Learning objectives:
The requirements for very tall buildings in previous editions of NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection permitted gravity feed systems, but did not adequately cover gravity feed tank refills. Proposed changes to address gravity feed refills have tentatively been accepted for the 2019 edition of NFPA 20. NFPA 14: Standard for the Installation of Standpipe and Hose Systems permits gravity feed systems. These systems reduce the reliance on power during a fire emergency and reduce the number of pumps required for fire protection systems. A five-zone system for a 1,900-ft high-rise building will be explored.
Provisions for automated testing of fire protection systems were added to the 2014 edition of NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems and have tentatively been accepted in the 2019 edition of NFPA 20. The design of tall buildings should implement the highest level of automation available at the time of the design. This can reduce testing costs and increase reliability by automatic reporting and allowing an increased testing frequency.
Design requirements for very tall buildings
The current and revised requirements in NFPA 20 for very tall buildings will provide a system that is fully operable even with any one pump, tank, or pipe section out of service. This is accomplished with the following general requirements.
Acceptable water sources for very tall building
The water supply must supply the following flow rates and total volume:
NFPA 13, NFPA 14, and NFPA 20 accept the following water sources:
Requirements for water-storage tanks in very tall buildings
NFPA 20 has the following requirements for water-storage tanks in very high buildings:
Pressure requirements for fire protection systems
NFPA 13 and NFPA 14 have the following pressure requirements:
NFPA 13, NFPA 14, and NFPA 20 accept the following pressure sources:
Refill requirements for water storage tanks in very high buildings
NFPA 20 has the following tank refill requirements:
The water supply to the refill valve can be:
Fire pump considerations
Gravity feed considerations
Cross-connections in very tall buildings
Design of a standpipe/sprinkler system in a very tall building
Consider a design prepared for a 1,900-ft very tall building that has mechanical floors with water storage tanks at levels third level below grade (LL3). The elevations of the water storage tanks mechanical floors provide sufficient pressure to gravity down-feed the four lowest fire protection zones. In addition, it is desired to increase reliability by using gravity feed systems wherever possible to reduce the need for electrical power to supply fire pumps in the event of a fire.
As previously noted, traditional fire protection designs in very tall buildings use fire pumps and tanks located below the standpipe/sprinkler zone they serve. This building is designed with gravity feed systems. The fire pumps also refill the next tank above each fire pump. The fire pumps and tanks are located within the zone they serve. The fire pump discharge pressure is selected to provide a minimum of 100 psi at the top of the zone and 25-psi minimum pressure at the tank refill valve.
The design has a five-zone combination standpipe/sprinkler system with four gravity-fed zones and one pumped zone for the same 1,900-ft-high building. Refill pumps are provided at the four lower tanks. The systems are designed to comply with NFPA 14 and the proposed revisions to NFPA 20 and include the design considerations that were previously mentioned. The tank and pump levels are also the same.
The design shows the same five-zone combination standpipe/sprinkler system with four gravity-fed zones and one pumped zone for the same 1,900-ft-high building, except with a refill pump at every other tank level. The systems are designed to comply with NFPA 14 and the proposed revisions to NFPA 20. The tank and pump levels are the same.
In all of the schemes of the design, redundant refills are provided for all tanks. The pumped system design will require duplicate refills from the municipal water, or a redundant tank for the tank on LL3. For the gravity-fed designs, one refill for the tank on LL3 is from the municipal water supply and the other refill is gravity-fed from the tank on 34. One refill for the tanks on 34, 59, and 84, is gravity-fed from the tank above. The other refill is pumped from below. Both refills for the tank on 101 are pumped from below. This requires a redundant refill pump on level 84.
In the gravity feed systems, standpipe/sprinkler zones one through four are gravity-fed from the tank above though a connection to a standpipe/sprinkler system riser. Dual express risers, each connected to a different tank compartment, provide redundancy. A cross-connection with an isolation valve is provided in the tank connections and at the top of the zone. One refill connection is from the standpipe riser (gravity refill from the tank above). A second refill connection is from a pump taking suction from a tank below. Additional tank-refill redundancy can be provided at a relatively low cost by providing an additional connection to a different standpipe to feed the tank-refill valve.
Where a tank is provided with gravity and pumped refill, a check valve is required in each connection to prevent the systems from cross-feeding. Gravity refill from the tank above will lower the level of water in that tank. Gravity down feeding for refill will eventually require operation of multiple refill pumps. It is, therefore, desirable for the pump refill to be the primary refill and the gravity refill to open below the level at which the pump refill operates.
The pump pressure requirements are similar between the pump design and the gravity feed design with refill pumps on each tank level. A check valve is required in the tank-refill valve connection to the standpipe riser and to the tank-refill valve connection from the fire pump below. Again, it is desirable for the pump refill to be the primary refill and the gravity refill to open below the tank water level at which the pump refill operates. If the gravity-refill pressure is higher, it will be necessary to maintain the valve in the cross-connection between the refill valves normally closed to allow the pump refill to operate first. The normally closed valve can be opened whenever the pump refill is impaired.
The pumped system requires a total of 10 fire pumps; five primary and five redundant. The gravity feed system with refill pumps on each level requires a total of seven fire pumps (four primary refill pumps, one redundant refill pump, one fire pump, and one redundant fire pump). A gravity feed system with refill pumps on every other level requires a total of six fire pumps (two primary refill pumps, one redundant refill pump, one fire pump, and one redundant fire pump); however, the pressure requirements for this design may exceed the currently available pressure ratings on valves and components listed for fire protection.
Design documents high-pressure pumps that are capable of refilling tanks at two different levels. Redundancy for the primary pumps is provided by a backup pump for each primary pump. A total of six pumps is required in this configuration: two to refill Tanks 2 and 3, two to refill Tanks 4 and 5, and two to supply the five-zone fire protection system. For tanks 1 through 4, additional redundancy is provided by the gravity refill.
The pressures in the design diagrams exceed 175 psi on multiple floors, requiring the use of pressure-reducing fire hose valves. The sprinkler systems will also require pressure-reducing valves on some floors, unless sprinklers and other system components with pressure ratings of 250 to 300 psi are used. Eliminating the need for pressure-reducing valves would require dividing the building into more vertical zones.
Gravity-fed systems in very tall buildings is a design approach that reduces the dependency on fire pumps during fire emergencies. Gravity-fed systems also reduce the number of fire pumps and the future costs of periodic testing and maintenance. Changes in the 2019 edition of NFPA 20 clarifies that gravity tank refill from a tank located higher in the building meets the requirements for a one tank refill source.
Gayle Pennel is a project director with JENSEN HUGHES specializing in fire protection system design and water supplies. He is currently chairman of NFPA 20 and a member of the NFPA 25 Committee. He has designed multiple high-rise, exhibition, warehouse, retail, and factory fire protection systems. Alex Popov is a fire protection engineer with JENSEN HUGHES with prior experience in fire pump testing for certification.
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