A costly problem with explosive atmospheres – how to avoid it?

A costly problem with explosive atmospheres – how to avoid it?

Atex Wolff i Wspólnicy sp.j. (GRUPA WOLFF)

Food manufacturing usually requires the use of powdered materials, which brings a hazard of explosion and fire. The Polish and European laws require from the owner of a production plant to prepare an Explosion Protection Document, which may become a costly problem for the plant owner. How to deal with that?

Within adjustment of one of the largest food industry plants in Poland (manufacturing instant products among other things) to the legal requirements for explosion safety, the so called Explosion Protection Document (EPD) had to be prepared. In the opinion of Jarosław Pusio, expert of the WOLFF GROUP for analyses, EPD is a sort of a roadmap for the manufacturer indicating explosion related hazards present at the plant. It happens often that such documents include proposals for technical and organisational solutions, which, when implemented, will help the employer to reduce the risk of explosion or limit its possible effects to the acceptable level. In justified cases, the proposed solutions may also refer to the methods of elimination or limitation of the sizes of explosion hazard zones set out at the particular plant. The employers are perfectly aware that adjusting these specific work areas to legal requirements is often very costly.

A document prepared for the aforementioned plant reflected many places where external hazard explosion zones (outside of machines, tanks, etc.) have been set out. The situation brought about serious problems for the plant owners, among which the most significant one was the necessity of replacing a series of equipment non-conforming with the binding law requirements with ones certified and identified as explosion proof (Ex class). This project consumed significant financial expenditures.

Issue of a recommendation to replace equipment operating in an explosion hazard zone with a new one is the simplest one from the point of view of an engineer or a group of engineers auditing the given system. However, such approach is not always optimal with regard to economy and technology.

An experienced company auditing a plant coping with a similar problem should first of all propose technical solutions enabling elimination or limitation of the possibility of explosive atmosphere origination, and – what follows – eliminate or limit the size of explosion hazard zones that have been set out. In most cases, tightening of the transport system, transfer points or equipment, or implementation of local dedusting system or ventilation may prove to be much more effective and will also positively affect other work safety aspects.

“Just to mention working conditions improvement by limiting the concentration of health-detrimental dusts in the place of work”, says Jarosław Pusio.

EPD is a sort of a roadmap for the manufacturer.

In accordance with the Regulation of the Minister of Economy regarding minimum requirements for occupational safety and health, related to potentially explosive atmosphere in the place of work , first of all the risk of the explosive atmosphere occurrence must be limited, and only afterwards focus should be put on limiting the risk of activation of ignition sources – which in this case means equipment replacement. These are preventive actions to be implemented first of all, but it must be remembered that they are not sufficient and will never completely eliminate the risk of explosion. Therefore, the actions must be supplemented with protective measures which minimise the effect of explosion, i.e. explosion suppression and/or venting or isolation. This is, however, a subject for a separate article.

We requested our expert to present some problems coped with in industry as well as a practical approach thereto by applying preventive measures.

Jarosław Pusio: “I’ll focus on three generally occurring situations which clearly show how to completely eliminate or limit the existing explosion hazard zones, which may significantly eradicate the necessity of replacing equipment with one conforming with the ATEX directive requirements.

Example 1.

In big-bag, tanks, screens or mixers loading/unloading systems flexible connections are often applied (made of textiles or plastic), which connect the fixed system elements with the vibrating or temporarily fixed ones. Typical examples of the places where such solutions are applied include: a tank connected to a vibrating board at the bottom, a mixer connected to a discharging hopper or a big-bag attached below a charging hopper. Use of flexible connections enables tightening of the powdered material transfer points and minimising vibration propagation from the movable elements of the system to the still ones. Unfortunately, owing to their structure, such connections may also be the potential sources of dust emission to the environment, which requires explosion hazard zones to be set out in their vicinity. It must be emphasised that the method of fixing the flexible connection does not always ensure complete tightness of the system. Most often flexible materials are fixed between two system elements with clamping rings tightened with a bolt or with a regular adhesive tape, which may loosen and simply disconnect as a result of vibration propagation. Also the very flexible materials may leak due to natural wear and tear (abrasion, cracks) and lack of systematic control of their technical condition.

Setting out of an Ex 22 explosion hazard zone in the vicinity of flexible connections may be fully eliminated or at least the zone size may be limited. Depending on the case, it is recommended to:

  • use of robust/reliable fixing for flexible connections (flanges, triclamps, etc.);
  • use of materials with proven resistance to abrasion and/or tensile forces;
  • use of two-layer flexible connections (the external material of the flexible connection should be transparent in order to control the tightness of the internal material of the connection);
  • carrying out systematic tightness checks of the connections as well as wear and tear checks of the applied materials; or
  • in atypical cases, e.g. pneumatic transport pipelines or vibrating boards in the bottoms of tanks/silos protected against the explosion effects, only such flexible connections should be used which will sustain the reduced explosion pressure (pressure originating in the protected device during explosion venting/suppression, for example), as confirmed with an adequate certificate.

    Each case of Ex 22 zone size limitation or complete elimination around a flexible connection requires an individual approach. In some cases, when flexible connections are not subject to tension and do not come in contact with an abrasive product, their reliable fixing (e.g. with a triclamp) and regular tightness control may be sufficient to eliminate Ex 22 zone. On the other hand, in the case of flexible connections subject to constant vibration or ones that get in contact with an abrasive product, use of a double flexible connection with improved durability parameters may prove to be necessary as well as reliable flange fixing and higher frequency of the connection technical condition checks. Only after long-lasting observations of the applied solution a decision on resignation from Ex 22 zone classification in that area may be taken. It may appear, however, that even such advanced protective measures will not ensure 100% system tightness and the only thing to do in such case will be reduction of the Ex 22 zone from 2 m to 0.5 m, for example.

Example 2.

Another type of area of potential dust emission from an engineering system, where Ex 22 explosion hazard zone needs to be set out, is the outlet of clean air from filtration units (filters, dedusters or vacuum cleaners). Aspiration systems installed at production plants (not only in food industry) are intended specifically for dust concentration reduction in powdered products transfer or loading points, or are responsible for cleaning and discharge of excess transport air from tanks loaded with powdered materials by a pneumatic system. Worth mentioning is the fact that application of an effective dedusting system may result in lowering of the explosion hazard zones classes in the area subject to aspiration and in the vicinity of that area. This is, however, not the subject of the problem described at this point. Aspiration systems, except for positive aspects, may also contribute to generation of Ex 22 explosion hazard zones. As a result of leaking of the filter bag, cartridge or cassette installed inside the dedusting unit, dust may be released to the environment through the air outlet duct. In other words, in the direct vicinity of the cleaned air outlet from the filtration chamber, explosive atmosphere in the form of a cloud of dust may be present, which will consequently lead to deposition of a dust layer on the neighbouring equipment, and this causes an explosion hazard. In some circumstances (e.g. draught in the room where the filtration unit is installed), the deposited dust may be stirred in the air and an explosive dust cloud will be created. Then, only an effective source of ignition is needed to initiate an explosion.

It must be considered that in filtration units air outlet areas technical solutions ensuring complete Ex 22 explosion hazard zone elimination cannot be applied. Regular tightness and cleanness control in such areas of aspiration systems will not be sufficient, either.

However, at the audited plant some inventive solutions were observed, which enabled elimination (change of location or limitation in size) of Ex 22 explosion hazard zones. For that purpose the following were applied:

a) taking the dust emission sources outside of the building

Air cleaned in filtration units is discharged by a duct running through the building wall to the atmosphere. Therefore, the location of Ex 22 zone changed. The zone was moved outside of the engineering premises, to an open space where there is no equipment or personnel. Removal of the zone from the engineering premises eliminates, among other things, the costs of replacing electrical equipment with Ex class equipment (certified in accordance with ATEX directive requirements).

b) two-level filtration of the air cleaned of dust

It must be mentioned that the PN-EN 60079-10-2 standard provides for alleviation of explosion hazard zones if two-level filtration is used. If cleaned air from one filter is forwarded to another filter, then Ex 22 zone does not have to be set out at the outlet of the air from the second filter. The zone will be closed in the internal space between the two filters.

At the audited plant such solution was applied, which means that at the outlet of cleaned air from the basic filter a cartridge filter was installed at the end of the outlet duct (second filtration level). Such secured outlet duct does not generate an external Ex 22 explosion hazard zone. Ex 22 zone was limited solely to the space inside of the duct. Also in this case the fact that there is no Ex 22 zone in the engineering premises enables elimination of the costs of replacing electrical equipment with Ex class equipment (certified in accordance with ATEX directive requirements), among other things.

First of all the risk of the explosive atmosphere occurrence must be limited, and only afterwards focus should be put on limiting the risk of activation of ignition sources.

Example 3.

Another type of area where dust emission may occur and explosion hazard zones will have to be set out are powdered materials batching/dosing stations. Batching equipment may be divided into open or partially enclosed, or ones from which dust is removed or not. In accordance with the PN-EN 60079-10-2 standard, depending on the applied solution, Ex 22 or Ex 21 zones are set out around the batching stations. However, inside of the batching equipment Ex 20 zone is established. The sizes of the external explosion hazard zones are determined individually, depending on the observed dust presence – usually, this is 1 m from the external edges of the batching equipment. In justified cases the size may be larger or smaller.

Large importance for classifying zones in such areas is the effectiveness of local dedusting system, material granulation as well as the method and frequency of cleaning.

For example, when batching larger size bulk products (e.g. cut pieces of dried vegetables), dust emission to the environment is negligible. On the other hand, if the material is powdered (grain size smaller than 0.5 mm), the following must be provided for limitation of dust emission and at the same time size of explosion hazard zones:

  • application of (effective) local dedusting to ensure adequate negative pressure in the space where the material is loaded;
  • the operations of materials transfer/dosing must be slow and easy, whereas the time of the operation to fulfil that condition must be determined experimentally;
  • when possible low-dust materials must be used (e.g. granulates);
  • cleaning needs to be made regularly (removal of dust from the floor and equipment) with the use of negative pressure equipment (e.g. ATEX classified vacuum cleaners);
  • substances crushing operations must be made in tightly closed equipment (mills, crushers, etc.), although this may generate the necessity of protecting the equipment against the effect of explosion.

Following the above guidelines adequately to the situation will enable limitation of the size of explosion hazard zones (e.g. from 1 m to 0.5 m, or in some cases even to 0) or change of the zone classification (e.g. from 21 to 22).

Above mentioned are only a few problems present in typical explosion hazard systems. However, they show that not always the simplest solutions (replacement of some of the equipment in this case) are optimal both from the engineering and economic point of view. Preconditions for developing the most favourable solution to limit the risk of explosion in the respective space to the acceptable level include:

  • a team of experienced engineers who know and understand not only safety regulations and standards but also the technology applied at the respective production plant;
  • cooperation and willingness to discuss on the part of the plant owners (or staff delegated by the owner).

Author: Jarosław Pusio