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DIESEL GENERATOR FUEL TECHNOLOGY AND TANKS
With a diesel generator, up to 3000 L of diesel fuel can be stored in a room. If needs are greater, you should consider a separate fuel storage room, which serves as a separate fire protection zone, or an underground fuel tank. In all cases, the tank fleet must be filled from outside the room. In the case of a tank or a set of tanks located in the room, it makes the most sense to use a 2-inch pipe and terminate the pipe on the outer wall or in the refuelling cabinet with a 2-inch so-called Snap-Lock nozzle.
For underground tanks, a 3-inch pipe is used, and refuelling is carried out with a similar 3-inch Kam-Lock terminal.
It is necessary to have a tank air conduit built out, which must be terminated with a vent nozzle equipped with a rain cover, also featuring a flame retardant netting. The refuelling truck operator does not necessarily know how much your tank can fit, so an overfill sensor plug must be installed next to the refuelling nozzle to which the refuelling truck operator connects the end of the overfill signal. The overfill sensor gives a stop signal to the fuel pump when the tank is full.
Both for indoor installation and for underground tanks, the tank must have a double shell. For indoor installation, tanks usually feature a visual leak sensor, but underground tanks use glycol or compressed air to detect a leak between the two shells and the leak alert must be transmitted to the building automation.
An underground tank also needs a counterweight under the tank or a weight plate on top of the tank to compensate for lift force, otherwise the tank will rise like a submarine from the sea at high water levels.
For example, a 10,000 L tank that on its own weighs 2,000 kg requires 8,000 kg of weight for balancing. The method and requirements for installation do not differ from the installation of underground tanks at filling stations, neither in terms of the tank nor the pipeline and signals.
The use of light fuel oil damages the fuel pump and forces oil changes twice as frequently and its energy value (cost per kWh) is much lower than that of diesel fuel. So, refrain from using light fuel oil in diesel generators – in the long run, it will be much more expensive!
VENTILATION SYSTEMS
Since the diesel generator itself does not drive around, cooling air is blown out of the radiator, not taken in! There must be sufficient air exchange in the generator room to expel the heat generated by the engine (radiator and engine housing) and the thermal radiation from the generator windings. In addition, we need more combustion air for the engine! We therefore need a vent for the intake of combustion air and cooling air, as well as for expelling warm air. The minimum here are vents that must not be smaller than the area of the heat sink! The incoming air vent should be even larger because combustion air for the engine is also taken from there. It must be kept in mind that we install ventilation grids in front of the vents, which must prevent precipitation from entering the room together with air flow. If air velocity in the vent increases to more than 2 m/s, some of the precipitation is also drawn into the room. Data on this is usually available from the manufacturer of the ventilation grid! According to the Eurovent standard, air velocity permitted for air intake in the direct surface of the grid is 2.7 m/s and water permeability at that speed is 2%.
Let’s continue the example using a 100 kVA diesel generator – the radiator measures 770 x 620 mm and radiator fan output capacity is 3.7 m3/s.
As a minimum, it is not recommended to use a vent smaller than 800 x 800 mm, but in this case the surface area of the vent is 0.64 m2 and air velocity in the vent is 5.78 m/s. At such air velocity, 10% of precipitation gets into the room! Air velocity of more than 5 m/s causes the ventilation equipment to make extra noise.
A normal vent would leave an air velocity of 2.7 m/s at which only 2% of precipitation gets into the room, and the surface area of the vent for the equipment in question must be 1.37 m2. Suitable vents are, for example, 1200 x 1200 mm.
Vents can be elongated or round, the surface area must match, and the rule that bigger is better applies!
If we do not want to keep the room at the same temperature as the outside temperature in winter, we install insulated ventilation valves on the inside of the vents. A diesel generator opens the valves through its automation, and the most common for opening are Belimo spring-return valve drives, so that the spring opens and the tension closes the valve – vents must be opened quickly when the equipment is started!
The heat sink must be connected directly to the outgoing vent, either by means of a duct or a soft connection. Otherwise, air recirculation will occur around the heat sink and the space around the device will quickly overheat. Ideally, the air outlet and intake are in opposite walls, or at least not in the same wall! In the case of vents placed in the same wall, recirculation easily occurs between the vents in the outer wall, and it is easy for a diesel generator that stopped working due to excess temperature on a hot summer day to cool down. Cooling is not required by the engine radiator alone, but also by the generator windings and the engine housing! In the event that the heat sink of the diesel generator is not attached to the device and has been moved out of the generator room (for example, a horizontal larger heat sink), we need to ensure sufficient air exchange to remove thermal load from the engine housing and generator windings. To do this, air exchange must be created with electric fans – in our climate, air exchange of 1 m3/s is enough to remove 10 kW of heat. For example, the engine housing of a 100 kVA diesel generator emits approx. 10 kW of heat and generator windings emit 8.5 kW of heat – air exchange is 1.8 m3/s.
EXHAUST SYSTEM
If cars didn’t have silencers, the ground and everywhere they went would be quite noisy, and the same applies to diesel generators. There are many options for silencers:
-9 dB resonator – used with another silencer and provides minimal damping to the exhaust system;
-29 dB residential area silencer – an effective silencer that is sufficient in most cases;
-40 dB critical silencer – extremely effective silencer for good results, for example in hospitals.
In addition, the exhaust from the collector/turbo of the engine exhaust must go into the silencer and from there out of the generator room. In exhaust systems, only acid-resistant steel (AISI-316) can be used for diesel engines, because when the exhaust cools to the dew point, sulphuric and nitric acid is formed as condensate!
The same standards apply to exhaust gas pipelines as to other combustion gases, but it is important to consider the temperature of exhaust gas and that the diesel generator flue is not a draft flue but a flue for compressed gases! As one solution, insulated modular flues covered with polished steel sheet can be used on the exterior facade of the building, but they must comply with the regulation for working with compressed gases – EN 1856-1:2003-06.
Tips for planning exhaust piping:
The simplest solution is to remove exhaust above the heat sink, through the wall and terminate the pipe 30 cm from the wall cut at 45°. The air coming out of the heat sink takes the exhaust with it and dissipates it.
The end of the exhaust pipe or flue opening must be at least 6 m from the nearest opening window.
The vertical flue must rise at least 80 cm above the edge of the roof parapet.
Horizontal flues must feature a pocket for collecting precipitation and an option for emptying it.
NOISE SUPPRESSION
If we have installed at least an -29 dB silencer on the exhaust piping, the most noise is quite obviously emitted from the ventilation outlet. The noise level in the diesel generator room is 100–110 dB and usually 90–95 dB is emitted through the vent, measured from a distance of 1 m! According to the Minister of Social Affairs Regulation no. 42 of 4 March 2002, published RTL 2002,38,511, “Standard noise levels of technical equipment in buildings and on the outdoor territory of buildings”, noise levels are clearly standardised! The standard levels are usually 35 dB at night and 45 dB during the day measured at a distance of 1 m from the window of the adjacent residential building. If it is an industrial area or only wolves in the forest can hear the noise, then noise suppression is not critical. If a diesel generator is installed in the immediate vicinity of a residential area or even on an adjacent plot, complaints about noise pollution are easily made, leading to precepts and fines. One option is to build noise barriers as we are used to seeing along motorways, but they are definitely not suitable for a residential area.
The best tried and tested solution is silencer boards that can be installed in vents. All kinds of filters impede the movement of air and can easily become clogged. Silencer boards are acoustic wool boards with a thickness of 100 or 200 mm and air gaps of 100 mm between the boards. The boards reduce the vent two or three times and since reducing air velocity in the vent is one of our goals, we need to increase the area of the vents two or three times!
The most efficient working length of the silencer board is 1500 mm and, yes, we need extra space in terms of dimensions. Extending the silencer board does not add much value, rather it would be reasonable to create two damping chambers with a 1 m of working part and a gap of 1 m between them.
REFERENCES
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