Fire Detection for Large Spaces

Fire Detection for Large Spaces: Integrating Aspiration Smoke Detection

Modern structures are complex, which makes them difficult to protect against smoke and fire. Features like large atria, vast open areas, or high ceilings can make the use of traditional smoke and fire detection methods impractical, ineffective, and difficult to maintain and operate.

Large, open spaces are prone to high airflow and smoke stratification (caused by the high ceilings and/or dilution of the smoke), making it difficult for smoke to reach spot-type smoke detectors and for conventional detection devices to operate properly. Additionally, significant effort is required just to test and inspect these devices in such spaces, per code requirements.

In general, fire detection in large volume spaces directly relates to the size of potential fires within the space. Building contents such as furniture, large trash containers, or merchandise displays typically burn with enough energy to push smoke to significant heights. However, low energy fires with a small heat-release rate are subject to stratification and can be difficult for ceiling or near-ceiling mounted smoke detection devices to detect.

What is considered a large space?

Large spaces are those with ceiling or roof heights extending tens or even hundreds of feet above the floor. Some examples are:

·        Spaces containing atria, such as hotel lobbies and shopping centers

·        Warehouses and distribution centers

·        Record/archive storage facilities and automated storage and retrieval warehouses

·        Manufacturing plants, such as automobile assembly plants

·        Convention centers, theatres, auditoriums, stadiums, and indoor sports facilities

·        Cathedrals, churches, temples, and mosques

·        Aircraft hangars and other large storage and maintenance areas

·        Transportation terminals, such as airport terminals and train stations

Regarding fire protection, large spaces can be classified into two basic categories:

Category 1: Large volume spaces with fire-load contents, generally of limited height, located on the main floor or other floors open to the large volume. These spaces tend to have moderate to large occupant loads. Examples: atria areas, single-story manufacturing areas, heritage buildings, religious worship buildings, and airport terminals.

Category 2: Large volume spaces with stacks or piles of combustible contents, which may or may not be in storage racks. These contents occupy a significant portion of the volume and height of the space that is most often one story. These spaces tend to have low to moderate occupant loads and a high value of contents. Examples: warehouses, distribution centers, automated storage and retrieval facilities, and library stack rooms.

Detecting Smoke in Category 1: Hotel

Atria, a popular architectural design feature, enhance the perception of light and space within structures but can be dangerous for occupants in a fire. One fire design challenge is compensating for the fluctuation of solar radiation that enters through large expanses of glass, which can often generate thermal gradients within the space and result in a hot gas layer near the ceiling or roof. This can vary from day-to-day, depending on thermal load and operations of HVAC or natural ventilation systems.

The base of the atrium is especially vulnerable. A fire at this location could potentially fill the unenclosed shaft with smoke, allowing fire to spread rapidly and transport heat, smoke, and toxic gases from floor to floor. More importantly, traditional technologies take too long to detect smoke in these large areas because smoke may not be buoyant enough to penetrate the ceiling layer of stratified environments. Aspirating smoke detection systems, however, can be designed to avoid the issues posed for ceiling-mounted spot smoke detectors due to a solar-induced hot layer.

Detecting Smoke in Category 2: Warehouses

Category 2 spaces require additional considerations beyond those previously discussed. Modern warehouses and storage facilities often rely on automatic sprinkler systems for basic fire protection. However, the potential loss of key supply chain facilities, high value storage, and irreplaceable archived storage can warrant the installation of early warning smoke detection for staff intervention before sprinklers are needed.

Arson and electrical/mechanical systems account for a significant percentage of warehouse fires, but aspirating smoke detection can provide early indications of such events and allow for manual fire suppression or depowering of equipment. STRUCTURE FIRES IN U.S. WAREHOUSES by Richard Campbell, published in June 2013 by the National Fire Protection Association Fire Analysis and Research Division, says that during 2007-2011, an estimated average of 1,270 structure fires in warehouses were reported to U.S. fire departments each year, with associated annual losses of 4 civilian deaths, 23 civilian injuries, and $188 million in direct property damage. According to Campbell:

·        Nearly one-fifth of these fires were set intentionally

·        Electrical distribution or lighting equipment was involved in 13% of fires

·        Electrical failure or malfunction was the leading factor contributing to the ignition of warehouse fires, as well as in contributing to direct property damage and to civilian injuries, representing 19% of the total in each category

Integrating Aspiration Smoke Detection

There are five basic design options for integrating an aspirating smoke detection system in warehouse and storage buildings. Selecting a sampling pipe configuration is dependent upon the warehouse application and internal characteristics (such as presence of high-bay racking, mezzanines, and voids) as well as the facility’s operational objectives and smoke detection objectives (performance-related).

1.     Near Ceiling Detection – sampling pipes are located close to the ceiling only

2.     Below Ceiling Detection – sampling pipes above a ceiling use drop down pipes or capillary tubes with sampling holes

3.     High-Low Alternating Detection – sampling pipes are located on the ceiling with alternating sampling holes on the ceiling and drop-down pipes

4.     Multi-Level Detection – this detection method applies to warehouses comprising high bay storage racks and utilizes more than one ASD system for ceiling and intermediate level detection.

5.     In-Rack Detection – the sampling pipes run horizontally or vertically between back-to-back racks at various heights. Drop-down pipes from the ceiling can also be used for in-rack detection.

In addition to these generic warehouse scenarios, additional aspiration protection should be considered in the following situations:

·        Office and control area: for aesthetic reasons, sampling in these areas is usually conducted via capillary tubes fed through the ceiling from the main sampling pipes, which are located in the ceiling void.

·        Ceiling void: it is especially important to protect ceiling voids containing cabling and/or any other equipment, due to an increased risk of fire. The spacing of the ASD sampling holes is again determined using the grid method.

·        Lighting areas: The ASD sampling pipes should not be placed underneath or in close proximity (<3.3 ft (1 m)) to lighting fixtures, heaters, skylights, or other heat-emitting objects.

·        Loading bay: ASD sensitivity should be lowered to account for the truck exhaust fumes regularly present in the loading bay environment.

Installation Scenarios

NFPA 72 provisions on the spacing and location of smoke detectors states that, “The location and spacing of smoke detectors shall be based upon the anticipated smoke flows due to the plume and ceiling jet produced by the anticipated fire, as well as any pre-existing ambient airflows that could exist in the protected compartment.”

Aspirating systems can be mounted wherever it is most convenient – on walls, columns or rack frames – but it’s recommended to mount the units in close proximity. For example, these units can be located in the center of a warehouse with the pipe network extending outward, toward the walls. This configuration minimizes the distance between detectors and reduces wiring costs.

There are many options for warehouse detection, including:

High-low alternating detection: this option is comprised of ceiling sampling holes that alternate with drop down pipes. This method is used to penetrate the hot air layer that may develop at the roof level, which allows for optimal sampling when the air at the roof level is either hot or cold. Consult local codes and standards to determine the correct length of drop down pipes for individual facility configurations [Figure 17].

In-rack detection: This may be used in warehouses with high-bay racking where stratification occurs, and/or localized detection is required. The air sampling pipe can have either a horizontal or vertical in-rack detection configuration [Figure 18]. Ideally, the detector is installed at the end of the rack at an accessible level. As a safeguard against mechanical damage from forklift trucks, sampling pipes should be located between back-to-back racks out of harm’s way.

Non-high-bay racking detection: In this case, ceiling drop-down pipes may also be used for in-rack detection. When installing drop-down pipes, here too, it is important to ensure that they will not be damaged by forklifts or be in close proximity to the stored goods. The drop-down pipes can be fitted to the sheltered side of the racking, fixed to the rack frame [Figure 19]. In-rack drop-down pipes normally have two or three sampling holes drilled in them to provide sampling at different heights within the racking system.

Multi-level detection: Additionally, this option can be used in high bay storage racking areas with expected high stratification levels and/or mezzanines (catwalks) consisting of solid flooring that will impede/delay the rise of the smoke plume. This design option requires one ceiling aspirating system and multiple intermediate level aspirating systems [Figure 21].

Benefits of Aspirating Smoke Detection

There are many hidden benefits of integrating an aspirating smoke detection system into the fire detection system design for a large volume space. It’s often a challenge to handle diverse placement options in fire and life safety designs, but aspirating smoke detection technology is flexible. It can accommodate a variety of horizontal and vertical sampling point locations and/or capillary tubes – providing effective coverage while minimizing architectural and aesthetic impact. Aspiration smoke detection systems also allow for more specific levels of warnings and alarms to allow building staff to investigate before emergency operations (such as evacuation) are implemented.

Aspirating smoke detection systems can also provide maintenance flexibility and cost savings for large spaces. Inspection and testing is easy, because an air sampling pipe network can be in the upper elevations of large volume spaces while the air sampling detector is installed in a remote accessible location. By comparison, spot-type smoke detectors located on high ceilings would require special procedures for inspection. Furthermore, aspirating technology performance is more consistent and reliable because it is not subject to building movement considerations of optical beam smoke detection devices.

Smoke detection Aspirating systems generally include a sensor of considerably increased level of sensitivity compared with a standard point or line optical beam detectors. Such “high sensitivity” aspirating smoke detection systems, are designated Class A systems in BS EN 54-20 and are often used to protect critical electronic equipment rooms, in which even a very small fire can result in unacceptable damage. Guidance on fire protection of such facilities is given in BS 6266.

Typically, a Class B (enhanced sensitivity) or Class C (normal sensitivity) aspirating smoke detection system is used in such applications, but it is important to seek advice from suppliers with sufficient experience and knowledge of aspirating smoke detection systems and BS EN 54-20.

For aspirating systems, specialist application guidance needs to be sought from the manufacturer.