How Long Does Diesel Last In A Tank?

Hospitals, utilities, diesel fleets, emergency management, telecom/data centers, facilities with backup generators - those who have been in operations or management for any of these kind of facilities know that the fuels we use today have changed from years past. If you’ve been a Bell Blog reader for any length of time, you’ve seen references to this over and over again.

How Long Does Diesel Last In A Tank? A big difference between then and now concerns diesel fuel’s storage life. How well it resists the kind of reactions and changes that all fuels go through over time in the real world. Back in the 1950s, the US Army reported that you could get 3-5 years (or more) out of stored diesel fuel. Now, without taking rigorous steps on fuel preventive maintenance, 3-5 years is out of the question.

What is the question at hand?

How long does diesel fuel last in a storage tank?

Though we’ve already established that it’s “not as long as it used to be”, asking this kind of question is really asking a different one altogether: how long will diesel fuel retain its optimal characteristics before it degrades or changes past the point that it performs differently with respect to what you want it to do?

We shouldn’t really say “degrades” here because that might imply that some fuels degrade while others don’t. The truth is, all fuel begins to degrade immediately after it is produced. But the process is slow, taking months and years before any change is noticeable. Kind of like we all start aging as soon as we’re born, right?

It Depends (On What?)

How long does diesel fuel last in a storage tank? As with many things in life, the best answer is It Depends. But depends on what? The cliff notes answer is that it depends on 1) whether there’s any biodiesel in it (and how much), 2) whether it’s got or develops any significant microbial presence, and 3) whether water is removed regularly.

The third one is integrally linked to the second one – all tanks build up water, all tanks have microbes in them, microbes love free water and need it to multiply. You do the math. If a microbial problem develops, it can destroy the fuel quality within a period of months. Luckily, this can be controlled (never eliminated, just controlled) with simple housekeeping and condition monitoring efforts.

Biodiesel content in the fuel matters because biodiesel increases diesel fuel’s ability to hold water and also increases the chances of the fuel developing microbial problems (because microbes like to feed on it).

And hiding in the background while we discuss these three possible factors is the problem of the cracked feedstocks in today’s diesel fuels. Diesel fuel oils and gasoline (plus other stuff like natural gas and naphtha) are the distillate fuels refined from crude oil. Residual fuels (like heavy fuel oil) are what’s left after the distillate is taken off. Refineries want more distillate fuels and less residual fuels because distillate fuels are what’s more in demand (and more profitable).

Back in the day when you could get 3-5 years of good storage life out of your diesel fuel, the refineries were only getting 55-60% distillate content from each barrel of crude. Now, they get 93% distillate content. But they only get this through specialized chemical reactions (cracking). They get more diesel and gasoline, but those fuels are less stable and more reactive.

The Answer?

So how long do today’s diesel fuels last in a storage tank? IF it’s a common on-road diesel (which will have 3-5% biodiesel content in it) and if it’s reasonably well taken care of (water removed monthly, fuel is monitored for microbes), you would expect to get 18-24 months out of it before you would expect to start seeing issues. Sometimes 12-18 months.

But that’s if everything goes well. Enough things don’t go well in everyday life that the recommended best practices for taking care of stored diesel always include incorporating diesel fuel stabilizers. And that’s in addition to good housekeeping and condition monitoring practices that you should be doing as a matter of regular business habit.

So how long does diesel fuel last in storage? 18-24 months. Unless it doesn’t.