Smoke, flame & heat detectors for high-risk environments
Published: 01 September, 2006
Brian Cummins is the MD of Proline, a company which has developed its UK manufactured range of FM-approved and UL listed linear detection cables, accessories -including optional interface units - which can be simply integrated with all central fire alarm control panels and building management systems.
Brian has over 35 years of experience in the fire protection industry and is a specialist in linear heat sensing technology.
“Proline’s core product range is its type ‘TH’ digital linear heat detection. As a combined detecting and heat sensing cable in one, it will monitor changes of temperature at the exact point of risk where possible, but is equally as effective for large area protection, providing that the distance between parallel rows of sensor cables do not exceed the Underwriters Laboratories listed maximum space of 9.5 or 10 metres,” he explains.
Although heat detection is sometimes viewed as the slowest form of response, this is applicable only to point type devices installed at the most convenient roof or ceiling height where it relies upon favourable airflows to get from the source of the fire to the detector such concerns and restrictions are not applicable to digital linear heat detection, he says.
“Because the linear heat sensing system uses a small diameter flexible cable it allows you to install it at the exact point of risk. For instance, if your risk is a bearing overheat on conveyors or escalators. Alternatively, a potential strike of lightning in a fuel/chemical storage tank and the possible ignition of gas leakage is between a secondary sill and a tank wall then you would install sensors cables around the circumference of the tank roof.”
The other advantage of linear heat sensing cables - particularly with digital cable - is the fact that you have a fixed alarm temperature, says Brian.
Whether you have one millimetre or one kilometre of this cable exposed to an overheat/fire condition, the alarm temperature is going to be exactly the same. There are other systems that are temperature / length dependent, so the shorter the length heated, the higher your alarm temperature.
Undetected rises of temperature
“All incipient fires and overheat conditions are very localised events; all fires start off within less than a one metre length, whether it is a flash fire or an overheat.
Therefore, you are only ever going to subject a very narrow band of air to an increasing temperature, whether it is a smoke or heat detector.
More importantly, for the majority of high risk areas, with the exception of the floating roof tanks where the primary risk will be from a flashfire, 80% of all industrial fires are undetected rise in temperature that ultimately reach the combustion stage,” Brian continues.
“What we are talking about with a linear heat sensing cable is the ability through the proximity of the sensor cable to that potential overheat to prevent the fire in many cases and not just detect it. The point is to detect overheat and allow action to be taken before that temperature rises to a combustion level.”
On the electronics side Proline cables produce a simple alarm signal and allow integration with any central system; when using a Proline optional interface unit, they are able to accurately determine where the alarm has occurred even along sensor cable lengths as great as 5,000 metres.
In an overheat condition there will be no visible flames and someone who responds to that alarm might not be able to see where that problem is occurring, but if he is told via the interface unit that the overheat is (for example) 57 metres away, the location could be pinpointed.
Operator-programmable outputs
Equally as important, whether it is an overheat or a fire, the Proline ZI-03 interface can be programmed at the site. Multiple alarm outputs ( maximum 50 per interface) allow sub zoning of a sensor cable length - from one metre to 5,000 metres in length. The operator can program the required number of outputs, with signals sent to an associated central fire alarm control panel.
The system can then determine exactly which associated extinguishing, ventilation or smoke extraction zone installed within the same area protected by up to 5000 metres of Proline sensor cable needs to be activated.
“I am certainly not saying that linear heat detection is the answer to everybody’s problems and can replace all forms of detection. It never will. If you consider a relatively clean environment and are looking at a potential source of risk, such as a paper fire, then yes, you would use smoke detection. If you are looking at a dirty, damp, dusty and corrosive environment with areas of limited access surveillance, then traditionally, that is exactly where you would install linear heat sensor cable.”
Brian explains that the advantage of a cable over any point heat detector is that although there are regulations which stipulate the maximum distance between two point type heat detectors, there is a probability that if you have the point detectors X number of metres apart, any fire will occur exactly midpoint between those two detectors. With a linear heat sensing cable this is not going to be the case, because it is equally as sensitive at any point along its length, he says. One can compare it with very many point detector installed right next to each other, but at a fraction of the cost.
This approach is less expensive compared to other forms of detection, maintains Brian.
“In terms of total area coverage, this system is sensitive as well as cost effective. I would never suggest that linear detection is maintenance-free. It tends to require more preventive maintenance than corrective maintenance. The physical appearance of a 3.5 or 4mm cable also suggests that these cables could be easily damaged. However, as part of a demonstration, I would invite someone to actually take a hammer and physically abuse the cable to illustrate its true mechanical strength,” he continues.
He illustrates his argument with the example of the Kings Cross Disaster of ‘89. When protecting escalators, there are many moving parts. In these situations the potential risk of actual damage may appear quite high. Heat and the transferred heat are exactly what the cable is looking for.
There should be no barriers between the heat source and the sensor cable. It is, however, still possible to braid the cable without affecting its operational sensitivity. In doing that you can greatly enhance the usability to withstand a lot of abuse. Where mechanical or overheat/fire damage to the sensor cable occurs, it is possible to just replace the damaged section - not the complete zone length reports Brian.
In the petrochemical industry the similar concerns apply to corrosion of the sensor cable.
Brian comments: “Our products consists of a particular high grade of industrial nylon that is extruded around the sensor cable. It can withstand specific categories of Salts, acids and alkali’s in different concentrations and temperatures. We suggest the minimum-operating lifetime for a sensor cable would be 10 years. In fact, the average lifetime is probably 20 to 25 years, but there are systems out there which have been operational for 40 years.”
The traditional applications for linear heat sensing cables are: Power Generation: cable tunnels, transformers, alternator pits, and electronic cabinets. In fossil fuel power stations, linear heat detection is used to protect or monitor for overheat on the bearings of the support rollers of
the coal conveyors that feed the power station. Petrochemical: the number one application is floating roof tanks.
Linear heat detection in most cases is not a system on its own, but an integral part of a much larger central fire and building management control system supplied by one of the larger fire alarm companies. With detection usually linked to a water or gaseous based extinguishing system. A signal from the linear detection system operates the extinguishing system.
An advocate of point detection
“We provide point smoke detectors and point heat detectors for hazardous areas,” Rowland Davies, marketing manager for Apollo explains.
“We have them certificated by the LPCB, which is one of the organisations that makes sure that detectors in these sort of applications comply with all the right standards and with the ATEX directive on equipment used in explosive atmospheres. We manufacture optical smoke detectors, multi-sensor smoke detectors and heat detectors.”
Apollo has traditionally been a specialist in point smoke and heat detectors According to Rowland, point detectors are a very reliable way of detecting smoke or heat. First and foremost a system must be reliable, secondly false alarms must be avoided as point detectors can react to phenomena such as dust as well as smoke. Thirdly, it must be affordable. Apollo detectors can be used in almost any environment, both hazardous and non-hazardous, he points out.
“The way you would choose a detector for an application such as an oil refinery or an offshore platform would be very much the same as choosing it for elsewhere. The question is where a point detector would be likely to be of most use. The next question would be: is that product suitable to use in an area that is potentially explosive? In that case you must use an intrinsically-safe detector.”
In case of an offshore application, you would need to confirm whether the product is suitable for use in marine environment. There are a number of bodies that provide marine approvals. For example, the Marine and Coastguard Agency in the UK, the UK Department of Transport, DNV in Norway and The American Bureau of Shipping. These agencies look at the product and the drawings and test it for safety in marine applications.
Maintenance is necessary once installed and it is absolutely essential to have a maintenance contract. A qualified maintenance company will come round at regular intervals to make sure that the system is working properly or will carry out any maintenance procedures when necessary.
Some environments are particularly dirty and will attract more dust and this means the maintenance intervals will have to be shorter than in a clean office area. If the detector is placed in a dirty area, it should at least be maintained quarterly or even more often than that.
“You can do some degree of cleaning yourself, but if you need to dismantle the detector in order to clean it properly, we believe it is better and more economical to return the detector to the factory. We have a service department that will clean and recalibrate the detectors, after which they will be returned to the customer.”
So how do you protect your facility in the best manner?
Rowland advises that every company employs a qualified fire engineer to produce a plan and a design for protection of your plant or offshore platform.
“Flame detectors are often placed in particularly hazardous areas. They will search for the radiation given from a flame, this means they can react very swiftly. However, they are considerably more expensive than smoke detectors, but they do a different job and do that very well indeed.”
Rowland feels that smoke detectors make up the most important part of reliable protection, because smoke, after all, is the earliest warning of combustion, heat develops later, he says.
Traditionally, ionisation detectors were used for many applications, but for various reasons these are no longer a favorite. The main reason was that they had a small radioactive foil. Nowadays people now tend to use optical and multi-sensor detectors.
Optical detectors are currently most popular.
“We are about to launch a new range of detection equipment and this includes an optical smoke detector. Flame detectors, which are nowadays almost all infrared, are looking for radiation from a fire. They are fairly bulky by comparison as well as quite heavy. You can get smaller point flame detectors, but in industrial use you would probably use the large version,” Rowland continues.
He points out that Apollo has to be able to supply a product that manufacturers of control panels and systems can easily incorporate. In the case of conventional products, it has always been easy as a conventional detector is a simple on-off system, to which almost anybody can adapt. It is quite different when you are talking about analogue or intelligent systems because all manufacturers have an individual protocol.
Because Apollo does not manufacture control panels, it has to supply information on the transmission protocol. After the panel manufacturer has completed the product, it is tested at Apollo to ensure compatibility.
