Ultra-Low Sulfur Diesel Fuel
Cold Weather Information
The Ultra-Low Sulfur Diesel (ULSD (S-15)) that we started to
receive in mid 2006 has shown some dramatically different cold weather
characteristics from the earlier High Sulfur (HSD (S-5000)) and Low Sulfur Fuels
(LSD (S-500)).
These new characteristics including higher temperature gelling, wax dropout,
icing, and difficulty in treating have in the first year and will continue into
the foreseeable future to provide some significant challenges to distributors
and end users during cold weather.
Due to these new characteristics users in areas of the US where they have not
seen cold weather problems in the past, are now and will continue to see serious
issues with gelling, wax dropout, and icing.
Here are the main issues known today:
Wax in diesel fuels – Paraffin wax is a natural and important part of diesel
fuel. This wax provides several beneficial characteristics including high energy
content (Btu’s), lubricity, stability, and viscosity. The negative
characteristics mainly revolve around cold weather operation and include gelling
and something new we refer to as wax dropout.
In HSD and LSD the wax characteristics were relatively well understood and
consistent. For example the “Rule of Thumb” used for adding kerosene (#1 diesel,
Jet A) to #2 fuel to lower Cold Filter Plug Point (CFPP) was that for every 10%
kerosene added to #2 diesel you would lower CFPP by approximately 5°F. An
example would be that a 50% blend would have improved CFPP by about 25°F.
However the new ULSD has had several important characteristics changed by the
new refining processes. The catalytic cracking and hydrodesulfurization
processes remove some of the wax, it alters the size and shape of the wax seed
crystals in the fuel, lowers the aromatic content of the fuel, removes a
significant amount of the Lubricity, and lowers the fuels ability to dissipate
static electricity by as much as 100 times.
The result of this is that the ULSD fuel actually will gel at a higher
temperature than the old LSD and HSD. This problem is made more difficult
because we can no longer use regular kerosene (#1 diesel, Jet A) for cold
weather blending. These fuels are considered High Sulfur and their use would
cause the end fuel to have sulfur content higher than the allowable 15 ppm. So
refiners have had to create an ULSD #1 specifically for winter blending
purposes.
There are a number of problems with this new fuel. First, it is currently very
expensive, ranging anywhere from $.30 to $1.00 more than regular kerosene,
second it is not available in all areas, and third this new ULSD #1 is not as
effective at lowering the Cloud Point (CP) and CFPP (gel point) of the fuel. For
example; ULSD #2 when blended with 10% ULSD #1 will lower the CFPP by only 2°F
or maybe 3°F. This means that a 50% blend would only improve CFPP by 10°F.
To make this problem even more difficult, many of the diesel fuel anti-gel
additive products that have been on the market for last 5 to 30 years have
little or no effect on ULSD. The change in fuel chemistry brought about by
changes in the Catalytic Cracking processes and the addition of
Hydrodesulfurization have rendered many of the most popular products nearly
useless in ULSD.
There is a new cold weather problem that the industry has not adequately defined
as of today. We are calling this issue “Wax Dropout”. Wax Dropout occurs when
diesel fuel is “cold saturated”. This where the fuel reaches a given temperature
and stays at or below that temperature for a given period of time. This time
period is usually between 48 and 72 hours or longer and the temperature can vary
with different batches of fuel. This past winter we saw this problem at between
5°F and 10°F.
When the fuel gets to the Wax Dropout temperature, say for example 8°F and stays
there for 48 to 72 hours, the wax will suddenly agglomerate and fall to the
bottom of the container. This wax plugs filters and fuel lines until it is
removed or until the fuel temperature is raised to a point where the fuel will
reabsorb the wax.
Again there is a further complication, in that the “old” HSD and LSD wax would
gradually start to reabsorb as the fuel temperature rose. With ULSD when wax
dropout has occurred the wax does not begin to reabsorb until the fuel reaches
fairly high temperatures, often above 40°F, 50°F or even higher. This can make
the process of getting an engine with gelled fuel to run properly far more
challenging than we have ever seen before.
In the fuel distribution and fleet operations businesses, we have relied on CFPP
as
a measure of winter fuel quality for many years. CFPP is a fairly complicated
test involving using a vacuum to draw a sample of fuel through a 45 um (micron)
screen within a given period of time.
When the HSD and LSD were most prevalent and most fuel filters were 10 um there
was a good correlation between CFPP and the temperature at which a standard fuel
filter would plug. For example you could be relatively certain that a fuel
testing for CFPP of -25°F would provide trouble free operation to -15°F to
-20°F.
However the relationship is much different with ULSD. A ULSD fuel testing
-25°F CFPP might have filter plugging problems at between -5°F and -10°F. Also
CFPP does not seem to be directly related to Wax Dropout. A fuel can test for
-15°F and still have Wax Dropout at 8°F.
Furthermore, OEM engine manufacturers have changed the media size of their fuel
filters. Where 10 um has been almost an industry standard, we now see 7 um, 5
um, and even 2 um filters today. This throws the whole relationship between CFPP
and winter operability out the window. For example fuel that is at the CP can
have filter plugging problems with a 2 um fuel filter.
The industry has not yet agreed on or developed testing methods to measure cold
weather operability with the new fuels and filters.
Until such time as the industry develops a test method for determining the
relationship between CFPP, PP, Wax Dropout, and filter media size for ULSD, we
suggest the following: For 10 um filters; Take the midpoint between PP and CFPP,
for 7 um filters, take the midpoint between PP and CFPP, then take the midpoint
between that number and the original CP, for 5 um and 2 um use the CP.
Water is more of a problem than ever before. Diesel and biodiesel fuels hold
water dissolved in them. The amount of water that ULSD is able to hold is
greater than that of HSD or LSD. One of the characteristics of fuel is that its
ability to hold water in solution diminishes as the temperature decreases. Fuel
delivered at 70°F with 200 ppm of dissolved water will as the temperature drops
begin to push that water out of the fuel into droplets. These droplets can be
seen floating in the fuel and as temperatures reach and go below 32°F those
droplets freeze becoming ice crystals.
As a result many of the cold weather problems where people believe they have
fuel gelling problem are actually a fuel icing problem. If you have operability
issues in temperatures above 0°F you should check to be sure that you aren’t
dealing with ice.
Customers are regularly reporting situations where they have no water in storage
tanks, no water in vehicle or equipment tanks, but they constantly have water in
filters and separators. This is due to the dissolved water falling out of
solution due to temperature changes.
Diesel Doctor
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