What Are The Nine Major Steps Of Designing Generator Fuel Systems?
When designing fuel oil systems for gensets, there are nine key considerations:
- Runtime criteria
- Fuel storage
- Fuel pumping
- Fuel cooling
- Fuel piping
- Fuel maintenance
- Fuel filling
- System controls
- Applicable codes and standards.
Comprehending each one’s requirements and challenges is crucial to understanding any fuel system design. Note that although inherent nuances exist, some of the same considerations underlying the principles of fuel oil design may also be applied to systems intended for other applications, such as oil-fired boilers. The final application will be dictated by the design criteria which are unique to each project.
One of the first steps of designing a genset fuel oil system is to define runtime specifications in case of power failure. Sometimes determined by a combination of relevant codes and owner specifications, the runtime — or how long the genset has to work without refueling during an emergency event — will set the standard for designing and running the oil. Life safety gensets, for example, are usually expected to serve emergency loads for a duration of 2 hours following a power failure. Important buildings, such as data centers, are usually required to sustain the load for 24 hours or longer, depending on criteria for site resiliency.
Fuel oil storage
Fuel oil can be stored in storage tanks above ground (ASTs) or in tanks underground (USTs). Each has both advantages and disadvantages, and it is critical to specify the appropriate type to ensure optimum design.
Fuel oil pumping
Gensets are equipped with gear-driven pumps in the engine’s common-rail which pressure fuel. The internal pump draws fuel from the tank behind it. Excess fuel that has not been pumped into the cylinders is returned to the tank. The pump has limited capacity in the fuel distribution system (piping, fittings, and filters) for priming and overcoming friction losses.
Cooling the fuel
Excess fuel is returned to the tank at the standard rail that is not pumped into the cylinders. The return fuel is at high temperature as it absorbs heat from the injectors and the jacket of water. As it mixes in the tank with cooler air, the temperature of the fuel supply begins to slowly increase.
For every 10°F increase in fuel temperature above 100°F, the horsepower of the engine is reduced by around 1%. The high temperature of fuel also reduces the ability to lubricate components of the engine fuel system. When the fuel temperature to be supplied to the engine reaches a certain level (typically 140° to 150°F), the genset must shut down due to the safety cutoff. It is especially troublesome if the capacity of the tank is fairly low (e.g., auxiliary tanks), and the temperature of the return fuel is not decreased.
Fuel transfer pipes
A non-metallic material, such as reinforced thermosetting resin pipe, is preferred for the design of underground site piping due to its inherent corrosion protection. Underground plumbing, consisting of a carrier pipe and a containment pipe, is almost always double-wall. With a leak detection device, the interstitial space between the pipes is monitored.
Usually, single-wall carbon steel is the fuel transfer pipes placed above ground in accessible areas. Remember that for aboveground applications even local authorities and insurance providers can need double-wall piping.
Fuel oil maintenance
The fuel oil consists of organic compounds that will slowly degrade over time due to biological growth, accumulation of water which formation of particulates. If uncontrolled, this deterioration could result in clogged filters or could have a negative effect on the generator engine combustion cycle. In a worse-case situation, the genset may be shut down by deterioration due to a safety cutoff.
Filling the tank
It is necessary to decide what type of delivery truck the fuel-oil supplier will be using during the design process. Fuel-oil trucks are either a type of gravity or a type of pump (i.e., fitted with an integrated fill pump).
Fuel system controls
Usually, fuel systems use UL 508: Industrial Control System Programmable Logic Controllers (PLCs) specified for controlling and monitoring transfer pumps, storage tanks, auxiliary tanks, polishing systems, filling systems, fuel inventory, leak detection, and other related subsystems and equipment. They provide networking capabilities for integration with a building management system, such as BACnet, Modbus, and local operating networks (LonWorks).