Fire Protection in the Danger Zone
The gas explosion at a food waste recycling plant in Oxfordshire recently may have been caused by a lightning strike, but images of the huge fireball serve as a stark reminder of just how dangerous some fires can be.
In particular, sites where gases, dust or flammable airborne particles hang in the air are a danger zone that requires special treatment. A recently published design guide from Switzerland’s Securiton offers valuable insight into the advanced methods needed to reliably protect such sites – and how design elements can be used to improve cost-efficiency.
What are classified hazardous areas?
In fire safety terms, a ‘hazardous area’ is a space in or around a building that can be said to have a potentially flammable and/or explosive atmosphere.
Oxygen, heat and fuel are commonly referred to as the fire triangle. When a fire ignites, a fourth element, the chemical reaction, makes up a fire ‘tetrahedron’. In built environments with the presence of flammable gases or vapor, combustible dusts, or volatile fibres in the atmosphere, this tetrahedron can trigger a rapid chain reaction. This means either a huge, uncontrollable blaze, or an explosion when explosive limits are exceeded. Anywhere where this violent reaction can take place is generally deemed a hazardous area by one of two universally accepted methods of classification: the Class/Division/Group system used in North America and based on NEC 500/504 or the Zone/Group system used in Europe based on IEC 60079.
Because a fire or explosion in such circumstances would likely be catastrophic – causing loss of life or serious injury and wiping out a business – it is crucial to have in place a suitable fire detection and protection solution which adequately addresses the specific hazard profile and associated fire risks.
Fire Detection Challenges
Due to the unique characteristics of fire danger and risk profile in hazardous areas, significant emphasis is placed on eliminating ignition sources and controlling ambient conditions to well below explosive limits. Potential ignition sources include any heated apparatus, or other moving and electrical machinery (including cutting and welding, dryers, furnaces, turbines), hot surfaces and sparks. Crucially, safety measures regarding ignition sources also apply to the fire detecting equipment itself.
Additionally, hazardous areas require detection systems that are both reliable and robust, as the detectors will have to work in environments that may be dusty, subject to extreme temperature changes, and may be partly outdoors; and they must provide fast, reliable feedback to teams on and off site, so that suitable response measures can be implemented from the very first hint of a problem.
Early Warning Fire Detection design
Obviously, some form of Early Warning Fire Detection (EWFD) is desirable to allow staff to react if there is a fire, but even these devices must be rendered safe so that they can’t produce a spark. One approach to this involves using expensive ‘Atex’ tested equipment that is specifically designed for hazardous areas. The other involves designing a system that places most of the equipment at a safe distance, with suitable firewalls between the electronic equipment and any explosive or highly flammable airborne substances. The latter approach is known as ‘intrinsically safe design’.
Fire detection products installed inside classified hazardous areas must have specific approvals/markings, such as ‘explosion proof’ or ‘flameproof’. These products are often bulkier due to extra casings, and come at a premium price. A lower cost alternative is intrinsically safe design, where Early Warning Fire Detection systems such as Securiton’s SecuriSmoke ASD can be installed remotely in a non-hazardous area near the protected classified zone. Sampling pipes or other sensing elements run into the danger zone, using easy add-on components such as inline flame arrestors and sampling pipe grounding clamps.
Linear heat detection systems are often also suitable for intrinsically safe design, although they may not produce quite as quick and reliable warning. In practice, and given that hazardous sites often include open air areas, a combination of methods are often used together.
Safe and effective detection methods
For example, a line-type heat detector that consists of a sealed cable or tube can be run through dusty, dangerous areas. The monitoring unit will be positioned in a safe area where staff have regular, easy access. Maintenance will not generally require access to the danger zone, and the cables or pipe itself will not only be resistant to the harsh environment, but are also suitably earthed and safe in terms of any live components.
A similar approach is possible with aspirating smoke detectors, as only their hollow plastic sampling tubes must run through the monitored zone. Aspirating smoke detectors likely provide the best early warning of a potential problem but will struggle to function effectively outdoors, or in the most polluted environments. For this, beam detectors are ideal, as they can monitor large open areas from positions on the periphery. The best modern beam detectors can monitor for both fire and smoke, and can distinguish the latter from atmospheric dust.
Networked for fast overview
Once a suitable design has been approved and installed, how the information from this array of devices is processed and visualised is critical to the success or otherwise off a protection system. In a critical emergency, clarity is key: the manager in the response room must have a visual overview of what the detectors are reporting in real time. As far as possible, staff on site should also have a clear, visualised idea of what is going on.
All of the detection devices highlighted above can be networked to an advanced fire alarm system or panel. Increasingly, however, detectors are also offering simple visualisation at a more local level.
Acting on information
The best way to react to an alarm in a dangerous area is by using a staged response. This requires that devices offer several levels of ‘alert’ and alarm, depending on heat levels or smoke concentrations being detected. At the lowest level of alert, the desired response is to get a suitable member of staff to the area being flagged as quickly as possible – and in the case of a hazardous area this will have to be very fast and also involve an element of caution. They can directly tackle a small incipient fire if it is merely smoldering; they can raise the alarm and start evacuations if necessary; or they can report that no action is needed so that the system can be re-set.
At the same time as maintenance or security staff go to investigate the initial alert, the system should continue to evaluate the situation and will raise the alert or alarm status accordingly if the smoke level increases. Ultimately, such systems are also able to sound a full alarm, activate suppression systems and automatically call the local fire brigade – but the aim of the staged response is to prevent the need for such drastic measures with successful early intervention.
When it comes to safeguarding critical but dangerous infrastructure and ensuring that heavy, valuable industry is also safe to the local population, fire safety professionals need to use every resource at their disposal. That means sensitive detection, staged alert and alarm levels, clear visualisation, fast information relays and a suitable response should the worst happen.
For more information visit www.securiton.com/
By Dominic Jeff, Technical Writer at Securiton