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First of all, an explosion is nothing but rapid burning up of a material within a few milliseconds. It is the reactive forces released by this, which spread into the surrounding area in the form of flame jets and shock waves that are especially dangerous.
The process occurs at a very high speed. Controlled action in the event of an explosion is therefore almost impossible, in contrast to a fire incident, which takes place relatively slowly. In this case, fast and correct intervention often prevents anything worse from happening.
If the bases for explosion protection are considered, only three preconditions have to be satisfied to cause a dust explosion:
On the one hand, a flammable medium in dust form, which is dispersed into a cloud, is necessary;
- On the other hand, oxygen must be present, in order that, with the dust, it can create an atmosphere that can cause an explosion;
-Finally, a small source of ignition is sufficient to ignite the mixture of air and dust. In this way, a fireball that is extremely dangerous for the area surrounding it can develop.
Preventing dust explosions
There are different approaches for preventing dust explosions. These are all aimed at reducing one of the three afore-mentioned factors, in order to minimise the probability of an atmosphere capable of holding dust in it, or of a dust explosion. Examples of these preventive measures are the inertisation of equipment and the avoidance of active sources of ignition. However, regular maintenance of equipment and machines and keeping surfaces clean also reduce the risk of a dust explosion.
In practice, however, there are numerous applications, in which one of the three factors cannot be reduced, or not to the necessary extent. The risk of an explosion cannot be absolutely excluded, even when all possible safety measures have been implemented. For this reason, it is important to make provisions for reducing the effects to a tolerable level in the event of an explosion. These provisions can be summarised under the general heading "constructive explosion protection".
Protective measures against explosion can be implemented in different ways. The most current methods are listed below:
-; Explosion-resistant method of construction: when implementing this, a facility must be constructed in such a stable way that it can resist an excess pressure of at least 10 bars.
Explosion suppression: in the event of an explosion this will be detected in the shortest time possible. An extinguishing agent will then be blown towards the explosion, smothering the flames; in this way the dust explosion is suppressed.
- Explosion venting: bursting discs divert the shock waves and flame jets resulting from a dust explosion safely into the surrounding area. The bursting disc is designed and manufactured in accordance with the process conditions prevalent in the facility. At a specific pressure, as low as possible, the bursting disc opens and releases its cross section, the so-called venting area. In doing so, this venting area must be of an adequate size to safely allow an explosion to escape.
- Flameless pressure relief: If equipment or components are situated inside the buildings or in areas, in which a free pressure relief and the effects associated with it would cause damage, a flameless pressure relief is used. This way, the shock waves and flame jets are held back by a flame-trap affixed to the upper side of the bursting disc, so that there is no danger to the surrounding areas.
- Explosion decoupling: in the case of an explosion, the components of the equipment are decoupled from one another and from the remaining process; e.g. by non-return explosion valves. It is ensured that that the explosion does not propel the flame into equipment, which is upstream or downstream. The dust situated there would otherwise ignite and therefore trigger a secondary explosion.
It is essential to know the rim parameters and process conditions as precisely as possible for the construction of the explosion protection. It is necessary, therefore, for example, for the calculation of the required venting areas, to take account of the data regarding the equipment to be protected and of the exact key data about the existing material.
Intelligent pressure relief
A free pressure relief can only be guided into free, secured areas, due to the shock wave and the flame jet. In-facility traffic routes, close building developments or even public roads pose a problem time and again in the planning and implementation of such protection concepts. Operators often have no option but to determine very large areas at risk with safety distances around the installation for pressure relief. This additional expense means increased operating costs for the operator. The areas designated as safety areas may also no longer be used operationally, as entering them is extremely dangerous and is not permitted. It is precisely those traffic routes, which are used in-facility and also publicly, where a free pressure relief cannot therefore be implemented. Flameless pressure relief or explosion suppression is ideal protection concepts for keeping the effects of an explosion as low as possible. For many facility operators, however, a controlled pressure relief is a practicable alternative.
Problematic interpretation of standard specification
The diversion of the flames and shock waves into uncritical areas is addressed, amongst others, in Standard EN 14491. In Annex E of it, the use of so-called deflector plates is referred to. These are affixed in such a way that the effects of the explosion are diverted at an angle of 45° to approx. 60°.
The Standard does not mention, however, that the deflector plates must be constructed and fastened at great expense due to the tremendous repulsive forces. For a container of 20 m3, these forces amount to 50 kN (5 to), with an impetus of 25 kNs. In addition, these elements represent an enormous barrier during normal operation and take up valuable operational space.
New type of pressure relief
Against this background, the Brilon company, Rembe, has developed a new type of pressure relief. Targo-Vent is an opening angle delimiter, developed especially for bursting discs, which diverts the pressure relief into defined areas.
Targo-Vent cushions the bursting disc dynamically, progressively and can, therefore, elastically absorb large kinetic forces itself. The damper absorbs the enormous repulsive forces of the energy from the explosion and guides the explosion flames in the required direction. Depending on the conditions of application the flame is diverted upwards at a defined angle of approx. 30° to 45°. In this way, the shock wave and the flame jets are diverted into uncritical areas. This, in turn, allows the operator to minimise the safety areas around the vent openings and guarantees safe use of traffic routes. A further advantage in daily practice is that during normal operation Targo-Vent is assembled in a space-saving way on the bursting disc. No expensive baseplates or reinforcements are required. The system itself is made of maintenance-free stainless steel and does not cause any further current expenses.
Targo-Vent is manufactured in all current bursting disc sizes, so that retrofitting in existing facilities can be implemented without any problems. The system, in combination with Rembe bursting discs and venting panels has been type-tested and authorised in accordance with the Atex Directive 94/9/EC (Atex 114) (FSA 13 Atex 1637). Targo-Vent is therefore suitable both as the initial equipment for new bursting discs and as retrofitting component for installations already existing. The user receives an intelligent explosion protection, which involves neither high costs nor complicated fitting. Shock waves and flame jets, which arise during a dust explosion, are guided into the right channels, so that people and machines are protected from devastating damages.
read more about TARGO-VENT
Mike MacClancy Rembe Ltd managing director explained why the company had made this move: “Since incorporating Rembe Ltd in August 2009, Rembe’s UK business in bulk solids explosion protection, flow control and process pressure relief has steadily grown in all areas and applications by offering an unprecedented level of service to all customers. We keep extensive stocks of an ever increasing range of products for fast response deliveries; 48 hours door-to-door at no additional premium charge is now becoming something of a daily occurrence. Servicing and commissioning is also now regularly undertaken from a growing mature staff. Our previous facility was simply inadequate for our needs.” Colworth Science Park is the former headquarters of Unilever and is still used by Unilever R&D. The new premises offer high-speed fibre optic web connection and many state-of-the-art business facilities. Rembe Ltd’s central London offices are being retained.
The exhibition allows customers in the Middle East the opportunity to meet with REMBE® and discuss how the REMBE® superior laser technology can help their organisation to improve process reliability and reduce operational costs.
REMBE® Business Development Director, Orhan Karagöz, said “The Middle East and Africa are important growing markets for REMBE® and exhibiting at ADIPEC is key to that growth. I’m pleased to say it’s been another successful event for REMBE® and we are already starting our preparations for next year”
ADIPEC 2014 takes place in Abu Dhabi from 10th – 13th November.
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.”