Explosion Protection for Humans and Machines

The topic of explosion protection originated in mining. Mixtures of methane and air that arise in coal mining and are explosive in a certain ratio were handled with controlled explosions until the second half of the century. But how does it work now?

Flaring off firedamp is no longer necessary due to a number of technical achievements and protection regulations. However, the topic of explosion protection has not lost its importance despite all this. It is now widespread not only in mining, but also in other industries, because explosive materials are also present there. Common examples include the chemical industry, during the production of crude oil or natural gas and in the food industry.

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An Explosive Mixture

Combined with oxygen, these substances create a “dangerous explosive atmosphere.” If hot surface or an electrical ignition spark occur, this quickly leads to a situation that must be prevented under all circumstances. This is because such an event has the potential to directly harm many people, not to mention the impacts on the environment or the production systems. Therefore, appropriate member states’ directives and the legislation based on it have now become well-established in Europe: the ATEX directives (Atmosphere explosible). These include the 1999/92/EC for plant operators and the 2014/34/EU (previously 94/9/EC) for equipment manufacturers. The most important equivalents of Europe’s ATEX on the American market are the appropriate articles for the “Hazardous classified locations” (HazLoc) of the NEC and CEC and the EAC Ex. Other important regulations include the EAC conformity process (Eurasian Conformity) for Russia, Kazakhstan and Belarus, which replaces the old GOST import processes and is very similar to the ATEX and CE.

Explosion Protection

A distinction is generally made between primary, secondary and tertiary explosion protection. The measures of the primary explosion protection are aimed at preventing or restricting the generation of explosive atmospheres. Secondary explosion protection measures are used to prevent the ignition of explosive atmospheres – i.e. to prevent potential ignition sources. The measures of tertiary explosion protection are used to mitigate impacts of an explosion, bringing them to near-harmless levels. As part of an hazard assessment, which must be performed by each plant operator, the operator must ask if – as part of the primary explosion protection – it is possible to replace potentially explosive material to prevent an explosion in the first place. If this is not possible, then the plant operator is asked to classify the plant depending on the hazard and to mark the access. The zone model is the most used method worldwide and is specified in 1999/92/EC. A classification into “Divisions” is often found in the U.S. and Canada.

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Explosive atmospheres are always present inside a tank – it is therefore an area of Zone 0. Control valves or exhaust vents are classified as Zone 1. Explosive atmospheres can occur here during normal operation. This does not normally occur in Zone 2 and, if it does, then only momentarily.

Ex Zones

The zone model classifies plant areas based on hazards present into Zones 0, 1 and 2 for gas atmospheres and 20, 21 and 22 for dusty atmospheres. As part of risk analysis, the plant operator must assess how often and for how long explosive atmospheres can occur in the different areas of a plant. Accordingly, the operator must divide her/his plant into these Zones. Zone 0 and Zone 20 are in this case the most dangerous ones (Fig.: Zone Description Table). Example: The Zone classification for a tank that is filled with liquid crude oil and includes a pressure switch might look as shown in our illustration.

Device Selection

All devices that will be used in Europe for explosive atmospheres in Zones 0 and 1 or 20 and 21 must be certified by a recognized body and must include a mark that is listed in the type test certification. This identification mark includes the required information for usage in explosive areas. It provides information about the equipment group and the category. With respect to the equipment group, operating resources are divided into two groups: devices for use in mines susceptible to firedamp (I) in categories M1 and M2, and devices for all other applications (II) in the categories 1, 2 and 3, with the appendix G for gas and D for dust. The category indicates the Zone in which the equipment can be used. In addition, the identification marking includes information about the type of protection, the gas or dust group and the temperature class if the device was tested in accordance with a standard. Several options generally exist to prevent an explosion. These were carefully developed during the last decades and taken into account in the respective standards. Different types of protection were defined for electrical equipment. However, certain types of protection are not appropriate for all zones. The Ex n type of protection, for example, can only be used in Zone 2. The Ex i type of protection (intrinsic safety), in contrast, is approved for equipment up to Zone 0. We are selecting intrinsic safety for our example. It is one of the most favored and widely used types of protection.

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Intrinsic Safety

The type of protection is based on the principle of energy limitation: Current, voltage and power values of an electric circuit, which get into the explosive area during measurement and control, must be low enough that they cannot generate sparks or get too hot. The intrinsically safe electrical circuits therefore consist of the intrinsically safe equipment and the associated equipment. The latter is installed outside the Ex zones. What this means for our example with the oil tank is: First, the intrinsically safe equipment, namely the sensor for the pressure switch, must be appropriate for installation within Zone 1. Second, the following device and the associated equipment that is connected to the sensor must ensure that no more energy reaches the sensor than it can handle without heating up. The level of permitted heating depends on the composition of the explosive atmosphere, which means it depends how high or low the ignition temperature is of the gas used. In addition, the energy limitation must ensure that no ignition spark can be generated, or that a possible ignition spark remains below the ignition energy of the gas used.

Verification of Intrinsic Safety

An additional major aspect must be considered for the “intrinsic safety” type of protection: the technical safety data. This exists for the intrinsically safe operating resources and the associated equipment. The information about Ui, Ii, Pi, Ci and Li discloses the maximum values that a device can absorb at the input without the risk that the protective function of the intrinsically safe electrical circuit will be nullified. The Uo, Io, Po , Co and Lo information indicates the maximum output values of the equipment. The comparison is used to ensure that no significant ignition sparks are generated and that the surface of the intrinsically safe equipment does not get hotter than acceptable for the approved used. The values must be compared to each other. The condition that is shown in the figure “Intrinsic Safety and Verification” is applicable in this case. This comparison is called “Verification of the Intrinsic Safety”. Like all other documents, it must be filed in the explosion protection document (see figure below).

WDP_116_Vorsicht explosiv_Ex-Zonen_02.png

Intrinsic Safety and Verification

Two Ignition Protection Categories

Many devices, especially so-called associated electrical equipment, which must normally be installed outside of the Ex area, often have an additional approval for installation in Zone 2. The markings then look like this:

Example of a marking for installation in Zone 2:

Ex II 3 G Ex nA IIC T4 Gc

Example of intrinsic safety marking, which means that the device may be connected with an intrinsically safe sensor or actuator in the Ex area if the “Verification of the intrinsic safety” permits this:

Ex II (1) G [Ex ia Ga] IIC

New ATEX Directive 2014/34/EU

Since April 20, 2016 the new ATEX directive 2014/34/EU has been valid. It includes new requirements with respect to the application areas, the concept, the provision on the market, the accreditation of the test bodies and market monitoring. The obligations of the manufacturer with respect to the equipment marking and the user information were expanded. The new ATEX directive was implemented as a national law with the new explosion protection product regulation dated January 6, 2016. “As a general rule, the new ATEX directive 2014/34/EU is an adaptation of the directive to the rather formal requirements of decision No. 768/2008/EC, without substantially changing the directive compared to the old ATEX directive 94/9/EC,”, according to the Berufsgenossenschaft Rohstoffe and chemische Industrie (BG RCI) [Mutual Indemnity Association of the raw materials and chemical industry]. The good news: All devices that were certified before April 19, 2016 are not subject to a new test. The 2014/34/EU directive only applies to new products.

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New, Unambiguous Regulation

In addition, it must be noted that plant operators can become “manufacturers” under certain circumstances and are therefore also subject to the new ATEX directive 2014/34/EU. This refers to the in-house production, which was in the past only viewed as installation. The gray areas in the old directive are more clearly regulated in the new one. According to PTB, all areas that could be theoretically sold because they are, for example, mobile must be viewed as in-house production. In addition to the hazard assessment and the explosion protection document, this also requires an EC conformity declaration and an EC type test certificate. The new explosion protection regulation defines §5 “General obligations of the manufacturer” this way: “If the manufacturer markets products or if the manufacturer uses them first for its own purposes, then the manufacturer shall ensure that they are designed and produced in accordance with the major health and safety requirements per appendix II of the 2014/34/EU directive.” This means that the respective documents must be complete before commissioning. The new ATEX directive specifies higher requirements for “recognized” bodies, with respect to test options and equipment, among other things, which must now be verified in Brussels. In addition, stricter requirements were established for market monitoring. The market supervisory authority will be strengthened. Each economic actor, including the dealer, must provide information to the market supervisory authority about from whom the product was purchased and to whom it was sold.

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