New Guidelines spacing

Often, many of which are on the subject we are surprised that the spacing of 30 feet ( 9.1 meters ) commonly used to smoke detection facilities does not represent a spacing listado.A often , many of which are on the subject we are surprised that the spacing

30 feet ( 9.1 meters ) commonly used to smoke detection facilities does not represent a list spacing. In fact, Section 5.7.3.2.3 of the 2002 edition of NFPA 72, National Fire Alarm Code , states: "In the smooth ceilings , spacing for type smoke detectors - they pay puntodeberán accordance with section 5.7 .3.2.3 ( a) to the 5.7.3.2.3 (E ) " .

Part (A ) states: "It shall be permitted to use the spacing of 30 feet ( 9.1 meters ) as a guide ," and annex of NFPA 72 adds , " spacing of 30 feet ( 9.1 meters ) is a guide for designs established ( ... ) based on the customs of the community fire alarms. "

To date, there is no research that proves that the metric used for spacing of smoke detectors is correct. All we have is positive anecdotal information that this pattern has met the objectives of community detection of fire protection through the years.

However, a recent study sponsored by the Institute for Fire Detection (IDF, for its acronym in English) shows that duct smoke detectors which are used to prevent smoke from a fire at a building site recirculated from an HVAC system to another-effectively served their purpose, and placement recommendations in previous editions of NFPA 72 were incorrect. Research shows that old patterns of location of duct smoke detectors in 6-10 wide elbow duct or opening, are not valid.

This research has had an impact on code requirements for the location of the detectors of the 2007 edition of NFPA 72, which is soon to be published. For example, the material in the Appendix Section 5.16.5.2 of the new edition, now states "when detecting side return required, that requirement must be fulfilled separately from those that monitor the supply side detectors. In order to be effective, smoke detectors return air ducts should be positioned so that there is no filter between them and the source of the smoke. Sampling tubes should be oriented to withstand thermal stratification due to the buoyancy of the smoke in the upper half of the conduit. This condition occurs when line speeds are low, buoyancy flow exceeds the inertia or when the detector is installed near the fire compartment.

A vertical orientation of the sampling tubes resists the effects of differential buoyancy. When a detector is installed in a duct serving a single compartment fire, when the buoyancy exceeds the inertia of the air flow in the conduit and the sampling tube can not be oriented vertically, then the effects of thermal stratification can be minimized by location detector sampling tube in the upper half of the conduit. Thermal stratification is not a concern when the detector is installed away from the compartment when smoke or fire is at or near the average temperature in the canal. "

In all other matters relating to the location of the smoke detectors, beamed ceilings and lattice ceilings were often the most difficult to assess. Inevitably, more smoke detection is supplied than necessary. Research again, this time sponsored by the Foundation for Research on Fire Protection, led to changes in the requirements for the location of smoke detectors type-point configurations in these ceilings. [1]

The 2007 edition of NFPA 72 code now effectively addresses crosslinked type ceilings, establishing in Section 5.7.3.2.4.2 (3): "For ceilings crosslinked or grooved type with straps or massive beams no more than 600 millimeters ( 24 inches) deep and not more than 12 feet (3.66 meters) center to center spacing should allow the following:

1. The spacing in smooth ceilings, including provisions allowed for irregular areas in 5.6.5.1.2 replacing "selected spacing" by "listed spacing."

2. The location of smoke detectors-point type on ceilings or on the bottom of the beams. "

The material serves as a support Annex explaining the effect on the geometry and reservoir contributes to the development of speed, temperature and smoke opacity in smoke detectors located within the boxes beamed ceiling or on the bottom of the beams, since "the smoke collected in the reservoir volume spills into adjacent boxes." Although ceilings crosslinked or grooved type created by braces or massive beams retard initial flow of smoke, they are "in increased optical density, temperature rise and gas velocities, compared to smooth ceilings free. "

These are just two examples of how recent research has affected the contents of NFPA 72. Based on the progress of the Foundation for Research of Fire Protection, will continue the positive changes in future editions of the code.

[1] Performance of smoke detectors with deep-level ceilings beams and configurations for deep beams boxes, available in www.nfpa.org/foundation

By Wayne D. Moore , PE, FSFPE