Monday, September 3, 2018

Designing Generator Fuel Systems (4)

Referring to the Designing Generator Fuel Systems (1), (2) and (3), let's summarize to form a whole picture.

The successful design of a backup generation system is critical to maintaining business continuity, sustaining critical operations, and life safety in the event of a serious power outage (see "Fuel system design checklist"). Without proper design and ongoing maintenance, fuel oil systems cannot meet the needs of the gensets they serve, and therefore, cannot guarantee the assumed reliability of the facility's backup power.


Fuel System Design Checklist
When designing fuel oil systems, remember


  • Provide foot valves (to maintain pump prime), anti-siphon valves (to prevent accidental leakage), and fusible link shutoff valves (for fire safety).
  • When calculating pump suction lift, assume the worst-case scenario (i.e., a nearly empty tank).
  • When calculating friction losses through the fuel oil distribution system, assume the worst-case scenario (i.e., viscosity corresponding to the lowest anticipated fuel temperature).
  • If fuel temperature is anticipated to fall below its cloud point, provide a means of heating (tank heaters, space heaters, pipe heat trace, etc.)
  • Ensure storage tanks are equipped with adequate ports to accommodate pipe connections, sensors, vents, switches, etc.
  • Using auxiliary tanks if necessary




Consider Using Auxiliary Tanks When

  • The main tanks are located more than 50 ft away from the gensets
  • The main tanks are located more than 12 ft below the gensets
  • The main tanks are located above the gensets

Fuel Oil Design Cheat Sheet

Refer to this cheat sheet for important considerations when designing a fuel oil system:


  • No. 2 fuel oil NFPA Classification: Class II
  • Genset fuel consumption: approximately 7 gph/100 kW rating
  • Atmospheric pressure: 30 in. Hg (mercury column)
  • Minimum recommended pressure at external pump inlet: 15 in. wc
  • Pressure of 2.6 ft of No. 2 fuel oil: 1 psi.




Runtime Requirements

Runtime requirement for emergency power supply systems, according to NFPA 110: Standard for Emergency and Standby Power Systems include:


Class 0.083: 0.083 hours = 5 minutes
Class 0.25: 0.25 hours = 15 minutes
Class 2: 2 hours
Class 6: 6 hours
Class 48: 48 hours
Class X: other times (application, code, or user dictated)



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