Dust explosion protection measures are essential for industrial companies today. We have contributed to making grinding systems safer today thanks to many years of research and further development of safety systems. All our hammer mills and all other machines are tested to be resistant to explosion pressure shock and flame penetration, up to an explosion overpressure of at least 0.4 bar. Our machines and the safety technology used comply with the necessary national and international specifications.
The pressure-shock-resistant design of appliances and machinery, the optimum design of pressure relief devices and the use of explosion decoupling of the supply and discharge channels play a decisive role in ensuring a safe system. Safety always originates in the system. Our concepts for system safety as well as the comprehensive documentation will assist you in communicating with the responsible authorities or experts. Regular inspections and the maintenance of your safety equipment allow us to guarantee the permanent functional safety of your plant.
We work together with renowned testing institutes (Notified Bodies) at home and abroad. This is where the machines and appliances we use, some of which we have developed ourselves, are tested for their safety and effectiveness under actual operating conditions.
We offer comprehensive advice and draw up explosion protection concepts that are individually adapted to your requirements. In doing so, we use different explosion protection systems from various manufacturers. Our objective is to create the optimum explosion protection concept for each customer.
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Hammer mills are among the most important processing plants in the bulk solids industry for the size reduction of grain, wood, biomass or other organic materials. However, due to their high rotor speeds, large kinetic energies and the processing of combustible dusts, they also represent typical sources of hazards for fires and dust explosions. An effective protection concept must therefore take into account both explosion protection and organisational and technical fire protection.
Explosion and fire protection for hammer mills is based on a holistic safety concept that combines preventive, constructional, detection-based and organisational measures. Only the coordinated interaction of these elements reliably prevents sparks, overheating, foreign bodies or dust deposits from leading to a fire or dust explosion. A modern, well-maintained protection system protects employees, machinery and buildings, minimises production downtimes and fulfils all legal and insurance requirements, making comprehensive explosion and fire protection not only essential for safety reasons, but also an important economic factor for trouble-free and efficient plant operation.
Hammer mills reach circumferential speeds of around 100 m/s. If foreign bodies such as metal parts enter the grinding chamber or if friction occurs due to wear, mechanically generated sparks can be produced. In addition, overheated bearings, blocked screens or smouldering material in the grinding chamber can lead to increased temperatures. This results in typical explosion hazards as well as classic fire scenarios such as smouldering fires or glowing nests. Dust deposits in pipelines, filter chambers or on machine surfaces further increase the fire load and promote the spread of fire or hot gases. As explosions or fires can spread via screw conveyors, filters, silos or pipelines, all system components – from raw material intake to product separation – must be taken into account and protection against flame propagation, combustion gases and sparks is a key component of the overall system.
The first line of defence consists of measures that prevent ignition sources and at the same time minimise the probability of fire development. The separation of foreign materials using magnetic separators, heavy-particle separators or the Tietjen feeding systems AGS and DA, which combine both, prevents the generation of sparks and reduces the mechanical load on the mill. Equally important is comprehensive sensor technology: temperature monitoring on bearings and in the grinding chamber, motor current monitoring for detecting blockages, as well as vacuum or level monitoring. These sensors must be ATEX/NEC 500 compliant and must automatically generate a signal or shut down the mill if limit values are exceeded. In addition, all pipelines and components must be reliably earthed in order to avoid electrostatic ignition hazards. From a fire protection point of view, a low-dust plant design, regular cleaning and constructional measures to prevent material accumulations are of crucial importance, as they reduce the fire load and significantly lower the risk of smouldering spots.
As preventive measures cannot guarantee absolute safety, additional constructional protection devices are implemented. Tietjen grinding systems are designed to withstand explosion pressures (typically 0.4 bar) so that important components such as mill housings, filters, screw conveyors and pipelines can resist an explosion. In addition, rotary airlocks and additional safety devices in the Tietjen feeding systems prevent the transmission of pressure and flame into adjacent plant areas. The A‑Vent air inlet valve, a type-tested protective system developed for the intake air supply of the feeding devices, closes within milliseconds in the event of an explosion and thus reliably prevents the spread of pressure and flames. Protective systems for flameless pressure relief, such as the Tietjen E‑Vent overpressure valve, open in the event of overpressure build-up, allowing the explosion to vent in a controlled manner into the surroundings and thus preventing damage to the mill and the grinding plant, while the integrated flame arrestor prevents flame propagation to the outside.
The detection of potential fire incidents is a central element of the overall concept. Spark detection systems in conveying lines prevent glowing particles from entering the filter. CO sensors or flue gas detectors in the exhaust air detect smouldering fires at an early stage, while rate-of-rise temperature detectors on filters and containers indicate glowing nests. Various systems are available for firefighting: automatic spark extinguishing systems, high-pressure water mist for cooling ducts, manual firefighting points and – depending on the plant concept – CO₂ inerting systems that inert the mill or the filter and thus extinguish smouldering zones.
A protection concept is only as effective as its consistent implementation in operation. Personnel must be trained to recognise malfunctions, to respond correctly to spark or fire alarms and to act appropriately in the event of a fire or explosion. Documented cleaning and inspection intervals are essential for detecting dust accumulations, wear or defective components at an early stage. Safety-relevant components such as A‑Vent, E‑Vent or rotary airlocks require annual inspections and thorough cleaning at least every three years. Repairs and interventions in the protection system must be documented through a release and control process.
In the case of a dust explosion within the grinding system, it is vital to discharge the resulting explosion overpressure (pmax e.g. 9 barg) in a controlled manner and consequently reduce it to a harmless level (pred=0.4 barg). When doing so, no flame may escape from the machines and appliances in an uncontrolled manner.
Structural dust explosion protection only works reliably if the components are professionally coordinated to work together with one another.
1. Explosion decoupling of the feed and discharge channels:
2. Pressure-shock resistant machines and system parts between the decoupling points.
3. Explosion decoupling of the supply and exhaust air:
4. Flameless explosion venting at the mill secondary container and at the filter system
5. Active explosion suppression by means of extinguishing bottles
Tietjen Verfahrenstechnik has been intensively involved in explosion and fire protection for grinding plants for decades. After the publication of the first ATEX directive in 1994, Tietjen began to focus on constructional explosion protection measures in hammer mills. As early as 1999, tests on explosion-resistant mills started at German test institutes. After the implementation of the ATEX directive into national law in 2003, explosion protection became mandatory for all mills.
