Safe working pressure for aluminium tube.

[SiddleyParticipant @siddley]

Compressed air ( or any other gas ) is potentially an enormous hazard – I have always had a very healthy respect for it – but I don't think it's too dangerous for a non-professional to work with if they research the subject and follow the right design procedures.

I used to build silencers for firearms ( legally ! ) and the pressures inside a 'can' ( as we called them ) are an order of magnitude greater than the ones mentioned here with the larger calibres of weapon. Sure it's a brief spike – the can isn't closed, obviously – but it's still a lot of gas energy to restrict.

Aluminium, stainless and titanium are all suitable materials if used correctly, although aluminium cans have to be designed carefully to avoid 'flame cutting' by the propellant gasses.

What I mean to say is that it's do-able ( high pressure gasses ) as long as you get clued up and follow good engineering practice.

Clive – my mate was the commander of an M109, I'm sure he thought he was Rommel 😀

[TerrydParticipant @terryd72465]

Diving bottles are indeed pressurised to around 2000 – 3500 psi, however they are made to extremely strict regualtions and have very many hours of development and specialised manufacturing techniques behind them. Aluminium bottles especially are susceptible to stress cracking and have to be tested regularly for cracks.

All of these pressure vessels are almost all professionally made to exacting standards, even air rifles, I would not trust home made devices to these pressures.

Best regards

Terry

[Clive HartlandParticipant @clivehartland94829]

I found working on the Americam M109 fulfilling, I rebuilt 2 engines, I often took out the engine and drive gear for running outside the gun on umbilical hoses and cables.

The sights, my main job were very easy to work on. I was always on the range when they fired and the accuracy was superb. I have seen shells lobbed down the turret hatches on target tanks at 2500mtrs range. Long distance accuracy, when the Regt. is split into three Batteries at different ranges the shells are fired at different times of a few seconds and they all come in on the target at once. 18 guns x 60lb shells make a big bang in a 50mtr target area.

By the way, the gun is made of aluminium armour, its two layers, soft inside and hard outside.

Clive

[Mark P.Participant @markp]

Thanks chaps food for thought.

Mark P.

[Andrew CarterParticipant @andrewcarter50444]

Mark

I guess you may not be intending to place the goods on the market, so CE marking might be of questionable worth, but the Essential Safety Requirements contained in the Pressure Equipment Regulations are well worth reading.

http://www.hse.gov.uk/pressure-systems/law.htm

Frankly, for home use, I'd be looking for a ready made cylinder. Sodastream perhaps?  You'd have to check the rating.

Andrew

Edited By Andrew Carter on 21/12/2012 22:31:29

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

In behalf of all the members, I have a prayer for you: Please do not make pressure bottle without you know the consequences you get when you or other people being injured by explosion and, we do not want to be involved in this when we were on to comment on self-generated pressure bottle.

Pressure testing of the pressure bottle will show all is ok and then the next all can go wrong when the pressure bottle is filled with air under high pressure.

Use the approved and tested pressure bottle from dealer.

[Ian S CParticipant @iansc]

Back in the mid 1950s, the people of Dunedin got a rather large fireworks display. A shed on the wharf went on fire late one night, no problem, well it was stacked with gas cylinders, and from where I lived on the hills on the other side of the harbour, we watched the cylinders shooting out ofthe fire, and then the sound of the explosion. Not quite the subject, but a few hundred cylinders exploded that night, I suppose it was similar to an artillary barrage, and the fire brigade just had to stand by and watch, from a distance. Ian S C

[shaun hillParticipant @shaunhill88399]

The vast majority of air rifle cylinders are not individually tested, the only tested cylinders are made by luxfor in the uk which are fitted to some makes of air rifle.

Any cylinder made for a air rifle under a 500cc capacity does not require to be tested, i have been making custom pcp air rifles for 25 years and each rifle i make i test the cylinder before despatch.

Regarding testing the bottles, quote, pressure testing will show all is ok then the next time all will go wrong, in that case what is the point of testing full stop. All diving cylinders are tested hydrostatically to a third above there recomended working pressure. they are all tested using water, the bottle is pressurised and a level mark put on the water gauge, the pressure is released then repressurised to the same pressure and the water gauge checked to see if the water level is in the same place, if its lower thats indicated the bottle as stretched and not returned to its original size, your bottle will be returned to you with several holes drilled in it.

Most of the german made pcp air rifles are made using alloy cylinders, some i have seen are frightning but they pass tests and are deemed safe, they are recommending now the cylinders to be replaced after 10 years, im sure a fellow member on this site will be along with pics to confirm what im saying.

All alloy gas bottles at BOC are scrapped after 10 years due to internal stress fractures forming, potentially lethal, all bottles are now tested at BOC ultrasonically to check for internal flaws, they still have steel bottles in service which where made in the 1940s.

My advice is to make your cylinder out of steel or titanium forget about aluminium. Dont think because a air rifle cylinder manufactured by a known company are bomb proof, most are made to the minimum spec required to save weight and have more air capacity, some cylinders ive seen would not pass a proper pressure test. until some sort of testing rule is bought out in the uk pcp air rifles can be made by anyone and any how, over the years ive seen some guns that in my opinion should not be in production, one guy in the far east was almost killed a few months ago by making a cylinder, the tube did not split the end plug came out, not enough thread in the tube, the requirement is only 6 full threads, ridiculous in my opinion.

[SiddleyParticipant @siddley]

You aren't the Shaun who makes the Spartan are you ? because if you are I bought a home made milling machine machine from you once – I'm Craig P's Section One RFD buddy from Derbys ( but now Spain )

Best looking air rifle on the planet, the Spartan.

( tried to PM you the above, but the PM function didn't want to work )

Edited By Siddley on 22/12/2012 16:47:02

[shaun hillParticipant @shaunhill88399]

Hi Sid

Yes it is i, How are you, im still making the spartans and mk3 airstreams occasionally when i get time. do you still have that old miller or did you dump it in the sea on the way to spain, lol

my e-mail address is ispairrifles@hotmail.co.uk

http://www.isp-airrifles.com

[SiddleyParticipant @siddley]

Small world – I'll drop you a line over the next couple of days.

I eventually ran out of space for the mill ( I was using it for the same jobs as you – inletting, it worked great for that and kept the sawdust and fibreglass away from the other mill ) and gave it to this kid who was just starting out in gunsmithing. Bet he's a multi-millionaire now, what with gunsmithing being such a well paid profession

[nigel jones 5Participant @nigeljones5]

looks more impressive than my BSA Metior !!

[SiddleyParticipant @siddley]

Posted by fizzy on 22/12/2012 21:11:36:

looks more impressive than my BSA Metior !!

I'm not going to embarass Shaun, but he's making air rifles that are of a quality which you would only otherwise see in the finest custom firearms. There really isn't anything that compares to them.

[JonParticipant @jon]

Howdo Sid quite correct on the burnout and done a few legally. Just to confirm Shauns quite correct.

I would go many stages further and say many tubes in production air rifles wouldnt get through the door of testing station, if there was such a test. Theres no requirement for a fixed cylinder to be tested.

From memory if go to a better aluminium stock holders and get off the shelf hydraulic tube 1 1/4" x 3/16" it has a pressure rating of 70 bar region (1015psi) Subject to the end plugs being adequate that vessel then should not operate above 676 psi.

Heres an end plug on right M30X0.75 max 10 full threads and rated not at a comparitive puny1000psi but 300bar (4500psi)

This actual tube would not have been tested, maybe just a sample now and again with the prototype. It will however being German have date, max fill pressure, TP, and manufacturer etched. Like other vessels that have similar specs and passed same standards, i have seen many cracked tubes in fact got two here now both within the 10 year life span. Point is no matter how big a company is, they are not fallible.

Same company Prelutech in Germany notice 200 bar working pressure, test pressure 350 Bar

Someone mentioned 232bar for diving bottles, the norm certainly the last 8 years is for 300bar, no one wants the old 200bar or 232bar bottles for love or money.

[SiddleyParticipant @siddley]

Jon – here's something to frighten you – prices of legitimate bottles being what they are in Spain guys here convert CO2 fire extinguishers to take HPA for air rifle use. I'm not sure how they get them filled, but someone must be prepared to do it because otherwise they wouldn't make them.

Talk about a hand grenade with the pin out…

Nice work on the end plugs. Although I operated mainly with firearms I got started on air rifles and still have a lot of affection for them, both as pieces of engineering and as accurate things to shoot. I'm working with air cartridge stuff at the moment actually – it's legal here and I always liked the system.

[JonParticipant @jon]

Has no one mentioned it to them, its as bad as those yanks slapping 1/8"bsp male threads in to what can be no more than 2mm thick aluminium Luxfer bottles holding 232bar back. Two have gone up.

Along same lines as Saun but have catered for the after market for twenty years. Remember you getting an RFD i worked for someone for 11 years doing high class stuff. Nothing like taking a hammer to a brand new Purdey o/u with customer standing over you. Dont miss it, glad to get out of it just a job.

[SiddleyParticipant @siddley]

The Spanish aren't big on health and safety – put it this way, the guy who makes the fireworks for the local fiestas here ( one about every two weeks ) keeps his home made black powder in filing cabinets in his basement

Traditional shotguns are Greek to me – I only ever worked on the 'combat' stuff, all black and scary. A lot of the really skilled guys seem to have trad shotguns as a background, I can see why.
With deac rules being what they are here I'm planning to get a deactivated side by side and convert it into an air cartridge double rifle. A bit silly, won't have any practical use, but it should be a fun project.

I used to come across the Luxfer bottles as military surplus – they were used as part of the cooling system in the MIRA thermal imager that was fitted to the MILAN anti tank missile. Very cheap but the liability involved in converting them for use with a Rapid or S10 wasn't worth it to me ( and that would have been a new end cap, not that crazy BSP modification )

[JonParticipant @jon]

Back on topic while i think of it.

Made some aluminium tubes to actually work at 232bar 15 year ago.

Alum high pressure after stating what i wanted 1 1/4" O/Dx approx 3mm+ wall threaded M27.5 x 1.5mm with O ring recess 28.2mm from memory. This tube started to bellow the ends where bored out to 28.2mm at 4800 psi.

Same in steel looking at 9700 psi of course done right.

Most of those German producers are running aluminium, no idea of the grade but 32 to 33 o/d with a wall thickness of no more than 2mm. In my earlier post out o/d of tube 32mm with an M30X0.75mm pitch thread = not a lot of meat at 29.2 bore size. Even if the o ring is at front of thread, the stress is still longitudonal and would still splay the tube ends due to the thread angle assuming the threads didnt strip.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

I'd be interested in a simple comparison of the material stresses between this part and the cylindrical section of, say, a 4" diameter model boiler running at 100ps with usual gauge materials.

Martin.

Rough sums answers are :

Copper tube 0.125 wall – 1600 psi tensile .

Copper tube 0.1" wall – 2000 psi tensile .

Aluminium tube – 2600 psi tensile .

Be aware though that knowing the shell stress is only a very small part of the information needed to decide whether a pressure vessel is safe or not .

We can talk more about this if anyone interested but just have a think about this meanwhile :

Two consideration never mentioned at all in model boiler design are (a) Failure mode – ie if it failed what would the sequence of events be and (b) Failure consequence – ie if it failed what would the damage be .

You could add (c) Failure probability and (d) Design(er) overall credibility .

Regards ,

Michael Williams .

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