Stored fuel is the life blood of business and government, yet it’s also one of the things most taken for granted by those that need it the most. Out of sight, out of mind, it seems. Diesel fuel is purchased and kept in inventory in storage tanks, generators and emergency management systems, waiting for the time when it will be called upon to be used. The expectation is that when the need arises, the fuel is going to do what it’s supposed to, and in turn, the engines and systems it powers will get the job done that they need to do.
So it can be a real shock when stored fuel fails to do what it’s supposed to, when it’s needed most. In fact, in an emergency, fuel issues are the #1 cause of equipment failure. And when you dig into these “fuel issues”, it becomes apparent that diesel fuel contamination is a primary problem that deserves both blame and consideration.
Solve diesel fuel contamination issues and you can prevent a lot of headaches. But contamination in diesel fuel is a broad umbrella. There are multiple culprits to look at. And they can all be interrelated as well – contributing to and supporting the development of each other in the fuel.
The fuels landscape is completely different than it used to be, for a number of reasons. Changes to fuel, driven by stringent environmental regulations, have given the marketplace fuels that need more care than before. Engine designs have evolved to meet more stringent emissions regulations, and these engines require a much higher level of fuel cleanliness. These changes make apparent the greater need for preventing and managing fuel contamination.
For contemporary fuels, there are three basic areas of contamination concern – water, inorganic debris (sand, dust, rust), and organic debris (products of fuel breakdown, microbial products and the waste products of fuel deterioration). These make up the bulk of the contaminants normally found in fuel and fuel oil.
Specific to “organic debris”, many times people mistakenly refer to microbial contamination as diesel fuel algae, when in fact, it has more to do with petroleum gums, varnishes and lacquers than it has to do with “algae”. Algae doesn’t grow in storage tanks (because it needs light to grow). But other kinds of microbes – bacteria/molds/fungus - they very much do grow and cause problems in stored fuel.
Fuel management and operations professionals who have been in the industry for a long time can readily see that there’s something fundamentally different about today’s fuels vs. the fuels from decades past. If they can’t see it in the fuel itself, they definitely see it in the problems that develop more readily than before. It’s important to have a clear understanding of how fuels have changed if you’re going to understand these fuel contamination problems and what to do about them.
Fuel used to be much more “stable” when you stored it than they seem to now. In the 1960s, the US Army did a study of fuel storage life. What they found was gasoline storage life could be expected to be 2-5 years. They also found that diesel fuels could be stored for 10 years or more without a problem.
Today, it’s not even close. The expected shelf life for common gasoline (E10) is 90 days. Diesel Fuel #2 degrades 26% in the first 28 days, and may be up to 95% if it has water in it. So fuel storage life is way, way down.
So what changed? Simply put, refineries are under pressure to produce more fuel from the same amount of crude, in order to meet ever-growing demand, they’ve had to develop new processes and methods to maximize that yield – processes like hydrocracking. In a nutshell, they break the larger molecules in crude oil into smaller molecules that can be turned into fuels like gasoline and diesel. So they get more diesel fuel and gasoline from each barrel of crude oil.
While this is a good thing, the fuels they produce in this manner are less stable and start out with large proportions of “unstable precursors” – molecules that will turn into sludge and gums and deposits and varnishes.
The move towards ultra-low sulfur diesel fuel (ULSD) in the last ten years has been great for the environment. Millions of tons of sulfur gases have been prevented from entering the atmosphere, and that’s good for things like preventing acid rain. But these ULSD fuels attract more water and are less resistant to microbes than the higher sulfur diesel fuels from before 2006. So when you’re linking this to the issue of fuel contamination, this fuel change means that today’s diesel fuels are far more likely to develop contaminants like microbes, biomass, sludge and water.
With these considerations in mind, consider all those thousands of old stored fuel tanks scattered across the country. Some have partial or trace amounts of old diesel fuel in them, and many of those have had newer ULSD added to them. Stored fuel like this doesn’t tend to be checked until there’s an apparent or urgent need to do so – like the threat of a major hurricane.
What do you think’s going to happen with those tanks and the fuel when we get the next hurricane blitz? They’ll go to use the fuel and find some serious fuel contamination problems.
Water build-up in diesel fuel tanks is a universal problem across the nation. Almost any stored diesel fuel left for any amount of time will end up with water in the bottom of the tank. And given the changes to the properties of ULSD fuels, it’s an even bigger issue than in the past.
How does water get into fuel storage tanks? There are lots of possibilities. Water enters fuel storage systems through tank vents, but unlike particle contamination, water is not necessarily driven by the level of fuel in the tank, but rather by fluctuations in environmental conditions like temperature and humidity. Outside air packed with water vapor travels in and out of the tank. In storage tanks, the water from the air condenses and rolls down the side of the tank when the air cools down in the evening.
Some of the blame for water in diesel can also be laid at the feet of the diesel engines themselves. In diesel vehicles, the temperature change comes from hot diesel fuel returning to the tank after being used to cool the injectors. Injectors get hot due to their tremendous pressures. The engine uses diesel fuel circulated from the tank to dissipate some of this heat. The now-hot fuel is then circulated back to the fuel tank. This temperature difference causes water condensation even in this vehicular environment.
Today’s fuels like to hold on to water. The common practice of blending low levels of biodiesel in conventional diesel also accelerates water problems, as biodiesel is hygroscopic and migrates toward any water presence in the fuel. Another culprit for increased water in today’s diesel is the fact that, in ultra low sulfur diesel, the loss of naturally occurring lubricants must be compensated with lubricity additives to protect the moving components of the engine that rely on fuel as a lubricant. These lubricity additives increase fuel surfactancy, which has an unintended effect of increasing the stability of water trapped in fuel. What this basically means is that adding of lubricity agents to diesel fuel makes it easier for water to become emulsified in that fuel.
All of these are good enough reasons to control the build-up of water in the storage tank; this is typically done by using some kind of concentrated fuel treatment. Unless the amount of water in the tank is substantial, in which case, the best course of action would be to pump the water out as part of a fuel PM program. Even the best chemical water-controllers for fuel have their limits. The best course of action for you would depend on the size of the tank and amount of water in it. This is where having a knowledgeable partner becomes a real advantage.
Water can be found within fuel as free water, dissolved water, and emulsified water. Dissolved water is dispersed in fuel molecule by molecule, and the typical diesel fuel can’t hold more than 200 ppm of dissolved water content. Once the amount of water exceeds the maximum level for it to remain dissolved, water will fall out of the fuel and form a fuel-water emulsion, with small water droplets suspended in the fuel. As the water content increases beyond this, the water will turn into free water that exists as a separate layer at the bottom of the tank.
Typical water contamination levels are able to fluctuate widely. Lab testing can show that a ULSD fuel can often have water saturation approaching 50 ppm at 50° and close to 200 ppm at 100°. This amounts to 1.7 ounces of water dissolved for every one hundred gallons of fuel. When fuel in the tank and airspace above it cool to 50°, the dissolved water holding capacity of the fuel is reduced to its starting value, and the calculated amount of water is dropped out of solution and into the fuel tank as free or emulsified water.
Storage of diesel fuel, especially ultra-low sulfur diesel and diesel fuel with small amount of biodiesel, for long periods of time also makes them more susceptible to contamination by microorganisms like bacteria and fungus.
Despite most best efforts, water inevitably collects at the bottom of the tank. This provides the necessary environment for microbes to grow and flourish in fuel – they live at the interface with the water and fuel, drawing their necessary elements and nutrients from both phases. Pretty soon you’ve got a microbial infestation that produces slimy “mats” which float on top of the fuel. The microbes multiply, excreting acids from their biological processes which both corrode the fuel tank and accelerate the breakdown of the diesel fuel, leaving you with a tank of nasty, poor quality fuel.
The earlier mentions about how fuels have changed become important to understand here. Today’s ULSD fuels have virtually no natural resistance to microbial growth because of the removal of the fuel sulfur. Sulfur, historically, has functioned to retard microbial growth – they don’t like to be around it. And it’s not just sulfur, it’s also the mandate reduction in aromatic content of ULSD. Microbes don’t like to consume aromatic hydrocarbons as a fuel source. If you reduce their proportion in the fuel makeup, you increase the amount of the things microbes do like to feed on.
So while ULSD fuels are much better for the environment, they are extremely prone to microbial growth that may happen very quickly. It is not uncommon to start with a completely clean and sterile fuel storage tank and see evidence of fuel microbial activity within 1-2 months.
As you can guess, microbial contaminations are most common in situations where the fuel is stored for long periods of time, and also more common in marine situations where the fuel tank is around water. How do you know if you’ve got an infested tank? You’ll probably notice rough running and poor performance with your vehicle or boat. Fuel filters will clog more often and (if you have a storage tank you can see the fuel in), you should be able to see slime floating on top of the fuel (along with foul sulfurous odors). All of these are strong indicators that the diesel fuel tank has a microbial problem.
How do they get in the tank? Microbes are all around us, so it’s not that difficult. There can be ingress from the environment through dust and water, or even fuel drops themselves. The microbes get into the tank from the environment (such as in the air that circulates in the tank, or attached to dust particles), or attached to water vapor. You just can’t keep them out.
Microbes grow in the fuel tank, multiply, and produce biomass and slime growths which will plug filters in the storage tank. So, if you’re a fuel or tank manager and you notice an increase in the rate of filter changes needed, that’s one sign you need to look closer at what’s happening in your tank.
But this isn’t the only thing to be concerned about. The problem is easily spread to vehicles and other tanks through fuel transfer. This is another reason why it’s important to deal with the problem when you find it. If it spreads to things like vehicles and equipment, filter plugging issue becomes a real problem because this shuts down the vehicle or engine, which takes the machine out of commission, and depending on what you need it to do, an extra added expense and headache.
Microbes are heavily linked to accelerated breakdown of fuel stability because, not only do they produce biomass, but they give off acids and biological byproducts that attack and break down fuel quality. This creates the same resultant problems as with unstable fuel, such as the problems that we saw as we talked about fuel instability from cracked feedstocks: engine deposits, black smoke and emissions, rough, inefficient engine operation.
This last problem is the biggest concern, perhaps, for critical use system. Critical use systems and emergency management systems – they’re different from fleets and those kind of users. They store fuel because it’s intended to be used in an emergency. There’s a high value placed on that stored fuel performing properly when it needs to. But because of the nature of these things, you can’t predict when that’s going to be. Emergency backup systems aren’t like fleets which have predictability in fuel usage. And that makes it more likely that, unless there are rules in place that force users to check and/or test fuel at given specific intervals, it’s very likely that this kind of emergency backup fuel may not have been checked in a while. Out of sight, out of mind. And that can lead to unfortunate situations like we’ve seen in the recent past.
When the fuel quality is degraded, it increases the chance of system failure and engine shutdown at critical times when it can least be afforded. This has been seen in the past, during Katrina, Sandy, and after the 2005-2006 hurricane blitz across the Southeast. Users in all of these times had emergency generators that would not start, or they tested the generators afterwards and found widespread system failures because the fuel quality had been destroyed by microbial growth in the tanks. And they didn’t even know it until it was too late. These ended up being huge costs, just as a result of microbes in stored fuel.
The last major problem we see centers on microbial presence in fuel systems over a long period of time being strongly associated with corrosion damage in storage tanks, commonly called microbially-influenced corrosion or MIC. In these scenarios, biological acid production from microbes attack and damage metal surfaces. Or they cause cathodic corrosion as they grow and flourish behind the protection of biomass formations. Some kinds of bacteria have been known to perforate a 5mm thickness of 316 steel in little more than one month. This may be a rather extreme case, but it does serve to illustrate that microbial corrosion can cause serious damage in shorter time than you might think. This damage is expensive and needs to be repaired when it happens, often running into the thousands per tank. If you’re in charge of purchasing or maintaining storage tanks, you know the scale of expense that we’re talking about.
Sludge buildup happens because diesel fuel forms heavy polymers that drop out and collect at the bottom of the tank, resulting in a sludge buildup in the storage tank itself.
This is a problem because, first, sludge like this represents energy value that’s not contributing when the fuel is burned. And second, sludge can plug filters and contribute to engine deposits if it happens to be consumed by another piece of equipment.
Organic contaminants in ultra low sulfur diesel can damage elastomer fuel filter seals and shorten fuel filter life. In this case, the organic contaminants are the result of free radical reactions in the diesel fuel. These reactions are accelerated in ultra low sulfur fuel due to the removal of naturally occurring antioxidants during the hydrotreating process for sulfur removal. As a result, a large number of peroxide molecules are generated. These peroxides promote oxidation and polymerization reactions and have a damaging effect on elastomer seals in fuel systems.
Not only are today’s fuel susceptible to formation of solids in storage as a result of their inherent instability, they can also be affected by particle contamination from other sources. These particles in fuel can be road dust, engine rust, wear particles or any other hard particles. Particle contamination gets into diesel fuel in multiple ways. The fuel itself can pick up particles during transit, even from gas pump to gas pump. Particles can also ingress through the tank vent. As the fuel tank is drawn down, ambient air is drawn into the tank, providing a source of particle contamination. Wear debris from fuel system components is another source.
Particle contamination gets into diesel fuel in multiple ways. The fuel itself can pick up particles during transit, even from gas pump to gas pump. Particles can also ingress through the tank vent. As the fuel tank is drawn down, ambient air is drawn into the tank, providing a source of particle contamination.
Historically, fuel cleanliness levels are ISO rated at 22/21/18, which means a particle count of 20,000 – 40,000 per ml for particles of 4 microns or higher, 10,000 – 20,000 per ml for particles of 6 micron or higher, and 1,300 – 2,500 per ml for particles of 14 micron or higher. But today, diesel fuel pump manufacturers are requiring fuel with ISO cleanliness counts of 13/9/6 or better at the injector. This difference represents a 1000-fold reduction in contamination between the fuel pump and the time the fuel reaches the injector system. Today, particles significantly smaller than 4 µ in size are potential wear contributors to important parts.
The storage problems that ultra-low sulfur diesel fuels experience mean it is more important than ever before for businesses, emergency backup and fuel storage entities to keep preventive maintenance in mind for their fuels. Today’s diesel fuels have enough problems that they cannot risk their fuel not performing exactly how they need it, when they need it.
There are options available, such as Fuel & Tank Services companies that will partner with them to both solve existing fuel and tank storage problems (like microbial problems, water buildup and sludge or biomass present in storage tanks). The best fuel and tank services partners take a more comprehensive approach to fuel and PM care, bringing to the table the essential hybrid combination of
Anyone who relies on consistent stored fuel quality to get the job done would be well to keep this in mind.
Problem: Buildup in the bottom of storage tanks. Takes energy value out of fuel. Burns incompletely if drawn into the engine, leaves deposits.
Which change is it caused by? Cracked fuel instability, microbial destruction of fuel quality
Who might experience it? Those that store ULSD.
Preventive and remediative measures: Fuel stabilizers will slow the formation of sludge. A good stabilizer treatment should have certain characteristics (you’ll want it to have): Antioxidant, Dispersant, and Metal deactivator. They should also have a low treat rate, at most 1:2000 . The most effective ones will be be less than that. If you have to use more, chances are you don’t have one of the better ones available. For best effectiveness, stability treatments have to be added early in the fuel’s life, because they work to stop chain reactions before they start. They won’t reverse chain reactions.
Anti-microbials can prevent the formation of sludge induced by microbial activity. Use an anti-corrosion tank treatment that also absorbs sludge. The best such treatments combine both of these benefits for better value.
Which change is it caused by? Removal of sulfur and aromatic from diesel fuel, yielding a fuel with less resistance to microbes.
Who might experience it? Those that store ULSD fuel and those that use fuel from infected tanks
Preventive and remediative measures: There’s a lot to be said for this, split into two categories.
Housekeeping – reducing conditions that might lead to microbial growth
For housekeeping measures to prevent microbial growth, control of water buildup in tanks is the biggest factor.
Existing microbes – essential steps to get rid of an existing problem
For existing problems, use biocides to kill microbes (water control additives won’t do it). You cannot kill microbes simply by removing water bottoms. You ESPECIALLY cannot get rid of a microbe problem through simple fuel polishing. YOU HAVE TO USE A BIOCIDE.
The best biocides should incorporate the following characteristics
One thing to note is that all biocides get used up. No biocide lasts forever. 30-60 days is the industry best.
If they don’t have an EPA registration number, they aren’t registered and it may be illegal to use it in your particular state.
We have a lot more information on best practices for solving these types of problems. If you or your customers think you may have this kind of issue, talk to us. We’ll be happy to help.
A biocide only works if it is applied properly.
You need to remove as much of the tank water as possible.
After biocide addition, fuel circulation is essential, as biocides must come into contact with microbes for it to work.
Biocides will take a certain time to work, so you must wait a minimum of 8 hours before using the fuel, to allow for complete kill and to allow dead microbial mass to settle. The dead microbes will still need to be filtered out.
Lastly, sometimes it’s worth considering a biomass dispersant to pair with the biocide to increase its effectiveness.
Fuel filtration is an essential element for managing and remediating fuel contamination. The need for filtration of diesel fuels goes all the way back in the 1930s or before, with a 1931 Caterpillar owner’s manual stating that 90% of diesel problems were due to dirt or water in diesel fuel. Today’s common rail fuel injection systems have critical filtration requirements due to increased operating pressures and tighter clearances for injectors and fuel pumps.
Today’s diesel engines generally have two filters to deal with the issue – a primary and a secondary. The primary filter usually has efficiency ratings from 7 µ to 25 µ. Secondary fuel filters are placed between the transfer and high-pressure injection pump. The secondary filters protect the high-pressure fuel pump and sensitive fuel injection components from particles that can cause wear damage. Typical ratings for secondary filters in high-pressure common rail fuel systems are in the 4 to 5 µ efficiency range.
Some diesel systems will also use filtration to deal with water, which is generally removed from fuel using a stripping or coalescing mechanism. A coalescing filter separates water by allowing it to pass into the media where droplets are captured and combined to form larger droplets which are finally released on the downstream side. As water droplets in low interfacial tension fuels form stable emulsions, both strippers and coalescers can become ineffective and cannot achieve required water filtration efficiency.
In terms of particle filtration efficiency, beta ratio is still a quantitatively way to define filter medium effectiveness to keep diesel fuel relatively free of damaging aware particles. Beta ratios are defined as the ratio of the number of particles upstream to the number particles downstream at a specific particle size. Beta ratios unfortunately are often inadequate measures of fuel filter performance, being derived from standardized multi-pass fuel filter tests while on-vehicle filtration generally happens in one single pass.
If you see yourself in any of this, it’s important to take stock and see an honest picture of where you stand. How prepared are you to deal with fuel contamination? Start by asking yourself these “fuel ready” questions.
Depending on what industry you’re in, you might have requirements handed down to you that require storage tank inspection on a regular basis. But for many, storage tanks may go years without being looked into. And while those tanks are sitting there, problems with water and sludge and microbes are growing inside. The longer they stay undetected, the worse the consequences.
Recommendation: Check tanks for water and sludge at least quarterly. Use water paste and a tank stick to gauge water depth in storage tanks. If you don’t want to do it yourself, contract out with a qualified group who can do it for you.
Just as with storage tanks, you may already be subject to requirements to do fuel testing at given intervals. Hospitals and critical use facilities typically fall under these requirements. For everyone else, the most common stance is a reactive one – if you don’t see problems in the present tense, everything is fine. You’ll deal with problems if you come across them. On the other hand, fuel sampling, even if it’s just once a year, goes a long way in identifying future problems and heading them off.Without fuel testing, you have no sure way of knowing these critical problems exist.
Recommendation: Sample the fuel in each storage tank for testing at least once a year. There are only a small number of tests that you need to pay attention to - the tests that tend to be required for critical use installations, meaning they are considered most relevant to ensuring that stored fuel will perform properly in essential situations.
Where to have tests like these done? There are plenty of testing laboratories available that can do these tests with a pretty quick turnaround.
Recommendation: Seek to establish relationships with a service provider company that has existing relationships with certified labs. Many times, such a partner can get the same tests done at a fraction of the normal “a la carte” cost you would have paid, and the company will also take care of packaging up the sample, submitting it for testing, and apprising you of the results.
Critical use facilities have certain specific fuel tests specified for them because there can be disastrous consequences if the fuel fails one or more of these specs while being called upon to perform in an emergency. Here are some of the most common ones:
Essential Test #1: Distillation A test that measures the combustion properties of the stored fuel and ensures the fuel burns properly in the engine.
If the distillation curve of the stored fuel is off, the engine will run roughly, make black smoke, or may not even start at all.
Essential Test #2: Sulfur Content Measure the sulfur content to ensure it does not exceed the legal limit. Off-road diesel used to be exempted from the 15 ppm sulfur cap applied to on-road fuel. That exemption has been phased out. Which means everyone, including municipalities and stored fuel professionals, will need to be monitoring the sulfur content of their stored fuel.
If the sulfur content is too high, it puts you at risk for violating environmental laws.
Essential Test #3: Water & Sediment A test that documents the level of free water and sediment present in a fuel sample. Excessive levels of either one indicates the fuel is unstable and also prone to microbial contamination.
If the Water & Sediment test fails, the fuel is probably unstable and may not be useable. It will burn poorly in the engine, and create black smoke and harmful deposits in the engine and injectors.
Essential Test #4: Cetane Index Measures the “combustion quality” of diesel fuel. Having the right cetane rating in diesel fuel is essential for any diesel engine to run properly at starting and in cold weather.
Inadequate cetane rating has significant effects on the running of the engine – rough and noisy operation, higher instances of black smoke. It also makes a diesel engine difficult to start, which is a death sentence for emergency generators and critical-use systems.
If you have microbes in your fuel tank, it’s inevitable that you’ll have big problems sooner rather than later. Fuel that’s been sitting around for years, such as in limited-use backup generators, is virtually guaranteed to have significant microbial growth in those little-used storage tanks.
Microbial growth in fuel storage tanks is the single biggest cause of fuel problems and destroyed fuel quality. They plug filters, change the pH of stored fuel, and cause damaging tank corrosion. Any time you have water present in a storage tank, microbial growth isn’t far behind.
Recommendation: Do a microbial test on a fuel sample once a quarter, to help head off destructive microbial growth in fuel storage tanks. ATP testing is the best practice because it will give you a quantitative measurement of the problem instead of a yes/no answer that may not be as useful. If the microbe levels come up high, take appropriate action. But be sure it’s the right action to take if you want to solve the problem.
If the fuel is tested and it fails one or more of these essential measures, it’s time to consider a solution.
In the absence of defined testing, if you notice changes in the way your equipment runs, that also signals that it’s time to look more closely at solutions. And if you have a positive microbe test, then you REALLY need to take action. But what action is the best to take?
Some people, like fuel polishers, insist that mechanical solutions, like filtering and polishing, are the only way to go. For them, fuel polishing will do anything – some have even claimed they can “polish” sulfur right out of diesel fuel. Other parties – we’ll call them ‘additive makers’ – want you to believe anything and everything can be solved by some magic potion you add to the fuel. They advocate for a chemical solution only for whatever problem they see. To them, chemicals will solve everything.
The real truth is somewhere in the middle. Some problems respond very well to chemical treatment while other problems can only be solved by a mechanical fix. And there are other problems that need the hybrid approach – a combination of both chemical and mechanical solutions that work synergistically with each other to produce the best results.
The diesel fuel that Fleet and Facility Managers rely on has changed dramatically in recent years, and so has the practice of maintaining fuel quality. New problems and solutions have evolved and forward-thinking professionals now know that they must adopt a fuel-specific preventative maintenance program.
We understand that everyone is being tasked to do more with less. There are lots of different options to consider and getting it right is the only option. After all, it could be a matter of life and death!