Day tank design decides whether a generator gets clean fuel at a steady pressure. A tank that is too small runs dry during a long outage. Pipework that is too narrow starves the engine at full load. Good design fixes both before the concrete is poured.
Quick Answer
Size the tank for at least 8 hours of full-load running.
Keep the tank above the engine fuel pump so gravity does the work.
Use a pipe diameter that keeps fuel velocity below 1 metre per second.
Fit a bund that holds 110 percent of the tank volume.
Add a low-level alarm and a manual shut-off outside the room.
What is day tank?
A day tank is a small fuel tank mounted near the generator that supplies the engine directly from a short, controlled pipe run.
It means the buffer between bulk storage and the engine, holding enough fuel for one shift or one outage.
day tank in practice
Typical sizing by generator load:
| Generator size | Full-load burn | Suggested size |
|---|---|---|
| 60 kVA | 12 litres per hour | 150 litres |
| 100 kVA | 20 litres per hour | 250 litres |
| 250 kVA | 48 litres per hour | 500 litres |
| 500 kVA | 95 litres per hour | 1,000 litres |
| 800 kVA | 150 litres per hour | 1,500 litres |
| 1,000 kVA | 190 litres per hour | 2,000 litres |
What does a day tank and fuel pipework installation cost in Nigeria?
A 250 litre fabricated tank costs about 180,000 to 320,000 naira in Lagos.
A 1,000 litre tank with a bund runs about 650,000 to 1,100,000 naira.
Fuel pipework and fittings add 120,000 to 400,000 naira depending on the run.
A low-level alarm and control panel add 90,000 to 180,000 naira.
A transfer pump set adds 250,000 to 600,000 naira.
Installation and commissioning take 3 to 7 working days.
How is day tank carried out?
- Calculate the full-load fuel burn from the engine data sheet.
- Choose a tank volume for at least 8 hours of running plus a reserve.
- Set the tank height and route so fuel reaches the engine by gravity.
- Size the pipe for low velocity and fit a filter and isolation valve.
- Build a bunded base and fit a high-level cut-off and low-level alarm.
- Pressure test the pipework, flush it and commission the supply.
Sources
Manufacturer fuel system manuals, NFPA 30 flammable liquid code principles, NIS 1000 electrical installation rules, Diesel storage and handling guidance
Why a buffer tank beats direct bulk storage
A buffer tank puts a small, known volume next to the engine. The transfer pump fills it from bulk storage, and the engine draws from a short line.
A day tank puts a small, known volume next to the engine. The transfer pump fills it from bulk storage, and the engine draws from a short line.
The arrangement also limits the fuel that can feed a fire in the generator room. That is a safety gain, not just a plumbing detail.
Sizing the tank
Start with the burn rate at full load, then multiply by the longest expected run. Eight hours is a sensible minimum for a commercial site.
Do not oversize the tank. Fuel that sits for months absorbs moisture and grows microbes, especially in humid Nigerian conditions.
Do not oversize the day tank. Fuel that sits for months absorbs moisture and grows microbes, especially in humid Nigerian conditions.
Gravity feed and tank height
Most small and medium generators use a mechanical lift pump that pulls fuel from the tank. Keep the tank outlet above the pump inlet.
If the tank must sit low, use a transfer pump with a return line and a pressure regulator. Never rely on a pump that can overpressure the injector pump.
Fit a foot valve or a anti-siphon break so the line stays primed after a filter change.
Pipe diameter and routing
Size the pipe so fuel velocity stays below 1 metre per second. Higher velocity stirs sediment and increases pressure loss.
Use seamless steel or fuel-rated reinforced hose. Avoid galvanised pipe, because zinc can react with diesel and clog filters.
Route pipes away from hot exhaust parts and protect them from vehicle traffic. Support them every 2 metres to stop vibration fatigue.
Leak containment and fire safety
Build a bund around the tank that holds at least 110 percent of its volume. Seal the bund and keep it free of water and rubbish.
Fit a high-level cut-off on the transfer pump and a low-level alarm on the tank. Both should report to the control room.
Place a manual isolation valve outside the generator room so fuel can be shut off without entering a fire area.
Commissioning and routine checks
Pressure test the pipework before filling. Flush the line to remove mill scale and swarf, then change the engine filter after the first run.
Check for air leaks by watching the sight glass and listening for a rough idle. Air in the line shows up as surging and hard starting.
Every month, drain water from the tank sump and check the bund. Every year, clean the tank and inspect the internal coating.
Related guides from this series
- Diesel Generator Fuel Tank Design and Storage Safety in Nigeria
- Generator Fuel Management in Nigeria
- Generator Fuel Tank Bunding and Spill Containment in Nigeria
About the author
Illuminate Engineering Limited designs and installs generator fuel systems for commercial and industrial sites across Nigeria.
Frequently Asked Questions
How big should a generator fuel buffer tank be?
Size it for at least 8 hours of full-load running plus a 10 percent reserve. A 100 kVA set typically needs a 250 litre tank.
Can a fuel buffer tank be installed below the generator?
Only with a transfer pump and a return line. Gravity feed needs the tank outlet above the engine fuel pump.
How often should a fuel buffer tank be cleaned?
Seamless steel or a fuel-rated reinforced hose. Avoid galvanised pipe and clear PVC, which degrade or react with diesel.
How often should a day tank be cleaned?
Inspect every month for water and sediment, and clean the tank every 12 months. More often if fuel quality is poor.
Conclusion
A buffer tank is a small part of the plant that decides whether the generator starts and keeps running. Size it for the real load and route the fuel properly.
Bund it, alarm it and label every valve. The design work takes a day; the reliability lasts for years.