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When To Treat For Cold Flow: Navigating Diesel's Seasonal Challenges
As winter beckons, a prudent eye turns towards the fuel tanks and the specter of cold flow problems that lurk in the...
The Fuel Pulse program from Bell Fuel & Tank Services was created to help customers navigate the complicated world of stored fuel testing.
Fuel Pulse offers customers the confidence of knowing how their fuel should be tested and what it means for them.
Fuel Pulse customers have a clear picture of what's important to know about the whys and the hows of the most important fuel testing their stored fuel needs – What tests should they do? What do these tests tell them? What DON'T they need to waste time and money on?
When it comes to stored fuel testing, in order to know what to do you have to know what’s important to pay attention to vs. what can be ignored. There’s so much information out there, it can all start to look like white noise.
Start with the basics. What about the fuel can tell us whether it’s in currently useable condition? What should we look at to tell us how long it’s likely to stay that way?
The Fuel Pulse program offers its customers a reliable road map to navigate through this complicated and technical world.
The Fuel-Pulse program helps customers more easily identify the tests that show both the current condition of the fuel and whether it’s likely to stay that way.
There are lots of standardized fuel tests you can consider, but the gold standard are the ASTM tests. They have procedures have been vetted and agreed upon by industry groups – the people who know best. ASTM tests have standardized procedures so they are reproducible, meaning that a test that can be done by different people in different locations at different times, and produce results that can be meaningfully compared.
In any discussion of fuel testing, you always start by recognizing that testing the fuel is about answering two kinds of questions:
1. Are the fuel’s properties what they should be right now?
2. How likely are the fuel’s properties to change in the future?
It's good to focus first on the fuel specifications - the properties of the fuel that relate to important areas of its functionality. ASTM D975 is what defines the minimum set standards for what diesel “fuel oil” should be.
Generally speaking, ASTM D975 tells us that an ideal diesel fuel is one which flows easily at all temperatures, is clean and free from foreign contaminants and separated wax, ignites readily and burns quietly, cleanly and economically. These descriptive ideals are reflected in the diesel fuel’s specifications.
If diesel fuel meets all of the D-975 standards, then what we just said will be true of it. Diesel fuel that meets these standards will be diesel fuel that gets the job done when you need it.
Each fuel specification matters because it’s a measurement of a property that contributes to the fuel behaving in a desirable way. Many times when a fuel does not meet a given specification, that’s when you may see symptoms of a problem in how that fuel is behaving, whether in combustion system you’re using it in or in the storage and distribution system it lives in.
So let’s run through the list of diesel fuel specifications - what they mean, how they’re measured, and what you’re likely to see happen if a diesel fuel does not meet that particular specification.
What are these properties that diesel fuel has to meet minimum standards on? Many of these are specifications you may have heard referenced in other areas. As we go through them, we’ll start to see how these properties really are important for making sure diesel fuel does what we need it to do.
Diesel fuel specifications (and specs for gasoline, too) can be divided into two classes: ones that influence combustion, and all the rest of them. In considering them, we should ask
First, the combustion-related specifications.
Density is the weight of a unit of volume, typically measured by a hydrometer.
Density is linked to the heating value of the fuel. Higher density fuel produces more smoke but more power (because it has more energy value per unit of volume).
The very first one we look at isn’t technically defined in the ASTM standard. So maybe you’re wondering why include this? Density is an essential property of diesel fuel, but it’s not defined by an ASTM test because it’s actually governed by the results of other standards that we’ll see later, like Distillation and Aromaticity and Cetane Number. If those tests are within spec, it means the density will be within ideal range, too.
282-338 deg C for 90% recovery
Volatility is expressed in terms of the temperature at which successive portions (%s) are distilled from a sample of fuel under controlled heat. Values are usually determined for 10% and 90% recovery, as well as initial boiling point. Distillation or boiling range influences other properties like viscosity, flash point and density.
Fuel refineries can try to hit distillation ranges by altering the amounts of low or high temp components blended into the fuel. Raising the back end (90%) temperature will raise the cloud point. Lowering the front end (10% or IBP) temperature lowers the flash point and raises the vapor pressure, which can result in vapor lock.
The 10% recovery temperature reflects how well the fuel will start to vaporize. The 90% recovery indicates the extent to which complete vaporization of the fuel may be expected in the combustion zone. Excessive high boiling components leads to engine deposits and more smoke. Diesel fuel with unsatisfactory distillation temperatures may be contaminated with other compounds like gasoline.
This has both physical distillation and simulated distillation (a shorter version of the test). They heat the liquid up until it turns into a vapor, where it will rise up in a column and start to cool. As it cools, it turns back into a liquid and is collected.
Distillation tests are looking for the temperature at which 90% of the original liquid volume is condensed back and collected. To meet the standard, No. 2 diesel distillation temperature can be between 282 and 338 deg C.
Distillation temperature matters for a couple reasons: It speaks to whether the diesel fuel is going to combust as fully as needed within the typical conditions of a compression ignition engine. If the temperature is too low or too high, it may not fully combust or it may burn too quickly or too early.·
Also, distillation temperature is actually related to some of the other properties of the fuel, like Cetane Value and % Aromatics. Keeping the distillation temperature within the required range helps ensure that the diesel fuel has the right composition of molecules in it, like the ratio of light end components vs heavy ends.
1.9 - 4.1 Mm2/S
Viscosity Is Resistance To Flow. Higher Viscosity Means Greater Resistance And Slower Flower. Viscosity Is Measured With An Apparatus That Measures The Amount Of Time Needed For A Fixed Amount Of Liquid To Flow Through A Capillary Tube Viscometer Immersed In A Temperature-Controlled Bath.
Fuel with viscosity that falls outside the specified range cannot be properly metered/injected into the combustion chamber. Changes in viscosity will alter the injector spray penetration rate, the cone angle and the droplet size distribution. If the fuel atomization is subpar, as such, you won’t get optimal mileage and performance and you’ll have higher emissions. Excessively high viscosity makes fuel difficult to flow in cold temperatures.
We’re all at least somewhat familiar with the concept of viscosity – the expression of internal friction of a liquid. Thick liquids like honey have high viscosity values, thin liquids like water have low values.
No. 2 diesel has to have a viscosity between 1.9 and 4.1 mm2/S. The question might be asked, why does viscosity matter for diesel fuel? Because viscosity affects how the fuel can be injected by the injectors (which can affect other things like emissions and engine performance).
40 in the United States, 45 in Europe and 50 in Japan
Index is a calculated value derived from fuel density and volatility (distillation). Illustrates the readiness of a fuel to ignite when injected into a diesel engine. Cetane number itself is calculated using a specialized engine, which is more expensive. Test results are compared to the index values of a high and low cetane reference fuel, and results calculated from that.
Diesel fuel with poor cetane number/index will have poor ignition quality. You’ll have less power, noisier and rougher running engine. Also linked to cold starting difficulties.
We’re familiar with the importance of cetane in diesel fuel. And 40 is the minimum value defined by the regulation in the United States. What we can also see in D975 is that there is an additional requirement that says, you either must have a Cetane Index test that also meets a 40 minimum or an Aromatic content test that has a maximum of 35%.
38 Degrees C for #1 Diesel; 52 Deg C for #2 Diesel
Temperature At Which The Fuel Has To Be Heated To Produce A Vapor/Air Mixture That Will Ignite When A Flame Is Applied. Flash Point Relates To Front-End Volatility Of The Fuel. Refineries Have To Choose The Right Light Refinery Streams To Be Included In The Fuel Blend So That The IBP Remains Sufficiently High Enough. For Pensky-Martens, A Fuel Sample Is Heated Slowly In A Covered Cup At A Constant Rate. At Regular Intervals The Cover Is Opened And A Flame Is Introduced. The Procedure Is Continued Until The Fuel Temperature Is High Enough For Flash Ignition To Occur.
Low flash point influences safe handling of the fuel. Has no significance on performance in a diesel engine. Variations in flash point will not influence autoignition temperature.
The minimum temperature at which something will ignite in the presence of a flame or ignition source. This is different from autoignition temperature, a higher temperature that speaks to how hot something has to be to self-ignite without any ignition source.
Flash point matters because of how diesel fuel works in a compression ignition engine. If the flash point is too low, as in gasoline (which has flash point of -45 deg F), it will prematurely ignite and damage the engine.
35% maximum aromatic content
Many times, it is calculated using an equation that factors in density and viscosity.
Excessive aromatic levels affect fuel emissions when it is combusted. Aromatic content also inhibits microbial growth, so putting a cap on aromatic content makes the stored fuel more likely to support microbial growth.
Aromaticity - the fuel’s aromatics content - can affect the distillation curve temperature, can produce harmful emissions, and can produce excessive ash. So you don’t want too high a content of them. They also inhibit microbial growth, which means limiting aromatics to make the fuel “greener” makes it more likely to have microbial growth problems.
Now, the other important specifications that don’t directly influence combustion, but which are important to stored fuel’s proper functioning over time.
D2500 or D4539/Yes
No stated value in the D975 specificatiojn
Cloud point = temperature below which wax crystals will precipitate out of the fuel, giving a cloudy appearance. CFPP (Cold Filter Plug Point) is the predictive temperature at which enough wax comes out of suspension that it blocks fuel filters and shuts down engine operation.
We know what this answer is.
ASTM D975 mentions both Cloud Point temperature and CFPP (cold filter plug point). But what’s interesting is they don’t give required values for those. In note J of the spec, they say “it’s not realistic to require minimum temperatures for fuel because they’re so variable”. The specification only gives guidelines, but not requirements.
2-3 mg/mL is typical
This measures the stability of the fuel i.e. ability to resist sediment formation when subjected to accelerated oxidative stresses like prolonged exposure to atmospheric oxygen.
A sample of fuel is heated to 95 deg C and maintained for 16 hours in the presence of continuous oxygen. Fuel is then cooled and solvent washed to recover gummy residues that may have formed. They may also measure the color of the fuel before and after using the D1500 test described below.
There’s also the D4625 test which takes 100 days (fuel held at 43 deg C) and is believed to correlate with ambient storage conditions for one year of storage. However, this takes too long to be used for routine quality control at the refinery level.
Excessive gum formation predicts lack of storage stability of the fuel.
Fuel instability is responsible for numerous phenomenon, including deposit format in injectors and combustion chambers, as well as subpar emissions products and engine operation.
D6468 or D1500 or NOF-21 or Dupont test
70% is the satisfactory cutoff
This test is used to predict how well a fuel will resist sediment formation upon longterm exposure to higher temperatures. Two 50ml fuel samples are heated to 150 degrees and held for 90-180 minutes with oxygen exposure. Fuel is then cooled and filtered. The filter pads are then exposed to light to measure their reflectance. Higher amounts of filtered insoluble yields a lower reflectance score, expressed as a % reflectance. Reflectance scores of 71+ are considered good. 62-70 is “fair”. Below 70 is marginal or worse.
The color of the fuel sample can also be logged before+after using the D1500 color comparison test. Typically, an aged fuel sample will end up with a higher (darker) color score.
Higher levels of filterable insoluble predicts fuel that is more likely to form gums and deposits in storage over time.
The Thermal Stability test is designed to illustrate a fuel's tendency to form sediments if exposed to elevated temperatures for an extended period of time. It does not have as much overt applicability to real world fuel storage as oxidative stability does. However, Thermal Stability does have predictive use to illustrate deposit formation inside of an engine, where these elevated temperatures exist.
D2709 or D1796/Yes
0.05% by volume, maximum
A 500ml sample of fuel is mixed with 500ml of water-saturated solvent in each of two graduated tubes. Samples are centrifuged to concentrate the water and sediment at the bottom of the tube. The sum is reported as the % of water and sediment.
Excessive water content can encourage growth of microbes and may block filters. Excessive sediment content may be a sign of fuel instability and can cause wear in injectors. Corrosion and wear in the engine and fuel injection system.
Water comes from any number of points in the distribution system. Sediment content can be from inorganic deposits (rust in bottom of the tank) or organic deposits (degradation of fuel, bacterial action, wax settling that does not dissolve).
Water And Sediment Content is defined in D975 and relates to the amount of contamination present in the fuel. It’s also been strongly linked to the presence of microbially-induced corrosion (MIC) present in storage tanks. So it’s an important test to pay attention to.
0.01% by mass, maximum
A small sample of fuel is burned in a weighed dish until all combustible material has been consumed. The unburnt residue is weighed and reported as a percentage of the original fuel sample.
All diesel fuel contains small amounts of ash-forming material like suspended solids and soluble organometallic compounds. These can cause damage in the close tolerances of fuel injection equipment. Contibute to increased deposit levels and abrasive wear of piston rings and other components in the high temperature zones of the engine (where the fuel gets burned).
On the face of it, this one seems pretty important. You want the fuel to burn cleanly and completely, without producing leftover ash which can damage areas of the engine. Ash itself comes from a combination of partially-burned carbon components (like aromatics) and inorganics (like metal content). Obviously you don’t want much of that if you’re burning diesel fuel in, say, a truck engine.
0.15% maximum for #1 kerosene, 0.35% maximum for #2 diesel fuel oil
Test measures the amount of higher BP, lower volatility components are present in the finished fuel (some entrainment of higher bp materials occurs during refinery distillation). Carbon residue in diesel is already low, so the test is run on the 10% residue from the laboratory distillation test to improve accuracy. A 4g sample of 10% bottoms is placed in a glass cooking bulb and heated at 550 deg C for 20 minutes. After cooling, the bulb is reweighed to determine amount of residue.
Carbon residue test indicates the tendency of the fuel to form carbon deposits, although it does not predict injector coking.
This is a measurement of the residue the fuel is going to produce at high temperatures, such as those you might find in a diesel engine. As with ash content, you don’t want a lot of that. A high score on this means you’re probably going to have combustion chamber deposits formed.
Measured on an HFRR machine, which wears the size of a wear scar left behind by steel balls immersed in the liquid and subjected to measured pressure for a defined period of time.
Unsatisfactory lubricity can predict the inability of the fuel to protect injectors and fuel pumps from premature wear. Fortunately, the addition of low-level biodiesel to diesel fuel has solved the lubricity problem.
Also refers to film strength. Lubricity is the ability for the liquid to lubricate essential moving parts in the engine. All of these diesel fuels are capped at an HFRR score of 520, in other words, they want to be lower than that.
15 ppm for ULSD
Typically measured by some kind of mass spectrometer, which can detect the weight % content of sulfur.
Excessive sulfur in the fuel can poison the emissions catalyst. It can also lead to big regulatory fines if detected.
Sulfur content is controlled by regulations to help the environment. We are all well familiar with the sulfur issue, and D975 is one of the places that spells out the requirements. 15 ppm for the S15 diesel fuels, 0.05% (which is 500 ppm) and 0.50% (5000 ppm) for the S500 and S5000 diesel fuel oils.
No. 3. A score of 1-3 indicates tarnish but is permitted. A score of 4 indicates corrosion and is not permitted.
A polished copper strip is immersed in a portion of fuel and heated for 3 hours at 50 deg C. It is removed, washed and compared with an ASTM standard chart. Copper is used because copper and its alloys are sensitive to attack by certain sulfur compounds, so they are a good indicator.
A failing grade on this test implies that the fuel is excessively corrosive and may attack metals in the fuel distribution and storage network.
This test seeks to verify that the diesel fuel isn’t going to cause corrosion damage by long-term contract with metal parts. The test itself is run by taking a strip of copper metal and immersing it in test liquid for a define period of time and temperature. The strip is then compared to a visual chart and given a grade to illustrate its resistance to corrosive change.
ASTM D975 lists a lot of specifications and they’re all important. But for stored fuel, they’re not all essential parts of best practice condition monitoring.
Fuel-Pulse customers have a guide to help cut through the jungle of options and focus in on what tests are going to be the most important for them.
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