Diesel fuel does not keep forever. In a standby generator tank it can collect water, grow microbes and break down over time until it plugs filters and stops the engine. Regular fuel testing, keeping water out and managing the tank as part of the generator's maintenance program are what keep stored fuel usable.
Why stored diesel degrades
A vehicle burns through its fuel in days or weeks. A standby generator may hold the same fuel for a year or more, burning only a little during tests. Over that time several things happen:
- Oxidation. Fuel reacts slowly with oxygen and forms gums and sediment. Heat and contact with certain metals speed this up.
- Water. Condensation forms as the tank breathes through daily and seasonal temperature swings. Water also gets in through loose fill caps, damaged vents and fuel deliveries. Being heavier than diesel, it settles at the bottom.
- Microbial growth. Bacteria and fungi live at the boundary between water and fuel. They form slimy mats, produce acids that corrode tanks and create sludge that plugs filters.
- Sediment. Rust, dirt and degradation products build up and get drawn into the fuel system.
Modern ultra-low sulphur diesel, and blends containing biodiesel, can be more prone to some of these problems than older fuels. Biodiesel attracts more water and can support more microbial growth, so the blend in your tank matters.
Cold weather and fuel
As diesel cools, waxes in the fuel start to come out of solution. The cloud point is the temperature at which wax crystals first appear, and below it filters can plug. Fuel sold in Canada is blended for seasonal conditions, but fuel delivered in summer and stored into winter may not suit the coldest days. For outdoor tanks and generators in unheated enclosures:
- Ask your supplier about the cold-weather properties of the fuel you buy, and plan deliveries with the season in mind
- Consider fuel heaters or heated enclosures where the set must start in deep cold
- Keep water out of the tank, since ice crystals add to plugging
- Use additives only on the advice of the fuel supplier or engine manufacturer
Testing fuel
You cannot judge diesel condition by looking at it in the tank. A fuel testing program usually includes:
- Bottom sampling. Drawing a sample from the lowest point of the tank, where water and sediment collect. A clear sample jar shows free water, haze and debris.
- Water checks. Water-finding paste on a gauge stick, or a sample from the drain, shows whether free water is present.
- Laboratory analysis. A fuel lab can test for water and sediment, microbial contamination, stability, cloud point and other properties, and compare results with the applicable diesel fuel specification.
Test at least on the schedule set by your generator maintenance program, and more often for large tanks, critical facilities or fuel that has been stored a long time. Keep the results with your generator records so you can see trends.
Water is the starting point. Most fuel problems in standby tanks begin with water. Removing free water regularly, and stopping it from getting in, prevents a large share of microbial growth and corrosion.
Taking a bottom sample, step by step
A simple bottom sample tells you more than any gauge reading. Follow your site's safety procedures for fuel handling, and have a qualified person do it if your staff have not been trained.
- Prepare. Use a clean, clear sample jar and a bottom sampler or the tank's drain. Have absorbent pads and a container for drained fuel ready. No ignition sources nearby.
- Draw from the lowest point. Take the first fuel out of the drain or the sampler lowered to the tank bottom. That is where water and sediment collect.
- Let it settle. Leave the jar for a few minutes and look at it against a light background.
- Record what you see. Note the date, tank, sample point, colour, clarity, any free water layer, any slime or particles, and the fuel level in the tank.
- Decide on next steps. Drain water until the sample runs clear, and send a sample to a lab if anything looks wrong or a lab test is due.
- Dispose of drained fuel and water properly. They are hazardous waste in most jurisdictions.
Reading the sample jar
| What you see | What it usually means | Typical next step |
|---|---|---|
| Clear and bright, no layer at the bottom | No free water or heavy sediment at the sample point | Record and continue routine testing |
| A distinct layer at the bottom | Free water in the tank | Drain water, find how it is getting in |
| Hazy or cloudy fuel | Water held in the fuel, or wax if the fuel is cold | Let it warm and settle, then retest; lab test if it persists |
| Slime, strands or a mat at the fuel and water line | Likely microbial growth | Lab test, remove water, plan polishing or tank cleaning |
| Dark particles or rust-coloured sediment | Degradation products, rust or dirt | Check filters, consider polishing and a tank inspection |
A sample jar is a screening check, not a lab result. Fuel that looks clean can still fail stability or microbial tests, so keep up the laboratory testing set out in your maintenance program.
Keeping fuel in good condition
- Drain water from the tank bottom and from fuel-water separators on a regular schedule
- Keep fill caps tight and locked, vents screened and fill boxes free of standing water
- Keep tanks reasonably full to reduce the air space where condensation forms, within the limits of safe filling
- Buy fuel from reliable suppliers and ask what blend and additives it contains
- Rotate fuel where practical, for example by using it in other equipment and refilling with fresh fuel
- Use fuel polishing (filtering and returning fuel to the tank) for large or critical tanks, either as a service visit or a permanent system
- Have tanks cleaned when sludge or heavy microbial growth is found, since treating the fuel alone will not remove it
- Use biocides and stabilizers only as recommended, and after removing water
Storage tank requirements
Fuel tanks for generators are regulated. Depending on the tank's size, location and type, requirements can come from the National Fire Code of Canada as adopted in your province, provincial storage tank regulations, environmental rules and the local fire authority. Typical requirements cover tank construction and listing, secondary containment, spill and overfill protection, venting, leak detection, separation from buildings and ignition sources, and registration of some tanks. Alberta, BC and Ontario each set these rules differently, so check with your provincial authority before installing, altering or removing a tank.
Inspect tanks and piping regularly for:
- Leaks, stains or fuel odour around the tank, piping and generator
- Corrosion, dents or damage to the tank and supports
- Water or fuel in the secondary containment space
- Working level gauges, alarms and overfill protection
- Clear, readable labels and placards
How fuel fits into generator reliability
Fuel problems often show up as a generator that starts and then stalls, or loses power under load. A load bank test can expose a restricted fuel system before an outage does. For the full maintenance picture, see standby generator maintenance.
How ehZee can help
Our diesel generator inspections include the fuel system: tank condition, containment, water and sediment, filters and records. We report what we find, point out where testing or cleaning is needed and can coordinate fuel sampling and follow-up with qualified contractors.
Common questions
How long can diesel be stored in a generator tank?
There is no fixed shelf life. Clean, dry fuel in a well-kept tank can stay usable for a long time, while fuel with water and microbes can degrade within months. Regular testing is the only way to know.
Will a fuel additive fix contaminated fuel?
No. Additives can help prevent problems, but water, sludge and microbial mats have to be physically removed by draining, polishing or tank cleaning.
Do we need permission to replace a generator fuel tank?
Often yes. Tank installation, replacement and removal may need permits, registration or notification depending on the tank and your province. Check with your fire authority and provincial regulator first.
Why do our generator fuel filters keep plugging?
Repeated plugging usually points to contamination in the tank: water, microbial growth, sediment or degraded fuel. Changing filters treats the symptom. Sample the tank bottom and test the fuel to find the cause.