Explosion protection for bulk solids
15 July 2015
All companies that process bulk solids have their own specific features, yet their plants all have certain components, although they may vary in arrangement. These include unloading stations, conveyors, elevators, screens, dryers, mills and filters. In this article, Johannes Lottermann of REMBE GmbH Safety + Control looks at each element of a plant on its own and describes the explosion protection it requires.
Using a specific case, we can illustrate why a comprehensive protection setup is always more cost-effective for the plant operator than protecting each part separately. Fig. 1 below shows an existing installation, and our subsequent discussion is based on this particular arrangement of elements.
Unloading and mechanical conveyors
Most production processes, for example in mills, mixed feed plants, breweries and power stations, start with the unloading of the “raw material”. After its delivery by road or rail, the material is often poured into hoppers. Organisational precautions can be taken, for example leaving the material to cool down for at least 15 minutes before it is unloaded. This substantially reduces the ignition hazard from hot brakes, hot exhaust pipes or catalytic converters. Earthing, too, can offer protection against spark discharge.
If movable objects are involved, such as trucks and railway wagons, it is vital to proceed with great caution, and regular staff training should therefore be provided. In addition, agreements can be concluded with suppliers, specifying that the material must be supplied without glowing embers, as this helps to ensure effective protection against explosions within the intake facilities.
In our example the supplied material is taken to a downstream elevator by a screw conveyor. Conveyors can differ from one another and have their specific designs, so that they require different protection methods. All these methods are primarily designed to reduce or even prevent ignition hazards that might arise from the conveyors themselves – by limiting the speed, ensuring appropriate combinations of materials and using a safety-compliant setup.
Open, uncased conveyor belts are considered to be the least hazard-prone, as the conveyed material is not usually stirred up and is not in direct contact with hot surfaces – unlike on trough chain conveyors and screw conveyors, which operate differently. Depending on the fineness, moisture level and dusting propensity of the material, the conveyance principle and speed as well as the connected plant sections, the zoning and the ignition risk assessment, it may be necessary to provide constructional protection through explosion venting facilities.
As a minimum, any explosions in adjoining plant sections must not be permitted to spread along the conveyors, and decoupling systems are therefore required. The general standard is to install ATEX-approved rotary valves, extinguishing barriers, quench valves or quick-closing valves.
If expert instruction is provided, the actual augers themselves can be converted into protection devices with a decoupling effect. One or two spirals of the auger need to be removed. Whether or not this is feasible should depend, in particular, on the flow quality of the material and the installation position of the auger. Ideally, the conveyed product should fill the entire cross section of the pipe in this area, thus plugging it shut. Under normal operation, it should replenish itself automatically, and in the event of an explosion it should act as a decoupler, so that it prevents the explosion from spreading into other parts of the plant.
The situation is different with trough chain conveyors where the open cross section is not filled with product and where the augurs are not really sufficiently hard, so that extinguishing barriers are necessary. These are controlled by pressure warning devices or infrared detectors which identify flames and explosions and cause an extinguishing agent cylinder to open. The extinguishing agent, which escapes within a few milliseconds, stops and cools down the flame.
Fig. 1: Example of standard elements in a production plant processing bulk solids
Outdoors, on the other hand, it may often be wiser and more cost-effective to use approved pressure venting devices, and we would recommend consulting experts for this purpose.
Elevators are some of the most widespread conveyance devices in the bulk solids industry. They usually allow the vertical transportation of large volumes to heights up to about 60 metres. At the same time they are a special source of danger in the event of an explosion, as both their operation and design are conducive towards explosions (“explosive mixtures” and “ignition sources”).
Moreover, unless an elevator is protected, pressure waves and flames can spread across several floors. Due to this increased risk, the Association of German Engineers (VDI) has set up an expert committee consisting of plant operators, elevator manufacturers and explosion proofing specialists who wrote a dedicated VDI standard. VDI Standard No. 2263, page 8 (sections 8.1 and 8.2) specifies explosion protection precautions for elevators, with advice on the dimensions of the pressure venting devices and the design of the explosion suppression systems.
In addition to bursting disks, which are normally used as standard pressure venting devices, the VDI standard also deals with flameless pressure venting options. This is because if the elevator is situated within a building, pressure must not be vented via bursting disks alone.
Based on the specifications of the VDI standard, three companies – REMBE, IEP Technologies and GreCon – have formed an expert committee for the development of a comprehensive protection system for elevators.
ElevatorEX, the result of this working group, offers a suitable solution for all types of elevators. Hazard-prone parts of the system such as the foot and head of the elevator, are equipped with maintenance-free pressure discharge devices, such as the Q-Box or bursting disks. The elevator shafts are protected by extinguishing agent cylinders, which are controlled by spark detectors. The system is suitable both for the first-time fitting and for the retrofitting of existing elevators.
Again and again, expert committees have discussed the special explosion properties of elevators with round shafts. They see such a model as hazard-prone, as it has a round cross section which offers more expansion potential for an explosion than a rectangular cross section. This increases the vehemence of the explosion – i.e. the reduced explosion pressure – by a factor of about 2.
It is of course advisable to take a range of further precautions, such as using extractors to limit the concentration of dust. However, this applies to all the parts of a plant, not just the elevators.
Fig. 2: Protecting a screen with a Q-Box
Except for self-igniting materials such as biologically active solids, silos do not normally contain their own sources of ignition. This means that they lack any significant components that might cause explosions. Nevertheless, there is the hazard that a source of ignition might enter the silo from upstream sections of the plant. Outdoor silos therefore need to be protected with bursting disks, while indoor silos must have flameless venting or explosion suppression facilities.
Depending on the material that is conveyed by the system, it is also possible to take preventative measures. One preventative option, for instance, is to combine spark detectors with spark extinguishers or quench valves, as this may prevent sources of ignition from entering the silo. However, dispensing with constructional precautions should be the exception and should therefore always be discussed with and assessed by the relevant experts.
The same is true for screens, destoners, etc. These are devices which do not normally have their own sources of ignition, so that constructional explosion protection is usually only required in a very small number of cases, despite an extremely high probability of an explosive mix within a screen.
Here, too, however, it is important to maintain a comprehensive perspective, as they can often be extremely hazardous, especially in combination with upstream dryers (e.g. spray dryers in the dairy industry, drum dryers in the timber industry and electric dryers in the starch industry). The tumbling and rotating movements of a screen may act as a trigger to glowing embers that were created in a dryer and which have often survived mechanical conveyors where they did not previously ignite.
The latest stage at which such embers may be broken down and then cause an explosion is in the screen. This is precisely the scenario which caused one of the biggest explosions in a particle board factory in South America in 2012, when six people lost their lives.
In such cases it is important to provide constructional explosion protection for screens, although this poses special challenges to safety engineers, especially within buildings. The pendular movements usually require dedicated solutions where any vibration-prone elements must be decoupled. Moreover, such solutions need to be based on flameless venting principles (see Fig. 2 above). Other protection methods, such as explosion suppression systems, are often unsuited, as the extinguishing powder cannot be distributed homogeneously within the screen on account of the built-in screen deck.
Fig 3: This add-on module for bursting disks saves valuable plant space
It is in the nature of a mill to have metallic parts that collide with one another at high speed, so that ignition sources are highly probable. Again, this is a closed container with a high concentration of dust which may include oxygen, making an explosion extremely likely. Many mill manufacturers therefore offer their machinery in a design that is resistant to explosion pressure (up to 10 bars).
Depending on the size of the mill, this may be very expensive for the operator. Alternatives would be flameless venting facilities. In either case a decoupler should be attached both above and below the mill. One particularly smart solution for the aspiration intake facility of the mill is to install a Q-Rohr LF.
This is a modified version of the Q-Rohr, well-known in flameless venting, which does not contain the bursting disk that is normally included. As a result, it is possible to aspirate air under normal operation, and the air can then be fed through the stainless steel mesh filter without any trouble. If an explosion occurs, this filter removes the resulting heat and protects the environment from the flames and the explosion pressure.
Aspiration and filter systems
The probability of an explosion in a filter system is higher than usual, and many filter manufacturers therefore include explosion protection facilities within their products. This is because sparks and glowing embers can enter the system together with aspirated dust from other parts of the plant.
One situation which may get particularly critical is the cleaning of the filter hoses themselves. This is when high concentrations of dust are formed which then lead to explosions in combination with the other known components, including the ignition source that has entered the hose. In indoor use filters are therefore protected through flameless venting, and in outdoor use through bursting disks.
If vehicle routes or thoroughfares are situated within the explosion venting range, the solution is to use smart add-on modules for bursting disks to deflect flames and pressure waves into non-dangerous areas.
Separate protection is possible, but only a comprehensive strategy is cost-effective
Fig. 4: Effect of Fig. 3 add-on module (left)
Based on the above, all parts of a plant can be protected separately. However, this method is not usually economically viable. Cost-effective solutions can only be achieved through a comprehensive treatment of the entire plant, covering all interaction between its parts and also any specific arrangements in the production operations. Professional explosion protection does, of course, have its price, but unprofessional over-engineering or insufficient protection are far more costly – not just in monetary terms. In the worst case, people may pay with their lives.
Independent experts therefore always recommend that plant operators work with experienced professionals who take a comprehensive approach and who implement an all-encompassing, fully customised protection system. Turnkey solutions never exonerate a prospective plant operator from ensuring efficient explosion protection. It therefore follows logically that experts should be involved in the process.
As we showed above, it is no great art to protect all the parts of a plant under a run-of-the-mill system that somehow suits the entire system. Real professionals, however, always start with an assessment of the actual need for explosion protection. As we saw above, not every system requires constructional precautions simply because an explosion-prone mixture runs through it. Yet this is exactly where it is good for explosion experts to separate the wheat from the chaff.
If, for instance, chain trough conveyors vary in length but are in principle identical in design and are used in several places in a chipboard factory, then it is important to enquire into the probability with which explosive mixes and potential sources of ignition will actually occur. If, for example, only rough, moist wood chips are conveyed prior to being crushed and dried, then no explosion protection is needed. Once they have been through the dryer, the explosion hazard rises, so that precautions need to be implemented.
Such a detailed process engineering analysis is equally worthwhile when assessing the explosion protection of elevators in mills, breweries and mixed feed plants. If the occurrence of an explosive mix is still likely on account of a high dust content, particularly in intake elevators, and if the explosion hazard is therefore high, e.g. through the presence of undesirable impurities such as pieces of pasture fences or mower fragments, then the hazard in the elevators is often substantially reduced once the cereal has been washed and cleaned in the tempering cells. This is where, in each instance, purely preventative or organisational precautions can provide adequate explosion protection.
Even if it seems obvious that construction measures are inevitable, it is worthwhile looking at the details. Money can be saved through small changes to the distances between containers, for example between dryers or mills and cyclones, and also by smartly engineering the relevant pipelines, taking account of the tested installation distances of decoupling systems.
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