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Protection from dust explosions

Supplier: ProDetec
08 November, 2010

Anything that has been transformed into fine particles can explode. Unfortunately, all too often it does.

The statistics speak for themselves. Dust explosions lead to considerable material damage and long production shutdowns. Every year there are also accidents with fatal consequences.

In order for a dust explosion to take place, the material must have a low moisture content, the dust particles must be of a certain size and the amount of fine particles per unit of volume must lie within certain critical values. An ignition source is also needed such as flames, sparks, embers or hot bodies.

The temperature needed for ignition is much lower than most people think. Almost all organic material in the form of a dust cloud will ignite at temperatures below 500° C - approximately the same temperature as a newly extinguished match.

The actual explosion does not normally occur in the conveying line in which the fine particles are being transported, but in high-risk zones such as filters, silos and cyclones. In other words, places where dust concentrations can vary considerably and easily attain critical values.

A good way to protect against fire and dust explosions is to ensure that ignition sources cannot end up in these high-risk zones. Firefly offers a unique protection system which detects and neutralizes ignition sources within milliseconds.

All fires and explosions have an ignition sequence

In industrial processes, ignition sources may be generated at many stages of the process or in machine components. Ignition sources exist in many different forms, but they all have one thing in common: they often cause fine particles to ignite. The consequences are costly, in the form of production downtime, mechanical damage and at worst a serious fire or explosion which can cause serious injury.

The most common sources of ignition are those which arise within the production equipment itself - when metal strikes metal, or by heat generated in the process. Mills, saws, fans, grinders, dryers and overheated bearings are the most common generators of ignition sources. It is also common for worn parts, as well as foreign objects such as stones, nails, nuts and other loose components to increase the risk of generating sources of ignition that could cause fire or explosion.

In order to prevent fires and explosions, one must have a good knowledge of the process and the process equipment. By analysing the potential areas where ignition can occur a comprehensive prevention system can be designed.

Detection down to the right temperature...

All fires and dust explosions have an ignition sequence. An effective means of protection is to neutralise these small amounts of energy before they manage to ignite the fine particles.

The most important component in the Firefly system is the range of patented infra-red (IR) detectors, which not only detects sparks, but also hot bodies with temperatures as low as 130° C. Conventional detectors available in the market are detecting light from a spark and are therefore incapable of detecting hot bodies with temperatures below 700° C.

The detectors have a 180° field of view. This means that a single detector is normally sufficient to cover the entire cross-section of a process conveyor.

Each detector has seven control zones. In order for the detector to trigger an alarm, an ignition source must be registred in three zones. This design provides a high level of security against false alarms, but also means that the detector reacts equally rapidly irrespective of the speed of the material flow. The alarm is triggered if the ignition source passes the detector.

Flame detection

Firefly has a range of flame detectors for different applications, in open areas and in enclosed volumes. In open areas the most reliable detector is the combined UV/IR flame detector. By combining IR and UV wavelengths the detector will efficiently recognise flames but discriminate other energy sources, such as lamps, sunlight and even arc welding. In enclosed volumes fire can easily become a big problem if time is given for the fire to grow. Therefore, Firefly uses fast acting IR flame detectors that will detect flames at a very early stage without the common problem of being daylight sensitive.

...and the best extinguishing methods available

The choice of extinguishing method is determined entirely by the process. Water is the most common extinguishing agent, provided the process permits its use. The water is sprayed under high pressure through our specially manufactured fullcone nozzles which distribute the water uniformly, generating a comprehensive extinguishing zone. The full-cone nozzles are robustly built and are insensitive to wear. They have varying angles of distribution and are selected according to the type of application.

Water mist as fire extinguishing medium is gaining ground and has proven to be very effective in fighting and controlling fires. It has a remarkable potential for suppressing fires and is causing minimal residual damage. Water mist is usually used in open areas for extinguishing developed flames. 

Extinguishing with steam is an efficient and convenient method, provided that steam is available. In some processes, gas is the most suitable extinguishing agent. Carbon dioxide and nitrogen are excellent extinguishing agents provided that the affected section of the process can be isolated. This is done with the help of Firefly’s fast acting valves.

Our product range includes a large number of specially designed valves in varying sizes with closure times down to 50 ms.

Another extinguishing method used in the Firefly system is mechanical diversion. When an ignition source has been detected, a diverting valve is opened which rapidly re-routes the material flow out of the process flow for collection in a container. The process itself does not even have to be stopped.

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Firefly Systems

A Firefly system can consist of several protection zones and Firefly has chosen a de-centralised system with a network structure, thus avoiding long cable runs and vulnerability due to remoteness between control unit, detection and extinguishing zones. With the Firefly network it is possible to connect several control units, CUE, to a master unit, MUE. The MUE is used to present information centrally. The Firefly solution keeps the installation cost to a minimum.

Some other benefits are:
- Flexibility
- Self-diagnostic function
- Battery back up in each unit
- User friendly

A service contract and training – Safety in the long term

A Firefly system is easy to install. The comprehensive documentation which comes with every system contains instructions as to where the components are to be fitted. Most customers prefer to have one of our service engineers on site at start-up to make a final test of system functions and to train the operators who will be responsible for the system.

The presence of ignition sources varies significantly between different processes. This means that some Firefly systems can be triggered at regular intervals. In some cases this frequency could be weekly and in other cases at much longer intervals. At all times however, the system must be ready for action and fully reliable. The best way of maintaining the system is by entering into a service contract. On a regular basis our service engineers will go through the system: testing the sensitivity of the detectors, checking the alarm functions, function testing valves and water nozzles and, most important of all, training the operators.

If the plant is expanded or altered, new risk situations can easily arise. Firefly should therefore be notified of any such changes. During visits, our service engineers can, when asked, discuss if the risk situation has been affected and can suggest measures to re-establish safety.