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CFPP (Cold Filter Plug Point) vs. CP (Cloud Point)
Cold Weather Operability in Diesel Fuels including ULSD
Traditionally the two main considerations for diesel fuel have been Cloud Point (CP) and Cold Filter Plug Point (CFPP).
Let’s start by defining the terms:
Cloud Point (CP) ASTM D2500 – This test determines the point where wax becomes visible in a fuel sample. This wax first appears as a floating cloudiness in a transparent fuel.
Cold Filter Plug Point (CFPP) ASTM D6371 – This test is a more complicated procedure involving using a vacuum to draw a 20cc fuel sample through a 45 micron screen within a 60 seconds.
There is usually but not always a spread between CP and CFPP of 2°F to 8°F.
CP is a first indicator of cold weather operability temperatures for diesel fuels. It is a visible indication of paraffin wax in diesel fuels. Prior to the introduction of Ultra-Low Sulfur Diesel (ULSD, S-15) into the US market, the importance of CP was often discounted by many due to fact that diesel engines could generally successfully operate at temperatures many degrees below the CP.
Up until the introduction of ULSD many if not most operators used CFPP to provide a reference temperature for cold weather operability with diesel fuels. This is however a complicated and imperfect test. As mentioned above, CFPP uses a vacuum to draw a sample of the fuel through a 45 micron screen within a given time. The point at which the sample fails to go through the screen in 60 seconds is the CFPP.
The main issue is that up until recently most fuel filters used a 10 micron filtering media. The significant difference 10 microns and 45 microns caused a disparity between the test and real world operations. However many in the industry felt that this differential was consistent and that provided a reliable guide for cold weather operability.
For example if you had a CFPP of -30°F, you could feel reasonably confident that you could operate to -20°F.
However three new factors need to taken into account due to changes in fuels and engines.
1. The new ULSD fuel does not appear to provide the same consistent differential between CP and CFPP as we had come to expect with High-Sulfur Diesel (HSD, S-5000) and Low-Sulfur Diesel (LSD, S-500).
2. The new phenomenon of Wax Drop Out (WDO) where under periods of extended “Cold Soak” (48-72+ hours) the wax in the fuel suddenly drops out of the fuel can happen at temperatures that can be above the CP. This problem appears at this time to be independent of CP or CFPP.
3. As diesel engines have become more sophisticated there has been a rise in fuel injection pressures. In order to obtain these higher pressures OEM’s have had to manufacture pump and injector parts to ever closer tolerances. Today many injectors have tolerances in the 2 micron range. These tight tolerances and the very high cost of making and replacing these components have caused manufacturers to use fuel filters with smaller media to protect these components. Where in the past fuel filters typically were 10 microns, today we are seeing filters of 7, 5, and even 2 microns.
This makes the problems associated with ULSD even more difficult. Cloudy fuel that would easily pass through a 10 micron filter can often plug a 5 or 2 micron filter. This makes correcting the cold weather operability issues of ULSD like hitting a moving target. Today you need to adjust your fuel treatment to reflect the engines and filter arrangements in your fleet.
We are now suggesting a formula based on both CP and CFPP. Take the difference between CP and CFPP, divide by 1.5 and add to the CFPP to get a safe operability number.
Example: CP = 8°F, CFPP = 3°F
The difference between 8 and 3 = 5, 5 x .75 = 3.75, Take the CFPP of 3 and add the 3.75 to it equaling 6.75°F. You could expect to reliably operate that fuel in an engine with a 7 to 5 micron filter at 6-7°F.
For those operating 2 micron filters we suggest using the CP of the fuel.
For those still able to operate with 10+ micron filters, we are suggesting a number half way between CP and CFPP.
It is important to remember that the traditional method of using Kerosene or Jet A to “cut” or blend with HSD or LSD to lower the CFPP and Pour Point (PP) is not as effective or reliable as it was in the past when using the new ULSD #1 to cut or blend with ULSD #2.
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Last modified: 04/06/09
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