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[steamboy]

Hi, Ive just made a new 4inch vertical boiler, but have a problem with the tube bulging at only 40psi, I used Peter Barlows formula to calculate the working pressure. which was 83.75 PSI. I stopped firing when I saw a slight bulge at 40 PSI, There was no bulge under hydrostatic test that I noticed. I don't think it would get a boiler pressure certificate in this condition do you. My only thought as to the bulge is the fact that when heated to Silver solder the boiler together the Copper is still very soft. and may cause the bulge in the centre of the boiler. Any thoughts please guys ?


Steamboy

[derekwarner]

Steamboy....whilst you offer little about the actual design of the boiler you have built, the Australian Standards and Regulations covering the acceptable design and manufacture of model steam boilers, and  approvals to use in a public area  is a complex issue

http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQjBAwAQ&url=http%3A%2F%2Fwww.aals.asn.au%2FAALS-Standards.htm&ei=YaXhVIzZJdji8AWYwYFw&usg=AFQjCNFM0h8n9JUd2hmeyww3LH_-28drIw

You will find many differing terms such as .......design pressure, test pressure and working pressure.....

FOS...[factor of safety] ...is not necessarily considered in the basic Barlow calculations....Derek

[AlexC]

Hi, Ive just made a new 4inch vertical boiler, but have a problem with the tube bulging at only 40psi, I used Peter Barlows formula to calculate the working pressure. which was 83.75 PSI. I stopped firing when I saw a slight bulge at 40 PSI, There was no bulge under hydrostatic test that I noticed. I don't think it would get a boiler pressure certificate in this condition do you. My only thought as to the bulge is the fact that when heated to Silver solder the boiler together the Copper is still very soft. and may cause the bulge in the centre of the boiler. Any thoughts please guys ?


Steamboy

Hi Steamboy,

That is not good to hear after all your hard work building your boiler.

I suspect you have used far to high a value for 'S' which would yield a higher allowable pressure for a given wall thickness... the result being a wall thickness which is to thin.

You have not provided any details of your design with regard to Material sizes used, type of material (copper, Brass etc) or the actual working pressure you are designing for/requiring.

As Derek has stated... there is no direct allowance for a safety factor or annealed strength variance in Barlows' formula... you need to adjust the value of 'S' to provide for these.

Assuming you actually wanted 83.75psi and the boiler shell was Copper... then the value for 'S' should be 3125psi and the wall thickness would need to be 0.054" (1.37mm)... giving a safety factor of 8 for the hoop stress in an annealed condition.

If you provide more details we can help... and calculate a safe working pressure for what you have actually built.

Best regards.

Sandy.

[steamboy]

Hi, I`m going to scrap this one and start again. However, The materials I used for the boiler were  copper,   with a wall thickness of 0.067" and a length of 4 inches. The S value was 10. the allowable material stress of the copper tube was 25,000psi The operating pressure I prefer to use is 50psi. So Barlows formula gave me the working pressure of 83.75 lbs. So do you think the S value is too high and what value do you think S should be. The 21 fire tubes were of copper as well. As you mentioned Barlows formula down`t allow an annealed strength of copper either.


Yes a shame to scrap it after a lot of work, but the next build hopefully will turn out a better result.


Steamboy.

[AlexC]

Hi, I`m going to scrap this one and start again. However, The materials I used for the boiler were  copper,   with a wall thickness of 0.067" and a length of 4 inches. The S value was 10. the allowable material stress of the copper tube was 25,000psi The operating pressure I prefer to use is 50psi. So Barlows formula gave me the working pressure of 83.75 lbs. So do you think the S value is too high and what value do you think S should be. The 21 fire tubes were of copper as well. As you mentioned Barlows formula down`t allow an annealed strength of copper either.


Yes a shame to scrap it after a lot of work, but the next build hopefully will turn out a better result.


Steamboy.

Hi Steamboy,

OK I am a little confused by your stated design values... If it is 4" long then what OD is the main barrel?... or did you actually mean to say Diameter rather than length?

You also mention an 'S' value of 10... this sounds like a misunderstanding of the requirement... where does the 10 come from? and how are you applying it?

The value 'S' in the formula should be the max allowable safe stress for the material... i.e the 25,000psi figure.
This being the tensile strength of Copper allowing for the annealed condition and elevated temperature.
However, you also need to allow for a safety factor... this is usually 8 for model boiler shells.

25,000 / 8 = 3125psi... So THIS is the value you should use for 'S' for copper boiler barrels.

Assuming you actually meant 4" outside diameter and with a wall thickness of 0.067" (1.7mm)
This would give you a maximum safe working pressure of 108psi... so 50psi would be well within safety limits.
The length of the barrel has no effect on the working pressure of the barrel.

If this is what you have then I cannot see why it would bulge at 40psi so something is clearly amiss with the build... I suspect the wall thickness may not be what you think it is.

The formulae for solid drawn tubes subject to internal pressure, which most regulations specify are : -

For wall thickness 'T' : -

T = P x OD / 2t...  This can also be written as T = P x OD / 2S

And for maximum safe working pressure 'P' : -

P = 2T x t / ID... this can also be written as P = 2T x S / ID

Where P = Max safe working pressure.

          T = Wall thickness of shell in inches.

          OD and ID = Outside diameter and Inside diameter respectively.

          t or S = The maximum allowable safe stress of the shell material in psi.

    For Copper in an annealed state and with a safety factor of 8 this would be 3125psi.

The internal fire tubes would have no effect on the strength of the outer barrel so they can be taken out of the equation for now, however, they are subjected to External pressure which requires an entirely different set of formulae and Length does make a difference for these.

As stated... clearly something is amiss in either the material measurements or in your calculations so I order to track down where the problem is, or where things went wrong for you... can you provide a full list of dimensions and material actually used... a drawing of the boiler would be useful if you have one.

I would be happy to help you with your design if you need it.

Best regards.

Sandy.

[ooyah/2]

Hi, Ive just made a new 4inch vertical boiler, but have a problem with the tube bulging at only 40psi, I used Peter Barlows formula to calculate the working pressure. which was 83.75 PSI. I stopped firing when I saw a slight bulge at 40 PSI, There was no bulge under hydrostatic test that I noticed. I don't think it would get a boiler pressure certificate in this condition do you. My only thought as to the bulge is the fact that when heated to Silver solder the boiler together the Copper is still very soft. and may cause the bulge in the centre of the boiler. Any thoughts please guys ?


Steamboy

Steamboy,
I find it hard to understand why your boiler that having a 16 S.W.G. ( .067" thk )  tube which can take a safe working Pressure of  104.6 P.S.I. will bulge at 40 P.S.I.

You say that it was hydraulic tested, but not at what pressure, did you test it to 2 x 87.35 p.s.i. ( 166.70 p.s.i. )  ?

If tested to that did you take it up to 166.70 p.s.i. all in the one go ?

You should always when doing a hydraulic test take the pressure up in 10 p.s.i. stages letting it down after each increase before reaching your final test pressure
did you do that ?

By not taking the pressure up in stages you can very certainly cause a bulge in the shell as you know at that stage the copper is very soft after the S/Soldering process.

Your boiler shell with a .064" thk shell can take a working pressure of 100 p.s.i. so if you didn't take your intial test up in stages that is probably when the shell bulged but you didn't notice it

Have you measured how much it is bulging ?

I wouldn't scrap it but have a talk with your local inspector for his opinion and if he will not pass it keep it for your own use as a test boiler as you are only wanting to use
 50 p.s.i. and then build another for use in public places if that is where you wish to use it.


While composing this post Sandy has beeten me to it  but I still think that you have pumped the boiler up in a single stage which has caused the bulge.

The calcs that Sandy quotes are the ones that I use from K.N.HARRIS'S book on model boiler construction.


George.

[steamboy]

Hi Thank you your help and replies, As you kindly pointed out, I should have taken the hydrostatic test in 10lb steps, something I didn`t know so I suspect that is why the bulging has occurred.


Overall it has bulged by 6mm in circumference which is its widest point.
 
Alex I did mean 4inches in Diameter drrr ! and as you say the working length has no affect on the working pressure of the boiler. I used a safety factor of 10 and not 8. Hence the lower maximum safe working pressure. I did check the shell tube wall thickness and it was as stated, so no problem there.


Thanks for your offer of help in design Alex , It is but a simple multi tubed shell and dos`nt really need to much thought to construction. Although I am going to scrap this model boiler I have learnt something along the way. When setting up for a hydrostatic test be sure to do it in 10 psi increments. And don`t rush it.


George would you know where I can get a copy of K.N. Harris`s book on Model Boiler construction.


Steamboy.

[derekwarner]

Steamboy.......what part/location of OZ do you reside?.....the AALS have member affiliates Australia wide...& as George has eluded to ..it would be good to touch base with a local club boiler inspector prior to your next build......

I am unsure what has happened to your 4" OD boiler barrel.....& there appears to be something fundamentally wrong here

Explaining a shell expansion of a model boiler by growth circumference is certainly unique and clouds the understanding further, however I see

It was 101.4 OD which was ~~318.4 circumference
If it has increased 6 mm in circumference it will now be ~~324.4 circumference  which equates to ~~1.9 mm increase on diameter

So does the boiler now measure ~~ 103.3 mm diameter?.......Derek

[AlexC]

Hi, Ive just made a new 4inch vertical boiler, but have a problem with the tube bulging at only 40psi, I used Peter Barlows formula to calculate the working pressure. which was 83.75 PSI. I stopped firing when I saw a slight bulge at 40 PSI, There was no bulge under hydrostatic test that I noticed. I don't think it would get a boiler pressure certificate in this condition do you. My only thought as to the bulge is the fact that when heated to Silver solder the boiler together the Copper is still very soft. and may cause the bulge in the centre of the boiler. Any thoughts please guys ?


Steamboy

Hi Steamboy,

Thanks for the update on your figures, they now make more sense.

Whilst I agree with George on the need to increase pressure in stages during Hydraulic testing this does not exactly answer the original question.

You stated (in your initial post) that you did not see any bulge when performing the hydraulic test (which is exactly where I would expect to see any deformation and is one of the primary reasons for the test being undertaken/required) so what pressure did you test it at?.
If you are intending to work at 50psi then it should be hydraulically tested to 100psi.

Once you have arrived at this test pressure it should be held there for 20 minutes minimum with no loss in pressure.

At this pressure there may be a small increase in diameter at the centre of the barrel, since it is unsupported and the material is still quite soft after soldering. The exact amount would be proportional to the length between the end plates... but I would not expect this to be more than 0.010" in total diameter at this pressure for a modest model size boiler.

Even if you took it quickly up to 2 x working pressure it would still work harden, although the final diameter may be slightly more...at perhaps 0.020" increase... due to the more rapid rise in pressure.

The act of performing the Hydraulic test, If done in stages, will cause the Copper to work harden and once the test has been performed there should be virtually no further change in dimension... even when fired (unless you run it without sufficient water in it).


You then state that it only showed a bulge during firing, and at only 40psi... which makes little sense to me since the hydraulic test would have work hardened the barrel at a stable level... and firing the boiler should make no difference.

You appear to be getting around 0.075" increase in diameter at only 40psi... how exactly are you measuring this expansion/bulge?
This leaves me suspecting the integrity/quality of the tube you have used for the outer barrel and suspect it may vary in thickness in some locations, or perhaps it has an unstated inclusion in it's material content causing this behaviour... where did you get it from?

For model boilers you should be using Copper of 'C106' grade.

I agree with George and Derek, take it along to your local club and let their boiler inspector look it over... I would not be in too much of a hurry to write it off at this stage.

You can download a PDF copy of the K.N. Harris book here: -

http://449511.internet-bookstore.com/


It has most of the general formulae for boiler making, however, it does not provide any formulae for internal tubes subject to external pressure (fire tubes, flu tubes etc.)... which is a major omission... but does suggest some general sizes for these... other than that, it is a great book to have on your shelf.

Best regards.

Sandy.

[malcolmbeak]

hi Steamboy


All very strange. If you are using different pressure gauges for the static and steam tests I would suspect the steam test gauge. Check one against the other .


Malcolm

[steamboy]

Hi, Derek, The circumference at it largest point is 327mm at the top and bottom of the shell the circumference is 321mm which I measured using a 50lb stainless steel trace wire and plastic coated, very firm and then measured the length.  I did not notice the bulge whilst caring out the hydro test, But it could quite well have been there and I just didn`t notice it as the shell hadn't been cleaned just then.


The copper tube I used was just a piece of scrap bought from a scrappy, I have used the same length of tube on another horizontal boiler which dosn`t bulge so have discounted the non uniformity of the tube.


I did use different pressure gauges, for my hydro and for the first firing test. My test gauge and another gauge which is a standard 3inch dial face. There is a small difference in the pressure which is not even over a pressure range, However it has a 2psi difference between 40 and 50 psi. So have discounted that, I think  George has found the problem of this bulge down to the fact that In my test It was not in the 10psi increments which he kindly pointed out.


Thank you Sandy for the link to K.N. Harris`s book when have a few minutes time I shall download it onto my reader.....And thanks to all of you for your increasing help


Regards,


Steamboy

[derekwarner]

steamboy...you have opened your own can of worms here

1. I do not believe any non-ferrous metals tube manufacturer in OZ would market substandard 99.9% pure copper product for any offcuts to go to a scrap metal merchant

2. Pressure gauges [general range] have a rated accuracy of +/- 1.6 % FSD [full scale deflection] ...so a 1000 PSI gauge when subjected to an indicated 100 PSI could actually be seeing anything from of 84 PSI to 116 PSI actual...by design selection, to measure 100 PSI with reasonable accuracy, the appropriate gauge scale is 1.75 of WP or 175 PSI FSD...so a 200 PSI gauge would be the nearest standard item

3. Your reported linear circumferences are conflicting/confusing between your posts by some 2mm

4. To accurately measure/gauge the deformation of a 4" diameter barrel, a pair of outside calliper's and referenced back to a vernier calliper is a far more acceptable way of understanding a dimension....if you experience variance's around the clock....just sum them & come back with the average 

Derek