Day Tank, Bulk Tank or Belly Tank: Fuel Storage for a Generator Set

In Short

A belly tank sits in the generator base frame, a day tank is a small working reserve near the engine, and a bulk tank stores fuel for long outages. Most standby systems combine them. Together they decide how long a diesel generator can keep running without a fuel delivery.

A diesel generator uses a lot of fuel at full load. Where that fuel is stored, how it reaches the engine and how it is kept clean decides whether a standby set runs through a long outage or stops after a few hours. Three kinds of tank do the job: the belly tank built into the generator base frame, the day tank near the engine, and the bulk tank that holds the autonomy for days.

This guide explains what each tank is for and how they are combined in a fuel system.

The Belly Tank: Fuel in the Base Frame

A belly tank is the generator's base frame built as a fuel tank: the engine and alternator stand on it, the fuel sits inside it, and the set carries its own reserve without any external tank. Single-wall belly tanks are bunded by the enclosure floor; double-wall tanks have their own interstitial space with a leak sensor. The capacity depends on the size of the set and on the autonomy the site asks for.

It is the simplest fuel system and the standard for canopied and containerised sets: one fill point, a level gauge, a low-level alarm and an overflow or a fill-limit device. Its limits are the autonomy the frame can hold and the weight it adds to the package.

The Day Tank: A Working Reserve Near the Engine

A day tank is a small tank installed beside the engine or inside the enclosure, fed from a bulk tank by a transfer pump and feeding the engine by gravity or by the engine's own lift pump. It keeps a working reserve at the engine when the bulk tank stands far away or below the engine level, gives the engine a steady suction head, and lets the fuel return from the engine go back to a tank that is near.

The day tank carries the controls of the fuel system: level switches that start and stop the transfer pump, a high-level alarm and an overflow back to the bulk tank, a low-level alarm, a leak sensor in the bund and, on critical installations, duplicated pumps and a manual fill connection. It is kept large enough to ride through a transfer pump fault.

The Bulk Tank: Autonomy for Days

The bulk tank holds the autonomy the site demands - often days of running - and stands outside the enclosure, above ground in a bunded double-wall tank or underground. It is filled from a road tanker, so it needs a fill point with a spill box, a tanker connection, an overfill prevention device and a level gauge the fuel supplier can read. Its outlet feeds the transfer pumps of the day tanks; its return takes the day-tank overflows.

Large installations put several sets on one bulk tank with a fuel transfer system - duty and standby pumps, a filter and water separator, and, with the fuel standing for months between runs, a fuel polishing system that circulates and cleans it. The tank is designed to the fuel storage rules of the country: bund capacity, distances, fire resistance, venting and leak monitoring.

Putting Them Together

A small standby set: a belly tank alone. A set in a plant room with a long autonomy: a bulk tank outside and a day tank in the room. A data center or hospital: bulk tanks, duplicated transfer pumps, a day tank or belly tank per set, fuel polishing and monitoring of every level. The fuel is the same diesel in each; what differs is the autonomy, the distances and the level of redundancy the site requires.

AXON builds belly tanks, day tanks and fuel systems for generator sets of any make - single or double wall, with the gauges, pumps, alarms and polishing units the installation needs - and the containers and enclosures they go into. Send the generator rating, the required autonomy and the site layout, and the proposal shows the tanks and the fuel system that fit.

In short

Belly tank: in the base frame, the simplest system
Day tank: a working reserve near the engine, fed from the bulk tank by pumps
Bulk tank: days of autonomy, tanker filling, bund and leak monitoring
Instruments: level gauge, high and low alarms, overflow, leak sensor, overfill prevention
Fuel polishing where the fuel stands for months
AXON builds the tanks and the fuel transfer system for any generator make

What to send with an enquiry

Generator rating and fuel consumption at full load, or the engine model
Required autonomy in hours at full load
Number of sets and whether they share a bulk tank
Site layout: distance and height between bulk tank, day tank and engine
Single or double wall, above or below ground, local storage rules
Monitoring and alarm requirements, fuel polishing yes or no

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Questions we are often asked

How big should a belly tank be?

It follows the autonomy the site asks of the set itself, within what the base frame can hold and the package can weigh. Longer autonomies go to a bulk tank. Send the rating and the autonomy and AXON prepares the proposal.

Does every generator need a day tank?

No. A set with a belly tank and no bulk tank has none; a set fed from a bulk tank nearby and above the engine may feed directly. A day tank is needed when the bulk tank is far away, below the engine, or shared by several sets.

What is the overflow for?

If the transfer pump keeps running after the day tank is full - a stuck level switch - the fuel must go somewhere other than the floor. The overflow pipe takes it back to the bulk tank; the high-level alarm reports it.

Why double-wall tanks?

A leak in a single-wall tank reaches the ground. A double-wall tank keeps it in the interstitial space and reports it with a sensor, which most storage rules now require for tanks without a bund.

How long can diesel be stored?

Without treatment, not as long as most people expect: water, microbial growth and oxidation degrade it over months. Standby fuel that stands for long periods is kept with additives, regular sampling and a fuel polishing system that filters and circulates it.