Liquid fuel storage requirements are based on the type of fuel, use, and whether the fuel is “combustible” or “flammable” as judged by the fuel's flash point. This article provides information on applicable design standards for diesel storage tanks, including best practices for sizing and installation. It also explains how diesel fuel blending affects classification and tank vent design.
Diesel powered generators (gensets) are used in many power generation stations for primary energy. In addition, emergency and standby diesel generator sets are used in many other locations, including coal and nuclear power plants, as well as industrial, commercial, medical, and educational facilities (Figure 1). This means that almost everywhere where electricity is generated, diesel fuel is stored.
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1. Diesel powered generators are regularly used for emergency power supply in commercial, industrial, medical and educational facilities. They are also used in power plants to provide standby power and black start capability. Courtesy: TAI Engineering |
While it may seem simple to fill a tank with diesel fuel, there are detailed storage requirements outlined in several codes and standards, including the National Fire Protection Association's NFPA 30Code for flammable and combustible liquidsand NFPA 110Standard for emergency and standby power systems. There are also a number of best practices for developing safe and reliable diesel fuel storage systems.
Diesel-Generator-Sets
A simple diesel-powered genset includes a diesel engine and an electric generator. The mechanical energy provided by the diesel engine spins the generator rotor to generate electricity in the generator stator windings. The diesel engine itself is an internal combustion engine with various subsystems such as the cooling system, the starting system, the speed control system, the lubrication system and the fuel system.
The genset typically has a control panel equipped with switches and indicators to operate the generator, such as B. the start and stop controls. In addition, it offers a range of displays for various parameters such as voltage, current and frequency. The control panel can also include functions to monitor engine parameters such as temperature, RPM, oil pressure and more. The control panel microprocessor can be programmed to read engine parameters and take corrective action including engine shutdown.
diesel fuel system
On many diesel engines, an engine-driven fuel pump supplies fuel to the fuel injectors through a fuel filter for combustion in the cylinder. The fuel injector is a precision component and has the ability to pump, meter and inject the correct amount of fuel into the combustion chamber. Fuel flows continuously in the supply line to the injectors and the excess fuel is returned to the fuel tank via a pressure regulator. The pressure regulator ensures that the correct fuel pressure is maintained at the inlet to the injectors.
Another fuel system design uses a slightly different arrangement, with the fuel injection pressure being generated outside the injectors by a high pressure fuel pump. With this arrangement, fuel is not continuously circulated through the supply line. Instead, a small amount of fuel is diverted during fuel injection and this diverted fuel is returned to the fuel tank. Due to the high pressure in the fuel supply, the fuel temperature rises and the bypassed amount is therefore passed through a cooler before being returned to the fuel tank.
Diesel fuel temperature must be controlled to a maximum of 66°C (150.8°F) to ensure injector nozzles do not clog due to coking and to keep fuel viscosity within specified limits. Similarly, cold weather applications require fuel heaters to maintain fuel viscosity and prevent injector clogging due to wax build-up.
Storage and delivery of diesel fuel
According to NFPA 30, storage requirements are based on whether the liquid fuel is “combustible” or “combustible” as judged by the fuel's flash point. The fuel flash point is the lowest temperature at which the fuel will ignite in the presence of an ignition source. NFPA 30 defines flammable liquids with a flash point equal to or greater than 100 F (37.8 °C) and flammable liquids with a flash point less than 100 F (37.8 °C).
The flash point of conventional diesel fuel is typically between 52.2°C and 95.5°C (126°F and 204°F). Therefore, diesel fuel is considered a flammable liquid. It is further classified as Class II if the flash point is below 140 F, or Class III if the flash point is above 140 F, depending on the specific fuel.
However, it is important to note that when diesel fuel is blended with ethanol (e-diesel) to reduce emissions, the blended diesel fuel has a low flash point of around 68°F (20°C). The blended fuel is therefore considered a flammable liquid requiring management of the associated fire and explosion hazards. For the sake of simplicity, this article only considers conventional diesel fuel.
Clarification of biodiesel
In the original article published in the April 2020 issue ofPERFOMANCEDiesel fuel mixed with ethanol was incorrectly referred to as "biodiesel". Whereas the traditional name for ethanol blended diesel fuel is “e-diesel”. The wording was changed in this online version of the article on April 2, 2020. The following clarification was also published in the June 2020 issue ofPERFOMANCEMagazine.
According to a spokesman for the National Biodiesel Board, "blending ethanol into diesel fuel produces an off-spec fuel under ASTM D975 specifications for diesel fuel." The spokesman said biodiesel is "a clean-burning alternative to diesel fuel" made from a wide range of renewable fuels resources, including soybean oil, animal fats and recycled cooking oil. Biodiesel can be used alone or blended with petroleum diesel.
“Fuel grade biodiesel must be manufactured to strict industry specifications to ensure proper performance. Biodiesel blends meet the specifications for legal diesel engine fuel (ASTM D7467). In addition, B100 (100 percent biodiesel blend) must meet the ASTM definition for biodiesel itself (ASTM D6751)," the spokesman said.
The flash point of biodiesel is over 200°F, well above the flash point of petroleum diesel fuel of around 125°F. “Tests have shown that the flash point of biodiesel blends increases as the proportion of biodiesel increases. As such, biodiesel and biodiesel/petroleum diesel blends are safer to store, handle and use than conventional diesel fuel,” the spokesperson added.
However, John Fischer, an engine consultant from Palatine, Illinois, spoke up in an emailPERFOMANCEwrote: “One is cautioned against using biodiesel for the type of application (primary or emergency/standby power) addressed in the article. Because the significant amount of on-site fuel is rarely used quickly and has a food aspect, biodiesel degrades faster than traditional (100%) petroleum diesel. And there are enough concerns to keep 'standard' diesel clean and free of water.”
Sizing of bulk diesel storage tank. The size of a bulk diesel tank can depend on a number of factors, including the Emergency Power Supply System (EPSS) classification in some applications. The classification is defined in NFPA 110 as the “minimum time, in hours, that the EPSS is designed to operate at its rated load without being refueled or recharged.” For example, a Class 48 EPSS is expected to operate at least operates at its rated load for 48 hours without refilling the bulk tank. If the rated load consumes 450 liters per hour, the large tank should be able to provide 48 hours x 450 liters per hour = 21,600 liters of fuel.
In addition, NFPA 110 requires that the actual size of the bulk tank be at least 133% of the amount specified by the EPSS class (or fuel economy equivalent sensor amount). In addition, the large tank should have a reserve volume of 5% above the maximum liquid level if it is to comply with EN 12285 – the European standard.
The fuel storage capacity of 133% offers the possibility to test the emergency diesel several times for maintenance purposes before the tank needs to be refilled. The tank refill cycle should be such that the fuel inventory does not fall below the minimum level based on the NFPA 110 EPSS classification.
Installation of bulk diesel storage tanks.Bulk tanks can be installed above ground, in a storage tank vault, underground (directly buried) or in a storage tank building.
Above ground tanks (Figure 2) shall be fitted with an emergency relief vent to relieve internal pressure if the tank is exposed to fire. Above-ground tanks must also have means of preventing spillage.
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2. Shown here is an above-ground, double-walled, secondary containment-type tank for spill containment, located off the property lines. Courtesy: Pxfuel |
Tanks listed as "above ground tanks" can be installed in a vault, and the vault can be either above or below ground. Backfilling is not permitted around the tank in a vault and sufficient clearance is required around the tank for inspection and maintenance purposes. Vaults should be fitted with means to admit fire suppressant, contain spills from vaults and sound an alarm if vapour/liquid is released.
Underground tanks and pipelines require external corrosion protection through cathodic protection or the use of corrosion-resistant material. The substrate and fill must be clean, non-corrosive, compacted sand or gravel.
Storage buildings are subject to minimum clearances from property lines, public pathways, and important buildings on the same property. Buildings and structures for storage tanks must have a minimum fire rating of two hours and be equipped with manual/fixed fire protection equipment. Leakage of liquid from public waterways/sewers or adjacent properties must be prevented and all vents must be outside the building.
Design standards for large diesel tanks.In diesel gensets, the bulk storage tank is generally an atmospheric tank designed and constructed to American Petroleum Institute (API) Standard 650Welded tanks for oil storage. Such tanks may be operated from atmospheric pressure up to 1.0 psi gauge (6.9 kPa), but must be vented to avoid a vacuum. Other design standards can be used, but the pressure limits should be checked to avoid deformation of the tank shell during service. The bulk storage tank can also be designed as a low-pressure tank according to API 620Design and construction of large, welded, low-pressure storage tanksor as a pressure vessel under Section VIII of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
Venting of large diesel tanks.Bulk diesel storage tanks should be equipped with a vent line to prevent vacuum formation or tank over-pressurization during tank draining or tank filling or due to changes in atmospheric temperature. The vent pipe must open into a safe area a minimum of 5 feet from building openings and a minimum of 15 feet from powered ventilation air intake devices.
The vent pipe should be sized per API 2000 standardVenting atmospheric and low pressure storage tanks, but in no case less than 1.25 inches inside diameter. Also see Table 23.6.2 in NFPA 30 for vent line sizes as a function of vent tube length and fluid flow. The vent pipe can be fitted with a U-bend and/or a strainer to prevent foreign matter from entering. However, these devices create flow restrictions that are exacerbated by clogging with dirt or nests of insects. Flow limitations should be considered in pressure drop calculations when estimating in-service tank overpressure/vacuum conditions that could otherwise damage the tank shell.
Storage tanks used for ethanol blended diesel fuel must be fitted with a flame arrester at the vent tube outlet. This is because diesel fuel mixed with ethanol is a flammable liquid with a low flash point.
day tank.The day tank is located between the bulk storage tank and the diesel engine. The fuel is pumped from the large storage tank to the day tank via the large storage tank feed pump. Fuel from the day tank is delivered to the diesel engine by the engine driven fuel transfer pump mounted on the engine assembly.
The day tank is required when the engine fuel pump is unable to draw fuel from the fuel tank due to distance or altitude issues. The day tank is also used to ensure efficient fuel flow to the engine by eliminating any externally induced head created by the position of the bulk tank or by the bulk storage feed pump. The day tank also serves as a heat sink to collect the hot, unused fuel that is returned from the engine via a cooler.
Using gear pumps for bulk fuel transfer to the day tank.The fuel transfer pump from the hold tank to the day tank is generally a positive displacement gear pump located above the hold tank. When the pump is started, the air in the suction line is removed and a vacuum is created which provides the buoyancy for the liquid in the tank to rise up the suction line. The total suction lift plus friction height in the suction line should not exceed 15 inches of mercury (inHg, 7.4 psi) for nonvolatile liquids. This value progressively decreases further to 10 inHg (5.0 psi) and below for volatile liquids due to their higher vapor pressure. A foot valve in the suction line keeps the pump primed and ready to run, eliminating the need to bleed air from the suction line when starting the pump. ■
—S. Zaheer Akhtar, PEis Senior Process Engineer atTAI-Engineeringbased in Owings Mills, Maryland.