How Long Does Diesel Fuel Last & Tips to Store it for Longer

How Long Does Diesel Fuel Last?

First, the quality of the diesel is not perpetual — diesel fuel can go bad over time. The fuel can only be stored from 6 to 12 months on average — sometimes longer under the best conditions. Generally, to prolong the life of the quality of stored diesel fuel, it should:

  • Have been clean in the first place;
  • Be kept cool at around 70 degrees Fahrenheit;
  • Be treated with biocides and stabilizers, and
  • Be maintained properly in accordance with NFPA

When the above conditions are not met or are not met adequately, three threats to the quality of the diesel may arise:

  1. Hydrolysis;
  2. Microbial growth; and
  3. Oxidation.

The presence of these three factors can shorten the lifespan of the diesel, and as such, you can expect the quality to degrade quickly after 6 months. Below, we discuss why these three factors are threats and provide tips on how to maintain the quality of the diesel and prevent the occurrence of these threats.


Unfortunately…Even Fresh Fuel Can Be Contaminated!

Clogged filters and rust on the filler cap are usually the first visible signs that you’ve got these organisms in the tank. But even a quick glance at your fuel can tell you if something’s wrong.

All of these samples (pic above) were taken from the middle of diesel tanks with various states of overgrowth.

If left alone, these organisms will continue to grow and you run the risk that your generator will shut down during an emergency.

With the combination of price and fluctuation factors, the trend has been to purchase diesel fuel in bulk when the price is relatively low and then store it. On-site storage is meant to save on costs and add an element of convenience. There are, however, risks associated with storing diesel on-site.

(actual diesel fuel samples from left to right: sludge; high water to fuel ratio; sludge; sludge, rust, particulates; particulates; few visible particulates but enough to clog the smaller fuel injectors on more modern generators; the far right sample has no visible particulates, but a lab report showed it had a severe presence of microorganisms. A visual inspection sometimes isn’t enough.

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Threat from Water

When diesel is exposed to water, it causes a hydrolysis reaction, meaning the diesel breaks down due to exposure to water. When condensation accumulates, drops of water can drip from the roof of the storage tank onto the diesel. The exposure to water creates a chemical reaction that — as mentioned — breaks down the diesel and makes it susceptible to the growth of microbes (bacteria and fungus).

Furthermore, if the tank is above ground, rainwater can pool on top of the tank. This pooling can cause the tank to rust and negatively impact the overall integrity of the tank. Pooled water is also attractive to all kinds of insects and will quickly become a breeding ground for additional bacterial growth. When rust occurs, this bacterial-laden rainwater can seep into the tank and negatively affect the diesel’s quality.


Tips to Prevent Water Contamination

There are a number of things you can do to prevent water from contaminating onsite stored diesel:

Short-term Management

  • Use biocides. Biocides will help prevent the growth of bacteria and fungus that can thrive in the water-diesel interface. Once the microbes are present, they multiply quickly and are hard to eliminate.
  • Use fuel treatment with demulsifying properties that separate water from fuel. Tanks available today are most likely equipped with a Fuel Water Separator (FWS) filter and the demulsifying treatments boost the performance of the FWS.
  • Top off the tank. If the tank is full, condensation is less likely to occur.
  • Check the tank for pooled water after rain. Maintain the tank’s structural integrity by checking it regularly, particularly after rain to remove pooled water on top of the tank.

Long-term Management

  • Check the tank weekly. Make sure you add it to your maintenance schedule to periodically check for water inside the tank and remove water that has pooled at the bottom of the tank.
  • Empty and clean the tank every ten years. A thorough cleanse every decade will help maintain not only the life of the diesel fuel but the life of the tank.
  • Cover the tank. One means of preventing rainwater from damaging the tank is providing a roof for the tank.
  • Invest in an underground storage tank. The initial expense may be more, but the long-term costs are less: it keeps the tank safer and temperatures cooler and the quality of the fuel will last longer.


Threat from Microbial Growth

Microbial growth, as mentioned, is often the product of conditions created by water interfacing with diesel fuel: microbes require water to grow. Microbial growth is a problem for many reasons.

On a performance level, it is problematic because microbes produce acids that degrade diesel fuel, clog tank filters due to biomass formations, restrict the flow of fluid, corrode the tanks, and damage engines. On a regulatory level, the increase in microbe growth today is the unfortunate result of an Environmental Protection Agency (EPA) mandate requiring less sulfur in diesel to reduce pollutants in the air, but less sulfur in diesel makes the fuel less stable and prone to microbial growth. Before EPA’s ultra-low sulfur diesel requirements, the sulfur found in diesel acted as a natural sterilizer — the chemical is deadly to microbes. Thus, when the natural levels of sulfur were left in the diesel, there was remarkably less microbial contamination.


Tips to Prevent Microbial Growth

Under the current regulatory environment, there are still ways to manage microbial growth. Here are a few tips to implement into your fuel storage strategy.

  • Keep water from contaminating the diesel fuel. With no water, the conditions for microbial growth are nil.
  • Use biocides. Because the risk to contaminated diesel fuel is high, it is important as a preventative measure to use biocides that prevent the growth of and kill any existing microbes.
  • Prevent or eliminate biofilm. Biofilm can grow at the diesel-water interface. It is a thick, sludge-like mass. Biofilm can reduce the effectiveness of biocides and encourage re-infection of microbial growth after fuel treatment. If biofilm existed before biocide treatment, the tank may need to be mechanically cleaned to fully and successfully eliminate the biofilm and to reap the full benefits of the biocide.


Threat from Oxidation

Oxidation is a chemical reaction that occurs when diesel fuel is introduced to oxygen, and this happens as soon as diesel fuel leaves the refinery. As such, diesel immediately begins to degrade and its lifespan is reduced. Oxidation has a negative effect on both the quality of the fuel and the tank storing the diesel.

Oxidation reacts with compounds in the diesel to create high acid values and to develop unwanted gum, sludge, and sediment. The higher acid value works to corrode the tank while the production of gum and sediment work to clog filters. If oxidized diesel fuel goes unnoticed and is inadvertently used, it can damage the diesel engine of your generator.


Tips to Avoid Damaging Effects of Oxidation

Due to the natural state of things and processes, oxidation cannot be prevented, but its harmful effects can be minimized or avoided. The following are some suggested solutions to this problem.

  • Keep the tank cool. The key to delaying oxidation is a cool tank — around 20°F is ideal but should not go higher than 85°F. A cooler tank can be achieved by either investing in an underground tank or by providing a roof (e.g., canopy) or some type of enclosure to both reduce exposure to the sun (we frequently encounter contaminated diesel here in Houston from the hot sun) as well as reduce exposure to water sources.
  • Treat the fuel. Additives, like antioxidants, and fuel stability treatments maintain the quality of the diesel fuel by stabilizing it and preventing a chemical breakdown.
  • Treat fuel, but treat it right. Do not use treatments or fuel additives that claim to work for both gasoline and diesel fuels. Gasoline and diesel are two very different fuels with different compositions that require specific technology. As such, how you treat the diesel should be specific to diesel, not general to any given fuel source.

In general, you want to establish a monitoring and maintenance plan for your diesel fuel tank storage system that incorporates all the above tips. A maintenance program is especially important when the stored diesel is for a standby or emergency generator. These generators don’t get to run very often and the fuel source is all too often overlooked in testing and maintenance. When an emergency situation presents itself you need to know everything will continue to work and a big part of that is keeping the fuel in good condition. You also want to establish a time frame to use up all the old stored fuel so the tank can be cleaned and fresh diesel can be added. Finally, your plan should ensure it meets all federal, state, and local standards and regulations. In doing so, you have already gone a long way to prolonging the shelf life and quality of your stored diesel fuel.