Emergency power systems are critical in a wide range of settings, such as hospitals, data centers, and other critical facilities. To ensure the reliability of these systems, the National Fire Protection Association (NFPA) has established standards and codes, including NFPA-110. This article focuses on paragraph 7.9.2 of NFPA-110, and provides an explanation of the intent behind the standard, and its significance in maintaining a reliable fuel supply for emergency generators.
NFPA-110, “Standard for Emergency and Standby Power Systems”, provides guidance for the installation, performance, maintenance, and testing of emergency power systems. Within NFPA 110, paragraph 7.9.2 focuses on fuel supply reliability.
“Fuel tanks shall be close enough to the prime mover for the fuel lift (suction head) of the prime mover fuel pump to meet the fuel system requirements, or a fuel transfer pump and day tank shall be provided.”– NFPA-110 7.9.2
NFPA-110 7.9.2 looks to ensure that the engine has a reliable fuel supply source by specifically requiring that the engine’s fuel source (fuel tank) be in close proximity to the engine. NFPA 110 recognizes that, if the engine-mounted fuel pump is unable to draw fuel from the source tank, the emergency generator will starve of fuel and shut down. The criteria for what is “close enough” is defined by the specific engine’s fuel pump lift capacity (suction head), which is published within the engine manufacturer’s data sheet (see related article). When the source tank is not “close enough”, the standard requires the use of a day tank, which of course must be located close enough to the engine to satisfy the original criteria in paragraph 7.9.2.
When the primary fuel storage tank is too far from the engine, the use of a day tank in close proximity to the engine is essential for several reasons:
- Fuel Lift (Suction Head) Requirement: The fuel lift, or suction head, is the vertical distance between the fuel source (tank) and the engine’s fuel pump. The engine’s fuel pump has a limited capacity to draw fuel effectively. By positioning the fuel day tank in close proximity to the engine, NFPA 110 ensures that the fuel lift is within acceptable limits for proper fuel delivery.
- Efficient Fuel Flow: Placing the fuel day tank near the engine minimizes the distance over which fuel needs to travel. This reduces potential pressure losses, and restrictions in the fuel supply line. Consider, for example, a new installation where the fuel tank is just close enough for the engine to draw fuel effectively. Most engines are equipped with fuel filters, which build up restriction as they approach the end of their useful life. Will the engine still be able to draw fuel when the fuel filter is less than clean?
- Fuel System Reliability: The proximity of the fuel day tank to the engine enhances the reliability of the fuel system. It minimizes the risk of fuel starvation, which can occur if the fuel supply line is too long or if the fuel lift exceeds the pump’s capabilities.
To summarize, in cases where the primary fuel tank is located at a considerable distance from the engine, or at an elevation lower than the engine’s fuel pump, NFPA 110 requires the use of a fuel transfer pump and day tank. The fuel transfer pump is designed to move fuel from the primary tank to a fuel day tank located near the engine (depending on suction head, the fuel pump may be mounted on the day tank, or may need to be installed by the source tank). The day tank then serves as the immediate fuel source for the engine, meeting the proximity requirements outlined in the standard.
By providing flexibility in fuel system design while upholding the fundamental objective of fuel supply reliability, NFPA 110 ensures that emergency power systems can operate effectively, minimizing the risk of power disruptions in critical facilities during emergencies.
For additional support:
On-Site Power Advisor can assist engineering firms in specifying fuel storage tanks, fuel delivery systems, and fuel quality maintenance systems to ensure that emergency power systems have a reliable fuel source. For further design support, please contact here.