Fuel Polishing, Tank Cleaning or Fuel Replacement: Which One a Site Needs
What is the difference between fuel polishing, tank cleaning and fuel replacement?Fuel polishing treats the fuel while it stays in the tank, circulating it through filtration and water separation. Tank cleaning treats the tank: it is emptied and the sludge, biofilm and sediment are removed from the floor and walls. Replacement removes the fuel altogether. Which one a site needs depends on where the problem is: in the fuel, on the tank, or in the chemistry of the fuel itself.
Three different jobs that get called the same thing
When a fuel test comes back bad, the next phone call is usually to someone who “cleans fuel”. What happens after that call varies a great deal, because three quite different jobs are sold under that description.
Fuel polishing works on the fuel. Fuel is drawn from the tank, passed through filtration and water separation, and returned. The tank stays in service and so does the fuel. A correctly designed polishing system also neutralises microbial growth and separates emulsified water as well as free water, rather than only removing what has settled. The glossary defines both kinds of water.
Tank cleaning works on the tank. The fuel is pumped out and held or disposed of, the tank is opened, and the deposits on the floor and walls are physically removed. It usually means confined space entry and it always means the tank is out of service for the duration.
Fuel replacement works on neither. The fuel is removed and new fuel is delivered.
Each one fixes a different problem. Choosing the wrong one costs money and leaves the problem where it was.
What polishing can fix, and where it stops
Polishing is the right tool when the fuel is chemically sound but contaminated. That covers most of what goes wrong in a standby tank:
- Water, both the free water lying at the bottom and the emulsified water held in suspension in the fuel. See where the water in a fuel tank comes from.
- Particulate, meaning dust, rust, and the sediment that forms as fuel ages.
- Early microbial growth, when it is treated with a biocide stage as well as filtered. Filtration alone removes the visible matter and leaves the organisms. The diesel bug article covers why.
There are two things polishing cannot do.
It cannot undo oxidation. Oxidation changes the fuel’s chemistry. Polishing removes the gums and sediment once they have formed and dropped out, and a stabiliser can slow further oxidation, but neither puts back the stability the fuel has already lost. A fuel with poor oxidation stability will keep producing new sediment after it has been filtered.
It cannot remove deposits that are attached to the tank. A thick sludge layer on the floor, or biofilm bonded to the walls, is not in the fuel stream in any useful sense. Circulation disturbs some of it, which is why the first hours of polishing a neglected tank often block filters faster than expected. But a heavy deposit will keep re-contaminating the fuel for as long as it is there.
What tank cleaning does that polishing cannot
Tank cleaning removes the deposits that polishing cannot reach, and it is the only one of the three that lets anyone see inside the tank.
That second point is undervalued. A cleaned tank can be inspected for internal corrosion, coating failure and the condition of the low point, and those findings often explain why the fuel went wrong in the first place.
Cleaning has two limits of its own.
It does nothing for the fuel. If the fuel that was pumped out is put back without being treated, the clean tank is re-contaminated on the day it returns to service.
And it does nothing about the cause. If water was getting in through a vent, a fill point or condensation, it will get in again. A tank that needed cleaning once and has had nothing else changed will need cleaning again.
For a standby system there is also the question of what feeds the generators while the tank is empty. On a site with a single bulk tank, cleaning means a temporary supply or a period without full standby cover, and that has to be planned rather than discovered.
When the fuel itself has to go
Replacement is the right answer when the problem is in properties that filtration cannot change.
- Oxidation stability has failed. The fuel is still forming gums and sediment and will continue to after any amount of filtering.
- The wrong product was delivered, or the fuel has been mixed with something it should not contain. No polishing system separates one hydrocarbon from another.
- The fuel fails its specification on a property that is chemistry rather than contamination. EN 590 and ASTM D975 both set requirements that a filter does not touch, which is covered in more detail in the article on testing stored diesel.
Removed fuel has to be handled and disposed of under local waste rules, which adds cost and time. That is a reason to catch problems while polishing can still fix them, not a reason to keep fuel that has failed.
How to tell which one a site needs
The decision comes from a test, not from looking at a sample jar. Fuel can look bright and clear and still fail on oxidation stability, and it can look dark and still be serviceable.
Take samples from the bottom of the tank as well as the middle. Most of what goes wrong in a tank shows up at the bottom first. Then read the results this way:
- Water and particulate high, oxidation stability acceptable, little or no microbial growth. The fuel is sound and dirty. Polishing.
- Microbial growth established, heavy bottom sludge, or filters blocking repeatedly once circulation starts. The tank is carrying deposits the fuel keeps picking up. Tank cleaning, with the fuel polished and treated before it goes back.
- Oxidation stability failed, or the fuel is off specification on a chemical property. The fuel is the problem. Replacement, and in most cases a tank clean before the new fuel goes in.
The test should also say how far the problem has gone. A first finding of water at the bottom of a tank is a polishing job. The same finding after two years of nobody draining it is often all three.
The order matters when more than one is needed
When a site needs more than one of these, the sequence decides whether the money is well spent.
- Remove or hold the fuel. Test it to decide whether it can be treated and returned or has to be replaced.
- Clean and inspect the tank. Record what was found at the low point and on the walls.
- Fix the cause. Vents, breathers, fill points, the drain, whatever let the water or the dirt in.
- Refill with fuel that has been tested. Either the original fuel after treatment, or new fuel with a delivery sample taken.
- Keep it that way. This is where a permanent fuel polishing and maintenance system earns its place, because it is the step that stops the first four from being needed again.
Delivering new fuel into a tank that has not been cleaned is the most common way to waste money in this sequence. The new fuel is contaminated by the old deposits within weeks.
The cheapest of the three is the one that prevents the other two
Tank cleaning and fuel replacement are both responses to a problem that has already happened. They are expensive, they take the tank out of service, and on a critical site they create a window with reduced standby cover.
Polishing on a regular schedule, with water drained and the fuel tested, is what keeps a tank from reaching that point. Sizing the polishing system to turn the tank over often enough is covered in sizing a fuel polishing system.
Polishing fixes fuel that is sound but dirty. Tank cleaning fixes a tank that keeps re-contaminating its fuel. Replacement is for fuel whose chemistry has failed. A bottom sample and a proper test say which one applies, and regular polishing is what keeps a site from needing the other two.
Standards referenced: EN 590 and ASTM D975 for diesel requirements. Read more on Diesel Fuel Maintenance Systems.