How To Design A Generator Fuel System

11 Oct How To Design A Generator Fuel System

Generator fuel systems are an important part of any generator set. They allow the generator to run smoothly and efficiently and help to extend the life of the generator. In this article, we will discuss how to design a generator fuel system that meets your specific needs. 

The 9 major steps of designing generator fuel systems

Gain an understanding of the prerequisites as well as the obstacles involved in the design of a genset fuel oil system.

Learning objectives:

• Get familiar with the nine most important factors to take into account when designing a fuel oil system for a generator set.

• Early on in the project, the proposed design should be reviewed by the authorities that have jurisdiction over the matter.

• When designing a system to handle fuel oil, it is important to keep certain rules of thumb in mind.

There are 9 primary factors to take into account when designing fuel oil systems for generator sets.

1. Runtime criteria
2. Fuel storage
3. Fuel pumping
4. Fuel cooling
5. Fuel piping
6. Fuel maintenance
7. Fuel filling
8. System controls
9. Applicable codes and standards.

To successfully design a fuel system, it is essential to comprehend the requirements and obstacles of each type. Despite inherent nuances, some of the same considerations underlying fuel oil design principles can be applied to systems intended for other applications, such as oil-fired boilers. The application will be determined by design criteria unique to each project.

1. Runtime criteria

Establishing runtime criteria during a power outage is among the initial steps of designing a fuel oil system for gensets (see “Runtime requirements”). Typically determined by a combination of applicable codes and owner requirements, the runtime—or the length of time the generator must operate during an emergency without refueling—sets the standard for fuel oil design and operation. For instance, life safety gensets are typically required to support emergency loads for two hours in the event of a power outage. Depending on site resiliency requirements, critical facilities, such as data centers, are typically expected to support the load for 24 hours or longer.

Because runtime criteria have a direct bearing on the required fuel storage capacity on-site, this factor must be investigated first. Manufacturers readily provide information on the fuel consumption of generator sets at various loads.

2. Fuel oil storage

Fuel oil can be stored in either above ground or underground storage tanks (USTs). Each has advantages and disadvantages, and it is essential to specify the proper type to ensure the optimal design.

3. Fuel oil pumping

Generators are equipped with pumps driven by gears that pressurize fuel in the engine’s common rail. Fuel is drawn by the integral pump from the external tank. Any fuel that is not injected into the cylinders is returned to the tank. The pump has limited priming and fuel distribution system friction loss overcoming capacity (piping, fittings, and filters).

External to the generator, there are typically two types of electric-driven fuel oil pumps: gear pumps and centrifugal submersible pumps, each with their own advantages and disadvantages.

Typically, gear pumps and centrifugal submersible pumps—each with specific benefits and drawbacks—are used as external fuel oil pumps powered by electricity.

• Gear pumps: These pumps, which can be either internal or external gear types, are mounted on a separate skid and are typically used for low-flow, high-pressure applications. Gear pumps, which fall under the category of positive displacement pumps, are appropriate when pressure requirements exceed 40 psi. In reality, there are gear pumps with pressure capacities greater than 2,000 psi. Gear pumps have a constant flow rate, and the horsepower of the motor affects how much pressure can be discharged.

• Submersible pumps: Even though the majority of the pump assembly is inside the fuel tank, submersible pumps, which are used for high-flow, low-pressure applications, still need enough clearance above the tank for accessibility and maintenance. Both suction lift and priming are not problematic.

When designing a fuel system, static lift and friction losses should be carefully considered. Pumps should operate sporadically to fill the auxiliary tanks rather than continuously by having their design flow rate two to four times higher than their peak demand.

4. Cooling the fuel

The excess common rail fuel that is not injected into the cylinders is returned to the tank. The return fuel is heated because it has absorbed heat from the injectors and water jacket. When it mixes with cooler fuel in the tank, the temperature of the supply fuel begins to gradually increase.

5. Fuel transfer pipes

Due to the inherent corrosion resistance of nonmetallic materials, reinforced thermosetting resin pipe is preferred when designing underground piping. Almost all underground piping is double-walled and consists of a carrier pipe and a containment pipe. A system for detecting leaks monitors the interstitial space between the pipes.

Above-ground fuel transfer pipes that are easily accessible typically consist of single-walled carbon steel. Local authorities and insurance companies may also require double-walled piping for above-ground applications. Suction pipes are sized so as to minimize frictional loss in the pipes and fittings. It is advised to maintain absolute pressure at the pump’s inlet above 15 in. Hg (mercury column). Centrifugal pump discharge pipes are sized based on the pumps’ dynamic head limitations, while positive displacement pump discharge pipes are sized between 8 and 12 feet per second.

6. Fuel oil maintenance

Fuel oil is composed of organic compounds and degrades over time as a result of biological growth, water accumulation, and particle formation. This degradation, if uncontrolled, could result in clogged filters, or could negatively impact the combustion process in the generator engine. In the worst-case scenario, the degradation could cause the generator to shut down due to a safety cutoff.

Degradation is not a concern for applications where fuel is consistently used and a fresh supply of fuel is regularly introduced, such as generators used in combined heat and power applications. In standby generator applications, fuel consumption is minimal as a result of limited runtimes caused by periodic testing. For these applications, a fuel maintenance or polishing system can be used to periodically treat fuel oil (usually on a weekly or biweekly basis).

The fuel is pumped through a series of prefilters, final filters, and a water separator as it cycles through the polishing system. Biocides can be manually introduced or automatically injected by the polishing system. Providing pipe connections at the tank and enlisting the services of a contractor who performs periodic fuel maintenance with a mobile or roll-up polishing system is an option if site or budget constraints do not allow for a permanent, onsite polishing system.

7. Filling the tank

During design, it’s important to choose the fuel-oil vendor’s delivery truck. Gravity or pump fuel-oil trucks (i.e., equipped with an integral fill pump).

Both types of trucks can carry USTs and ASTs at lower elevations. High-altitude AST filling requires pump trucks. Gravity trucks fill USTs best. A gravity-type truck won’t work if the fuel tank is higher than the truck. Gravity trucks can fill ASTs at a higher elevation using a remote fill system with an integral fuel transfer pump. Remote fill systems have gauges and sensors to assist and alert the operator.

Fuel oil could be needed in an emergency, so extreme measures must be taken. In the event of a city-wide power outage, when everyone needs fuel, a fuel oil vendor may promise one type of truck but not have it. Fuel oil system design must consider extreme situations.

8. Fuel system controls

Fuel systems typically use programmable logic controllers (PLCs) listed by UL 508: Standard for Industrial Control Equipment to control and monitor transfer pumps, storage tanks, auxiliary tanks, polishing systems, fill systems, fuel inventory, and leak detection, as well as other subsystems and equipment. For building management system integration, they provide BACnet, Modbus, and local operating networks (LonWorks) communication capabilities.

For mission-critical applications such as data centers, the control system typically employs dual independent PLCs and dual power inputs to eliminate single points of failure. During the design phase, the architecture and sequence of operations for the fuel system control should be thoroughly examined. The entire scope of work associated with fuel systems (equipment, controls, startup, and training) should ideally be handled by a single, specialized vendor.

9. Codes and standards

City, state, and federal authorities regulate the storage and system design of fuel oil due to its combustible nature and its detrimental effects on the environment in the event of a leak. Design requires careful consideration of a multitude of factors. For instance, the Office of the Illinois State Fire Marshal requires a 10,000-gal AST to be at least 25 feet from the property line, whereas the Chicago Building Code requires it to be at least 30 feet. Compliance requires that the worst-case scenario be considered.

Code mandates maximum fuel storage, tank construction, spill containment, location relative to buildings and property lines, fire suppression, and building height restrictions. Due to inherent fire risks, the requirements are especially stringent for applications involving indoor fuel storage. Frequently, the requirements listed in different codes and standards vary considerably. Table 2 lists Chicago’s requirements for a 10,000-gallon outdoor AST. To avoid surprises later, it’s best to approach the authority having jurisdiction early in the project to review the proposed design, preferably with fire department representatives.

So there you have it. The basics of how to design a generator fuel system. This is just a starting point, and there are plenty of other factors that you will need to consider when designing your system. But if you follow these guidelines, you should be well on your way to having a functional, safe generator fuel system.

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