Things, or better facts, exist that have a high durability even today. Explosion protection is such a subject, because it is based on unchangeable physical laws for the origination of an explosion.
Mine damps – that is what they were called by miners. These mixtures of methane and air are generated when mining coal. They are explosive in certain mixing ratios. Through the second half of the 19th century it was common to flare off mine damps – by simply bringing them to an explosion in a controlled manner. It was a life-threatening task for the miners. This is fortunately no longer required due to a number of technical achievements and protection regulations. It is no coincidence that explosion protection had its origin in mining. However, it is now also applied in other industries, because explosive materials are also present in their processes. Common examples include the chemical industry, during the production of crude oil or natural gas or in the food industry.
These materials generate a “hazardous explosive atmosphere” in combination with oxygen. If a hot surface or an electrical ignition spark is added for the ignition, then a case develops 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 legislation based on it have now become well-established in Europe: the ATEX directives (Atmosphere explosible). These include the 1999/92/EG for plant operators and the 2014/34/EU (previously 94/9/EG) for equipment manufacturers.
The most important equivalents of the European ATEX on the American market are the appropriate articles for the “Hazardous classified locations“ (HazLoc) of NEC and CEC and the EAC Ex. Other important regulations include the EAC conformity process (Eurasian Conformity) for Russia, Kazakhstan, Belarus, which replaces the old GOST import processes and which is very similar to the ATEX and CE.
As a matter of principle, a differentiation is 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 the tertiary explosion protection are used to limit the impacts of an explosion to a harmless level. 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 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 it is specified in 1999/92/EG. A classification into “Divisions” is often found in the U.S. and Canada.
The zone model classifies plant areas depending on their hazards into the Zone 0, 1 and 2 for gas atmospheres and 20, 21 and 22 for dust atmospheres. As part of a risk analysis, the plant operator must assess how often, and for what time periods, explosive atmospheres can occur in the different areas of a plant. Accordingly, the operator must classify her/his plant into these Zones. Zone 0 or Zone 20 are in this case the most dangerous ones.
All devices, which will be used for explosive atmospheres in Europe in the 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 marking includes the required information for the use 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 firedamp endangered mine workings (I) in category 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 shows the Zone in which the equipment can be used. In addition, the identification marking includes information about the type of protection, for the gas or dust group and the temperature class if the device was tested in accordance with a standard.
As a matter of principle, several options exist to prevent an explosion. They were diligently developed during the last decades and considered in the respective standards. Different types of protection were defined for electrical equipment. However, certain types of protection are not qualified 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.
The protection type is based on the principle of energy limitation: Current, voltage and power values of an electric circuit, which get into the explosive area during measurements and controls, must be low enough that they cannot generate sparks or get too hot. The intrinsically safe electrical circuits therefore consists of the intrinsically safe equipment and the associated equipment. The latter is installed outside the Ex-Zones.
This means for our example with the oil tank that: First, the intrinsically safe equipment, namely the sensor for the pressure switch, must be qualified for the installation within Zone 1. Second, the following device and the associated equipment, which is connected to the sensor must ensure that the energy that gets to the sensor is not too high to keep it cool. The level of permitted heating depends on the composition of the explosive atmosphere. Which means, it depends how high or low the ignition temperature of the existing gas is. 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 exist 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 is 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 justifiable 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”. As all other documents, it must be filed in the explosion protection document.
The calculation with reduced capacities and inductances is special in this comparison. It can be performed in a simplified manner. On request, WAGO provides a calculation tool free of charge for the verification of the intrinsic safety. The calculation can be saved or it can be attached as a printout to the explosion protection document.
It is important to mark the intrinsically safe electric circuit. For example, through a labeling of all components and devices participating in the intrinsically safe electric circuit. If a color-based marking is selected, then the standard requires light blue.
Two Ignition Protection Categories
Many devices, especially so-called associated electrical equipment, which must normally be installed outside of the ex-proof area, often have an additional approval for the installation in Zone 2.
Text: MARLIES GERSTKÄMPER-OEVERMANN | WAGO
Photo: ©ISTOCK.COM, WAGO