Drilling into munitions and containers filled with something particularly nasty should be done only after some thought. We have been making holes or helping others to do it, for over twenty years so we have given it an awful lot of thought (claret and a good pipe help). Doing it for that long means you learn a thing or two.
Survival instincts say that when you plan to make a hole in the wall of a 155mm packed with CWA it’s the agent and explosive burster you worry about. After all, those wee devils will hurt quite a bit if you get it wrong. What folks often forget is that there is another potentially fatal variable when dealing with sealed containers;
Now the pressure we are talking about here is not the trickle of sweat down the back, kind of pressure you might feel straddling a ticking chemical munition but proper mechanical engineering, Boyle’s law, N/mm², type of pressure. Make this engineering mistake and it could lead to a rather embarrassing incident which you may find hard to recover from ie death!
It is a rather foolish operator who ignores the amount of energy contained in a pressurised cylinder. Especially when that energy would be enough to puncture or displace PPE milliseconds before a fountain of agent sprays from the crack.
Now before anyone starts yawning this is not an engineering article with equations and hard sums. It is just a cautionary note based on twenty years’ experience. If you read this and think you understand the complexities of pressure in sealed containers – you don’t. I will say it again – you don’t! Unless of course, you are a Professor of Mechanical Engineering from MIT – in which case fair play you probably know something about the subject…
Seriously if you have an operation where the stuff below applies and want to chat it through – contact us.
For the geeks out there: if you disagree with some of the numbers don’t contact us – just don’t!
Anything written here is for guidance only and to emphasise that pressure is something to be very wary of. You want a detailed engineering study? Pay us some cash and we will happily write you a doctoral thesis!
A Cautionary Tale…
Hold on you say. Just hold on a cotton-picking minute – you might protest that a lot of chemical weapons aren’t pressurised and you would (in the main) be right. Let me address that objection, however, with a short story about a project we were involved in.
A few years ago some 1-tonne cylinders of HF were discovered. Since no-one wanted them it was decided to dispose of them using normal industrial processes. When the first couple were connected up to the emptying equipment nothing came out. Hmmm, they thought, that’s strange as the cylinders were known to be full. So another one was connected up and jings crivens (Scots exclamation) the pressure gauge needle nearly flew into orbit as it shot up to 800psi. HF cylinders are normally in the 15 to 30 psi range so something was badly wrong. After a careful investigation, it was determined that the PTFE liner had deteriorated and that the HF had reacted with the steel. This reaction produced an awful lot of hydrogen which created an internal pressure on the verge of rupturing the cylinder. The PTFE had also blocked up the valves – hence the reason nothing came out when connected up. As if this was not enough of a bad day, the HF had pitted the casing to the point that only a couple of millimetres of steel was left in some places. Big gulps and stand well back…
Now the really, really scary bit about this whole incident is that there was no way of telling from just looking at the cylinder that anything was amiss. If you have munitions and cylinders that have been lying around for a while then unless you know for certain, it is safer to assume there is some internal pressure and act accordingly.
The other thing about pressure is that a little can go a long way in the right (or perhaps I should say wrong) circumstances.
- The humble blue plastic barrel that you see all over the world produces a considerable bang about 60 psi.
- A 55-gallon oil drum explodes along its seams at between 60 and 100psi. If you are standing close enough then it has the equivalent of a ¼ of the force of an RG4 grenade. Standing next to these when they blow would make more than your ears pop and that’s before you fill it with something toxic.
The good thing about these thin-walled containers is that you can often see they are bulging but you don’t know whether they are about to explode or happily sit there. If you hear pinging though, run boy run.
Thick-walled though – watch out. We are not just talking about 2-inch thick metal walls here – a fire extinguisher is deemed thick-walled. When you have that sort of target it could be sitting at 400psi plus and you would not know it. No bulging, no pinging – no indication whatsoever.
As a side note. Just for a giggle, we decided to pressurise inert 155m artillery shells to find out at what point they burst open. It was a slow week. 11,500 psi. 11,500psi – imagine drilling into that and finding even 10% of that pressure trying to escape out the hole.
This is stressing me out!
And of course, all this is before you actually want to do anything with them. Open a valve, unscrew a lid, drill a hole – these actions could seriously spoil your day if that sucker is under any significant pressure.
Now as mechanical engineers we know that there is a good way to drill a hole in a sealed vessel and some very bad ways to do it. It is all to do with stresses – well two in particular – Hoop and Longitudinal.
In engineering terms, stress is the applied load divided by the cross-sectional area of the material. In the case of a munition or storage container that means the internal pressure – which causes a force on the walls; divided by the area of a cross-section of the complete circumference of the wall. Don’t get too hung up on this – it will do you no good!
Now you might think – hey the pressure inside the target is going to be equal so the stress in the material will be equal. Ah my poor, naive non-engineer, if only it were that simple. Think of a party balloon and the old trick of sticking a skewer through it without making it burst (for the ignorant Balloon Science – How to Push a Skewer Through a Balloon). It is much, much easier to do this by sticking it through the material at the ends rather than in the middle. This my friend, is the difference between Hoop stress and Longitudinal stress.
Without going into all the mathematics (you will just have to trust me on this), the Hoop stress is double the Longitudinal stress in thin-walled containers but can be up to a whopping 5 times in thick-walled. What that means for you is that most targets, just like the balloon, will be a lot less likely to burst open if you drill the ends rather than on the sidewall.
Could it get any worse?
Hold on though it gets worse. Drilling a hole in the material – even a tiny one causes the stress to concentrate around that point and makes it more likely to burst open. And the stress in the material does not increase by just a little bit more – nope it increases by 3 times more.
There’s a fair bit of engineering mathematics behind this which we won’t go into here. Back to the balloon example – when it is inflated it will sit quite happily containing the pressure but put a pin in the side and suddenly BANG!! That is stress concentration. It’s the same reason why bike frames are weaker if they have been dented.
So what? As mentioned above, this post was not designed to be a detailed engineering article – just some information to make you aware of the issues surrounding internal pressure. When dealing with munitions the stress (pun intended!) of the job can often mean we focus on the obvious hazards we can see or know about and forget about ones that could be lurking literally under the surface.
If nothing else, take away from this blurb that pressure is something any operator needs to think about before they open or make holes in any sealed container. Whether you are using Valent equipment or the hardware store battery drill, internal pressure can really spoil your day. The violent force caused by a rupture or the fountain of toxic material that will follow are risks that could be present unless you can confirm or know for certain otherwise.
Remember most containers will give no indication that there is internal pressure or not.
That’s why at Valent we ensure;
- Our equipment is designed to contain likely internal pressures as drilling takes place or they can be used remotely or a combination of both.
- If we suspect really high pressure we do our sums and then try to drill through the ends rather than sides or find some way of mitigating the pressure risks.