But what makes wood material and wood handling processes so special in terms of explosion hazards?
This question can be answered quite simply: Considering requirements of dust explosion phenomenon:
- combustible dust
- air / oxygen
- effective ignitions sources
nearly all wood handling installations have the “perfect mixture” for dust explosions to occur.
Using the example of a fiber board plant, where all typical wood industry-related equipment such as silo´s, conveyors, screens, mills, dryers, cyclones and dust collectors are found, an explosion hazard is even more likely, as all the above-mentioned equipment creates wood dust causing explosive atmospheres. Additionally, given mechanical moving parts as well as drying processes, ignition sources are easily generated. In Autumn 2012, this deadly combination led to one of the most severe wood dust explosions in history in South America, where 5 people lost their lives and the entire plant was shut down for more than 5 months.
How to protect against explosion hazards in wood handling installations?
The explosion safety concept for such plants typically is made up of a combination of explosion prevention measures (to reduce the likelihood of explosion) and explosion protection measures (to reduce the effects of an explosion to an acceptable level).
Explosion Prevention means taking measures to prevent the formation of explosive dust clouds as well as avoiding ignition sources by dedusting, housekeeping, grounding, proper maintenance and/or spark extinguishers.
We know that even if all preventative measures are applied (especially with regard to the latter), this approach might lead to misapplication of spark extinguishers which
- might not work if particles are large;
- cannot suppress an explosion;
- are only addressing the ignition risk arising from small, hot particles; and
- do not prevent ignition sources from tramp metal or hot surfaces.
That is why protective measures also have to be applied in most wood handling installations. They typically apply one of three approaches:
- explosion resistant design (simple explanation: make equipment so sturdy it will withstand explosion overpressure of up to 10 bar)
- explosion pressure venting (simple explanation: pressure and flame relief by applying a predetermined breaking point on the installation)
- explosion suppression and (simple explanation: a rapid fire extinguisher that stops the flame )
- plus: Explosion isolation (simple explanation: Prevent flame and/or pressure propagation to down or upstream units)
Due to minimal maintenance requirements and low invest costs, passive explosion protection approaches such as explosion pressure venting is the most commonly used in wood handling facilities. The fact that these burst panels can even be combined with flame-trapping mesh materials allows various applications to be protected by so called flameless vents.
As with any comprehensive safety concept, even a fully protected plant can only be secured when all relevant persons, situations and conditions are taken into account. In practice, this means that plant management in the wood handling industry has to be aware of the explosion risk in general, implement available explosion safety measures and educate plant personnel. The awareness of the need for combustible dust explosion safety has to be raised so that catastrophic events are not likely to endanger health, lives and business objectives such as profitability, continuity and productivity.
Therefore, a risk analysis should be carried out to identify the hazards and to allow the implementation of appropriate safety measures.
As an aside: The “butterfly” effect!
When conducting a risk analysis, all circumstances have to be taken into consideration – this last example impressively shows that even small “bugs” can influence the explosion risk of plants:
Several saw mill operators carried out a risk analysis and decided not to protect their installations that handle wood chips with normally high humidity content. These conditions changed following a pine beetle infestation that led to numbers of dead/dried trees. After a long period of quarantine, these trees were purchased at a low price and brought to the sawmill. Due to the pine beetle infestation, the resulting wood chips lots were drier than usual and many explosions occurred in the saw mill plants, lead to long downtimes and several injured people.
According to TRGS 720 / TRBS 2152 “Hazardous explosive atmospheres”, the employer must determine and assess the risk of his employees as part of his obligations in accordance with the German Occupational Safety and Health Act [Arbeitsschutzgesetz] (including the Ordinance on Hazardous Substances [Gefahrstoffverordnung] and the Ordinance on Industrial Safety and Health [Betriebssicherheitsverordnung]) and implement the necessary safety measures. In accordance with this, he must check in the first stages of the hazard analysis whether there exist combustible materials and whether the formation of explosive atmospheres in hazardous quantities should be anticipated.
Explosion Prevention versus Explosion Protection
Although the legislative authority gives precedence explicitly to safety measures to avoid hazardous explosive atmospheres through substitute combustible materials, the experienced reader knows of the practical relevance of this preferred preventative measure. A baker simply needs flour and sugar to bake, a power station burns coal and sawdust naturally arises in chipboard factories. All these materials are capable of causes dust explosiv atmospheres. As a result, the explosion danger is essentially a given in all of the above examples.
So if hazardous explosive atmospheres cannot be safely prevented, the employer must assess the probability and duration of the occurrence of hazardous explosive atmospheres and the probability of the existence or arising of effective ignition sources. This stage of the assessment is commonly known in practice as “zoning” (see Table 1).
But what is frequently forgotten when implementing explosion safety measures in dust handling facilities, such as dust collectors, is the fact that the classification of hazardous places in terms of zones in accordance with TRGS 720 (1) 7. is ultimately only down to the so-called prevention of ignition sources.
Digression: Risk-based, probabilistic approach
In principle, the ignition prevention measures to be taken should make ignition sources ineffective or reduce the probability of it being effective. Consequently, the scope of explosion prevention measures complies with the probability of the occurrence of hazardous explosive atmospheres (zone). This probabilistic concept is based on the comparative assessment of the generally accepted residual risk (RREx), which arises from a combination of the severity (AS) and the probability of an explosion (PEx):
RREx = AS x PEx
In the case of an explosion, an unaccepted measure of damage is essentially anticipated. In consideration of the fact that the probability of an explosion is characterised by the probability of the existence of a hazardous explosive atmosphere (Pg.e.A) and the probability of the occurrence of an effective (of the thirteen in accordance with EN 1127) ignition source(s) (Pw.Z.),
PEx = Pg.e.A. x ∑ Pw.Z.
the following central requirement results:
RREx ~ Pg.e.A x ∑ Pw.Z. = const.
For this reason, in the practice of explosion protection, when applying the preventative measure of “Avoidance of Ignition Sources”, hazardous areas are only categorised into zones from these previous contexts in order to avoid ignition sources as follows
§ In zone 2 and 22: Ignition sources which can constantly or frequently occur.
§ In zone 1 and 21: As well as the ignition sources stated for zone 2 and 22, ignition sources which can occur occasionally, e.g. in foreseeable disturbances to a working material.
§ In zone 0 and 20: As well as the ignition sources stated for zone 1 and 21, ignition sources which can occur rarely.
By implication, this emphasises that the zoning is completely irrelevant in the case of the application of explosion protection measures, which reduce the effects of an explosion to an uncritical degree. The effects of an explosion in zone 20 are ultimately no more or less hazardous than those in zone 22.
In practice, for the aforementioned example of a dust collector system (see Fig. 1) which is protected with a flameless venting device and an explosion isolation flap valve, only measures to avoid ignition sources, but not to prevent ignition sources are obligatory. In the raw gas / dirty air section of the filter, which is normally classified as an hazardous place zone 20, also a rotary air lock of equipment category 3D could be used if this was also inspected and approved to be pressure shock resistant and flameproof. (Author’s comment: In all likelihood, most cases deal with identical devices, which are then only put onto the market with a different label).
However, a look into systems which are protected in practice shows that all (possible) stops are pulled out to apply preventative measures such as avoiding ignition sources, despite the existence of consequence-limiting measures.
In exaggerated terms, for dust collecting systems for example, in which often the (comparatively higher probability of) external ignition sources require measures of explosion protective measures, operators purchase and install any little explosion-proof equipment, even though the burst panel is fitted at the enclosure and already offers the legal safety level required. With regards to the comparably low probability of ignition within the design parameters of working equipment (see for example EN 13463-1 introduction), such “concepts” are reduced to absurdity. For example, a manufacturer recently applied for his silo discharge screws of equipment category 1D to be considered a unique selling point, although most of today’s silos are already protected by using explosion venting devices. So who does it surprise when the introductory cost-benefit issue of explosion safety is presented in light of such upwardly-forced investments?
It is beyond any question that only an “appropriate” mix of preventative and protective measures can lead to a consistent explosion safety concept. According to the interpretation of the author, the “freedom” of the designs of this “appropriate” explosion protection mix is meant in TRGS 720 / TRBS 2152, when the legislative authority speaks of “suitable combinations of preventative and constructive measures in accordance with expert judgement”. This interpretation is supported in the more precise interpretation of the European Directives 94/9/EC (ATEX 114) and 1999/92/EC (ATEX 153). According to these, all necessary measures must be taken to ensure that the workplace, the work equipment and the relevant connection devices are designed, constructed, assembled, installed, maintained and operated in a way to minimize the risk of explosions:
In view of equation 1, if the effects of an explosion are limited to an uncritical degree using explosion protective measures, an acceptable residual risk arises virtually independently of the probability of occurrence, with reference to the risk matrix, recognised by the professional industry and tried-and-tested in operational practice, of the VDI series of guidelines 2263 “Dust fires and dust explosions: Hazards, assessment, protective measures” (see Fig. 2).
What explosion protection can learn from explosion prevention
Although an explosion could essentially lead to catastrophic effects and death in any “zone”, similar to preventative explosion measure, in which the scope of measures is aligned as described to the “probability” (frequency and duration) of the occurrence of hazardous explosive atmospheres, the question of the requirement of a risk-oriented approach is raised in conclusion for protective explosion measures as well. The example of an system protected using explosion suppression, but the protective system of which was deactivated at the point of explosion, illustrates - if only in the approach - the necessity of such a reliability concept.
In the context, it becomes clear that a risk-oriented categorisation of protective explosion measures must also consequently occur with regards to the “probability” (frequency and duration) of the occurrence of effective ignition sources. In comparison with preventative explosion safety measures, with which an explosion is not permitted in principle, an impact-related categorisation must also take place, which considers the expected measure of damage.
A first approach to this is already stated by TRGS 721 / TRBS 2152-1, whereby the affected measures in “areas with explosion impacts exceeding the usual degree” in scope and type must be taken into account.
In areas, in which meeting places, corridors with dense traffic, residential buildings and larger office premises are in the hazardous area, only non-manipulatable or non-deactivatable, protective systems should be allowed to be used. Furthermore, with passive explosion protective systems, which are not normally installed and checked by the manufacturer, operators should consider the compliance with test requirements as per §§ 14 and 15 in connection with Appendix 4 Section A Number 3.8 of the BetrSichV.
On the basis of the experience of the author in the relevant expert committee activities, the development, coordination and validation of suitable assessment standards within the bodies of experts requires a considerable degree of work and time. For this reason, details of assessment standards for the categorisation of constructive protective measures and autonomous protective systems has not been entered into.
In this article, the contexts of preventative and protective explosion safety measures could be shown clearly, transparently and with regard to German and European legislation. It was comprehensively shown that an appropriate explosion safety concept, which is based predominantly on the use of protective measures (most-common example: explosion venting in connection with explosion isolated decoupling), permits the forgoing of additional preventative measures that become more cost-intensive. If ignition sources in explosion-prone systems cannot be avoided in operational practice with sufficient safety, then a safety-technical and economically reasonable combination of preventative and protective measures can be used according to professional discretion. In doing so, it is the operator’s responsibility to adjust the scope of preventative safety measures which purely reduce the probability of occurrence to their own requirements for a reliable yield and trouble-free value added.
North Rhine Westphalia’s Minister for Economic Affairs, Garrelt Duin (SPD), was greeted by REMBE®’s managing director Stefan Penno on site in Brilon on 27/08/2013. The Minister who has been in office since 2012, visited REMBE® headquarters together with Brilon’s mayor Franz Schrewe (SPD) and other SPD delegates. During discussions with Stefan Penno and the REMBE® workforce the Minister was not only shown through the shop floor but also wanted to learn about the details of bursting disc manufacturing. He showed great interest and appeared enthused by his visit to the company site and REMBE®’s innovative potential. REMBE® is investing in total around 2 million Euros in new production processes and warehouse systems as well as software at its Brilon headquarters. Structural expansion to accommodate the growth of the company is also planned.
Plenty of reasons for REMBE® to celebrate
More than ever has REMBE® the reason to celebrate. This year the annual REX convention again as with every year culminated with celebrations and this year it was to celebrate the company’s 40th birthday at the same time. From the 4th to the 6th of September 2013 REMBE® greeted 70 international participants from Europe, USA, Asia and South America at the REX convention, a most important meeting for all REMBE® safety experts who represent REMBE® and their bursting discs and explosion protection safety systems all over the world. There were presentations of the latest technical developments and customised global business strategies combined with hands-on sesssions. Of course with REMBE® things don’t end there. “REMBE®’s success story is very impressive” complimented Minister Duin during his visit. A success story that is certain to continue in the future.
This is guaranteed by a patent-pending, friction-free measurement of the centripetal force. The C-LEVER direct gravimetrically measures the product stream at the sensor without any mechanical components such as rotary belts, bearing or other moving parts. This offers a step-change in measuring accuracy of ± 0.2 % and a turn-down ratio of 20: Product characteristic changes have negligible adverse on the repeatable accuracy.
The steeply mounted gravimetric slide sensors prevent the product’s adhesion, which eliminates the zeropoint shift as a source of error. Costly pneumatic cleaning becomes history. The compact size means it is easier to install. Lower costs, fewer spares, less downtime.
Due to the seemingly endless sources for potential ignitions it is not hard to imagine the difficulty in eliminating them all. Removal of the fuel is impossible as this is what is being produced by the process in the plant that we are considering protecting. Removal of Oxygen is feasible but can only be achieved through vary expensive processing. The security limits here are also rapidly reached and the risk of a dust explosion cannot be excluded. Therefore a constructive and efficient explosion protection regime is required and essential.
NOTE* Contact KERSTING GMBH SAMPLING + GROUNDING, a REMBE® ALLIANCE company and refer to 'Grounding Systems
Protection is crucial.
Safety valves protect the refinery processes against hazardous overpressures. These measures are crucial as high temperature differences could result in major pressure fluctuations taking place in the plants.
Safety valves: Pros and cons.
Safety valves provide numerous advantages, opening when pressure becomes too high and automatically closing after venting. However, an immense drawback is that safety valves are very maintenance-intensive and present the disadvantage of not providing liquid tightness. In turn, this results in huge material wastage and rising emission rates.
“It’s the volume that counts.”
“When operating several large plants worldwide, it’s the volume that counts”, says Stefan Penno, Managing Director of REMBE® GMBH SAFETY+CONTROL: “Incessant material wastage weakens the value-added chain, pollutes the environment and demands increased maintenance. Now that’s costly.”
Economic damage and environmental contamination.
A study conducted by the Netherlands Organization for Applied Scientific Research (TNO) substantiates this conclusion: 22-27% of all leakages, defects in liquid tightness result from flange connections. Just in the Netherlands this results in an annual raw material loss of approximately 106,000 tons of gases and liquids. With a calculated average price of 700 EUR/ton, this results in a loss of 75.000,00 EUR/year. When adding the leakage losses resulting from shaft sealing, this makes an additional 383.000 tons/year resulting in an amazing total of 268.000.000.00 EUR for the Netherlands alone. The American Environmental Protection Agency (EPA) has discovered that 40.000 tons of volatile organic compounds (VOCs) alone were annually emitted in the USA. Besides the economic damage, the environmental contamination issue should not be ignored.
Robust bursting disc for sensitive process areas: KUB®
REMBE®, the specialists in process safety and explosion protection for nearly forty years, recommend all refinery operators to revise their opinions. “The sole implementation of safety valves doesn’t make sense in every case. There are certain sensitive process areas, where you are truly better advised to implement robust bursting discs”, states Penno further. REMBE® has specialised in laser technology, and developed a unique two-layered REMBE® Reverse acting buckling-pin bursting disc (KUB®). Its utilisation in those types of demanding processes has successfully proven itself for years.
The BT-KUB® mechanism is based on the buckling-pin principle by Leonard Euler. As opposed to other bursting disc manufacturers, REMBE® waives any mechanical scoring. This is due to the fact that response pressures are accurately defined by Euler’s buckling-pins, which are positioned by state-of-the-art laser technology. "Once our bursting disc is installed, it needs no further maintenance and reliably safeguards the process/plant. Only in the event that the disc has responded, must it be replaced; the timeframe can vary, after a year, a decade or never”, Penno explains. “In light of recent events, it would be wise to inspect the process safety and efficiency of existing as well as newly constructed refinery plants. By installing supplementary bursting discs, money can be saved long-term and thus, counteracts bankruptcy; at least in regard to operating and maintenance costs.”
“Sauerland secures Nutella production”
One example of this came from Andreas Hansen and Dr. Francesco Petruzzelli, employees at REMBE®’s Italian subsidary, who described competently and with Italian spirit how they managed to win an important contract with the multinational company Ferrero: “Mamma mia,” both exclaimed, “the Sauerland has secured Nutella production!” Outside, REMBE® engineer Roland Bunse demonstrated high explosives to an amazed audience of visitors. After the exciting explosions he remarked impassively: “Well that certainly wouldn’t have happened with a REMBE® bursting disc.”
Safety guaranteed by tradition.
Managing Director Stefan Penno emphasises how important the location of Brilon is for the company’s international success: “Our Sauerland roots represent the security that we guarantee our customers.” The fact that production takes place exclusively at the German plant in Brilon sets this traditional company apart from its competitors. REMBE® China manager Ben Liang is convinced: “The ‘Made in Germany’ stamp of quality is a door-opener, especially in Asia.”
Yet this sense of tradition is only one side to REMBE®’s success. Because we in the Sauerland don’t just know where we come from, we know where we’re going too. It is here that our REMBE® engineers are working diligently on new innovations. It comes as no surprise, then, that there have been attempts to copy patented REMBE® inventions abroad, to no avail of course. Friedhelm Kesting has been the plant manager and REMBE® employee for more than 35 years. He perhaps best represents the combination of groundedness and international vision typical of REMBE®. He confidently leads the international REX delegation through the factory building in Brilon, proudly showing the brand new laser equipment, custom-built for REMBE®’s specific needs, introducing workers to the guests, and answering questions. Friedhelm does all of this with typical Sauerland aplomb, and all in English. To sum up the REX convention: everyone got along wonderfully. And it wasn’t just the Chinese visitors who continually praised the climate: “The fresh air here is simply marvellous!”