Filter blocking tendency. Is Your fuel struggling to squeeze through the gaps?

Fuel filter blocking is a serious cause of fuel starvation and fuel component failure.

My lab frequently receives fuels and fuel system components from failed machinery and vehicles to determine the root causes of issues. Companies developing products to address fuel system problems often seek my input during product development. While dedicated fuel testing labs assess fuel batches against specifications, these tests often result in pass/fail outcomes. However, they may not cover common fuel issues like engine oil, Adblue, fuel bugs, and engine coolant, which can still pass despite their presence. This is why we conduct tests beyond specifications to diagnose problems.

Today, I’d like to introduce a test called filter blocking tendency (FBT), which assesses the likelihood of fuel causing a filter blockage. While it usually yields a pass/fail result with varying specifications and limits, some customers come to us after their fuel fails tests elsewhere, seeking solutions.

This article aims to provide a deeper understanding of FBT beyond pass/fail outcomes and how you can use test results to improve fuel storage and usage.

What is the Filter Blocking Tendency (FBT) Test?

Think of the Filter Blocking Tendency (FBT) test as your first day in a new job. In a new job, you want it to go as smoothly as possible and not end up setting fire to the photocopier, accidentally shredding that important document or pressing that fire alarm button precariously close to the door release button. You want the day to almost go by without being noticed. This fuel test is much the same where the fuel in its new work environment of a fuel filter wants to go through without causing a ruckus—in this case, clogging your filters. Conducted according to international standards like IP 387 or ASTM D2068, the test involves pumping a fuel sample through a designated test filter at a regulated flow rate and temperature. The differential pressure is monitored and recorded as the fuel squeezes through the filter. Importantly, the test concludes either when 300ml of the fuel sample has passed through the filter or when the differential pressure reaches 105 kPa, whichever comes first. This eventually leads to the calculation of the FBT index. The lower the index, the better.

🔍 Interesting Fact: Did you know that biodiesel is more prone to high FBT compared to standard diesel? This is often because biodiesel can contain higher levels of fatty acid methyl esters, which can solidify and block filters at lower temperatures. This is one reason why FAME based biodiesels are less popular than traditional petroleum diesels despite their environmental accolades. As you move from petroleum to biodiesels either HVO, GTL or FAME based the importance of testing becomes ever more important.


Interpreting the Results of the FBT Test

Once you’ve got your FBT index, it’s time to put on your detective hat. A low index score is like getting an A+ on an exam—your fuel-blocking issues tendency is top-notch and unlikely to cause filter issues. A high score? That’s when you need to start asking questions and maybe even call for a parent-teacher meeting with your fuel supplier. While the index gives a numerical value that can be compared to standards for a pass or fail, it’s like a medical test—it offers a chance for a deeper diagnosis than just “you’re fine” or “you’re not.”

Understanding FBT Index Values: What’s Normal and What’s Not, Including EN590

The FBT index limit can vary depending on the type of fuel and the specific application, but there are some general guidelines to consider:

  • Perfect FBT (<1): The fuel hardly makes an impact on the filter. You can almost guarantee the fuel if it stays in good condition will not cause filter issues.
  • Normal FBT Index (1-2): This is the golden zone. Your fuel is well-behaved and unlikely to cause any blockage issues.
  • Moderate FBT Index (2-6): This is the yellow light zone—proceed with caution. While not alarming, it might warrant some checks or preventive actions.
  • High FBT Index (6+): Red alert! A high FBT index means immediate action is required to identify and fix the issue before it causes real problems.

EN590 (white diesel) Guidelines

The guidelines for diesel fuels that adhere to the EN590 standard are quite specific. Filter Blocking Tendency values should not exceed 2.54 in the test. This stringent limit is in place to ensure high-quality fuel that’s suitable for the range of conditions diesel engines might encounter. However, for some red diesel specifications, a result under 6 is considered normal. Hence, it’s important to confirm which specification you are using to see if a pass or fail, but the same general guidelines above still apply on how likely to cause a blockage. Just different specs cut off at the cautionary stage and others at the serious stages.

Note: Always refer to the latest manufacturer guidelines or industry standards for the most accurate and application-specific limits, as these may change after writing the article.


đź“š Case Study: A logistics company was experiencing frequent filter blockages across their fleet. After a standard pass/fail test showed no issues, they turned to an investigative lab. An FBT test revealed a high index, prompting further analysis. It was discovered that the fuel had high levels of microbial contamination, likely due to poor storage conditions. The company implemented a biocide treatment and upgraded their storage facilities, leading to a significant reduction in filter blockages and maintenance costs.

Case study courtesy of Oil Analysis Laboratories in Colwyn Bay, UK


Causes of High FBT and Corrective Actions

So, you’ve got a high FBT index. Don’t panic, it’s not the end of the world—or your engine, hopefully. High FBT can be a result of many Saturday-night-party-gone-wrong scenarios for your fuel: uninvited contaminants like water and microbes, a cold snap turning wax into annoying crystals, or just age catching up and causing gum and sediment formation.

Now, for the fixer-uppers:

  1. Filtration: Think of upgrading your filters as going from standard definition to 4 or even 8K—everything gets clearer (and cleaner).
  2. Fuel Additives: Consider these the vitamins of the fuel world; they supplement and improve the overall “health” of your fuel.
  3. Improved Storage Conditions: Treat your fuel like fine wine—store it well, and it will behave well.
  4. Pre-Heating: In cold weather, pre-heating your fuel is like giving it a warm cup of cocoa before sending it out into the snow. It helps prevent those pesky wax crystals from forming.

In summary, FBT is more than just a pass/fail number. It’s an in-depth analysis that can guide you towards the cause of the problem, saving you time, money, and maybe even your sanity. Cheers to smooth running and clean filters!