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Explosion Protection – part three

World Cement,

Aspiration and dust collectors

The probability of an explosion in a dust collector (DC) is higher than usual, and many DC manufacturers therefore include explosion protection measures within their products. This is because sparks and hot embers can enter the system, together with aspirated dust from other parts of the plant. One situation that may get particularly critical is the purging of the filter hoses themselves. This leads to high dust concentrations. In indoor use, DCs are therefore protected through flameless venting, and in outdoor use through explosion vents. If vehicle routes or thoroughfares are situated within the explosion venting range, the solution is to use smart add-on modules for explosion vents to deflect flames and pressure waves into non-dangerous areas.

Separate protection is possible, but only a comprehensive strategy is cost-effective

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 all-encompassing, fully customised explosion safety solutions. Turnkey solutions never exonerate a prospective plant operator from ensuring efficient explosion prevention. It therefore follows logically that experts should be involved in the process.

Practical examples

As has been shown, 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 process. Real professionals, however, always start with an assessment of the actual need for explosion protection. As seen, not every system requires explosion protection on systems 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, 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 atmosphere 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 barbed wire 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 safety.

Even if it seems obvious that explosion protection measures are inevitable, it is worthwhile looking at the details. Money can be saved through small changes to the distances between vessels, e.g. between dryers or mills and cyclones, and also by smartly engineering the relevant pipelines, taking account of the tested installation distances of decoupling systems.

This is part three of a three-part article written for World Cement’s February issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the December 2015 issue here.

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