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.

11/28/2017


Learning objectives:

  • 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.

Figure 1: Four gravity feed zones, one pumped zone, refill pumps on four levels.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.

Figure 2: Gravity feed standpipe/sprinkler system riser diagram with high-pressure refill pumps on two levels.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.

This figure shows enlarged tank details from Figure 1 and uses the legend from Figure 1. The normally closed valve between the pumped refill and the gravity-refill cross-connection can also be seen.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. 

Figure 4: This photo shows pumps in a high-rise building arranged to operate in parallel. Pump suction piping does not conform to the current edition of NFPA. All graphics courtesy: JENSEN HUGHES

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.

Figure 8: Electric motor vertical turbine fire pump.

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.

Figure 9: Electric motor fire pump in a riser room.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.

Figure 10: Electric fire pump controller with a transfer switch (right side) and jockey pump controller (left side).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.

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