Why is the water droplet size in a Water Mist System important?

Smaller droplet sizes result in a more efficient use of the water. Water Mist works by flashing of the droplets to steam, resulting in reduction of the Oxygen content of the remaining air and cooling of the fire by causing loss of energy (steam production) and continued application of the water mist droplets. Smaller droplets have more surface area which increases the heat transfer rate from the fire, the water flashes to steam rapidly. In addition to this heat absorption and resultant cooling, the steam displaces oxygen, at the fire, thereby accomplishing extinguishment.

In general, High Pressure Water Mist Systems discharge water in smaller droplets than do either Intermediate Pressure or Low Pressure Water Mist Systems. Because of the use of Dual Media, the Securiplex FIRE-SCOPE®2000 System produces a water mist in the 150 – 200 micron range which is in the same order of magnitude as most High Pressure Systems.

We have been asked to propose a Water Mist System for a room containing three (3) Emergency Diesel Generators. The room volume is 22,000 ft3, which exceeds the total flooding approvals. How would Securiplex propose to protect this hazard?

In order to choose the best System, several questions must be resolved:

  • Ceiling height should be approximately 16 feet (5 meters) because water mist becomes less to non-effective at ceiling heights above 5 meters. Intermediate or level located nozzles may be required to retain effectiveness.
  • Is FM Approval/Listing required for system? SECURIPLEX Systems are “scalable” and will provide effective temperature and fire control in Total Flood Applications and can be used in spaces greater than their Listings.
  • Is there a sprinkler system installed in the Space? the Building? Some AHJ’s require that the required sprinkler system be retained in enclosures with alternate protection systems. Some AHJ’s will not consider Local Application Systems acceptable as replacement for a required sprinkler system.
  • Is there a reliable water and/or air source available? Use of Facility Water and/or Air as a Hybrid or Plant Services System is a very cost effective way of providing a fire protection system to protect a hazard.
  • Do Diesels run individually or all at the same time (or, is the hazard and individual Generator or all Generators)? A Total Flood System protects all simultaneously; a Local Application System can be designed to protect one or all (or any combination in between) units simultaneously.

We would propose a Self Contained, Local Application System protecting a single unit (possibly for 10 minutes flow to provide the option for protecting a second unit in the event of activation for the first) unless there is adequate water pressure and flow for a hybrid system (it is rare that adequate air flow and pressure are available!). We would suggest leaving the sprinkler heads in the space, but making them activate at much higher temperature or making this space a Pre-action.

We would propose a Self Contained Total Flood (low pressure) System, if there is no requirement for FM Approval for control and cooling only (4-5 hour rated fire wall enclosure) and the ceiling height was no more than 18 feet.