Professional explosion safety has its price, but unprofessional overengineering or poor protection can be much more expensive in the long run. In the worst case, people will pay with their lives. Therefore, it is recommended to work with experienced professionals who establish a holistic view of the situation, and create a comprehensive, individual protection concept for the facility. Even the purchase of turnkey plants never relieves the future operator from the duty to implement a sufficient explosion safety. Initially, experts always assess the necessity of explosion safety. Because not every facility or every part of a facility must be equipped with constructive actions without an assessment. In the following we will have a closer look at several stations of bulk material handling and processing.
Unloading bulk material
The respective bulk material is usually transferred into bulk containers from a truck or freight wagon. This is where organisational/preventive actions are mostly sufficient: The risk of combustion from hot brakes or exhaust systems is effectively countered through a wait and cooling time of 15 min before the discharge. Grounding systems additionally protect against spark discharges. For moving objects such as trucks or wagons, the staff must work carefully and have received the appropriate training. In addition, agreements should be made with suppliers to ensure that the raw material is delivered without any glowing embers.
There is no standard concept for explosion safety on conveyors, as the designs and functions are highly diverse. However, there are actions that avoid explosions – without the need to install protective systems. These include limited conveying speeds, suitable material pairings and safety-compliant designs. Open belt conveyors are relatively uncritical. The material is not swirled up and does not come into contact with hot surfaces. This is different with chain and screw conveyors: depending on the degree of fineness, moisture content and dust formation probability of the bulk material, the conveying principle and conveying speed of the connected system parts, there could be a need to implement explosion protective systems such as explosion pressure venting devices. It must be ensured that explosions that occur in neighbouring parts of the system, are prevented from propagating through other connected conveyors. For this purpose, isolation systems such as Atex-certified rotary valves, chemical barriers, quench valves or quick-acting slide valves are used.
Explosion safety for bucket elevators
Bucket elevators represent a special source of risk, because they facilitate the negative explosion prerequisites (explosive mixture and ignition sources) through their function and design. Additionally, pressure waves and flames can propagate across several floors through unprotected elevators, and cause severe damages.
VDI Guideline 2263, Sheet 8 (8.1 and 8.2), describes explosion safety measures and provides information on the dimensioning
of explosion venting devices and the design of explosion suppression systems. The particular challenge with elevators is that the protective systems installed must be as light as possible and at the same time achieve the maximum venting efficiency.
The Q-Ball E (E stands for elevator) from Rembe is an example of a flameless explosion vent designed specifically for elevators. At 25 to 50 kg, depending on its size, the Q-Ball is very light compared to other available systems that easily weigh 100 kg and more. This does not just make the installation easier, but also reduces the demands regarding the attachment and strength of the elevator.
Risk of an ignition source entry into silos
Silos do not usually contain any kind of their own ignition sources. This eliminates a decisive parameter that can trigger a dust explosion. However, there is a danger of combustion which is caused by a possible ignition source entry from upstream facility components. Outdoor silos must therefore be protected with explosion vents, and silos inside buildings with flameless venting devices or explosion suppression systems. Depending on the type of bulk material, preventive actions can also be implemented, such as the installation of spark detectors in combination with spark extinguishing systems or quench valves.
Screens and mills
Screens can often be extremely endangered, especially in combination with upstream dryers. Glowing embers in bulk materials which have survived in mechanical conveyors without igniting may subsequently be broken open by tumbling or vibrating movements and subsequently trigger an explosion. Explosion protection is therefore necessary, but not always easy to implement indoors. The solution: mostly individual, vibration-isolated systems based on a flameless explosion venting device. For the explosion protection of grinding systems several manufacturers have devices which offer an explosion pressure-resistant design (up to 10 bar). Depending on the size of the facility, this represents a cost-intensive investment. , Explosion isolation should always be provided above and below the mill in order to protect other facility components. The Q-Rohr LF is designed for the mill’s suction opening. It is a version of the Q-Rohr where the usually existing rupture disc is missing. The mill thus can suck in air during the normal operation, which is routed through the stainless steel mesh filter without any problems. If an explosion occurs, the stainless steel mesh filter protects the environment from flames and pressure by removing the heat.
Filter: Increased danger during the pulse-cleaning
The risk of explosion is particularly high in filter systems. Many manufacturers therefore offer products with integrated explosion safety devices. It becomes particularly critical when the filter elements are pulse-cleaned. The very fine dust can then spread evenly in the filter, and in combination with a present ignition source, lead to an explosion. Indoor area filters are therefore protected with flameless venting devices, outdoor filters with explosion vents. If transport or traffic routes are located in the vicinity of the explosion venting, add-on modules for explosion vents such as Targo-Vent are used to deflect the flames and pressure wave into non-critical areas.
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