Designing gravity feed fire protection systems
Gravity-fed standpipe and sprinkler systems are viable alternatives to traditional pumped, bottom-fed standpipe/sprinkler systems in very tall buildings.
- Learn how to do design gravity feed standpipe and sprinkler systems.
- Understand why automated testing should be incorporated into the design.
- Assess why one fire protection system should be specified over another.
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:
- A water-storage tank must be sized to provide the full fire protection system demand.
- NFPA 14 requires 500 gpm for the first standpipe riser and 250 gpm for the next two standpipe risers or a total of 1,000 gpm with three or more standpipe risers. The demand must be maintained for 30 minutes, for a total of 30,000 gal.
- For ordinary-hazard Group 1 occupancies, NFPA 13: Standard for the Installation of Sprinkler Systems requires a density of 0.16 gpm/sq ft over 1,500 sq ft plus a 250 gpm hose stream that must be maintained for 60 minutes, or 500± gal for 60 minutes, for a total of 30,000 gal. Most very tall buildings are light hazard and the water-supply demand is less than the standpipe demand.
NFPA 13, NFPA 14, and NFPA 20 accept the following water sources:
- NFPA 20 requires redundant water sources in very tall buildings with each capable of providing the fire protection system demand. The following are acceptable water sources:
– A reliable water utility with adequate volume and pressure.
– Water-storage tanks.
– A pond, river, lake, swimming pool, or water-storage reservoir. Note: Seasonal variations in water levels and the potential for introducing biological matter that may cause corrosion or clogging and particulate matter that may cause clogging should be accounted for before using a raw-water source.
Requirements for water-storage tanks in very tall buildings
NFPA 20 has the following requirements for water-storage tanks in very high buildings:
- If one water tank is used, it must be compartmentalized into a minimum of two equal halves.
- If multiple water tanks are used, they must be sized so that a minimum of 50% of the full fire protection system demand is available with any tank out of service.
- Each tank and tank compartment must be provided with redundant automatic refills, with each refill capable of refilling at the maximum fire protection system flow rate. Note: This permits taking a tank compartment out of service while maintaining capacity for the full fire protection demand.
- Each tank should be provided with an overflow sized for the maximum refill rate and discharging to a safe location. A large-volume intake is needed close to the top of the tank to meet the necessary overcapacity. The overflow can be piped to the tank below, allowing the overflow from one tank to another.
Pressure requirements for fire protection systems
NFPA 13 and NFPA 14 have the following pressure requirements:
- NFPA 14 requires 100-psi residual at the hose valve at the top of the standpipe.
- The desired pressure in NFPA 13 is dependent on the design criteria and the pipe sizing and must be adequate to supply the fire protection system demand.
- The pressure cannot exceed the rated pressure of any system component. Gravity feed systems and variable-speed pumps minimize the static to residual-pressure differential and are beneficial to assist in complying with this limitation.
NFPA 13, NFPA 14, and NFPA 20 accept the following pressure sources:
- A reliable utility water source that provides the required pressure.
- An appropriately sized fire pump taking suction from reliable utility water.
- An appropriately sized fire pump taking suction from a compartmentalized water-storage tank.
- A compartmentalized water-storage tank (or multiple tanks) with sufficient elevation above the system to provide the required pressure.
Refill requirements for water storage tanks in very high buildings
NFPA 20 has the following tank refill requirements:
- Each tank and tank compartment must be provided with an automatic refill valve with each refill valve capable of refilling at the maximum fire protection system flow rate.
- The water supplies to each refill valve must be connected to a different standpipe or express riser.
- The water supplies to the refill valves must be cross-connected and provided with isolation valves.
The water supply to the refill valve can be:
- From a water utility water source with adequate pressure to operate the refill valves.
- Pumped through a fire protection system below.
- Pumped directly from a refill pump.
- Gravity-fed from a tank located above the refill valve.
- A reliable domestic-water supply that has adequate volume and pressure.
Fire pump considerations
- Each fire pump must provide adequate volume and discharge pressure to provide a residual pressure that meets the fire protection system demand.
- A redundant pump (or alternative water supply) is required if serving a fire protection zone that is beyond the pumping capacity of the fire department.
- The fire protection system cannot have static pressure that subjects any system component to a pressure above its listed maximum working pressure. Variable-speed pumps minimize the static-to-residual-pressure differential.
- Fire pumps used for tank refill must only provide sufficient pressure to operate the refill valve.
Gravity feed considerations
- The water source must provide adequate volume and pressure to provide a residual pressure that meets the fire protection system demand.
- A water-storage tank must be elevated a minimum of 231 ft above the top of the zone to provide 100 psi at the top of the zone.
- Redundancy is provided with dual express risers, with each riser connected to a different tank or tank compartment.
- A cross-connection with an isolation valve is required at the top (tank level) and the bottom (top of zone) of the dual express risers.
- The fire protection system cannot have static pressure that subjects any system component to a pressure above its rated (listed) pressure.
Cross-connections in very tall buildings
- When tanks provide gravity feed to a fire protection zone, dual express risers with independent tank connections made to different tanks or opposite sides of a divided tank are required.
- Dual express risers must be cross-connected at the top and bottom with isolation valves in the cross-connections.
- When fire pumps provide the water supply to a fire protection zone, dual fire pumps with a suction connection to different tanks or opposite sides of a divided tank are required.
- Both the pump suction and pump discharge must be cross-connected with isolation valves in the cross-connections.
- When one refill is pumped and the second refill is gravity-fed, the isolation valve in the cross-connection between the two fill sources should be maintained in a closed position to allow the pumped refill to operate first.
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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