Water Mist Systems

Water Mist Systems

There are multiple manufacturers of water mist fire protection systems, with each manufacturer having a variation on system operation and performance. The system features listed below typically vary between the manufacturers systems and contribute to the systems performances:

• System operating pressurization
• Means of pressurization
• Operating pressure high and low limits
• Piping delivery network
• Water droplet propulsion
• Water droplet size
• Listing and performance for fire hazard types

Guidelines for Water Mist Systems
Water mist system fire protection design, performance, testing and inspection criteria have been developed by the National Fire Protection Association (NFPA) Technical Committee on water mist fire suppression systems. The NFPA Technical Committee developed NFPA 750 Standard on water mist fire protection systems.

NFPA 750 – Water System Standard
NFPA 750 classified water mist systems into the following categories, based on operating pressures within the piping distribution network.

High Pressure Systems: Operating pressure within piping distribution will be greater than 500 psi.
Intermediate Pressure Systems: Operating pressure within piping distribution will be greater than 175 psi, but less than 500 psi.
Low Pressure Systems: Operating pressure within the piping distribution will be less than 175 psi.

NFPA 750 – Classified Nozzle Types & System Requirements

  • Automatic nozzles (closed head)
    • Wet pipe system: nozzles are installed on piping network pressurized by water.
      Preaction system (Double-interlocked and Single): nozzles are installed on piping network pressurized by gas with a detection system installed in the same area as the nozzles. Activation of the detection system will operate a control valve pressurizing the piping network with water.
    • Dry pipe system: nozzles are installed on piping network pressurized by gas. The loss of pressurization in the piping network will activate a control valve allowing water to flow into the piping network and out of the activated nozzle.
  • Non-automatic nozzles (open head)
    • Deluge system: control valve and independent detection system installed in the same area as the mist nozzles. All nozzles flow when the control valve is activated.
    • Multi-functional

 

NFPA 750 – Water Mist Standard Applications
NFPA 750 recognizes four types of water mist system applications:

Types of water mist system applications

Water Mist System – Alternative Design for Sprinkler and Foam Fire Protection

Hangar 25 – Burbank, CA

The image above is an example of where a High Pressure water mist fire protection system is used as an alternative design to achieve fire protection. The project area of protection is 51,350 square foot airplane hangar with a floor-to-ceiling height of 50 feet. Normal usage of the facility includes housing of fueled jets.

The standards for fire protection system design for this hangar structure (NFPA 409) would have required the installation of under-wing (low-level) foam protection with an overhead wet sprinkler system.

Construction and Design Obstacles Avoided

The project posed several unique construction and design hurdles which impacted both cost and construction methods:

  1. The project site location did not have an adequate underground piped water supply to support a foam and wet fire protection system (meeting the design requirements for NFPA 409). To meet the water supply demand for fire protection system (required by NFPA 409), the project would have needed to install a new underground water supply whose connection point was over 400 yards away, or installed a stored water reservoir capable of providing an adequate main and reserve supply of water to satisfy local fire jurisdictions.
  2. Installation of a under-wing (low-level) foam fire protection system would have required the installation of a containment system capable of capturing all foam system run-offs.
  3. A project objective was for the facility to achieve a LEED certification. The installation of the foam system would have been a negative technical merit against the LEED certification process.

 

Project Approach and Merits

  1. Water demand for the High Pressure water mist system is substantially less than the requirement of the foam and wet system. The existing underground fire water supply source was adequate for a High Pressure water mist fire protection system, thereby eliminating several costs:
    1. Construction cost to trench, backfill and pave 400 yards.
    2. Installation cost of a new piped fire protection water main supply.
    3. Alternative cost for on-site water storage with main and reserve capacity. In addition to the cost for the water storage there were space restrictions at the property which would not have accommodated sufficient water storage.
  1. Misting system water run-off did not require the installation of a costly containment solution, which was associated with the low-level foam system. Projected construction cost of the containment system alone surpassed the cost of the High Pressure water mist fire protection system.
  2. Water misting system reduced fire protection water demands by up to 90% water conservation, compared to the foam and wet system water demands. Additionally, removing the foam system from the facility reduced the hazardous material signature of the project and aided in the project achieving a Platinum level LEED certification.

Single zone of the High Pressure water mist system being discharged

Water Mist System Design

  1. High Pressure water mist system.
  2. Hangar is a designed as a Zoned Application System, per NFPA 750.
    1. Mist distribution is throughout a predetermined portion of a space of the enclosure using open heads.
    2. The hangar area is separated into three zones.
  3. Per NFPA 750, each zone is designed using non-automatic nozzles; open head deluge system with separate control valves for each zone.
  4. Independent detection system using fast response combination ultra-violet/infrared flame detectors; used to activate each zone separately.

 To see the entire system discharge test:  High Pressure Water Mist System Testing (VIDEO)

Upcoming blog article: Alternative Water Mist Applications for Sprinkler and Carbon Dioxide applications.