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Technologies And Machines In Explosive Atmospheres

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hes april 18 16Technologies And Machines In Explosive Atmospheres

In everyday language, we may understand several things and physical processes under the word explosion. While the explosion of a heated boiler may cause similar or sometimes even bigger damage than a rapid fire, in we still distinguish between chemical and physical explosions. (Read More)

 

While the latter is a rapid physical property change of a material (immediate change from water to steam with extreme volume expansion), chemical explosions are basically an extreme rapid generation of fire (with propagation speed above 100 m/s).

Explosion-protection deals with the prevention of chemical explosions. Therefore, as the elimination or reduction of an unwanted oxidation process, explosion protection has naturally the same basic principles as the basics of fire protection. An explosion takes place if all of the following three factros are present at the same time on the same place.

  • Explosive material
  • Oxidizing material (simplest is the oxygen in the air)
  • Effective source of ignition (which can be open fire, electrical or mechanical spark or even the outer surface of an overheated machine part)

In order to minimize the risk of explosion, we need to eliminate the thee factors from the above triangle. It is well known, that mixtures of gases have an upper and lower explosive limit, i.e. an explosion can only take place if the air-mixture ratio lies within these limits. Hence, if the mixture is richer or leaner, the technology remains safe even if there are ignition sources present. Another way is to replace or limit the amount of oxygen by adding nitrogen, carbon-dioxide or noble gases to the technology. Again, this will keep the air-mixture ratio within the safe limits, while requiring proper sealings, proper gas-sensing and adequate protection system to react upon malfunction. On the other hand, proper selection of the process materials and thus replacing explosive substances with non-explosive ones will need to be considered and chosen when appropriate. The standard EN 1127-1:2011 defines these measures as primary prevention.

It is well understood that substitution, inerting or limitation of concentration may be impractical in several processes or technologies. Therefore, another risk reduction method is necessary. Secondary prevention deals with the elimination and control of effective ignition sources. The EN 1127-1:2011 defines 13 different types of possible ignition sources:

  • Hot surfaces
  • Flames and hot gases (including hot particles)
  • Mechanically generated sparks
  • Electrical apparatus
  • Stray electric currents, cathodic corrosion protection
  • Static electricity
  • Lightning
  • Radio frequency (RF) electromagnetic waves from 10 kHz to 300 GHz
  • Electromagnetic waves from 300 GHz to 3 PHz (3x10^15 Hz)
  • Ionizing radiation
  • Ultrasonics
  • Adiabatic compression and shock waves
  • Exothermic reactions, including self-ignition of dusts

It is important to mention that in the case of electrical apparatus, the standard explicitly declares that an extra low voltage (ELV, e.g. less than 50 V) equipment is designed for personal protection against electric shock and is not a measure aimed at explosion protection. Electrical sparks generated at even lower voltages can have sufficient energy to ignite an explosive mixture. It is requested that electrical apparatus shall be designed, constructed, installed and maintained in accordance with the relevant standards.

Under industrial conditions (which means all applications excluding subsurface mining), the IEC and thus the European system defines three risk levels for potentially explosive areas:

  • Zone 0 (for combustible dust Zone 20): explosive atmosphere is present under normal operation for long time or permanently (>1000 hours/year)
  • Zone 1 (for combustible dust Zone 21):  explosive atmosphere may possibly present frequently or for longer times under normal operations (10…1000 hours/year)
  • Zone 2 (for combustible dust Zone 22): explosive atmosphere is present rarely, for short time and under malfunction (<10 hours/year)

It is an accepted way to categorize a workplace to Zone 1 / 21 where employees spend more than 50% of their working time. It must be mentioned that the North American system (NEC 500) defines only two “divisions” for hazardous locations. This may sound as an easier approach, but it also has severe (financial) consequences for owners and operators of hazardous technologies and plants.

The question is, what type of equipment may be operated in which zone. The answer comes from various sources. In the European Union, the ATEX (2014/34/EU) directive and its harmonized standards (several books from EN 60079 and EN 80079) define the requirements against equipment to be used in potentially explosive atmospheres. Countries accepting the IEC regulations have very similar systems. According to this system, for locatons with the highest risk of possible explosion, equipment must be of the highest level of protection. This is designated in the ATEX with II 1 GD category, or very similarly in the IEC system with EPL Ga or Da. Such equipment has adequate protection that even under two simultaneous malfunctions, they cannot pose any risk of ignition. The next protection level (II 2 GD or EPL Gb / Db) is usable everywhere except from Zone 0/20. They are constructed in a way to remain free from ignition sources under one possible malfunction.  For equipment to be operated in Zone 2/22 (category II 3GD or EPL Gc / Dc), the requirement is that they do not present any source of ignition under normal operation.

It is usually not an easy task to decide if any equipment may pose a risk of possible ignition either in normal operation or even under several simultaneous faults. The ATEX requires manufacturers to carry out conformity assessment procedures to their products and in case of serial or batch production also to their manufacturing processes. Depending on the equipment category (hazardous location rating) and level of possible risk, an involvement of an external institute (Notified Body) becomes necessary.

The certification requirement is valid for both electrical and non-electrical (i.e. machines with mechanical functionality independent of their drive) equipment and the basis of the certification must be a proper and detailed technical documentation including a comprehensive risk analysis.

There are optional ways to complete this process, a Notified Body can also be a partner to find the most suitable way for the assessment.
In theory, the conformity assessment is task of the equipment or machine manufacturer. Theory might sound easy, but reality poses problems: some machines have no known manufacturer, after several years or decades of operation, they no longer have the minimum required documentation available. Despite periodic maintenance, not all maintenance reports are to be found and several replaced sensors may not be identical with the original built-in devices.

Very often, such non-conformities are discovered only after technology upgrade or modification, which causes a start-up delay for several weeks or months.

For the operator or site owner, this may be an unnecessary barrier interfering profit-oriented operation, but it is important to know that he takes the biggest risk: he operates the hazardous technology, his employees work with hazardous substances, he has the highest responsibility for ensuring safe workplace and healthy conditions. In order to fill in the gaps of missing information and required documentation or invalid protection due to improper maintenance, the site owner may naturally choose to become equipment manufacturer.

How can a Conformity Assessment Institute (Notified Body) help the manufacturer? As we have seen before, in several cases a Certification Institute needs to be involved in the conformity assessment process for machines intended to be operated in hazardous locations. As the name says, a Certification Institute issues a Certificate of Conformity for products or processes. They cannot be directly involved in the product development or design! However, they can serve as an external assessor to evaluate if the product meets the relevant sections of the standards and if not, to point to its weaknesses. This helps designers and manufacturers not having full and comprehensive knowledge on the field of explosion protection to avoid conformity assessment procedures ending with negative results (and thus either resulting in unusable product or requiring further modification/improvement/development over the planned budget). And even more, it prevents manufacturers from having mistakenly identify their own products as ‘conform’ (for category II 3GD equipment) and install them with possible sources of ignition in potentially explosive locations!

When does the operator become manufacturer? All the above seems easy to follow until the purchased machine is used without further modifications: the original manufacturer creates the design, manufactures the necessary parts and assembles them into a working machine, submits the required documentation for approval and takes the responsibilities of a manufacturer. By means of the laws, however, the operator becomes manufacturer – and takes all of his responsibility – upon major modification either regarding the original functionality or of the method of protection on the machine (which might even mean just a replacement of a sensor or limit switch by something different to what the original manufacturer built in). In this situation, the complete modified machine needs to be documented according to the requirements posed by the standards and must be assessed as a whole entity together with its new operating and safety manuals, new marking (i.e. new data plates with the new manufacturer’s name and address) and protection type and level. For the cases illustrated in the Table before, it even must be re-certified by a Notified Body before taking into operation again.

In case of further interest, we are glad to support your request for hazardous area compliance in terms of product and site in general.

Mr. Árpád Veress
MD of ExNB Certification Institute
(NB 2684)
+36309660223
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
http://www.exnb.eu/