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Design criteria for dust ignition fans

Author : Charles Halstead, group technical director, Halifax Fan

08 February 2010

As with perhaps the more common gas ignition fans, the design of Dust Ignition Fans is now covered by EN 14986:2007. This European standard sets out the design and manufacturing standards required to reduce the risks of industrial fans becoming potential ignition sources in gas and/or dust laden atmospheres.

Design criteria for dust ignition fans
Design criteria for dust ignition fans

Generally, the process of supplying such fans begins with an enquiry, including a technical specification. It is often at this stage where things become problematical. The first problem is identifying correctly the Zone and Category of equipment that is required. The zoning of areas is a subject in its own right but once the zone has been assigned, the relevant category of equipment can be selected.  The table below gives a very brief summary of zone and category assignment.  It should be noted that the category is a function not only of the possibility of an explosion, but also the consequences of an explosion.  In some cases it may be that Category 1 equipment is specified for a Zone 1 area, (perhaps an installation in a heavily populated area) or conversely for a remote, unmanned installation, a lower category of equipment may be used.

The important criterion regarding dust is the presence of a potentially explosive mixture. For airborne dust there has to be sufficient dust in suspension that a potentially explosive mixture is present. A fan handling “dusty air” in its normal operation is not by default Zone 20, a common misconception.  Dust may well be present, but in many cases NOT in a sufficient concentration for the mixture to be explosive.  For many dust types, were one able to look inside a Zone 20 mixture, the concentration of dust would be so high that one would not be able to see one’s hand in front of one’s face.

A good example is illustrated by a dust handling system, consisting of inlet from plant, cyclone separator, bag filter, fan and discharge duct.  Up to and including the cyclone Zone 20 might apply, since large quantities of dust will be present in normal operation.  Beyond the cyclone, Zone 21 may be applicable, depending on the efficiency of the cyclone and possible failures allowing large amounts of dust carry over.  The clean side of the filter, including the exhaust dust and fan internals may be classified Zone 22, as there is only a small risk of dust carry over in sufficient quantity (the filter bag failing coincident with failure of the cyclone).

In this example, although the fan will handle some dust continuously since even filtering will allow some carry over of very fine particles (fines), the correct, internal zoning of the fan would be Zone 22 Cat 3D.  There would also be an associated temperature rating corresponding to the ignition temperature of the dust handled.  This sets the maximum allowable surface temperature of any part of the fan, in order to prevent auto ignition in the presence of a dust cloud.
 
It is due to the fact that the amount of airborne dust required to give a hazardous mixture is so high, that Zone 20 fans are not included in EN 14986; Zone 20, would require a Cat1 fan. This would be so designed that in the event of failure of all the in-built spark minimising features, an explosion would be retained within the casing.  For gas applications this is achieved by designing the case to withstand 10 Bar internal pressure and fitting inlet and outlet flame arrestors to prevent flame propagation down connected pipe work.  This is not possible for dust hazards as the flame arrestors would quickly become clogged.  During the drafting of the standard, fast acting shut-off dampers with corresponding control systems were proposed.  However, this was not pursued due to the extreme complexity of these and the control system, as well as problems in testing.  It was decided that due to the exceptionally limited requirements of true Zone 20 fans, these should be removed from the scope of the standard. 

In the above examples, we have concentrated on the internal zoning of the fans.  By this, we mean the zone inside the fan case. However, often we need to consider the zoning of the actual area in which the fan is situated.  This may be the same or different from the inside of the fan.  It is important for any fan supplied with a casing that the Zone and required Category of equipment for both inside and outside the fan case are specified to ensure that the correct equipment is supplied.  Upon receiving this, the manufacturer must then consider any leakage that may occur. 

The EN standard suggests that unless the fan case is certified gas-tight there should be no more than one category difference between internal and external rating.  The question of the gas tightness of the case is itself worthy of some explanation.  A standard industrial fan, say of steel construction with welded joints, whilst in operation may not appear to leak, will in practice be far from “gas-tight” when subjected to a pressure test.  To achieve true gas-tight construction, no penetrative bolting into the case can be used, flanges require reduced hole pitches and radially split fan cases require special attention, not to mention the main shaft seal.  A gas-tight fan would typically be subjected to a pressure test, where the inlet and outlet flanges are blanked off, a test procedure is applied to the case, (typically working pressure or fan maximum static pressure, plus an agreed margin) and after a suitable settling period, the pressure monitored over time.

In the case of, for example, a fan with Zone 21 Cat 2 internals intended to be sited in a safe area, it would be more economic to certify the externals, including the motor, for Zone 22 Cat 3 rather than manufacturing a 100% tested gas-tight fan case.

In actual construction there is little difference between an ATEX dust and gas fan for equivalent categories.  Common design features include :

  •   Rigid fully welded (for fans rated at 11 kW and above) fan cases
  •   Gas tight if more than one category difference between inside and outside
  •   Impellers subject to over-speed test at 115% design speed or designed for 2/3 yield stress
  •   Suitable material pairings to prevent spark generation in the event of failure
  •   More accurate balancing
  •   Reduced vibration
  •   Positive impeller location
  •   Control of internal clearances
  •   Testing – including full run test of the complete fan assembly
  •   Certification and marking.  As we have seen, fans often have a dual rating for the internal enclosure as well as the  external environment in which they are situated.  In this case, separate ATEX nameplates, or some means are required to show clearly the degree of protection for these separate zones.
Additional features for dust fans are really limited to the provision of continuous vibration monitoring on Zone 21 Cat 2 fans.  This is due to the possibility of uneven dust build up on the rotating parts causing increased out-of-balance forces and possible failure.
Other features that would be desirable for ATEX dust fans, though not necessarily mandatory, are :
  •  Backward Inclined or Radial Impellers to give self cleaning properties – used when a dust hazard is present inside the fan case. For efficiency reasons, a Backward Curved Impeller may be the first choice.  However, due to the dead area on the trailing face of the impeller blades, this design is subject to material build up. Often this in itself does not cause excessive un-balance, due to the even nature of the build-up.  Problems occur when any residue is shed from the impeller. This tends to be uneven, leading to imbalance, vibration and damage if not addressed.
  •   Direct drive via flexible coupling is preferred to vee-belt drives. This removes the somewhat uncontrolled heating that  may occur from old and or poorly tensioned vee-belts
  •   Access panels for internal inspections.
In conclusion, working with EN 14986 ATEX Dust Ignition Fans does not present any additional complications above those for their Gas Ignition cousins.  There still needs to be dialogue between purchaser and supplier to ensure that the correct category of equipment is supplied, bearing in mind the dual zoning internally and externally of enclosed type industrial fans.  Careful consideration should be given to the amount of dust present in gas streams, in regard to its’ potential hazard but a fan handling dust as part of its normal application is not necessarily a Zone 20 application.





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