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FIA Technical Briefing

by Brian Sims

Often, fire detection in large open spaces has proven to be somewhat challenging. Although BS 5839-1 provides clear guidelines for detection at the ceiling, some designers have been embarrassed to receive customer complaints of detection failing to respond to a fire outbreak despite the systems installed being compliant with the Code of Practice. Robert Yates and Peter Massingberd-Mundy investigate.

The cause of poor fire detection system performance can often be attributed to stratification. This is where the fire has insufficient energy for the resultant smoke plume to rise to the ceiling because it has cooled to the temperature of the surrounding air and lost buoyancy, spreading out in a layer below where the detection equipment is sited.

The challenge for system designers, then, is to predict when and where stratification may occur. In practice, it’s almost impossible to reliably predict the height at which the smoke layer will form because it depends upon the size and energy of the fire and the temperature gradient in the protected space.

That gradient can in itself be influenced by many factors such as differing weather conditions, the time of day, building occupation and heating control, etc. As a consequence, it’s often simply argued that, as the fire develops, the heat output increases, hotter smoke will break through any thermal barrier and ceiling-mounted detectors will eventually operate.

In those spaces where smoke stratification is considered to be a risk, BS 5839-1 (and other similar Codes of Practice) has, for many years, recommended that a layer of several beam detectors is installed, with the detectors spaced sufficiently close to each other such that the rising smoke plume is unlikely to pass through the layer undetected. In reality, this recommendation is rarely followed because the number of beam detectors needed is prohibitively expensive.

Instead, many fire protection system designers instinctively apply the use of angled beam detectors in the expectation that they will be obscured by the smoke layer no matter the height at which it happens to form.

Research project

This alternative approach using angled beams was first included in BS 5839-1 in 2013, but no specific guidance was provided as to where to place the beam detectors because such an approach had never been fully researched.

On that basis, the Fire Industry Association (FIA) accepted a proposal from some of its member companies to invest in a research project purposefully designed to investigate the effectiveness of angled beams and provide data to support guidance as to where they should best be positioned.

The research project was sponsored by the FIA in conjunction with Laluvein Consulting Ltd, Fire Fighting Enterprises Limited and Xtralis (UK) Ltd (part of Honeywell), all of them member organisations of the FIA.

Using software called Fire Dynamic Simulator (FDS), the FIA (working in partnership with fire science researchers from the BRE’s Centre for Fire Safety Engineering at the University of Edinburgh) modelled a series of fire scenarios in a 25 metre-high space to simulate the effect of key variables such as fire size and temperature gradient on the characteristics of the smoke layer.

The BRE’s Centre for Fire Safety Engineering exists to equip tomorrow’s leaders in the field with the skills they require, support today’s fire safety teams with multidisciplinary research, provide first-class education in fire safety engineering and structural fire engineering, deliver fire safety consultancy services to industry and other consultancies and disseminate information about advances and research in fire safety engineering through courses, symposia and publications.

For those readers of Risk UK unfamiliar with FDS, it uses complex computerised mathematical modelling systems to simulate and predict the way in which smoke travels and how it rises within a building. Moreover, it includes models that can be used to predict the activation of smoke detectors (including beam detectors and aspirating smoke detectors). These models were fundamentally improved as part of the project to accurately model angled beams (of which there were over 100 included in the simulations).

Useful insights

As is the case with all such analysis, the absolute accuracy of the results cannot be relied upon until it has been verified. Despite this caveat, the results provide useful insights into the relative performance of detectors in different positions, while the visualisation afforded by SmokeView sheds some light on how the smoke layer develops.

The first finding was that the tight spacing (¼ height) recommended in BS 5839-1 (for interstitial beams intended to detect the rising column of smoke) shouldn’t be relaxed. For example, if the interstitial beams are installed at 12 metres they need to be spaced every 3 metres – requiring five times more devices than with the 15-metre spacing recommended for the ceiling-mounted beam detectors.

The second finding was that the predicted response of angled beam detectors passing through a stratified layer of smoke (resulting from a small fire when there’s a temperature gradient) is similar to the response of beam detectors positioned on a ceiling (to the same small fire when there’s no temperature gradient to cause the smoke to stratify). This is an important finding as it confirms that angled devices are likely to be effective.

One observation, and which differs from common/intuitive thinking, is that the horizontal velocity in the smoke layer reduces as the layer spreads. The research project indicates that the smoke layer doesn’t continue to stretch outwards across the room indefinitely, but instead tends to deepen around the centre and become more dense. When considering a design with angled beams, it’s therefore important that due attention is afforded to the extent – and speed – of the spread of the smoke layer.

Applying the research

The findings thus far are reflected in the current advice contained within BS 5839-1 for installing angled beam detectors, and particularly in the note under Clause 22.5d.

One additional recommendation from the research which hasn’t been reflected in BS 5839-1 as yet is that, when installing angled beams, they’re best deployed in a criss-cross arrangement. This is specifically intended to ensure that the distance to a beam at any given height isn’t in any way excessive.

From this relatively short overview of the research underpinning the current recommendations for angled beams, it’s clear that further analysis and validation is needed. In anticipation of this, the FIA has agreed to sponsor a second phase of research into the challenges of detecting stratified smoke.

The FIA’s Fire Detection and Alarm Council’s primary purpose is to find solutions to such technical issues on behalf of the fire industry. Council is comprised of roughly 30 individuals from FIA member companies, with selection conducted through an election process. The majority of those on Council have a senior background, an extensive range of experience and a deep knowledge and understanding of both British and European Standards.

All of this means that technical guidance documents can be created to the highest industry standard based on the FIA’s research and partnerships with leading universities, the British Standards Institution and other standards bodies.

The FIA acts as the springboard between industry and research at top Higher Education institutions, providing the industry with solutions and guiding standards by way of scientific evidence. The research helps in leading the FIA’s training and qualifications in fire detection and alarm systems.

Specialist qualifications

On that note, the FIA can now offer four new specialist qualifications: one each for the design, installation, maintenance and commissioning of fire detection and alarm systems. The qualifications have been carefully developed over time by a range of experts in the field to raise the level of professional knowledge and understanding of those working within the sector.

These qualifications have been produced in consultation with industry leaders and employers, matching the needs of the industry with what learners really need to understand. The FIA has worked with reference to the National Occupational Standards, current UK legislation and published standards, along with Codes of Practice and industry Best Practice to give learners the opportunity to expand their knowledge and understanding in a format that’s in-depth and delivered under expert guidance.

Robert Yates is Technical Manager at the Fire Industry Association. Peter Massingberd-Mundy is Technology and Expert Practices Manager at Xtralis (a Member Company of the FIA)


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