Are angled beam detectors an effective solution to smoke stratification blind spots?

Fire detection in large open spaces has often proved to be challenging.

BS5839-1 provides clear guidelines for detection at the ceiling and this may be perfectly sufficient for many applications. However, a few designers have been embarrassed to receive customer complaints of detection failing to respond to a fire despite the system being compliant to the code of practice.

The cause of such poor performance can 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 is sited.

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

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

In spaces where smoke stratification is a risk, BS5839-1 has long recommended that beam detectors are close enough to each other so a smoke plume is unlikely to pass through undetected

In spaces where smoke stratification is considered a risk, BS5839-1 (and other similar codes) has, for many years, recommended that a layer of several beam detectors is installed, spaced sufficiently close to each other so a 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 designers instinctively apply the use of angled beam detectors, in the expectation they will be obscured by the smoke layer no matter the height at which it forms.

This alternative approach (using angled beams) was first included into BS5839-1 in 2013 but no specific guidance was provided as to where to place them, because such an approach has never been fully researched.

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

The research

Using software called Fire Dynamic Simulator (FDS), the FIA, in partnership with researchers of fire science at the University of Edinburgh modelled a series of fire scenarios in a 25m high space to simulate the effect of key variables such as fire size and temperature gradient on the characteristics of the smoke layer.

For those unfamiliar with FDS, it uses complex computerised mathematical modelling systems to simulate and predict the way that smoke travels and how it rises within a building.

Moreover, it includes models that can predict the activation of smoke detectors (including beam 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.

The findings

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

The first finding was that the tight spacing (¼ height) recommended in BS5839-1 (for interstitial beams intended to detect the rising column of smoke) should not be relaxed.

For example, if the interstitial beams are installed at 12m they need to be spaced every 3m – requiring five times more devices compared to the 15m spacing recommended for the ceiling mounted beam detectors.

The predicted response of angled beam detectors passing through a stratified smoke layer was similar to that of beam detectors on a ceiling

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 is a temperature gradient) is similar to the response of beam detectors positioned on a ceiling (to the same small fire when there is no temperature gradient to cause the smoke to stratify).  This was an important finding as it confirms that angled devices are likely to be effective.

One observation, which differs from common/intuitive thinking, is that the horizontal velocity in the smoke layer reduces as the layer spreads.

The research indicates that the layer does not continue to stretch outwards across the room indefinitely, but instead it tends to deepen around the centre and to become more dense. It is therefore important when considering a design with angled beams that consideration is given 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 in BS5839-1 for installing angled beam detector – particularly in the note under clause 22.5 d).

One additional recommendation from the research which has not been reflected into BS5839-1 yet is that when installing angled beams they are best deployed in a criss-cross arrangement (see diagram below). This is intended to ensure that the distance to a beam at any height is not excessive.

The future

From this short overview of the research underpinning the current recommendations for angled beams it is 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 Fire Industry Association, the UK’s largest trade association in the fire protection sector, not only contributes to writing best practice guides and revising British Standards, but also is a powerful investor in scientific research into the behaviour of fire and a wide range of other issues including smoke detection and the capabilities of fire detection and alarm technology.

This project was sponsored by the FIA in conjunction with Laluvein Consulting Ltd, Fire Fighting Enterprises Limited, and Xtralis (UK) Ltd (part of Honeywell) (all member organisations of the Fire Industry Association).  The FDS modelling was undertaken by Ben Ralph from University of Edinburgh, Fire Safety Engineering.

To learn more about optical beam detection, visit the FIA’s ‘Resources’ section on the website, where a vast technical digital library can be accessed.

Peter Massingberd-Mundy attends the FIA’s Fire Detection and Alarm Council along with many other representatives from Member companies to find solutions to technical issues on behalf of the fire industry.  

The Fire Detection and Alarm Council comprises roughly 30 individuals from FIA member companies, and selection is through an election process. The majority of those on the Council have a senior background and an extensive range of experience and a deep knowledge and understanding of British and European Standards, meaning that technical guidance documents can be created to the highest industry standard based on the FIA’s research and partnerships with leading universities, BSI, and other standards bodies.

The FIA provides the springboard between industry and research at top Higher Education institutions, providing the industry with solutions and guiding Standards with scientific evidence.  Our research also helps lead our training and qualifications in fire detection and alarm systems.

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

Our qualifications have been produced in consultation with industry leaders and employers, matching the needs of the industry with what learners really need to understand.

We’ve worked with reference to the National Occupational Standards (NOS), 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 is in-depth – but delivered under expert guidance.

Learners will gain valuable insights into legislation, fire science, British Standards, as well as the required documentation and theory for their specialist area, whether this is Design, Installation, Maintenance, or Commissioning.

To find out more, visit the FIA website or call the office to speak with a member of the team.