Generator Fuel Systems Affect Uptime
Whether for a data center, health-care building, or 911 call center, a great deal of time and effort are put into the design of the emergency backup generators, switch gear, and UPS equipment that supply emergency backup power. Often very little attention is given to the design of the fuel oil supply system necessary to run the generators. A badly designed fuel system or poorly maintained diesel fuel can cause a power interruption just as certainly as a failure of the downstream equipment.
“All too often, we see a Tier III data center designed with a Tier II or Tier I fuel oil system,” said Doug Nakano, president of Fuel Oil Systems in Hayward, CA. “So much effort is directed to the design of the electrical system, cooling system, and delivery system; it would be a shame to fail to design the fuel oil system to the same Tier classification.”
Typically, transfer pumps are energized when the belly tanks or day tanks trip a low-level switch. “If there is a problem with the transfer pumps, you want to know it while your belly tanks are still full so you have time to fix the problem,” Cosgrove said. At Tier III and Tier IV facilities, Preferred Utilities energizes the fuel transfer pumps (and proves oil flow) whenever a generator is running. In addition, pump sequencing controls automatically energize a lag pump if the lead pump fails to prove oil flow. Duplex pump sets are sized for 100 percent redundancy so one pump can meet the needs of all connected generators, allowing the other pump(s) to be down in an emergency without interrupting service. Because generators use the fuel oil for cooling, their fuel usage rate is higher than their fuel consumption.
Poor fuel quality is one of the most common culprits when diesel generators fail to start. Diesel oil is a refined product that begins to break down as soon as it leaves the refinery. Water in the fuel or water penetrating the tank through leaks or by condensation exacerbates bacterial growth, which forms a film at the water-oil boundary and can clog generator intakes.
Proper design can minimize water infiltration. However, water can enter the fuel system by condensation through fuel tank vents or can even be delivered with the fuel. Permanently installed filtration systems can automatically treat one or several tanks. These filtration systems pass the fuel through a series of strainers and filters, remove any water in the fuel and store it, and inject a chemical stabilizer. Portable filtration units and fuel oil filtration services are also available. Because scheduled generator tests alone rarely consume enough fuel to keep an entire tank of oil turned over, the need for fuel oil filtering and dewatering cannot be overemphasized.
Fuel oil system design must comply with local, state, and national fire codes, including National Fire Protection Association (NFPA) 30. In the event of a fuel leak or a fire, oil pumps are required to shut down and all valves are required to be closed. In addition, many jurisdictions require fire safety valves that sense a fire and automatically shut off. Anti-siphon valves are often required to contain leaks in the event of a pipe rupture at a low point in the system. Monitored double-wall containment piping may be required when fuel piping is routed underground or within building interiors.
- Fill boxes are required to have overfill alarms and automatic shutoff.
- Main storage tanks must include level monitoring, high and low alarm points, and leak detection.
- Pump sequencing controls need to energize fuel pumps, prove flow, start lag pumps, and automatically rotate lead and lag pumps periodically. Return pumps may need to be energized whenever transfer pumps are running to prevent overfilling tanks.
- Day tanks require level monitoring, venting, and leak detection. In addition, temperature monitoring may be required to prevent overheating from generator return oil.
Oftentimes the design of the fuel oil system for the diesel generators is an afterthought in the overall facility design. Many design engineers are under the mistaken impression that the generator manufacturer will handle the fuel oil system. Sometimes little thought is given to the fuel oil system until after the generators have been ordered.
Installation of a typical fuel oil system can affect as many as six construction trades. “An underground fuel system may have mechanical, plumbing, electrical, concrete, excavation, and sometimes civil engineering work involved,” said Doug Nakano of Fuel Oil Systems in Hayward, CA. To help ensure that the fuel oil system is coordinated across all affected trades, underground fuel oil systems should be specified in Division 23 and above-ground systems in Division 2 of the new Construction Specification Institute numbering system.
Because of the nature of the design work and the many trades affected during construction, finding a supplier that will provide single-source responsibility for the fuel oil system, including design, supply, start-up, and compliance with all applicable codes is often the best way to ensure system reliability. Some of these suppliers are licensed contractors that will provide installation as well.
The Uptime Institute’s Tier classification system is a much-needed effort to quantify and standardize levels of reliability and maintainability for the different requirements of facility owners. Although written with an eye toward data centers, the philosophy behind the approach can be easily extended to other critical facilities, including health-care facilities, emergency-response facilities, and other sites that must meet a higher standard of reliability with their backup power generation.