test, the volume of test contamination added to the test system was recorded. This data became known as “Apparent Dirt-holding Capacity”. However, as the ISO4572 standard did not specifically demand filtration manufacturers or laboratories to advise how the data was to be presented to the consumer, many manufacturers simply advertised the data as the “Dirt-holding Capacity” leaving out the word “Apparent”. As might well be understood, the volume of contamination added during the test cycle has little or nothing to do with the actual volume of contamination retained by a test filter.

Reporting this information in such a way gave the consumer false and misleading information about the filters real ability to retain contamination. There are some products available today that still represent data in this manner.

The ISO4574 test method is no longer used as the AC Rochester Division of General Motors Corporation ceased manufacturing the test dust during the late 1990’s. Test procedures have changed considerably as have the test dusts that are used to obtain dirt-holding capacity information.

4. Current Test Procedures

The popular view held by many is that the dirt-holding capacity of a filter should be a relatively easy factor to determine; surely the filter element is tested according to a known method or standard, drained and then weighed to see how much contamination has been captured? However, this could not be further from the truth. Filter elements, including diesel fuel filters are NEVER weighed following a test in order to determine the actual retained dirt-holding capacity. Tests used today require a series of complex calculations to be completed in order to accurately assess the effective dirt-holding capacity of a filter. Figure 1 represents one of the calculations used for dirt-capacity.


Figure 1: Typical Dirt-holding capacity calculation

There are three common test methods used today by most manufacturers to obtain and report the dirt-holding capacity of a diesel fuel filter. This includes fuel filters used in engine applications and those that are used in large bulk transfer and fuel bowser applications. The common test procedures are as follows:

  1. ISO16889: 2008
    Hydraulic fluid power — Filters — Multi-pass method for evaluating filtration performance of a filter element
  2. ISO19438: 2003
    Diesel fuel and petrol filters for internal combustion engines — Filtration efficiency using particle counting and contaminant retention capacity
  3. SAEJ905: 2009
    Surface Vehicle Standard — Fuel Filter Test Methods

Times have changed in the way that dirt-holding capacity data is reported, however even today with these more stringent methods in place, the terminology used in the reporting of the data can be somewhat misleading. In most cases, the dirt- holding capacity of a filter tested today is known as “Retained Capacity”. Again it is important to note that the filter is not weighed to determine how much contamination is actually retained. The definition of Retained Capacity is as follows:

“mass of the specific particulate contaminant effectively retained by the filter element when the terminal element differential pressure is reached”

The word effectively has been highlighted to reinforce that the data is a calculated number rather than an actual weighed dirt capacity.

3.1 ISO16889:2008

Most fuel filters used in bulk diesel filtration and bowser applications use this test method for measuring dirt-holding capacity. When reviewing the dirt capacity of a filter element or comparing products to each other, the following should be considered in relating to this test method.

  • This test method is a multi-pass test designed specifically for testing the efficiency rating of hydraulic filters.
  • A bulk diesel fuel filtration application is almost exclusively a single-pass proposition, meaning that the filter has one chance to capture contaminants. Testing the filter under this test method gives the filter multiple chances at capturing the contaminants.
  • The method states that: “This test is intended to differentiate filter elements according to their functional performance but is not intended to represent performance under actual field operating conditions”.
  • The method uses ISO12103-1-A3 Medium Test Dust (ISOMTD). This test dust does not correlate to the natural particle size distribution found in #2 diesel fuels and will provide misleading results compared to actual in service applications.
  • The test fluid used is MIL-H-5606 oil and during testing the oil is required to be maintained at 15cSt. Diesel fuel viscosity is approximately 3cSt. Test contamination injection rate is ~10,000 x higher than in real world applications.
  • A constant flow rate is used with no pressure or flow pulsations. These conditions are known to negatively impact filter performance especially at higher differential pressures.
  • The flow rate is determined by the filter supplier and can be manipulated to lower levels, which has been shown to increase dirt-holding capacity.
  • Terminal differential pressure is determined by the filter supplier and can be manipulated. Many filters are known to unload contaminants at higher differential pressures, which effectively reduces the dirt-holding capacity. Terminal differential pressures can be lowered to provide a higher dirt-holding capacity.
  • This test method will typically exhibit an increase in dirt capacity of 10% to 40% when using ISOMTD over the values measured with ACFTD in the previous ISO4572 multi-pass test method.

3.2. SAE J905:2009

Most fuel filters used in bulk diesel filtration and bowser applications use this test method for measuring dirt-holding capacity. Consumers should take note of the following in relation to this test method.

  • Some fuel filters tested using this method are sold into bulk diesel filtration applications. Any dirt-holding capacity data being represented cannot be compared to data obtained from the ISO16889 test method. These tests are completely different to each other and consequently so too will the results be different.
  • The test is a single-pass test designed specifically for efficiency rating of fuel filters, which does provide a better representation of real world applications.
  • The test uses ISO12103-1, A2 Fine Grade test dust, which best represents the natural particle distribution in diesel fuel.
  • The test fluid used is SAE J1696 which is close to that of #2 diesel fuel.
  • The revisions of this test method prior to 2009 used a non-dispersant fluid.
  • The current 2009 revision uses fluid with significant dispersancy. Filtration efficiency and dirt-holding capacity will be significantly lower than if the same filter was evaluated by the older method. Results using the current method are more realistic than earlier test results and are more repeatable and reliable. Consumers should understand which particular version of test results are being assessed.
  • A constant flow rate is used with no pressure or flow pulsations. These conditions are known to negatively impact filter performance especially at higher differential pressures.
  • The test is designed specifically for engine fuel filters however vibration is not used to simulate an operating engine. Filtration efficiency and dirt- holding capacity have been shown to reduce in the presence of vibration.
  • The flow rate specified in the test method is “200 LPH or to a flow rate as agreed upon by the supplier and user.” Consumers should note that the flow can be varied and that lower flow rates have been shown to increase dirt-holding capacity.
  • Terminal differential pressure is determined by the filter supplier and can be manipulated. Many filters are known to unload contaminants at higher differential pressures, which effectively reduces the dirt-holding capacity. Terminal differential pressures can be lowered to provide a higher dirt- holding capacity.

3.3. ISO19438:2003

Some fuel filters intended for engine based diesel filtration applications use this test method. Consumers should take note of the following. The test method is essentially a combination of both previously noted test methods.

  • Like the ISO16889 test, this test procedure is a multi-pass test, yet it is designed for efficiency rating of engine fuel filters.
  • The method uses ISO12103-1-A3 Medium Test Dust (ISOMTD). This test dust does not correlate well to the natural particle size distribution found in #2 diesel fuels.
  • The test fluid used is MIL-H-5606 oil and during testing the oil is required to be maintained at 15cSt. Diesel fuel viscosity is approximately 3cSt.
  • Test contamination level of 50mg/l should be adhered to however; upstream gravimetric levels up to 100mg/l may be used to shorten test times. Such levels are vastly higher than in actual real world operating applications.

An additional common test method, SAEJ1985 Fuel Filter – Initial Single-Pass Efficiency Test Method, also exists. Some filter manufacturers have been known to report the dirt-holding capacity for their products according to this test method. It is important to note that this standard DOES NOT provide guidelines or procedures for how to obtain or report such data. Information on dirt-holding capacities using this standard should be avoided. The SAEJ1985 procedure is used to test the initial single-pass efficiency of a fuel filter, and is only designed for products meeting a specific flow rate range.

7. Conclusions

Diesel fuel filter element manufacturers use dirt-holding capacity data as a means of advising potential consumers how well their product performs against the competition. Essentially it is used as a marketing tool. Advising how much contamination a filter holds is something that the average consumer can readily relate to. A higher dirt capacity is obviously better than a lower one for a comparable product. However, even with the most modern test methods in place a certain degree of interpretation and manipulation remains with key variables that can all lead to false and misleading information being presented to the consumer.

When reviewing the dirt-holding capacity of a diesel fuel filter, consumers should be aware that ALL of the points noted within this paper regarding the current test methods will directly impact on the real world dirt capacity of a filter. The most important point to note is that the dirt-holding capacity of any diesel fuel filter tested under any of the test methods noted, will in no way correspond to the actual dirt capacity of the filter in service or give any indication of its expected service life following installation. As further evidence of this, some of the test methods actually include such disclaimers.

Comparing products between each other is naturally something that many consumers will endeavour to do. However, it is important to remember the flexibility and variability in the test methods. In order to accurately compare one product to another, all of the test variables that impact dirt-holding capacity must be known and documented, and an accurate determination can ONLY be made when filters are tested side-by-side. If consumers are unable to obtain such information from potential suppliers in order to make an informed judgement, the dirt-holding capacity being advertised should be avoided as determining criteria for product selection.

13. References

  1. ISO4406: 1999 Hydraulic fluid power – Code for defining the level of contamination of solid particles.
  2. ISO16889: 2008 Hydraulic fluid power — Filters — Multipass method for evaluating filtration performance of a filter element
  3. ISO19438: 2003 Diesel fuel and petrol filters for internal combustion engines – Filtration efficiency using particle counting and contaminant retention capacity
  4. SAEJ905: 2009 Surface Vehicle Standard — Fuel Filter Test Methods
  5. SAE J1985 – Fuel Filters – Initial Single Pass Efficiency Test Method
  6. ISO/TS 13353:2002 Diesel fuel and petrol filters for internal combustion engines – Initial efficiency by particle counting
  7. ISO4020 – Road Vehicles – Fuel Filters for Diesel Engines
  8. JIS D1617 – Automotive Parts – Fuel Filters for Diesel Engines Test Methods