Explosion Venting Wall Load Interactions with Bulk Materials

[pbc825]

I'm not sure if this is the right forum to be posting, but I think this is as close as I'm going to come.

I'm working on a material estimate for several bins which require explosion venting as they store food products. The bins hold aggregate materials with reasonably large particle sizes (nuts and nut hulls). We have not yet engaged a bulk materials expert, but will if/when the project is awarded. In the interim, I've built a monstrous spreadsheet to calculate wall load profiles in accordance with Eurocode for silos (BS EN 1991-4:2006) considering a wide range of lateral pressure coefficients, internal friction angles, and wall friction coefficients. These bulk material property values were not provided by the client. The bins all employ inlets and outlets concentric to the centreline of the silo (which is axisymmetric). We have engaged an explosion vent expert who has provided some guidance for explosion vent pressures (Preduced = Pred).

My question is this: Should the explosion vent pressures (Pred) be considered to add to the bulk material wall loads (specifically normal pressure on the wall and hopper) or should the events be considered mutually exclusive of one another? I won't taint the responses with my thoughts just yet. I'm hoping a bulk materials expert or anyone with experience is able to read this and shed some light on the subject.

 

[TehMightyEngineer]

I'm not an explosion or bulk materials expert, nor have I stayed at a Holiday Inn recently, but I would fully expect a reasonable load condition being an explosion above a partially filled silo in the space above the bulk material. This would cause an outward pressure on the walls and a downward pressure on the bulk material which also gets partially transferred to the walls along with the normal lateral load from the bulk materials.

Thus, I'd expect to include some partial loading of the bulk materials on the walls.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[pbc825]

Thank you for your reply TehMightyEngineer. Much appreciated.

For a non-porous product, I agree completely and I'm sure I could lump the surcharge into the wall load profile calculations. However, what if the bulk material was highly porous and the gas pressure in the blast wave was able to propagate almost freely to the walls and hopper of the silo? I tend to think peanut hulls would be pretty porous. I'm also thinking the bin walls would momentarily be subject to normal pressure from both the bulk material and gas pressure specified by the explosion folks. Thoughts on that scenario??

 

[miningman]

I also stayed at a Holiday Inn last week , but my thoughts are that surely the pressures from a blast would be several orders of magnitude greater than those generated by normal wall loads. Is this query not a little bit academic??

 

[TehMightyEngineer]

"I tend to think peanut hulls would be pretty porous."

You're right in that I would expect if you raised the pressure slowly the pressure above a layer of shells would be equal to air pressure below that layer; resulting in no net force external to that layer. However (and this is definitely beyond my expertise) wouldn't a blast wave inherently involve unequal pressures? As the blast wave hits the mass of shells it's pushing on the top of the mass but the air pressure inside the mass of shells hasn't increased significantly; thus you'll have an instantaneous period where the shells are being forced downward and thus they push down and out on the silo walls.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[pbc825]

wouldn't a blast wave inherently involve unequal pressures

It surely does, and the pressures are in fact quite high, but for mechanics of materials design of the silo, one considers Preduced (Pred) as specified by the explosion vent supplier as a uniform pressure. This occurs after the vents are open and spewing out hot fire for several hundred feet (Yikes!). I understand this all occurs within something in the range of a few seconds.

As the blast wave hits the mass of shells it's pushing on the top of the mass but the air pressure inside the mass of shells hasn't increased significantly; thus you'll have an instantaneous period where the shells are being forced downward and thus they push down and out on the silo walls

I tend to agree, but it would depend on the permiability of the product. Tough to say without understanding some of the test results (or hearing from an expert

my thoughts are that surely the pressures from a blast would be several orders of magnitude greater than those generated by normal wall loads. Is this query not a little bit academic??

They are not entirely academic. The maximum normal pressure for this particular vessel from bulk material during discharge is about 40kPa(g). Preduced defined by the explosion vent technical expert is about 60kPa(g). Understand that the silo is fitted with large explosion vents which rupture during a deflagration event.

Thanks again all. I'm going to try two scenarios and report back.
1) Considering free flow through the porous bulk material.
2) Considering an impermeable layer of bulk material and surcharge pressures at the top of the bulk material.