Selecting fire pumps
The key for fire protection engineers is to understand the requirements of both NFPA 20 and NFPA 70 to properly choose and configure a fire pump so that the fire protection systems can serve their intended use.
- Understand the two primary types of fire pumps: electric and diesel.
- Learn about the codes and standards and define specification of these systems.
- Determine how to select the best pump to meet the intended usage.
Power for fire pumps is critical in the design of a properly operating fire protection system. Without power, the building loses the ability to have an effective fire suppression system. The building’s fire/life safety system also cannot control or extinguish a fire, thereby negating the benefits of the fire protection system. Therefore, careful consideration in the selection of pumps and power supplies is critical to the operation of the fire protection systems.
As fire protection engineers, our team often selects fire pumps for various designs. Due to the size, magnitude, and building height of the projects, municipal water supplies often are not capable of providing the required pressures to meet fire protection system (automatic sprinklers and standpipes) demand. Therefore, fire pumps are specified to boost pressures that are needed for these systems to protect the building and its occupants.
This team’s first choice in selecting fire pumps is to use an electric-driven pump. An electric-driven pump is easier to design, is easier to maintain on a regular basis, and does not require external fuel to operate the pump. It lends itself to a cleaner and more efficient system.
Diesel-driven pumps are very reliable and have their place in the design and installation of fire protection systems. However, they require fuel storage tanks of combustible liquids to be stored in or near the pump room, and require ventilation of combustion products and a means to replenish fuel used during operation, which includes frequent run tests. Sometimes it is just too difficult to locate a diesel-driven pump inside a building due to these considerations, especially when the design requires pumps be installed within a tower due to pressure zone requirements. Diesel-driven pumps are a good choice when the pump is located at the base of the building near the exterior wall or in a separate pump house to accommodate the refueling operations and the ventilation of combustion exhaust. When installed inside a building or midway up a high-rise tower, they are difficult to design and install.
An electric-driven pump does not require a combustion-driven engine to start to operate the pump. As long as power is available to the pump, when the pressure drops in the system, the electric-driven pump will start. The key is to provide a reliable source of power to the pump, under both normal and emergency conditions. For an electric-driven pump, power is the key to the reliability of the pump and therefore the fire protection system. Electric power is easier to run through the building, especially within high-rise towers where multiple pressure zone pumps are located. Getting the power there is easier than getting diesel fuel.
Codes and standards
Codes and standards governing fire pumps recognize the importance power plays in the operation of these electric-driven fire pumps. NFPA develops many standards and guides on how to design and install fire protection systems. NFPA 20: The Standard for the Installation of Stationary Pumps for Fire Protection outlines the requirements for the design and installation of fire pumps. When a fire pump is required due to system demands, often NFPA 20 is the referenced standard. NFPA 20 provides specific details for the use of both diesel and electric-driven pumps, including the power supply requirements for electric pumps.
Chapter 9 of the 2013 edition of NFPA 20 provides specific requirements for electric drives for fire pumps. It outlines the requirements for both normal and alternate power. It is clear that the normal power source be continually available and arranged in one of five methods. These include:
- A utility service connection dedicated to the pump
- An on-site power production facility dedicated to the fire pump
- A dedicated feeder connection derived directly from the dedicated fire pump service
- A feeder connection that is part of a multi-building campus-style arrangement meeting certain conditions
- A dedicated transformer connection directly from the service meeting Article 695 of NFPA 70: National Electrical Code.
NFPA 20 requires an alternate source of power when the height of the building is beyond the pumping capacity of fire department apparatus or where the normal source is not reliable. If a backup diesel-driven or steam-driven pump is provided, an alternate source of power is not required. Also, many of the model building and fire codes require an alternate or secondary source of power be provided for all pumps serving systems in high-rise buildings. Per NFPA, this source of power is considered emergency and should be available within 10 seconds of loss of normal power. The emergency source of power is required to be available for at least 8 hours.
One of the things that often gets overlooked when dealing with emergency power to fire pumps is the power requirements for the controller and pump from the backup source. The backup source is typically an on-site generator. NFPA 20 requires the pump to run at up to a locked rotor current, which can be up to six times the full load current. If the generator is sized to handle only the full load, there is not sufficient power available to drive the pump to meet NFPA 20 requirements. Because most pumps are of a significant size (150 to 250 hp), this oversight can be drastic in the overall performance of the system. The generator needs to be sized to handle the required start-up load, not just the running load.
Most electric-driven pumps that require backup power will have transfer switches specified that are integral with the controller itself. The transfer switch is a component of the controller, and the two act in unison to operate the pump under both normal and backup power conditions. When normal power is lost, the transfer switch senses this loss of power and allows the controller to switch to emergency power from the generator. The transfer switch in essence transfers fire pump power from normal to emergency.
As mentioned, the power requirements for a fire pump have an impact on the design of the electrical systems mostly attributed to the requirements of dealing with six times the full load current. The impacts include coordination of sizing the standby generator to handle the starting in-rush current and all other emergency loads while still meeting the voltage drop allowed during these conditions at the fire pump motor.
The normal power distribution raises similar concerns. Can the utility handle the high inrush current while maintaining the minimum voltage drop allowed at the fire pump motor? Typically the answer is yes, because of the stoutness of the system. When dealing with customer-owned medium-voltage distribution, voltage drop becomes an issue when the customer-owned transformer losses have a definite impact on that voltage drop, especially when the transformer selected is closely sized at 125% of the full load amps for the fire pump motor. When locked rotor occurs, the transformer may become saturated, and as such the voltage drop is increased across the transformer. As a rule of thumb, for a 50 hp fire pump, a 100 kVa transformer should be specified. For a 100 hp fire pump, a 300 kVa transformer should be specified. Modeling the distribution system for motor starting analysis is recommended to properly size the transformer.
Use of reduced voltage starters can lessen the impact of generator power. Many types are available, ranging from primary reactors to wye-delta closed or open type to autotransformers. Each type has its advantages and drawbacks, and the more efficient ones will cost more to install. These reduced voltage starters can decrease the inrush current anywhere from 400% to 150% of the inrush current. Regardless of the type of starter, their use can help reduce the impact on the overall generator system when emergency power is required to supply the fire pump. However, when solid-state starters are used, care must be taken to size the generator based upon the across-the-line inrush because these starters have a required bypass, which removes the ramp starting from the circuit. NEC 695.7(A) exception removes the voltage drop limitations for the emergency run mechanical starting but doesn’t remove the requirement for the generator to be sized to start the pump for across-the-line locked rotor current. The benefit of the reduced voltage starter is to lessen the demand on the system for normal inrush current.
NFPA 20 requires electrical installation methods to comply with Article 695 of NFPA 70. One of the key considerations in protecting the reliability of the fire pump installation is protecting the feeder circuits to the fire pumps. NFPA 70 requires electrical services for fire pumps to be routed outside of the building, or if routed inside the building to be installed under not less than 2 in. of concrete beneath a building or encased within concrete or brick not less than 2 in. thick. This is to provide a means to protect the service feeding the pump from damage by fire or other physical injury.
The requirements for supplying power to fire pumps are very stringent. This is due to the fact that the code recognizes that a fire pump is an essential element of the fire suppression system. The installation, including the power supplies, has to be very reliable for it to operate under adverse conditions. Often these stringent requirements, coupled with the power demands on both the utility and emergency power sources, make the use of electric-driven fire pumps cost prohibitive, driving the design solution to diesel-driven or other types of fire pumps. But as mentioned, there are times when you simply cannot use a diesel-driven pump, and the best choice is electric.
So how does a designer or installer apply these code requirements to the buildings that don’t specifically lend themselves to providing electric power to fire pumps, especially multiple fire pumps distributed throughout the complex? How does the size and configuration of the building impact the ability to apply the code requirements of NFPA 20 and NFPA 70? In some instances, some consideration can be given to alternative methods that are allowed by code; other times, one must merely consider how to apply the code intent to the building being designed. Following are some suggestions for applying code requirements to the powering of electric-driven fire pumps in large, complex facilities.