It's a real shame when a storage tank fuel of expensive fuel "degrades" and starts to lose its quality.
Whether it's from oxidation or hydrolysis or a reaction to acidic byproducts of microbial contamination, this type of fuel loss costs businesses and users many millions of dollars every year.
The most visible sign that something is wrong is when the fuel changes color. Normal undyed diesel fuel is a beautiful amber-green color. The same fuel which has started to degrade will darken. This is because heavier components of the fuel blend are no longer dissolved in the fuel, but are coming out of solution and floating freely in the fuel. They have a darker color which turns the overall color of the fuel darker. Ever see tar and asphalt? Those are examples of heavier petroleum molecules which are obviously dark.
Storage Tank Symptoms
In addition to a change in fuel color, you can tell fuel in a storage tank is losing its storage quality if you assess changes in the normal amount of water accrued in the storage tank, a greater than normal sediment content of drawn fuel samples, and any slimy or abnormal coatings on surface and tank walls. The latter can be indicative of microbial present.
Many times, stored fuel users don't even know there's a problem until they notice changes in the performance or behavior of the engines that are using the bad fuel. Excessively clogged filters, black smoke and lower-than-normal RPMs at full throttle are all signs that the combustion quality of the fuel is not what it should be.
Reversing Degraded Fuel
This is somewhat of a trick question because it's pretty much impossible to reverse bad fuel in this manner. There are some "fuel treatments" that claim to be able to do this. If you see one of those, we would recommend going in the opposite direction pretty quickly. Prevention is the critical aspect here - it's much less costly to treat the fuel to protect it than it is to fix the problems caused by bad fuel left to its own devices in the storage tank.
This post was published on November 19, 2013 and was updated on April 21, 2014.