A question regarding design for explosion 4

[ThomasH]

Hi all,

I have worked with explosion loads and the design for that purpose on and off for several years. But last week a got a odd problem.

A client has a building and the process inside can explode if things go bad. The current solution is venting areas, hatches in the wall that will open for a certain pressure, releasing the explosion. It is a standard approach so nothing strange so far.

But now they need to install a chicken wire fence in front of that wall, due to other reasons.

Question 1: Can the chicken wire fence reduce the pressure on the venting areas so that the hatches in the venting areas don't function as intended?

Question 2: Will the chicken wire fence be damaged by the explosion?

Does anybody have any ideas or experience regarding this? Any reference would be appreciated.

Thomas

 

[miningman]

I would suggest either the OP, or the client needs to review photos or preferably review actual blast sites to recognise the futility of this question.

 

[JLNJ]

I suppose it depends on the openings in the "chicken wire". I picture chicken wire as nominally 1" diameter hexagonal openings with very thin wire. I doubt something like this will change the pressure profile. If it's more like a screen with small openings, there might be more to think about.

The only time I have seen something even remotely like this is a wire "catch" at overhead door openings in a blast-resistant exterior wall where the door itself is not blast-capable.

 

[human909]

I would suggest either the OP, or the client needs to review photos or preferably review actual blast sites to recognise the futility of this question.

What makes you think this question is futile?

Most likely the chicken wire will have negligible effect on the pressure gradient and thus the activation pressure of the explose release hatches. I'd expect the chicken wire the be largely unharmed by the explosion but there are a huge number of variables. I'm presuming this is a typical process dust explosion rather than a pile of TNT sitting in a hardened bunker. Obviously they are different scenarios.

P.S. I 'm not an explosion expert but I certainly do deal with structures to contain and safely vent them. In fact I spent all today finishing up a design for the strengthening and venting of a vessel so that it would be about safely contain and vent an explosion.

 

[ThomasH]

Like I said in my first post, I have worked with explosions for many years but this was a new problem. I don't know if it is futile but I don't want to start with that approach .

I would say that a shock wave can be compared to a sound wave. And In my experience chicken wire does not reduce sound so I don't think it will reduce the shock wave either. At least not significantly.

But will it be damaged by the shock wave? That I think is the challenge to answer.

Thomas

 

[LittleInch]

Got a drawing / sketch?

I guess the issue is whether anything gets in the way of the blast (plastic bag etc).

Any significant velocity will just blast thin wire like that out of existence.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HTURKAK]

IMO, the use of wire mesh to catch the fragments rather than to reduce the shock wave.. typical chicken wire mesh in general woven mesh with 2 mm wire size having diagonal openings 30-50 mm size .

 

[human909]

"Explosions" can obviously come is a variety of sorts. Given this is a process building that has venting as a solution to mitigate the damage I'd be guessing that the "explosion" is a sub-sonic which referred to as a deflagration. Most likely of the form of a dust or (less likely) a volatile liquid/gas combusting.

https://en.wikipedia.org/wiki/Dust_explosion

So I wouldn't be too worried about significant velocities unless the chicken wire is directly in the relief panels and I wouldn't be thinking of the event as "like as sound wave". As long as you have a path for the rapidly expanding gasses to escape to limit over pressures and nobody is in the way of the flame front then you can quickly mitigate many of the risks.

Of course I could be wrong with my assumptions. But you don't usually have 'hatches' to mitigate supersonic detonations in buildings. ;-)

Any significant velocity will just blast thin wire like that out of existence.

But we likely aren't talking high velocities or even a 'blast'.

 

[3DDave]

To have a shock wave one needs to move the mass of air on one side of the shock at a higher than sonic velocity into the air on the opposite side, so not only is there sound there is mass transfer and that means aerodynamic drag. However, it's a short duration event, which means the inertia of the wire will resist movement in parallel with whatever material resistance there is.

I'd expect the wire will not be blown out of the opening, but would not be surprised if it is dished. All the chicken wire I know of has very low resistance to forming, but maybe you are referring to welded wire mesh that could be tensioned to better take the load, perhaps with springs like a trampoline.

 

[r13]

o have a shock wave one needs to move the mass of air on one side of the shock at a higher than sonic velocity into the air on the opposite side, so not only is there sound there is mass transfer and that means aerodynamic drag.

Share the same thought. The chicken wire will cause turbulent flow due to drag.

 

[LittleInch]

Is it little chicken wire or big chicken wire??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

The bigger issue with explosions is not necessarily the shock wave, but all the debris that comes with it. The debris will damage or deform the the chicken wire.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[dik]

projectiles will likely breach the chickenwire... other than that I don't see an issue.


Because blast pressure rise will be so quick... turbulent flow only should be expected.

Dik

 

[rb1957]

how much stuff can chicken wire (of any grade) resist ? not much ? flying debris will almost certainly breach the chicken wire.

but the OP's question was more would the chicken wire degrade the performance of the blow-out panels. It would have a slight impact of the Cd of the panel, but probably not significant.

later question … Would the chicken wire be damaged by the aero shock, with no debris ? I guess I'd be surprised if it was … but who cares ? surely if you had an explosion you'd replace everything (and not everything but the chicken wire) ?

another day in paradise, or is paradise one day closer ?

 

[ThomasH]

Hi again,

I spoke to the client yesterday and there are some uncertainties regarding how this will work. Or perhaps I should say, how they want it to work.

First you have the venting areas, hatches, that will release if an explosion should occur. The chicken wire should not reduce the pressure on the hatches so they don't release. My current assumption is that the chicken wire will have a very small impact on that issue.

The purpose of the chicken wire is that after the explosion there may be a fire, so the building will be filled with foam to prevent the fire. And since the hatches are open the foam will leak out so the fire safety system won't work as intended. But "somebody" has claimed that chicken wire will stop the foam from leaking out.

I don't think the chicken wire will have a negative impact on the hatches opening, it will probably not significantly reduce that pressure. But if the chicken wire will be damaged from the shock wave or debris or whatever, I don't know. It may be so that there is nothing in the building that can become a "projectile" but can you guarantee that indefinitely? Rhetorical question . There is also a question if the scenario fire, then explosion is possible.

The client has some homework to do .

Thank you for your thoughts.

Thomas

 

[LittleInch]

I was being somewhat tongue in cheek with my earlier post, but it's amazing where posts lead you.

I never knew there were so many different options about "chicken wire" - e.g.

https://www.wirefence.co.uk/types/chicken-wire/

Everything from 50mm holes to 13mm and wires from 1.2mm to 0.7mm diam. So something more for the client to determine.

And that's before you get into welded mesh or perforated sheets.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[12345abc6ttyui67]

Put a (very sturdy) hinge on the panel - blows open/upwards due to blast, hinges closed under self weight / gravity when the blast has subsided (to prevent the foam leaking out).

If retaining the foam is safety critical (to prevent escalation due to subsequent fire) I don't think I'd want to be counting on some chicken wire.

 

[rb1957]

"But "somebody" has claimed that chicken wire will stop the foam from leaking out."

"somebody" is smoking "wacky tabacy".

self closing blow out doors are a good suggestion … I'd suggest springs … but whatever these'll impact the pressure protection of the hatches and would need to be studied … how quickly do they open ? does the partial opening (as opposed to the current design's unrestricted opening) of these new hatches affect their Cd.

I think a bunch of analysis (unless you can see your way by hand waving) and (I'd prefer) a test.

Thought … what sort of fire protection system (ie where your foam is coming from) survives an explosion ?


another day in paradise, or is paradise one day closer ?

 

[ThomasH]

Hi again,

I just got information from the client that they will proceed with the project in another direction.

I think that the uncertainties regarding several things made the concept less attractive..

Anyway, thanks for the input. Interesting discussion.

Thomas

 

[EdStainless]

While not my field I worked in a building with this issue. Our relief panels were in the roof or very high on the walls. Our bigger problem was interlocking the foam system to assure that it didn't go off with any people in the area.

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