Explosion safety covers a broad range of applications. To decide whether and to what extent precautions are required, you must initially conduct a systematic risk assessment. The first step is to gauge the probability that an explosive atmosphere might be created and determine whether any potential sources of ignition exist. Next, the effect that an explosion might have has to be classified. Both assessments can be merged into a matrix. The resulting indicators tell you whether or not the plant or parts of it have to be protected and if so, how. A green box means that no precautions are necessary. A red box, on the other hand, signifies that precautions are urgently needed. The higher the numerical rating, the more extensive these precautions should be.
How explosions occur
An explosion requires the availability of oxygen from the air, a source of ignition and a combustible substance. In the event of a dust explosion, the distribution of dust is another factor. However, not every dust-air mixture is explosive. What matters is the mixing ratio. Specific explosion limits have been identified for every common type of dust, and within those limits an explosive mixing ratio can be expected. The lower explosion limit is the minimum concentration required to create explosive atmosphere. The upper explosion limit specifies the point at which the mixture becomes too rich, so that the atmosphere is no longer explosive. There also needs to be an effective source of ignition (TRBS 2152 Part 3, “Hazardous explosive atmospheres – avoidance of the ignition of hazardous explosive atmospheres”, page 2). Among the common sources of ignition are hot surfaces, electric sparks and glowing embers arising during the process.
Explosion safety precautions
Depending on the results of the hazard and risk analysis, a number of proven precautions need to be taken. They are divided into explosion prevention and explosion protection. Preventive precautions are designed to prevent explosive atmosphere and therefore to reduce the probability of an explosion. Wherever possible, combustible substances are replaced by substances that cannot produce an explosive mix. In addition, it is possible to overlay the substance-air mix with inert gases. This has the effect of lowering the content of oxygen from the air, so that no explosion can occur.
Preventive precautions concentrate on the avoidance of effective sources of ignition. This includes, for instance, the use of suitable equipment to protect the product flow from impurities and monitor the system earthing with a view to preventing electronic discharges.
Protective precautions involve reducing the impact of a possible explosion to a more moderate level, so that the resulting damage is less severe. Conventional venting using explosion vents, flameless venting, explosion isolation and explosion suppression are just a few examples. This kind of explosion safety is a vital necessity in virtually all plants for various reasons:
- It is in the nature of the relevant processes that there can almost never be absolute or complete avoidance of effective sources of ignition.
- Inerting tends to be too expensive and/or impossible due to the characteristics of the processes involved.
Other preventive precautions may be helpful in certain respects, though they cannot normally eliminate the risk of explosion completely.
If a plant is situated outside a building or if parts of it are next to an outer wall, one frequent safety precaution is to install explosion vents. Such precautions are usually applied, for instance, to stationary silos, filters and elevators located outdoors. If an explosion occurs, the explosion vent protects the system by opening. This reduces any overpressure within the vessel and the explosion is released to the outside. As virtually no two processes are the same, there are numerous types of explosion vents which differ in shape and material as well as in their resistance to temperature, pressure and vacuum. Even processes with complicated hygiene requirements can be equipped with explosion vents today. The EGV HYP explosion vent from Rembe, for example, was highly successful in passing the EHEDG cleanability test. This test determines the in-place cleanability of plant components where this is a practical requirement for manufacturing absolutely hygienic products.
If a plant is situated within a building, however, explosion vents are not suitable for pressure relief purposes, as the safety area around them is inadequate to relieve the emerging dust and flames. Such an arrangement would pose an enormous safety risk to humans and machinery. This problem is often solved through the use of vent ducts, which channel the spread of an explosion to the outside. The disadvantage, though, is that these ducts rule out process-optimised plant design and are usually very expensive: the greater the distance between an explosion and its source, the higher the pressure which the vent duct and the plant need to withstand. This results in higher (manufacturing) costs per vent duct.
Flameless venting, by contrast, is an option that is both economical and effective. Different manufacturers employ different flameless venting technologies. The special mesh filter that is used in Rembe’s Q-Box or Q-Rohr cools down any flames efficiently, preventing both flames and pressure from emerging, and ensures particulate retention. The typical increase in pressure and noise that accompanies an explosion within a building is reduced to an almost imperceptible minimum, thus protecting both people and machinery. In addition to the special stainless steel mesh filter, the Q-Rohr and Q-Box each have an explosion vent with an integrated signalling system that alerts the process control system if the explosion vent opens.
In a production facility, the individual parts of the plant are always connected by pipelines. The purpose of explosion isolation is to ensure that the pressure and the flames cannot propagate, so that any adjoining parts of the plant are protected. A distinction is made between active and passive isolation systems. An active system is alerted to an explosion at an early stage when the explosion begins to develop. This is done using sensors or detectors which register the rise in pressure or the formation of flames and respond by activating the relevant isolator, e.g. a quench valve.
Passive isolation differs in that it responds purely mechanically to the spreading or loss of pressure on account of its structural characteristics. This also applies to explosion valves. During normal operation, an explosion valve in a pipeline is kept open by the flow. If an explosion occurs, the valve is closed by the spreading of the pressure front, thus effectively preventing the pressure and the flames from propagating any further.
Explosion suppression is another constructional precaution in addition to the methods mentioned so far. It means eliminating the explosion at its onset. This is made possible by detectors with sensors which register the presence of sparks or flames and immediately trigger the opening of tanks containing an extinguishing agent (also installed in the system). A highly effective extinguishing agent is released within milliseconds, nipping the explosion in the bud. If required, an explosion suppression system can also be used for explosion isolation purposes.
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