Two 20,000-gallon underground tanks in the process of being buried for Kaiser Hospital in Antioch. These tanks will provide four days of fuel for  generators and dual-fuel fired boilers.

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.

This data center project includes two 25,000-gallon underground storage tanks, redundant pumps, redundant power supplies, and completely redundant piping paths to eliminate all single points of failure in the fuel oil supply system.

The Uptime Institute’s Tier Classification and Performance Standard defines performance-based outcome requirements for different levels of reliability and availability of data centers. This four-tier standard encompasses the entire data center. So when a data center is designed to be a Tier III facility (Concurrently Maintainable Site Infrastructure), for example, the design of the fuel oil system feeding the diesel generators should reflect the same standards for reliability, redundancy, and maintainability. (Note: The Uptime Institute considers the facility’s local generators to be the primary source of power for a data center. The electric utility grid is considered an economic alternative provider.)

“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.”

Fuel oil diagram.

Designing a fuel oil system to meet the intent of Tier III or Tier IV classifications requires a careful integration of fuel oil hardware and controls, combined with a healthy respect for Murphy’s Law. Starting at the diesel generator and working back to the main storage tank, it’s important that each generator has a certain amount of fuel nearby in either a belly tank or a local day tank that the generator can draw upon without external pumps. “We design for at least four hours of fuel immediately available to the generators,” said Dennis Cosgrove, fuel oil product manager for Preferred Utilities. “This gives the operators at least four hours to correct any problems with the fuel oil supply system before it causes an interruption.”

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.

Preferred Utilities fuel control enclosures installed by Fuel Oil Systems at a Bay Area facility. Because a diesel fuel system involves so many trades, proper coordination is essential to a successful fuelsystem installation.

Debate continues regarding the use of positive displacement fuel oil pumps versus submersible pumps in the main storage tanks. Submersible pumps are lower in cost and have the advantage of always being primed since they sit in the tanks. For critical facilities, these advantages are outweighed by the difficulty in getting access to submersible pumps for maintenance or repair. “On the Gulf Coast, our generators are there mainly for storms,” said Sean Smith of Cougar Sales of Houston, TX. “During a storm, I don’t want to be outdoors, in a manhole, trying to work on a pump.”

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.

A typical fuel oil control system for a large data center may contain hundreds of I/O points interfacing with pumps, tank gauges, level controls, flow monitors, and leak detectors. A combination of indicating lights and color touch screens provide a graphical interface for the operators.

The design of fuel oil tank, pump, and piping systems for critical facilities pales beside the complexity of the fuel oil monitoring and control systems for these facilities. Typical inputs and outputs to be monitored include:
  • 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.
Finally, this entire fuel oil control system needs to communicate with the plant’s building automation system (BAS) or fire-suppression system. Because of the large number of points to be monitored, this is often done via Modbus, Ethernet, or Bacnet.

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.