BAUM PNEUMATICS

Explosion Safety, Fast Recovery Design Approach

Baum Pneumatics high-efficiency cyclone

PHOTO: BAUM PNEUMATICS

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 preferred 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. For 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: close clearance between rotor vane tips and feeder body, multipocket isolation, and optimized vane thickness. All three parameters are passive methods and do not require a trigger event.

Passive prevention and fast recovery criteria 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 parts and results in a low-maintenance dust collection approach. With advancements in engineering tools, highly efficient cyclone designs can deliver collection efficiencies equal to baghouses. On the other hand, baghouse design requires a buildup of fine dust to act as a filter. As a result, it separates and stores the fine particles inside, which contrasts with a cyclone design that has a constant outflow of fine particles separated from the airstream. From a fire safety point of view, fine particles for any given material have a higher dust deflagration index (KSt) value due to higher surface area and better surface chemistry. Therefore, the severity of an explosion in a cyclone is less compared to baghouse design. In addition, due to the filtering mechanism, baghouse design carries significantly more 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 criterion to ensure fast recovery in a potential dust explosion is ensuring 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 computational fluid dynamics (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 explosion scenario in a high-efficiency cyclone and test the effectiveness of venting design, the following extreme conditions were selected: ● Particle concentration of 1 gram per liter with an ideal distribution ● Fine wood flour particle size (3-80 micron) ● KSt 100, 200, 300 (Bar-m/second) ● 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 a high concentration while the middle section has low particle concentration. Ignition sources in these calculations also insert a high energy amount to ensure an explosion that might be a rare event in a real-world scenario.

Chemical composition of wood flour was assumed to be a mixture of methane, hydrogen, carbon dioxide, water, ash and carbon 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 (maximum explosion pressure) measured in a Siwek explosion test chamber according to NFPA 69 and ASTM-E1226. Explosion vent panels are designed to pop open at 100 millibar (mbar) of pressure. To capture the time of vent activation, a very refined time step of 10e-6 sec is used, allowing us to visualize and track the pressure wave originating 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 to ensure that the design can withstand the max pressure due to explosion.

To see a timestep video of this explosion simulation and more information, please visit: www.baumpneumatics.ca/explosion-venting.

Author: Ehsan Tootoonchi Mechanical Engineering 604-445-3394 etootoonchi@baumpneumatics.ca