Effectiveness of Sprinklers Not Under A Surface?

8/7/2020

I am looking for opinions on the effectiveness and likelihood of a timely operation of sealed sprinklers in open spaces where they are not installed below ceilings or obstructions.

For example, operation of sprinkler under open-grate platforms, or where used as pilot heads around external transformers. Many thanks.

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7 Comments

Paul

8/7/2020 10:22:22 am

Without a surface (for heat to collect) a sprinkler head would be very ineffective (in general). The heads would only fuse if it were directly above an open flame. It does have it's specific use. In the case where the idea is that it is positioned above an open flame as mentioned, usually in conjunction with an overhead system. (Flue spaces for rack storage, open grates, etc)

Phil W

8/9/2020 01:17:59 pm

Many thanks for the reply. It bears out my own thoughts really; I guess a head will operate at it's designated temperature, but that temperature would necessitate the head to be sufficiently close to the heat source. I've had conversations with water mist nozzle manufacturers previously on the same subject and they described specific nozzle testing where the distance from a soffit was increased; naturally they saw a delay in the operating time of that nozzle with the increasing distance.

8/7/2020 10:32:25 am

Paul is right on. Some designers might remember how it was common practice to add "heat collectors" (a.k.a. "pie tins") or square pieces of sheetmetal above sprinklers (such as in a new tenant space prior to ceilings being installed), thinking they would collect heat around individual sprinklers, or even thinking an intermediate-level sprinkler does the same thing. But heat collectors was never an acceptable method and is now specifically prohibited in NFPA #13 as they have been proven to be virtually ineffective.

Pete D.

8/7/2020 02:16:37 pm

For the applications you described, a deluge system activated my smoke or heat detection would be the preferred fully functional fire protection system to design.

Franck

8/10/2020 03:22:43 am

As indicated above, sprinkler heads need a "real" ceiling to activate the thermal sensible element with the collection of heat.
If the heads are located too far from the ceiling, this will be, in the best case, delayed with activation of more heads and possible overtax of your system, in the worst case, result in no activation of the heads.
There is no evidence of efficiency with metallic plates, even with 2 ft x 2 ft plates.

A study performed in 1990 (“A Study of the Utility of Heat Collectors in Reducing the Response Time of Automatic Fire Sprinklers Located in Production Modules of Building 707,” prepared for the U.S. Department of Energy Rocky Flats Plant by Hughes Associates, Inc. (HAI)) stated the following:
• Heat collectors with the edges turned down around the side produced a dead air space and the sprinklers had longer response times than sprinklers with a flat heat collector
• The heat collector must be in the plume to be effective. If the centerline of the fire is more than 1 to 2 feet from the edge of a flat heat collector, a standard-response sprinkler may take longer to respond, regardless of its thermal sensitivity.
• If a fire is midway between two sprinklers, the sprinklers may not respond at all (regardless of the size of the heat collector) because the sprinklers are not exposed to the convective heat flow of the ceiling jet.
• Quick-response sprinklers activate more quickly than standard-response sprinklers only if the fire was directly underneath the heat collector.

See also Minnesota State Fire Marshal "Quick Response" April 2011 issue (https://dps.mn.gov/divisions/sfm/programs-services/Documents/Quick%20Response%20Newleter/QR0411HeatCollector.pdf)

There are anyway some exceptions to this rule.

1. Heads installed sometimes below open grate platform such as in a turbine hall. There are not provided to activate alone, but installed to provide an additional protection in case of 3 dimensional fire (which could be the case with oil flowing from the turbine deck down to the ground) - See FM data sheet 7-101 - Steam Turbines and Electric Generators for more details. NFPA 850 and 804 are less self explanatory.
2. Heads installed as pilot sprinkler system. The aim is not to activate all the heads in a given area, but to activate at least one head to operate the deluge valve. There is some chances that at least one head will be installed directly in the fire plume and thus activate your system.
3. New sprinkler heads are under development, called Thermal Radiation Activated Sprinklers or RAS (studies performed by Factory Mutual) and they may partially compensate the absence of ceiling.

Phil W

8/14/2020 10:11:08 am

Thanks for your compehenive reply, very interesting.

Dale Morris

8/10/2020 10:52:30 am

Just had this real life scenario involving cable trays that are 4 feet wide. In some areas, where the cables in the cable tray exit the electric room, the trays are rather packed, so the sprinkler is more likely to activate, with the cables serving as that "hard' surface. But towards the end of the run, the cable trays are still 4 feet, but do not have much cable in them, and so they are more open than filled. We still opted to provide sprinklers under there, as we know down the road, the intent is for more equipment to come on board, and the trays will become more full as the years go by. The GC showed on a sketch a pendant sprinkler next to the tray, and I told him it had to go under. He asked about those heat collectors, and I had to tell him not allowed.

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