Boiler thickness and pressure

Although I don’t want anybody to rush out and make a boiler from a bit of copper foil and I’m not knocking any of our club boiler inspectors who do a very good job keeping the hobby affordable some may not like the following but it is based on fact.

There seems to be an opinion that some of these old designs that new ones are having to be based on were a bit over the top as far as materials were concerned and the other side of the argument has not yet come up with any figures to back up why these thick boilers are still what we should be using apart from they have been like that for ages when boilers were put together with a paraffin blowlamp and pile of coke.

Over the last few months I have been party to a friends alterations to a published boiler design, mainly to bring it upto date with current requirements eg separate feed to pressure gauge but also to get a working pressure of 120psi which is the point of this post.

The original design which is by a published author with at least a dozen model designs published most of which have been in ME at some time. It is a 5” OD boiler x 10swg in copper and silver soldered, drawn tube for the barrel and with a working pressure of 70psi.

Now my friend does not have the facilities to take on the boiler build so a professional builder (member of the assoc of prof boiler makers) was asked to come up with a new design that met the 120psi requirement which is a 70% increase in the original designs working pressure.

So what’s on the drawing for this new boiler?

3mm rolled and soldered copper barrel. So not only down from the 3.25mm (10swg) but also a joint right down it which from the calculations looks to be about 0.8 as the joint allowance, the higher temperature would also have been figured in to go along with the higher working pressure so all in all the old design was about twice as thick as it needed to be.

Now there is also the fact that this was for a traction engine, as mentioned last night by someone this makes the barrel a structural chassis so make an allowance for that. Now there is also the cyclic effect of a big compound cylinder stuck on top so again make an allowance for that in the calculations. Oh and did I mention it’s a ploughing engine with a longer barrel than most traction engines not to mention a big old casting an about 70m of cable slung below that will weigh in at around 12kg. And best not even think about the loads when it’s got a scale plough hooked to it.

So a boiler sitting comfortably between a pair of loco frames should have a very easy life. This seems to also be borne out by the stress analysis posted today where the 16swg barrel did not fail, only the unstayed end.

SDL's figures would give a thickness of 0.070" for the original 70psi design pressure and seamless tube

Where this new boiler has had changes is in the staying and flat plates, something that Fizzy mentioned was where the stresses were in his home experiments. I won’t go into too much detail about the changes but the main one is to do away with girder stays which do appear on these old hand me down designs and go for radial rod stays between the top of the firebox and crown of the boiler. Much like the Australian code requires

Well something to think about, we have moved on from riveted and caulked boilers and ones using sifbronze so why not move on from these heavily over engineered designs that people seem to want all boilers to be like. You moan enough about the cost of copper well if you are using twice as much as you need to who is to blame.

That’s enough for now I’ll hide behind the sofa for a while but may have a few more tip bits to add. Hopefully this will make for civilised discussion and get people looking at boiler designs in a new light rather than just following whats already been done

J

Edited By JasonB on 24/10/2014 14:42:10

Jason,

The FEA I posted, if you only consider the tube section, is little more than an "easy" hoop stress calculation as mentioned by Andrew. The information was just to give some indication as to what might be happening in a pictorial form, it should not be used for any justification of design etc.

Mark

The safety factor of 8 does seem rather high to me? However, in at least some of the calculations it appears to be used to account for, at least in part, the difference between ultimate tensile stress and yield strength. So presumably if one uses yield strength one can use a lower safety factor, whatever that might be?

Regards,

Andrew

Andrew,

I would think 8x factor also takes account of the reduced strength due to temperature etc. I do not know what the reduction is for copper but mild steel is reduced to 40% at 400 degrees C. I am certain the early posts mentioned it took account of temperature?

Mark

From SDL's posting the temperature is taken account of by a separate allowance and the safety factor remains the same at 8.

J

I'm not familiar with boiler codes, only some unfired vessel codes.

Over the last few decades, required design factors in the codes have tended to reduce somewhat, reflecting improved metallurgy, application of quality systems in manufacture, the use of more sophisticated stress analysis etc.

Sometimes the lower design factors are allowed only in very limited circumstances, but there has been a general reduction in required design factor.

A code with higher design factor typically uses more material, but requires less/simpler analysis, maybe less testing, and perhaps requires pressure test to a larger multiple of MWP.

The saving in materials from a lower design factor code may be offset by costs of extra testing, additional stress analysis, fatigue life evaluation, and 3rd party inspection costs.

Both approaches have merit - and both should result in equipment that is fit for purpose. I feel that some of the contrasting opinions here reflect this. It is important not to try to 'cherry pick' aspects from both approaches - that would lead to major problems.

Edited By David Jupp on 24/10/2014 15:46:30

The martin Evans book that I took this from has good worked examples and certainly the tube thickness is what is in the Northern association guidelines. Note these design guidelines are separate from the testing book. This should not stifle developments but few boiler inspectors are going to have the experience to review FEA but this should not stop people adding blind bushes for fixings of door runners etc

Please spare a thought for your Boiler inspectors, many do a thankless job, at the end of the day its them that has to sign off and pass things. At our club for many years the boiler inspectors thought they were covered by the club insurance, It was only when I raised a question in writing during the review before the last code change that it was made clear that the club needed separate indemnity insurance for the boiler inspectors otherwise there assets are on the line. as you could imagine we took this out promptly. This insurance is shown on the southern fed form but historically the implications had not been understood.

Steve

Hi guys, I know I have come this thread a bit late, but heres my two cents worth! Most burst calculations are based around Barlows equation. If I calculated this I would use UTS & apply my temperature correction on this value before calculating further. I would take a nominal ambient temperature of around 24 degrees C & calculate my temperature offset from that. Then plug that into Barlows equation. I note that some of the equations quoted seem to incorporate a temperature factor into the entire equation. This may well be one reason there is such large descrepancies. Regards R