Mechanical Design Criteria of Fans for Explosive Atmospheres: Clearance of Rotating Parts

Section 4.4.1 of the standard EN14986:2017, known as the design of fans working in explosive atmospheres, describes how industrial fans should be designed to make them suitable for placement on the EU or UK market, in explosive atmospheres. This standard provides guidance on design, construction and testing of industrial fans, and includes compulsory requirements for the clearance of rotating parts to prevent ignition.


Clearance between Rotating Elements and the Fan Casing

The most important safety feature of any industrial fan construction, to minimise ignition, is the clearance between the rotating elements and the fan casing. ATEX fans are designed for use in potentially explosive atmospheres where the presence of flammable gases, vapours or dusts can post a serious safety risk if not managed correctly. Having sufficient clearance between rotating parts, such as the fan blades and the housing reduces the likelihood of friction, sparking or contact that could ignite in the event of hazardous gases or dusts being present. This is clearly stated in BS EN 14986 at clause 4.4.2.

 

“ The minimum clearances between rotating parts such as the impeller and fixed parts, such as the fan casing, shall be at least 0.5% of the relevant contact diameters (diameter of the rotating part at the point where it can contact a stationary part) of the finished component, but shall not be less than 2mm in the axial or radial directions nor need be more than 13mm.”

 

The fan manufacturer should design and construct the fan ensuring that the clearances are well maintained under all conditions covered by the intended use. Clearances may change with rotation, temperature and due to vibrations or belt drive tension. Clearances should be adhered to in application considering potential casing distortion, ductwork and pressure drops that may affect. Rotating parts refer to the components within the fan that are designed to move in a circular or rotational motion to generate the required airflow. These parts are critical to the fans operation, as they create the necessary pressure difference to push or pull the air through the fan and distribute through the system. Rotating parts include the fan impeller, shafts that connect the fan motor to the impeller, bearings that support the rotating shafts and couplings that connect the motor and the impeller shaft.

 

Ignition is caused either by rubbing friction, such as between a rotating impeller and a stationary piece of metal, or by impact of two metal objects. The principle of ensuring clearance, reduces the likelihood of any contact between two non-permissible materials and as a result, rubbing friction. Research has shown that in metal-to-metal contact, the properties of the more readily oxidation metal, normally determine the degree of ignition hazard. The hardness, melting point, ignition temperature, specific heat conductivity and brittleness of the metals all play a role, in that they determine the size, duration, temperature and heat capacity of the incentive sparks. The concept of permissible pairings is another key aspect of industrial ATEX fan design.

In addition to the primary function of ensuring safety and reducing the risk of being a source of ignition from friction or sparking, adequate clearance helps to prevent wear and tear on components, extending the life of the fan, it ensures minimum resistance and interference to ensure optimal performance, prevents contamination when handling corrosive or abrasive substances and reduces vibrations and noise during operation to deliver a balanced fan system. If you’re unsure about the type of explosion proof zone you require, permissible pairings that will ensure safety in your gas application, and you have the hazardous area classification to hand, give our technical explosion proof fan experts a call or email [email protected] and we’ll talk you through the concepts, jargon and if we have the necessary information, we’ll supply a fan suitable for your application. Download ATEX Certified Fans Literature here.