Water contamination is one of the most common and damaging issues in diesel fuel storage and use. Diesel—particularly modern ultra-low sulfur diesel (ULSD) and biodiesel blends—is hygroscopic, meaning it naturally attracts and absorbs moisture from the surrounding air at a molecular level. This makes water ingress almost inevitable over time, especially in stored fuel.
Water enters diesel in three primary forms:
- Dissolved (invisible, fully in solution within the fuel).
- Emulsified (tiny suspended droplets that make the fuel appear cloudy or hazy).
- Free (separate liquid layer that settles at the tank bottom, since water is denser than diesel).
Main Sources of Water Contamination
Human Error or Poor Handling Practices Simple oversights like leaving fill caps unsecured, exposing drums or portable tanks to rain, improper storage, or careless refueling can allow direct water entry.
Condensation (by far the most common source) Temperature fluctuations cause fuel tanks to “breathe.” As the fuel and tank cool (e.g., overnight, during seasonal changes, or in varying weather), warm, humid air is drawn in through vents, breathers, or fill pipes. This air cools upon contact with the colder tank walls or fuel surface, reaching its dew point and condensing into liquid water droplets. These droplets accumulate and mix into the fuel. This process is exacerbated in:
Above-ground tanks (greater temperature swings).
Partially filled tanks (larger air space and surface area).
Humid climates or environments with high day-night temperature variations.
Contaminated Fuel from the Supply Chain or Delivery Water can already be present when you receive the fuel. Causes include:
Insufficient settling time at the refinery or during transport.
Shared pipelines or transport systems that introduce moisture.
Contaminated supplier storage tanks. Excess dissolved water may exceed saturation (especially as fuel cools), causing it to “fall out” as free water that settles in your tank.
Leaks or Compromised Tank Integrity External water enters through:
Damaged seals, cracked tanks, faulty vents, loose fill caps, or degraded spill containment.
Rain, snow, pressure washing, groundwater seepage, flooding, or even accidental drainage of collected rainwater into the tank.
Need help? See https://dieselcraft.com/portable-fuel-polishing/
Recent Comments