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May 20, 2015

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Selecting The Right ASD for Your Application

Xtralis will be showcasing the VESDA Vli Aspirating Smoke Detector on stand D950 at FIREX International 2015
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When: 16-18 June 2015
Where:
ExCeL, London

By Peter Massingberd-Mundy of Xtralis, Hemel Hempstead, UK

The early warning capability of Aspirating Smoke Detectors (ASDs) is well established and regularly proven by the hot wire performance tests conducted in numerous telecommunication and computing centre applications across the world.

Their reputation for providing early warning has been underlined by the publication of EN 54-20:2006 which includes three sensitivity classes (A, B & C) where Class A detectors are required to be 40 times more sensitive to smouldering smoke than a “normal” sensitivity Class C detector. However, it is not widely appreciated how much smoke is needed to trigger a “normal” sensitivity detector because few people have personal experience of the standard European fire tests (TF2-TF5) or the US equivalents.

In parallel with this deprivation there is an increasing tendency to use “lowest possible” sensitivity detectors in order to minimise the risk of nuisance alarms. However, in the majority of applications a higher sensitivity system, such as a Class B ASD system, will operate without nuisance alarms and will inevitably provide earlier detection of a threatening fire event. This paper aims to illustrate the alarming amount of smoke present in the test room, to present reasons why there is a common misconception that smoke detectors are too sensitive and to emphasise the benefits of early warning.

The paper concludes that early warning should be considered for many more environments and outlines some of the recent product innovations which provide cost-effective early warning without nuisance alarms in smaller and larger areas.

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Introduction

There was a time when every paper about ASD started by describing how they work. While that is no longer the case (because it is a well- established technique), it is still surprising how many misconceptions remain about how they really operate and how much more sensitive they are than some of the traditional technologies such as beam or point type smoke detectors.

So much attention is focused on the importance of minimising nuisance alarms that there is an inevitable tendency towards using less sensitive detectors rather than trying to achieve the earliest possible detection of a potentially threatening fire event – ideally using pre-alarms and staff warning to initiate early investigation and intervention before starting disruptive evacuation procedures

Standard and reduced test fires

The standard test fires, TF2-TF5, as specified in various parts of EN 54, are conducted in a relatively small room and are surprisingly large – producing flames that reach >1m high and smoke layers at the ceiling which are over 1m deep! Fast forward videos can reveal in a matter of seconds just how much smoke and flame are present at the end-of-test – that moment before which all approved detectors should have responded.

In comparison, the reduced test fires specified in EN 54-20 for Class A & B Aspirating Smoke Detectors are significantly smaller. For example the flame heights in a TF5A test typically reach about 30cm and the end-of-test for a TF2A fire is only 0.05dB/m compared to 2dB/m in the standard TF2 fire. Very few people have experienced the vast difference but again, fast forward videos can give a quick insight.

Real-world expectations

There is a general expectation in the field that smoke detectors will respond to the first signs of visible smoke. This is usually based on personal experience with domestic ionisation smoke detectors – which have had an annoying tendency to go into alarm at the first sign of burning food! Of course, there is plenty of evidence (and lots of material on the internet) to illustrate how ionisation detectors can have a very poor response to smoke from a slow smouldering fire.

Furthermore, it is also well documented that photoelectric/optical smoke detectors can be very insensitive to flaming fires. However, the fact remains that for the average man-on-the-street, the size of the standard test fires is unknown and there is a perception that they are much smaller than they actually are.

Very few of us have experience of a fire detector operating in anger – alerting us to the danger of a threatening fire event. However, by contrast, many of us have some experience of a nuisance or unwanted alarm – emptying a building at the most inconvenient time possible!

Unfortunately, these two perceptions lead to a misconception that smoke detectors are generally too sensitive and this view is increasingly fuelled by campaigns to reduce nuisance alarms.

Reducing nuisance alarms

An easy but imprudent way to reduce the likelihood of a nuisance alarm is to ensure that the smoke detector is as insensitive as possible – which has some obvious, and some less obvious consequences. Despite this it is an uncomfortable fact that many installers will look to install the least sensitive detection system possible – using the cheapest solution with maximum spacing – which meets the product approval and installation codes, and reduces the likelihood of future visits to investigate and resolve unwanted alarms.

The obvious consequence of using low sensitivity devices is that they will respond to a fire later than a more sensitive system. Vital minutes can be lost during an emergency.

Less obvious is the fact that many detectors incorporate drift compensation algorithms to counter the effects of contamination. For example many detectors become more sensitive over time due to contamination, and the drift in signal is suppressed or compensated to reduce the likelihood of nuisance alarms.

The challenge of understanding and assessing the effects of such compensation algorithms on the response to slowly developing fires and to high background levels are discussed in a separate paper[1] which illustrates that it is rarely possible to distinguish a drift in signal due to contamination from that caused by a high environmental background.

A more effective way of reducing nuisance alarms is to make effective use of the earliest possible warning of a fire event by signalling it as a pre-alarm or local technical/informative signal. This can provide time for those local to the alarm to investigate and intervene before the event escalates to a scale that warrants evacuation and the summoning of the fire & rescue services.

However, while many smoke detectors provide a pre-alarm function which is generally an indication that the system is approaching an alarm condition (e.g. 75%), an early warning signal provides an indication that normal conditions no longer prevail. This is an important distinction and is why the first level of alarm on VESDA detectors is called “Alert”.

Early warning in practice

While it is clear that Class A and B ASD systems are required to be significantly more sensitive than a Class C ASD system it is often commented that they are too sensitive for normal environments and suffer from a high nuisance alarm rate. However, experience using VESDA detectors over many years[2] indicates that the majority of installations, commissioned using default thresholds or using AutolearnTM (which sets the alarm thresholds based on a 14-day analysis of the normal environment), are Class B sensitivity (or higher), and nuisance alarms are not an issue. Of course there are challenging environments where background levels are unusually high or where there are processes or activities that occasionally lead to nuisance alarm phenomena being present – but these are rare. In fact one important trick to address the more challenging environments is to increase the number of detectors so that each is sampling from fewer holes and can consequently be configured with higher alarm thresholds while still achieving the Class B (or higher) sensitivity at each sampling hole.

In addition to the inherently higher sensitivity offered by Class A & B ASD systems (at each sampling hole individually!), the “cumulative effect” which is a hallmark of the technique ensures that ASD systems are inherently capable of providing early warning where it is most valuable. Specifically they are particularly effective at detecting small amounts of smoke emanating from slowly developing fires. Such smoke lacks the thermal energy needed to carry it towards the ceiling in a dense plume but instead has time to migrate throughout a space over the course of several tens of minutes. Such dispersed, low density smoke entering several sampling holes is detected by an ASD detector long before it is of a density that might trigger a detector monitoring for smoke at a single location (point or sampling hole).

This natural sensitivity to dispersed smoke, coupled with the inherently higher sensitivity available in Class A & B ASD systems, means that they are the ONLY sensible option for applications where the early detection of a fire is needed – i.e. detection of an incipient smoke condition long before it might be considered to be a damaging or life threatening fire.
This realisation of true early warning inevitably means that there are a few situations where tracing the source of an event can be challenging – particularly where the event is transitory. In such situations the ability to review the event using a comprehensive trend graph which is easy to retrieve from non-volatile memory and/or is available in real time is a valuable feature provided on many ASD systems. However the trend is only really useful if it presents an absolute measurement and the prevailing alarm thresholds. This is not the case with all ASD systems.
Innovations

Innovations in the field of ASD systems have been interesting – some are technology-led and look to exploit the advances available in the field of detection while others look to address the challenges of maintaining a highly sensitive measurement of smoke in challenging environments. For example, several ASD manufacturers are promoting the benefits of detection chambers incorporating multi-criteria/dual-wavelength technologies to provide improved rejection of nuisance alarm phenomena, while others are promoting the robustness of their products in particularly “shocking” environments. It is for the market to decide if these innovations are really necessary for the provision of reliable early warning of fires.

At Xtralis our focus remains on providing the earliest possible reliable detection of fires with minimum nuisance alarms – detection that is not compromised by contamination or adaptive algorithms and is provided within a comprehensive portfolio of products which can address the needs of all but the most challenging environments. For example, all products within the VESDA portfolio incorporate our unique clean-air barrier technology which keeps contamination away from the critical optical components within the chamber. The VLI incorporates patented fail-safe intelligent filtration technology to maximise the longevity of the product in industrial environments where contamination is the major risk to effective smoke detection.

More recently we have launched the VEU which incorporates a revolutionary detection chamber using imaging technology to achieve unprecedented stable and consistent measurement of smoke down to 0.001%/m. This new chamber can also be used to analyse the particulate present and provide targeted detection of particular risks. For example one licensable analytic is called DieselTraceTM which is specifically targeted at applications where the diesel engine exhaust particles are abnormally present. The primary objective is to provide an actionable response to activate ventilation when diesel particles concentration exceeds a certain limit. Another recent addition to the Xtralis portfolio is the VLQ which is a very cost effective solution for small (single room) areas providing four Class A sampling holes in a convenient ceiling-mounted package. Other innovations are in the pipeline and have been revealed at various exhibitions. Visit www.xtralis.com for the latest information.

Conclusion

ASD systems are the only sensible choice for any application where the earliest possible warning of a fire event is needed because:

  • They can be approved with enhanced (Class B) and very high sensitivity (Class A) which means that a single sampling hole is required to detect fires significantly smaller than those detected by Class C ASD systems and point type detectors
  • The majority of environments can be successfully protected by Class B ASD systems without undue risk of false alarms and many environments can be effectively protected by Class A systems
  • The natural cumulative effect of ASD systems (whereby they are increasingly sensitive as smoke spreads and enters more sampling holes) makes them particularly effective in areas where the early sign of smoke is diluted through a space – as they are able to detect levels of smoke that would never trigger a point type detector measuring the smoke density at a single location.
  • Event logs (available on many ASD systems) provide factual information about any event – whether a nuisance alarm, an incipient incident or a major smoke event – which is invaluable during the investigation of these cases and can enable optimum setting to be established for each particular application.
  • The Pre-alarm and warnings signals (available on many ASD systems) can be used to summon an appropriate response to the earliest possible indication of abnormal conditions within an area – long before the conditions become life threatening and require a full fire response/evacuation. Time is a valuable asset during the early stages of a life threatening emergency.
    Most importantly, a wide portfolio of detector styles and types is now available offering a variety of coverage and features – from a basic “point in a box” product to provide remote sampling at a single location to multichannel detectors which can identify which sampling hole is exposed to smoke.

[1] Massingberd-Mundy & Vayeda “A practical method for testing the performance of smoke detectors with drift compensation to slowly developing fires and to high background levels” AUBE14, Duisburg 14-16th October 2014.
[2] Massingberd-Mundy “Understanding the normal capability of Aspirating Smoke Detection” International Fire Protection, November 2008.

For more information please visit Xtralis on stand D950 at FIREX International 2015 – register here

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trv8mike
trv8mike
May 22, 2015 10:39 am

Very interesting and enlightening article. Having seen test fires for detection at LPC many years ago, I can vouch for the size of some of them, particularly the burning foam and flammable liquid ones!
Hope to come and see the new product at FIREX.