Speechless.  With that approach I suggest you continue to be frightened.

Research “hoop stress”  “longitudinal stress”  “buckling stress”. Then use the strength of copper.  Also look at operating ranges for pressure gauges and factor of safety.

Paul.

Thank you Duncan ! At least by being a pedant you are showing your paying attention ? Noel.

Mains water pressure depends on where you live. My in-laws had <15 psi some days. The upstairs taps were not impressive, and the electric shower often wouldn’t work because of low pressure trip.

According to Aussy code, a boiler shell from 1.2mm copper at 100 psi has a max diameter of 3.5″

I’d be more concerned with the end plates. If these are also 1.2mm thick the required stay pitch comes out at 0.41″, which is unlikely.

The Aussy code agrees well with UK standards.

On 24 February 2024 at 12:04 Hopper Said:

Bear in mind that mains water pressure is 80psi and copper pipe is made to withstand that with a safety factor of about 6 or so (it varies). For instance 1/2″ copper pipe Series L with a wall thickness of 1.0mm is rated with a max working pressure of 1200psi.

The process of silver soldering the copper boiler will have annealed it so that 1200psi figure for drawn tube should be halved to about 600psi . Then by the time you get to the OP’s 4″ pipe it drops to about 250psi. The OP’s tube is probably class M which is thinner so at 4″ annealed that would be 200psi.So already at about 1/6th the value and that is for tube, if the end plates are the same thickness and not adequately stayed you will have to drop that figure some more.

It does sound thin to be wanting to run at 100psi, take a well known example like a Minnie boiler 2.5″ dia 1.6mm thick, back and tube plate 2.3mm thiock and a WP of 50psi

Explain WHY ! Since you have asked the question I make the possibly foolish mistake of assumption that you do not understand the physics  of a boiler and what happens when one fails catastrophically ? I have 2 books that explain what happens in graphic detail – horror ! Alan Mc Ewans book on “Historic Boiler Explosions” is worth reading, in fact in my humble opinion it should be required reading for anybody thinking of building a boiler, it will save them asking questions about why we need rules about the making of pressure vessels. Noel.

PS. The second book is “Locomotive Boiler Explosions” by C. H. Hewison.

PPS. If the OP really wants to know the answer to the question then He is welcome to use my hydraulic equipment to conduct the experiment – so long as it fails below 10,000 psi !

On 25 February 2024 at 08:34 BOB BLACKSHAW 1 Said:

I wish I didn’t start this post, now I’m running the boiler at 100 lb which I’m not. The question was what point would it explode, not that I’m going to do its a curiosity thought. Explain why a cycle tyres can be pumped up to over 100 lb safety 120 lb with some .All my engine’s  I make from bar stock rum on a nebuliser so not much pressure needed on a boiler.

 

Well, your initial post did not say anything like that. It appeared to say that you were doing something potentially dangerous – whether or not you understand it to be so. You post clearly said you took the boiler to 100 psi in steam. Now you are backtracking. If you took more trouble to explain yourself clearly, then people would not come to (possibly) wrong conclusions.

Briefly, bicycle tyres are designed to take that sort of pressure, while your boiler clearly isn’t.

A quick calculation using the formula given in the Harris book gives a safe working pressure of just over 70 psi, and, and you would expect it to fail under cold hydraulic pressure at a bit less 600 psi. Even so, as others have said, 1.2mm would be an unusually thin shell for a 4″ boiler.

On 25 February 2024 at 08:34 BOB BLACKSHAW 1 Said:

… The question was what point would it explode, not that I’m going to do its a curiosity thought. Explain why a cycle tyres can be pumped up to over 100 lb safety 120 lb…

 

Difficult to predict exactly because there are so many factors that reduce the value.

The Copper pipe we lay our sticky paws on is generally intended for plumbing: water, liquids, vacuum and light steam pressure.  Much thicker tube is available for building full-size steam locomotives etc but I’ve never seen any!   Bob’s 1.2mm pipe is very thin, so not intended for high-pressure.

Burst pressure depends on the material, how hot it is, the thickness of the pipe wall and the diameter of the pipe.   Room temperature  values for plumbing pipe are available, for example here on ‘The Engineering Toolbox‘.   Max values for 4″ pipe: Type K=3415psi, Type L=2865psi, Type M=2215psi.

If an engineer was designing a 4″ dia copper pipe deliberately to explode, the apparatus would have to provide at least the pipe’s burst pressure, and more if it has to burst quickly.   A deliberately weak pipe or diaphragm that bursts first in an emergency is a safety feature often found in high-pressure chemical plant.

However, as accidental bursts are never wanted, the engineer has to solve a much harder problem; what size pipe is needed to remain safe in all operating conditions over a working lifetime?   Factors include:

• Annealed copper is roughly half the strength of drawn copper.     As anything done during manufacture that could anneal drawn copper will significantly weaken it, the designer should take this into account.  Soldering, brazing, welding and incidental heating all create weak areas .  Unless something is specifically done to bolster these, the designer should assume annealed burst pressure, not drawn burst pressure.    So Type M 4″ drawn copper pipe, assume burst at about 1100psi.
• At this point a multitude of variables like solder vs brazing vs riveting vs welding, operating temperature, stress raisers, and fatigue etc intrude.  Thus, it’s usual to apply rule of thumb safety factor determined by experiment and experience.  For 4″ Type M pipe, this gives a safe working pressure of 213psi, far less than the burst pressure.
• As joints weaken structures, I’d not personally be happy running a boiler made from thin-walled 4″ Copper pipe at 100psi.   Using  thicker 4″ pipe improves the safely margin.   Much depends on the details though.  Fairly obviously soft solder is weaker than brazing, and brazing is weaker than welding.   Rivets, done properly, are stronger than brazing, but weaker than welding.    The form and quality of the joint matters too, which is why boiler inspectors look closely at them.  Bad design, poor construction, and sloppy maintenance all reduce strength, often in difficult to predict ways.  Hence safety factors, of 2:1 or higher.
• The best way to find out what the max pressure a real boiler will take is a hydraulic pressure test.  The pump applies enormous pressure without storing much energy in the boiler, allowing pressure to be increased until something breaks gently!     Not storing energy during the test is important because energy determines the power of the explosion, not pressure.   Compressed air shouldn’t be used to test a boiler because it contains a fair amount of stored energy, and steam is much more powerful again.

Although big boilers are seriously dangerous, the small ones built by Model Engineers have an excellent safety record.  They contain less energy, tend to be over-engineered, only operated at moderate pressures, and aren’t not worked hard for years on end.  Further, Copper is forgiving in that it’s more likely to bulge and release steam slowly through a tear than go off like a bomb.  Still need to be treated with great care though – a faceful of super-heated water mixed with the contents of a red hot firebox would ruin your day!

As for tyres, they’re made of reinforced rubber, thick enough to take the design pressure.  They are quite strong!  I’ve seen a bicycle tyre explode whilst being filled at a garage.  Went off with a loud band, but tore rather than disintegrated – no-one hurt.  Never seen a car tyre fail with a bang but I’ve read lorry tyres and bigger have killed and maimed several.   They contain a lot of compressed air…

Dave

Continuing:

Modern design codes for pressure vessels are based on yield stress, or more correctly proof stress. This ensures you don’t get gross permanent distortion even during hydraulic testing. SOD is quite correct that annealed copper has a much lower proof stress than hard drawn.

As a teenager I expermented with blowing up plastic bottles with a fridge compressor (at the end of a long extension lead and behind a Lexan screen) I can’t remember what the big 10″ industrial pressure gauge went up to (probably a couple of hundred psi) but the PET drinks bottles were fine at max. With no gauge I could not keep a fitting in the neck. The old white PVC Fairy Liquid bottles were more interesting. They expanded as the plymer chains aligned then stopped before finally busting at around 100 psi. Very similar to how a soft copper boiler would behave but with much more expansion.
It might make a good demo at a ME club (using water or other incompressible fluid) if you can find a pvc bottle.

More recently we had a instrument that pressurised a small (0.5l) pyrex bottle to displace liquid. The pump was a tiny diaphram pump run from a DC motor and would only produce about 2 psi even when run at twice it’s rated voltage. The basic cetification argument was that even if you connected the bottle to 100psi “shop” air you would not come anywhere near the 250 Bar-litre limit (3.5 B-l at 100 psi). The safety argument was the bottle would never see more than 1 psi in normal operation and the bust pressure would be at least 100 times this. A secondary safety “feature” was the push-on silicone hoses would blow off the plain tube fitings at about 5 psiI.
Then someone wanted confirmation of the bust pressure of the bottle. The maker would not say. So I asked the in-house lab for the three oldest, most battered bottles they had. There were two 0.5l and one 1 litre. They were scratched and had been autoclaved many times. Tested with water the caps (heavy thermoplastic) failed before the glass on the 0.5l. The 1 litre failed at around 300 psi. This was higher than I expected.. A new 1l failed at >400 psi.
The existing advice not to used chipped or scratched bottles was retained and everyone was happy.

On the quiet the boss and I “failed” a 2l bottle on air inside several nested triwall cardboard boxes in an empty closed waste skip. The results were very impressive.