H&S - making sure demonstration equipment is safe?

[julianf]

I have been asked to build an air powered cannon to shoot squash balls from a bike pump.

Its a very simple device - a 42mm copper barrel, some end feed copper fittings, valves etc, but then the accumulator is a bit of soil pipe - this is the bit that concerns me.


The copper end feed stuff will be rated to 10bar plus, as, of course, its main purpose in life is to carry pressurised potable water, however, soil pipe is not specifically designed to be pressurised.

So, my question is -

Is this then unsuitable at any pressure (as its not got a rating) or is it a case of "If its tested to 6bar, it will be safe at 2 bar" - what about cumulative damage?

Or should this part be removed entirely, and replaced with somthing that is rated to higher pressure (eg wide bore mdpe water pipe - rated to 12 bar)

The idea of this item is to demonstrate (ie staff use only) trajectories etc in physics classes, but it is essential that it can pass the risk assessment rating, otherwise it can not be used.

Thank you,
Julian

 

[the snooper]

without doubt get something that has been independantly certified that it can hold pressure

 

[addjunkie]

Migh i suggest you source a suitable pressure vessel, for example from a dead air compressor. Im all for making things and ingenuity. But HS will kill you, so play them at their own game. A pressure vessel is designed to hold pressure, will also have safety valve to prevent over pressure, therefore will pass risk assesment.

Are you sure about the 10 bar rating of copper pipe, and fittings.

If you ever see something go with pressure, it aint pleasant. Ive seen a tyre go with 2 bar knocked the guy blowing it up over.

Also seen an air hose go at 15 bar, was not good.

 

[BrokenBiker]

If in doubt then stop and think. If it has to hold pressure then it has to be rated to hold pressure. Compressed air isn't something to play with.

A lot of people think that a low psi number means low pressure but if it's contained in a large vessel then it could be quite substantial.

If it isn't designed to hold pressure don't use it.

 

[julianf]

Are you sure about the 10 bar rating of copper pipe, and fittings.

Nope - it was just an off-the-top of my head thing, considering most PEX is rated to about 12bar (cold)

So...

Ive just looked up the first site i came to -

Conex Delcop End Feed fittings - Soldered

Working Pressure (Bar)
0°C to 30°C - 25
65°C - 25
95°C - 19
110°C - 16

So, 25 bar when soft soldered.

 

[MattH]

Why don't you use pressure rated 4" ABS solvent weld pipe and fittings? It will easily do 10Bar dependent on the rating and often much higher.
Used a lot in ponds and aquatics but it is proper pressure rated stuff.

 

[julianf]

I think that would be a good idea. As above, the copper stuff is rated way above what it will be used at, so its just the receiver / accumulator that's the (initial) problem.

The receiver is currently about 1m of 110mm soil pipe, which seems to work fine (as in the size) so somthing of similar (or smaller) would be fine.


So, if all the component parts of the build are rated to way in excess of the operating pressure, where's my next stumbling block?

 

[rtbcomp]

Be careful with pressure and plastic pipe - some types will shatter if they are knocked whilst under pressure and as soil pipe isn't designed for pressure I wouldn't risk it..

 

[the snooper]

What about something like an old fire extinguisher? Those are pressure rated...

perfect idea, especially Co2 with a ready made valve on it

 

[julianf]

That may (or may not) work so well, as the device needs a high volume at a lower pressure.

At present the receiver is plumbed to the barrel with 22mm copper. Even if i remove the valve from the cylinder, i think the bore may still be restrictive.


The barrel of the device is 40mm ish by aprox 1m. So its a question of getting enough volume of air into the barrel before the projectile (a squash ball) leaves the end. If the flow is too slow, the ball will have left the device before the receiver empties.

Something with a 3/4bsp thread (19mm ish bore) would probably be acceptable, but much less would, i think, be restrictive - although maybe not?

I dont know what thread CO2 cylinders have?

Obviously a propane tank would work, but im not going down that path, as that would be a whole new can of solicitors....

 

[penfold]

Why don't you use pressure rated 4" ABS solvent weld pipe and fittings? It will easily do 10Bar dependent on the rating and often much higher.
Used a lot in ponds and aquatics but it is proper pressure rated stuff.

Definitely ABS, as it fails safe whereas PVC tends to shatter into shrapnel, especially if cold.

 

[pedrobedro]

Throw bag launcher cannons are made of plastic pipe and they fire a weighted bag and line great distances.

 

[Tiggy_dorset]

I havn'nt read the whole thread, but is this little pressure tank any use for your contraption?

http://www.mig-welding.co.uk/forum/threads/what-shall-i-do-with-this-thing.32912/

 

[captain-destructo]

howabout 28mm copper?


edit!!!!!!

just re read the original post

make it all out of 42mm copper

 

[Parm]

Why don't you skip the accumulator and just connect to a compressor line with valve. It take it your looking for a sudden blast of air to shoot the projectile?? This way the main bit comes pre tested etc etc.

Generally be very carefull with this as your are potentially messing around with ballistics type stuf where barrels need to be proof tested as part of commissioning. Don't know the exact regs here, but If you make such a machine you are most definately in to the realms of the PUWER regs.

It's one thing to make this at home for a bit of fun but it's altogether more serious and involved when it's for someone else and money is exchanged

 

[captain-destructo]

just thinking you could use one of these to charge the cannon




built in preasure relief valve that way the tank on the cannon cant be over filled by over excitable children

 

[Fergus]

I hate to be a killjoy, but aren't you being asked to build a barreled weapon? 42mm keeps you under the 2" limit for smooth bored cannon/shotgun but if you are over 12ftlb muzzle energy I'm guessing that would class it as a cannon and put it into section 2.

 

[penfold]

in order to exceed the 12ftlb limit he would need to be launching the balls at over 130kmh; that's quite an ambitious target.

 

[Fergus]

That's only 80mph, slower than your average tennis serve. Lose the barrel and make a twin wheel ball launcher and it ceases to be an issue.

 

[Turbo]

Could I suggest the air tank of a lorry as a pressure vessel.