Explosion Venting of Filters for Dust/Air Separation (e. g. in Grinding Systems)

Bag filters, cartridge filters and electrostatic precipitators for dust/air separation which’ function is inherently connected with the creation of an explosive dust/air mix, have, if no other protective technique is used, to be protected by constructional explosion protection.

If equipped with explosion vents, the pressure shock resistance of the filter construction has to comply with the applicable standards. These standards demand a pressure shock resistance slightly above p_red. The value p_red expresses the highest possible value vent-mitigated deflagration pressure may reach. The interdependency of the mitigated deflagration pressure and the explosion venting system’s sizing and effectiveness is reflected by the equations for the calculation of vent area (vent sizing) requirements the norms provide.

In case of dust explosion scenarios in which a fire could be the consequence of the deflagration occurrence, it is recommendable to install self reclosing explosion vents, in order to avoid possible ingress of air oxygen via the open venting orifices.

In practise, wrong explosion venting arrangements can often be found.

E. g. explosion vents positioned in front of filtration elements, in configurations that hinder the venting.

Often, the recoil (reaction) forces and the flame length of an exiting blast are not taken into account correctly.

Deflagration-induced pressure is only slightly mitigated, before it affects the clean air compartment(s) of the filter. As a result, under certain conditions, the penthouse covers could be thrown off and the fan on the clean air side damaged.

Even more important than avoiding possible flame- and pressure effects that stem from a deflagration in the filter and could affect the grinding equipment, is avoiding possible propagating flame fronts and pressure shocks that stem from a deflagration in the grinder. Their propagation could accelerate, on its way through the duct from the grinder to the filter.

Therefore, explosion isolation has to be installed between grinder and filter.

A simple although in this case adequate form of explosion isolation is the installation of a so called explosion diverter. In case the filter had self reclosing explosion vents, this explosion diverter would also have to have the self reclosing feature, in order not to spoil the effect.

A correctly designed and correctly installed explosion diverter inhibits the so called flame jet ignition in the filter against which, if it occurred, explosion protection would be ineffective.

In order to avoid damage from accelerated flame front- and pressure shock propagation to both, explosion diverter and duct, and in order to prevent flame jet ignition in the filter, the acceleration has to be avoided or largely mitigated.

Thorwesten Vent‘s concept 1 & 2 for single stage air/dust separation and special versions of this concept for grinding systems with multi-stage air/dust separation enables the design of systems in which propagation will not occur.