Red Diesel – What Is It?

Red Diesel – What is it?

Generators burn red diesel fuel which is also know as Heavy Oil.

Diesel fuel for road use has a large percentage of tax added to the cost per litre. Diesel Generator fuel is termed RED DIESEL because a RED dye has been added to show that it’s TAX FREE when sold and used in the UK, and is NOT for road use. It is commonly used in all Diesel Generators, unless Bio Diesel is used. Because generators are not road vehicles. Of course you can use any diesel fuel in your diesel generator. Should you use regular diesel fuel from a retail pump then you are paying additional tax. Only do so in any emergency because retail diesel is approximately twice the price due to extra TAX.

Diesel Generator fuel systems must supply the diesel engine with a continuous and clean supply of fuel. Bulk fuel is usually stored in a large tank, and the fuel transferred to a smaller day tank near the engine by means of an electric motor-driven pump or by simple suction. The diesel generator system must be located and constructed in accordance with good safety practices and local codes.

Any flexible non-metallic lines used to route the fuel inside the building should meet Fire Regulations. Check for any possibility of trapped fuel lines or sharp protrusions that could, over time, puncture the fuel lines. Any air entering the fuel line can cause the diesel generator to fail.

Diesel Generator – Fuel characteristics

The following additional information describes certain characteristics and their relation to engine performance.

A. Cetane Number: This index ignition quality is determined in a special engine test by comparison with fuels used as standards for high and low cetane numbers.
B. Sulphur: Since the advent of high detergent oils, sulphur content has become less critical. A limit of 0.4% maximum is used for engines, without reducing oil change periods. Oil change periods are reduced with higher sulphur fuel.
C. Gravity: The measurement is an index of the weight of a measured volume of fuel. Lower API ratings indicate heavier fuel which has a higher calorific value.
D. Viscosity: This factor is a time measure: flow resistance of a fuel. Some low viscosity fuels are lubricants; a viscosity which is too high makes for poor fuel atomisation, thereby decreasing combustion efficiency.
E. Distillation: This involves the heating of crude oil to relatively high temperatures. The vapour which results is drawn off at various temperature ranges. Producing fuels of different types. The lighter fuels such as gasoline come off first, and the heavier fuels last.

Other points to follow:

F. Flash Point : The lowest temperature at which the fuel will give off sufficient vapour to ignite momentarily when a flame is applied.
G. Pour Point: This denotes the lowest temperature at which fuel will flow or pour.
H. Water and Sediment: The percentage by volume of water and foreign material which may be removed from the fuel by centrifuging. No more than a trace should be present.
I. Carbon Residue: Percentage by weight of dry carbon remaining when fuel is ignited and allowed to burn until no liquid remains.
J. Ash: Percentage by weight of dirt, dust, sand, and other foreign matter remaining after combustion.
K. Corrosion: To determine corrosion, a polished copper strip is immersed in the fuel for three hours at 122° F (50° C). Any fuel imparting more than a slight discoloration should be rejected.

Important Reminders

Remember to order as heavy a fuel as your diesel engine and temperature conditions permit. But check the manufacturer’s recommendations before ordering your diesel fuel.
Fuel costs can represent approximately 80% of total operating costs for an engine.

Don’t be caught out!

It is good economics to look closely at the largest cost first and remember that deliveries of fuel for a diesel generator, when less than 500 litres, are prone to a surcharge. Better to have a larger capacity fuel tank that avoids small deliveries of less than 500 litres each delivery. A regularly maintained Generator will give up to 30 years of standby life so think ahead and get a suitably large tank. You don’t want to have to upgrade it down the line when a bit of forward thinking would help.

Bulk Storage – Diesel Generators

The quantity of Red Diesel fuel stored must also be regulated, as fuel stored for long periods will sediment, and this applies to all diesel generators. A regular check of the BS standards of your fuel supplier should be carried out to ensure that your fuel is always of the correct standard and grade.

Red Diesel Generator External Fuel Tank


A large capacity storage tank is desirable to encourage bulk purchases and minimise dirt contamination. Maintaining a diesel generator tank and keeping it full minimises condensation, particularly if the fuel is seldom used. It may be located either above or below ground level.

The storage tank fill tube should be located for convenience and safety of the filling operation. A vent must be provided to relieve air pressure created by filling and prevent a vacuum forming as fuel is consumed.

Contaminant Build Up

Silt and water builds up over time in diesel generator fuel storage tanks.
A means of periodically drawing water and sediment from the tank of the diesel generator must be provided. Over time both bacteria, silt and water vapour contaminants build up in the red diesel generator tank.
Ground settling must also be taken into account when installing the tank to ensure that the drain cock remains at the lowest level. In underground tanks, water may be removed by pumping through a tube placed down the fill pipe. Avoid seasonal settling by burying the tank below the frost line and check local winter weather conditions to ensure the depth is the safe and correct depth.
The day tank is situated in the belly of the diesel generator. If unused, then the separate bulk tank must be located nearby to the diesel generator. Ready to supply to the engine mounted transfer pump.
Copper bearing steel tanks are preferable but black iron tanks and fittings are satisfactory. Galvanised fittings or tanks should be avoided because of possible reactions with fuel impurities clogging the fuel filter. In all cases, tanks for diesel generators must be bunded (double skinned) to allow any leakage of fuel to be retained within the bunded area.

Diesel Generators – Fuel delivery to the engine

The delivery line for carrying the Red Diesel fuel to the diesel generator engine-mounted fuel transfer pump, and the return line for carrying excess fuel back to the tank, should be no smaller in size than the fittings on the engine. If the distance from the diesel generators fuel tank to the engine exceeds 30’ (9.15 m), or if ambient temperature is extremely low, larger fuel supply and return lines should be used to ensure adequate flow.

Also, beware of pinch points!
Every diesel generator has fuel lines, and these should be regularly checked to ensure that they are not kinked , twisted, or trapped as any restriction can cause the generator to hunt and or fail. Also, these can cause wear and reduce the life of the diesel generator fuel line. Check and check again.

Fuel overflow

The fuel line is constructed from either:

  • steel
  • black iron pipe
  • copper tubing
  • galvanised pipe
  • SWA fuel hose pipe

As a cautionary note: we do not advise any zinc bearing alloy be used. The overflow line from the day tank (or, if no day tank is used, the fuel return line from the engine) should be of the same material and one size larger to allow free flow.

Fuel return line

The return fuel line from the diesel generator tank should enter the top of the tank and contain no shut-off valves. Avoid dips in this line so that air may pass freely and prevent any vacuum in the fuel system. The fuel suction line should be positioned to remove fuel from a point about 2” (5.1 cm) above the bottom. If possible, at the opposite end of the tank to the return line. If the fuel line enters the top of the diesel generator’s tank, a pipe should be provided inside the tank to extend the line to the proper distance from the bottom.

Joint cement should not be used in any part of the system. Fuel may be affected. All connections must also be made without any dependence on gaskets. A length of flexible fuel line should be installed between the pipe from the fuel source (bulk storage or day tank), to the engine fuel inlet and return to prevent vibration damage to the pipes and fittings.
Diesel Generator fuel tanks must be installed and tested correctly at the time of commissioning. Certainly, then you can be sure of decades of trouble free use. Remember a diesel generator isn’t just for Christmas it’s for life.

Diesel Generators Auxiliary Tanks

Diesel generators auxiliary or “day tanks” are desirable if the main fuel tanks are located more than 50’ (15.25 m) from the engine, or located above the engine, or are more than 12’ (3.65 m) below the engine. Total suction head should not exceed 12’ (3.65 m). Although they will not aid the engine in fast starting, they do offer a convenient and ready storage of additional fuel. Diesel generators day tanks also provide a settling reservoir, so water and sediment can separate from the fuel.
The auxiliary tank is located so that the level of the fuel is no higher than the fuel injection valves on the engine. The tank should be close enough to the diesel generator’s engine to minimise total suction lift.

Tank Filters

Clean fuel filters ensure maximum engine life and dependability. Anything else will endanger these characteristics. The engine filter protects the fuel injection pumps and nozzles. Thefuel-tank-pipe incoming fuel must never bypass these filters.

Primary filters with 0.012” screens are available to extend the life of the engine filter and the transfer pump. Water and sediment traps and filters can also be included upstream of the transfer pump. Pump performance must not be restricted.

In warm climates with large bulk storage, the diesel fuel may require full filtering every six to twelve months. Its important to renew the fuel supply every 2 years. This removes water, scale and bacterial growth.


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