We at Baum Pneumatics Inc believe that along with passive preventive measures, fast recovery after a controlled explosion incident should be a top priority in engineering design criteria of pneumatic conveying devices. Passive strategies are prefered when it comes to explosion safety scenarios as they are automatic in nature and do not require an action from other parts of the system. As an example Baum Pneumatics Fire Locks (also known as Isolators) benefit from this strategy to stop a flame front coming into the feeder from any direction. This bidirectional passive isolation is the result of three main design parameters, i.e. close clearance between rotor vane tips and feeder body, multi pocket isolation, and optimized vane thickness. All three parameters are passive methods and do not require a trigger event.
Passive prevention and fast recovery criterias are also applied to the Baum Pneumatics high efficiency cyclone design. High efficiency cyclones separate dust particles from air flow by taking advantage of centrifugal forces. This unique design eliminates the need for any moving part and results in a low maintenance dust collection approach. With advancements in engineering tools highly efficient cyclone designs are available that can deliver collection efficiencies equal to baghouses. On the other hand baghouse design requires a build up of fine dust to act as a filter and as a result separates and stores the fine particles inside which is in contrast to cyclone design that has a constant outflow of fine particles separated from airstream. From a fire safety point of view fine particles for any given material have higher explosibility index or Kst due to higher surface area and better surface chemistry therefore decreases the severity of an explosion in cyclone compared to baghouse design. In addition, due to the mentioned filtering mechanism baghouse design carries a much more significant amount of fine dust (fuel) than cyclone design which can become suspended in air with any vibration or impact.
Handling accidents and mishaps in a safe and efficient way is part of a well thought out strategy especially in an environment where efficiency matters and interruptions can be costly. A key criteria to ensure a fast recovery in a potential dust explosion is to ensure that the pressure buildup remains well below the enclosure strength or Pred which can be achieved by effective venting calculation. In addition the placement of the explosion vents should reduce or eliminate the back draft and reintroduction of oxygen into the enclosure.
Venting calculations are in accordance with general guidelines from standards such as NFPA-68 or VDI 3673. Baum Pneumatics CFD calculations on dust explosion can show the effectiveness of such calculations and provide alternative ways of achieving higher safety based on specific hazard scenarios.
to simulate a worst case scenario of explosion in a high efficiency cyclone and test the effectiveness of venting design the following extreme conditions were selected
- Particle concentration of 1 (g/L) with an ideal distribution
- Fine wood flour particles size (3-80 micron)
- Kst 100, 200, 300 (Bar-m/sec)
- Pmax of 8.7 (bar)
- Ignition source placed at the center of dust cloud
It should be noted that in real world events concentration varies inside the cyclone with particles moving close to walls at high concentration while the middle section has low concentration of particles. Ignition source in these calculations also inserts a high energy amount to ensure an explosion which both might be a rare event in a real world scenario.
Chemical composition of wood flour was assumed to be a mixture of CH4, H2, CO2, H2O, Ash, and C in different phases of solid, liquid, and gas. Mass fraction of chemicals along with reaction rates were adjusted to achieve the same Kst and Pmax measured in a Siwek explosion test chamber according to NFPA 69 and ASTM-E1226. Explosion vent panels are designed to pop open at 100 mBar of pressure. To capture the time of vent activation a very refined time step of 10e-6 sec is used which allows us to visualize and track the pressure wave originated from the point of explosion. This 100 mBar pressure front is shown by a 80-100 mBar contour and is an indication of explosion panel activation (pop) when the pressure front reaches an explosion panel. After vents are open high pressure gases can exit the enclosure. Gas velocity and pressure inside the cyclone are monitored along with the pressure exerted on walls. The max wall pressure during simulation will be the basis of Finite Element Analysis (FEA) to ensure that the design can withstand the max pressure due to explosion.