Boiler pressure

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Hello all. I’ve made several copper boilers in the past and checking the safety valves on them that they blow at the pressure needed. Experimenting with the springs I want them to go at 80 lb and 100 lb as that is the max on the gauges. The 100 lb the valve is almost there with a slight tug with pliers. My question is what pressure would explode a 1.2mm copper boiler with stays approximately 9 inches long 4inch dia silver soldered if the safety valve was faulty. I could pump up my racing cycle tyres in the day at over a hundred lbs ,so I’m just curious about the boiler, when steam is released at 100 lb, to me looks frightening.

Thanks Bob

[noel shelleyParticipant @noelshelley55608]

The first point would be that Your gauges should have a full scale reading of one and a half times the maximum pressure so a 100psi gauge should be used on a boiler of 65psi boiler max working ! Any of the good books on model boilers will give you the calculations, eg Model Boilers and Boilermaking by KN Harris. But using safety factors of 8 or 10 you SHOULD be looking at pressures in excess of 500 and may be as high as over a 1000 psi. The skill of the maker and the materials used will be the governing factor ! NEVER test a boiler with air ONLY fluids eg water!

For only 1.2 copper, without doing the sums I would not exceed 50psi ! Good luck. Noel.

[Paul KempParticipant @paulkemp46892]

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.

[noel shelley]

On noel shelley Said

 NEVER test a boiler with air ONLY fluids eg water!

 

Just being pedantic (who, me?) air is a fluid.

 

[noel shelleyParticipant @noelshelley55608]

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

[HopperParticipant @hopper]

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.  Other series and diameters vary though. Just a ballpark figure to show there is no need to panic about 100psi with copper.

But I would still refer to the old books mentioned and make sure for the diameter you have etc. The KN Harris book on model boilermaking is all over the net to download the PDF. (Been out of print for years I believe.)

Safety valve should lift on its own without help from pliers, and should be set about 10 psi above the normal working pressure you plan to run the boiler at.

A small hand pump to pump the boiler up with water to twice its working pressure would make a reassuring real-world test. The boiler inspectors used to do that with full sized industrial boilers when I were a lad. Just a simple piston with a long hand lever to operate it and two check valves are all it is. Once the boiler is full and all air bled out using a hose connected to mains/feed water line etc, it takes surprisingly little hand pumping to raise the pressure to 200 psi from there if all is valved off tight.

[duncan webster 1Participant @duncanwebster1]

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.

[Paul LousickParticipant @paullousick59116]

Without seeing details of the boiler I would guess that the boiler would not be approved under the Australian Miniature Boiler Safety Committee (AMBSC) Code Part 1 for copper boilers.

For a start, the barrel thickness should be 1.6mm, not 1.2mm

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Speechless, by Paul how odd. The miniature gauge has 120 lb ,the point of a safety valve is for as it says safety that’s was the point of trying different springs.

[Paul KempParticipant @paulkemp46892]

For sizing a good quality industrial gauge the recommendation is the system pressure should fall between 25% and 75% of full scale deflection (FSD).  For system pressure you should read the safe working pressure of your boiler.  Miniature gauges cannot really be considered good industrial quality so the figure Noel quotes is more appropriate.  On industrial standards your 120psi FSD gauge could be considered acceptable for your 80psi boiler but not for the 100psi unit (100psi is @83% FSD of the 120psi gauge).

Yes, a safety valve is designed to provide a level of safety.  However there is a little more to it if you want to be safe!  The safety valve must limit the pressure in the boiler to a maximum of 10% above the safe working pressure of the boiler, when the boiler is steaming at its maximum rate of steam production.  From your original post asking at what pressure your boilers would fail catastrophically (explode in your words) it seems doubtful you know what the safe working pressure of your boilers actually are?  Adjusting a safety valve to control pressure at a level above the safe working pressure is not really going to give a safe situation.

A secondary function of the safety valve apart from lifting to limit the boiler pressure is actually also closing with a minimum hysteresis to ensure the stress in the boiler is not constantly changing over a large range, introducing fatigue.  Therefore the rate of the spring becomes important.  You can adjust a low rate spring to allow the valve to lift as desired by applying more compression with a large hysteresis or you can adjust a high rate spring to lift at the same pressure with less compression (see Hookes law) with a lower hysteresis.  The actual spring rate you adopt is dependent on many factors and influenced by the valve design – but as you find my previous comment odd I am sure you know this.  Fundamental point is for the safety valve to provide the desired level of safety you first need to understand the design limits of your boiler and the resulting safe working pressure and maximum pressure applicable.

Paul.

[Hopper]

On 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

[BOB BLACKSHAW 1Participant @bobblackshaw1]

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.

 

[noel shelleyParticipant @noelshelley55608]

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 !

[JasonBModerator @jasonb]

There  is also a difference between Run as a display engine and Run to do work. As I recently posted I can blow into my engines and get them to run but they would stall as soon as I apply the slightest load. So there is a need for a boiler to produce a certain pressure depending on what it is  intended one will need hardly any pressure the other may need several 100 psi

Also note it is psi not just the pounds this is where stays etc come in as they are needed depending on the area in square inches that the force in pounds is applied to. And likewise the area of the piston as a big piston will not need as much pressure as a small one to do the same work.

[BOB BLACKSHAW 1]

On 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.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Yes I did take it to 100 lb in steam, so that is considered to be highly dangerous, best keep to a nebuliser.

[BOB BLACKSHAW 1]

On 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

[SillyOldDuffer]

On SillyOldDuffer Said:

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.

It’s more complicated than that Annealed copper and hard drawn have the same UTS, as the annealed is stretched it becomes hard. However, the ductility is higher, so that when the annealed is ready to go pop, it will be a bigger diameter and thinner than the initially hard, and so will go at a lower pressure. Not by a lot!. The end plates and stays are probably the weak link. Once upon a time someone reported in ME testing a boiler to destruction, perhaps the archive divers cann find it. At our club we once pumped up a loco type boiler which had no side stays. The deflection was severe, but we couldn’t get it to actually burst. The hand pump wouldn’t go any higher fighting against the pre-existing leaks.

[duncan webster 1Participant @duncanwebster1]

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.

[Dave HalfordParticipant @davehalford22513]

Bob

Re your bike tyre pressure. You are forgetting that to produce steam at 100psi the water temperature has to be way over 100C which has an effect on the boiler materials.

As an extreme example you can pump up a 2litre pop bottle to over 100psi of air and all you get is the thicker base dimples changing shape. Fill one with 100psi of steam and the result won’t be funny.

[Robert Atkinson 2Participant @robertatkinson2]

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.

 

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Apologies to all that replied, I made a big error, my boilers I made are 66 mm in dia.I got mixed up I have a long length 2 meters  at 3 1/2 inch dia 16g which I haven’t used, I thought that was what I made them from. Put it to old age, yes 4 inch at 1.2 madness.

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