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Press Releases The ATEX 95 Guideline The ATEX guideline 94/9/EC makes a manufacturer responsible for analysing whether its product will be suitable be use in a potentially explosive environment and whether that product has its own potential ignition sources. Table 1 describes the environment and allocation to equipment category. Equipment Group I is for mining applications. For non-mining applications Equipment Groups IIA, IIB and IIC apply. For the description of an explosive gas atmosphere Table 2 shows the temperature classes that apply. It should be noted that for dusty explosive environments the maximum surface temperature, which is marked on the equipment, has to be lower than 2/3rds of the ignition temperature of the dust. Allocation of equipment to a particular category is based on risk assessment results, to which parallel measurements and tests must be carried out to quantify risk areas such as surface temperatures. Individual elements of the risk assessment are supplemented by experiences from customer discussions or after-sales procedures. Risk assessments must contain hazard identification risk estimation, plus risk evaluation and reduction options analysis. Some instances of equipment abuse may not lead to ignition danger, and for some mechanical devices in categories 2 and 3 (see Table 1) and electrical devices in category 3 most evaluations can be carried out and certified with a declaration of conformity by the manufacturer. For category 1 mechanical devices and category 2 electrical devices the equipment's suitability has to be stated with EC-type examination certification by a named body (see ATEX guidelines for these). Apart from the fundamental suitability of the equipment further steps described in 94/9/EC have to be carried out. These are:
Danger of Ignition by Flexible Couplings Shaft couplings have the primary task of transferring energy as mechanical power in the form of torque and rotation speed. Couplings are designed to connect machines with varying shaft misalignment. The use of rigid couplings within a shaft misalignment assembly would lead to relatively high bearing loads on the connected machines. Torsional elastic couplings are therefore commonly used to connect the electric motor to a centrifugal pump. Pump drives are often used in the chemical and petrochemical industry in explosive environments, so the pump drive and coupling must be regularly checked for suitability. So far, no flexible coupling constructions have been identified that are free from the danger of potential ignition sources. Flexible couplings used in explosive environments therefore fall clearly into the range of applications under the 94/9/EC guideline and are defined as equipment within the machine guideline 98/37/EC. With the Neupex plug-in coupling (see Picture 1) the torque is transferred via elastomer flexibles located at the circumference. There are spaces between the flexibles into which the fingers of the opposite coupling engage to form the drive. The elastomer flexibles are made out of Perbunan and the metal parts are manufactured from GG 25 material. These couplings are additionally manufactured with spacer tubes or flange connections. Ignition sources of flexible couplings used in explosive environments are indicated in the 94/9/EC guideline. For the Neupex coupling the following ignition sources are to be considered for normal and special operating conditions: Hot Surfaces
At the Flender test centre the coupling temperature was tested with a thermal image camera and thermometers to measure the surface temperature for normal operating conditions and predicted overload conditions. Two sizes for each coupling design were examined in combination with the available elastomer flexibles. Loads were gradually increased until in extreme cases the following loads were applied:
The temperature rise was measured at low and high coupling speed and at individual and variable load combinations. In relation to the ambient temperature a maximum temperature rise was measured as 20°C. During conscious destruction of the elastomer flexibles a maximum temperature rise was likewise measured as 20°C. If the standard design of Neupex coupling is operated with metal-to-metal contact, abrasion erosion takes place, resulting in higher running temperature when the two halves operate at different relative speeds. With this operating condition, inadmissibly high temperatures and strong sparking could arise. In order to determine if any wear of the elastomer flexibles takes place, a suitable inspection interval must be maintained. Only through the planned maintenance and the exchange of worn flexibles can the coupling be prevented from becoming an ignition source. Full details to ensure the correct operation of the Neupex coupling are given in the Flender operating manual. With Neupex type DS coupling, no metallic contact arises after elastomer flexible wear has taken place, so that in this situation there is no danger of ignition. Regular examinations of the type DS coupling are not necessary either. Misalignment of the coupling parts could be caused by movement of the connected machines after slackening of the foundation bolts and would therefore be very rare. Sparking To investigate sparking risk the Neupex coupling was operated without the elastomer flexibles in an explosive atmosphere provided by combining H2 and air. With metal-metal contact and components operating at the same speed, no sparking was observed. Sparking could be seen when the coupling parts rotated at high relative speed between the two coupling parts. Static Electricity To evaluate static electricity danger, the elastomer flexibles and paint from the Neupex coupling were examined. With Perbunan flexible a possible static charge flows, since the electrical resistance is sufficiently small. In Neupex couplings using DS Perbunan or Polyurethane, the elastomer flexibles made of PU have a very high electrical resistance. They could not however, be electrostatically loaded as tests by ion bombardment showed. For paint in the explosion group IIA and IIB a thickness of 2mm is allowable and for explosion group IIC a thickness of 0.2mm is allowable. Results of Risk Assessment and Experiments At the Flender test centre a risk assessment based on the pattern in Table 4 was carried out, with participation from the engineering, quality and after-sales departments. Based on the risk assessment and test results the limits of the Neupex coupling for explosive environments were defined. For non-mining applications the Neupex coupling can be used in category 2 and 3 environments (zones 1 and 2). Dependent on the maximum ambient temperature, temperature classes T4 to T6 are permissible. For mining applications the Neupex coupling will fulfil category M2. Flender certifies the conformity with the necessary declaration and additional certificate issued by a named body can be provided where these are required. The outside diameter of the coupling can be CE-marked where required. The marking of the Neupex coupling is accomplished by customised machining so the couplings can be delivered in accordance with guideline 94/9/EC within a few days. As well as Neupex couplings, Rupex pin and bush couplings and all-steel Arpex couplings can also be delivered in accordance with guideline 94/9/EC, as can several Flender gearbox types. Flender will also shortly be able to provide the hydrodynamic Fludex fluid couplings to this guideline.
Table 1: Description of environment and equipment characteristics for use in non-mining applications
Table 3: Example of risk assessment
ENDS For further media information please contact Bridget Summers, Footprint, PO Box 4, Colton, Leeds LS15 8WX. Tel: 0113 251 5697. Fax: 0113 251 5351. E-mail: bridget@fim.org.uk |
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