[
One of the drawbacks in relying solely on carbon monoxide fire detectors is their slower performance in detecting open flaming fires. Bernard Laluvein examines how heat enhanced CO detection can respond to the smaller amounts of CO emission which these fires produce.
Carbon monoxide fire detection has developed over the last few years and can provide a good alternative to smoke detectors in certain applications. This article demonstrates how CO fire detectors can be enhanced by heat sensing to further increase their performance in a wider range of fire scenarios, and why such detectors should be an integral part of the fire engineer’s detection toolbox.
CO fire detectors are ideally suited to the detection of slow, smouldering fires which have a limited supply of oxygen. As such, they are able to provide a very early warning of poisonous fires in life risk situations such as sleeping accommodation. These detectors have also provided solutions in many applications where, due to permanent or temporary environmental conditions, the use of optical smoke detectors would give rise to unacceptable numbers of false alarms. Typical examples of this are hotel bedrooms where high levels of steam from the adjacent bath/shower room occur, student quarters where burning toast and steam from boiling kettles are common events, and theatre or entertainment production stages where stage smoke and pyrotechnics are regularly used are typical examples.
Importantly, since their introduction in the late 90s, standards have been introduced that enable the performance of CO detectors to be assessed against a set of minimum acceptable requirements. In the UK, BRE introduced LPS1265 and in 2004 ISO released an international standard, ISO 7240-6, against which third party approvals can be obtained. In addition to relevant environmental testing, these two standards include the smouldering test fires TF2 (smouldering (pyrolysis) wood fire) and TF3 (glowing smouldering cotton fire) used in the smoke detector standard, as well as a new deep-seated smouldering cotton test fire.
Heat-enhanced performance
In open flaming fires where the supply of oxygen to the source of the fire is not restricted, a more complete combustion occurs which limits the production of CO in favour of CO2. For this reason, CO fire detectors are less suited to the detection of open flaming fires that involve liquid fuels and which develop rapidly, such as those which may be started by arson. These types of fires, however, usually involve a significant increase in temperature which can be measured and used to increase, temporarily, the sensitivity of the detector to CO. The heat-enhanced CO detector is then able to respond to a smaller amount of CO emission.
Fire tests TF4 (flaming plastics (polyurethane) fire) and TF5 (flaming liquid (n-heptane) fire) are good indicators of the improved performance of heat-enhanced CO fire detectors in flaming fire situations – compared to CO only fire detectors. The response of a typical heat-enhanced CO detector to TF4 and TF5 is shown in Figure1 and Figure 2 respectively. The m and y smoke characteristics of the fires are also given in the graphs and it can be seen from these that the heat-enhanced CO detector, in both TF4 and TF5, gives an alarm before the end-of-test conditions as specified in EN 54-7. The availability of a high sensitivity setting permits an even faster response in areas where there is a low risk of unwanted alarms from prevailing environmental conditions.
Applications
Currently, Tyco Fire & Security, including ADT, is installing around 15,000 heat-enhanced CO detectors yearly. It is worth analysing where and how these detectors are being used to get a better appreciation of the benefits they offer to fire engineering.
Cumbria is a major tourist area with many hotels and boarding houses. It also has a large number of retirement homes, all of which makes it ideally suited for fire detection solutions utilising CO detector technology. Recognising this ADT, in conjunction with the Cumbrian Fire and Rescue Service, took the technology to all interested parties for approval. This involved carrying out presentations to senior fire officers and fire prevention staff, as well as to the managers, owners and occupiers of properties with life risk considerations. The numbers of bedrooms associated with these risks presented the fire service with such a potential for unwanted alarms that they very quickly accepted the use of CO fire detection in sleeping risks and other ‘difficult environments’ which suited the use of CO, and where smoke detection carried a high risk of generating unwanted alarms.
Over the last three years, ADT has installed in excess of 2000 units in the Cumbrian region alone. The types of premises and risk where these have been used include:
– Derwent Water Hotel, an 80 bedroom hotel in the Keswick area where heat-enhanced CO fire detectors have been fitted in all its bedrooms, leading to a dramatic reduction in unwanted alarms.
– Ehen Court, a sheltered housing scheme incorporating 35 flats for the elderly and Leaming Bar apartments, part of a national hotel chain where heat-enhanced CO fire detectors have been fitted in the flat living area and bedroom(s) together with a heat detector in the kitchen.
– Greystone House in Carlisle, a 25 bedroom home for the elderly, where a multiple technology approach has been adopted using heat-enhanced CO fire detectors in the living areas and bedrooms, supplemented with enhanced optical detectors in the escape routes and heat detectors in the kitchen.
– The theatre at Carlisle Leisure Complex, where heat-enhanced CO fire detectors have been used in the stage and backstage areas, where there is a high risk of unwanted alarms due to stage smoke and pyrotechnics, and a fire risk from the storage of garments and theatrical props.
– Copeland Council maintenance depot and workshops where dusty environment and maintenance activities had given rise to an unacceptable level of unwanted alarms. Optical detectors have been successfully replaced by heat-enhanced CO detectors.
– BNFL, the Sellafield nuclear powered station on the coast of Cumbria, uses heat-enhanced CO fire detectors to improve the reliability of the fire detection system in certain plant and machine rooms.
– A number of schools in the region were also fitted with heat-enhanced CO fire detectors in science laboratories and areas of craft work, such as woodworking and metal working, where smoke detection would not have been appropriate due to the risk of unwanted alarms.
– Heat-enhanced CO fire detectors have also been used in a warehouse and distribution centre for motorcycle garments and accessories, where unwanted alarms occurred due to dust from wooden packaging and from the movement of goods. In this application, heat-enhanced CO detection was also appropriate for a slow smouldering fire risk where garments are stored in bulk.
In other parts of the UK, the policy of using heat-enhanced CO fire detectors is being progressively adopted by major hotel companies. A couple of years ago, the De Vere hotel in Maidstone installed them in all its bedrooms and lounge areas, removing a serious false alarm problem. Other areas which have benefited from such detectors are prisons and university campuses. In Scottish prisons, for example, smoke detectors have been replaced with heat-enhanced fire detectors in every prison cell, almost eliminating unwanted alarms triggered by smoking. The campuses at Aberdeen and Leeds Metropolitan universities are interesting cases, where unwanted alarms from optical smoke detectors in corridors was reaching three figures per term. This was traced to steam escaping from adjoining bathroom/shower rooms, and to kitchens from burning toast and boiling kettles. The replacement of optical smoke detectors with heat-enhanced CO ones was immediately effective in removing these unwanted alarms. Practical fire tests were also conducted using ‘bin fires’ to demonstrate the effectiveness of the new detection system in the corridor areas.
As for any other detectors, compliance with performance standards is an essential part of ensuring end-user confidence in the product they are purchasing. Together with data supplied by manufacturers, it also gives designers of fire detection systems validated information on product characteristics, which should be taken into account when designing systems. Following the introduction of its LPS 1265 standard for CO-only fire detectors, BRE has recently introduced a new standard for the LPCB approval of carbon monoxide/heat multisensor fire detectors: LPS1274. This standard uses flaming fire tests TF4 (Flaming plastics (polyurethane) fire) and TF5 (Flaming liquid (n-heptane) fire). It is satisfying to see that the work done jointly by the UK industry and test laboratory has also been adopted by the International Standards Organisation, which is in the process of preparing a new standard
(ISO 7240-8), strongly based on the UK document. In Europe, the situation is somewhat confused by the current proposal to establish a generic standard that would encompass all forms of multi-sensor devices. In its current draft form, the European document has been conceived to detect smoke as the main indicator of fire and so would penalise other forms of multi-sensor technologies which do not incorporate at least one smoke sensor. It is clearly in the best interest of fire, protection to give fire engineers all suitable tools available to design effective solutions, and it is not the purpose of standards to preclude this happening. For this reason and in my view, the approach currently being taken by the European Standards Organisation, CEN, needs to be reviewed.
There is little doubt that, in specific situations such as those described above, heat-enhanced CO fire detectors provide the best solution. They are not, however, a panacea and they have limitations. Unlike smoke detectors, they do not provide a measure of smoke density which can be related to visibility, an important parameter when considering the ability of building occupants to escape. Also they are unsuitable for the detection of the early stages of the pyrolysis of material with low carbon content, such as the pyrolysis of electrical cable. However, heat-enhanced CO fire detectors have, undoubtedly, the ability to provide the right fire engineering solution where the fire risk lends itself to the production of CO and heat as main indicators of a fire, and where other form of detection may lead to unacceptable levels of unwanted alarms. It is important that they are seen in the context of an overall fire engineering strategy and are used where they bring demonstrable benefits in term of improved rejection of unwanted alarms, or improved detection of specific fire risks.
Bernard Laluvein is strategic product manager at Tyco Safety Products. He is a member of the British Fire Protection Systems Association fire detection and alarm executive, chairman of the BFPSA working group on fire detectors, convener of the ISO TC21/SC3 working group on CO and heat multi-sensor fire detectors, and convener of the CEN TC72 working group on CO fire detectors. He has written in FSE on the characteristics and advantages of fire detectors based on sensing carbon monoxide (October 2000 p23), and on applications were such technology would be appropriate (January 2001,page 20).