Boiler Design – issue 4765

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, all I can say is the the UTS is in the plastic range, and there is no elasticity in the elastic range. When I started work, high tensile bolts were not in general use, and when you unbolted things, the same bolts were used to bolt them up again. It soon became clear to me, that used bolts would stretch a little bit more every time they were done up again, and the stretching would accelerate, to the point it was clearly obvious, and the tightness of the bolt could not be made to the original, and it would eventually fail trying to even reach any sort of tightness at all. The UTS tells you the point that a material is most likely to fail. When I did my testing on the Tesometer, you could feel the stretching takin place, as the effort used to turn the handle was getting les as it was approaching the UTS.

Regards Nick.

[noel shelleyParticipant @noelshelley55608]

I take a dim view of being referred as a LUDDITE or the use of other derogatory terms ! May be this thread has run it’s course and should be closed ?  Noel.

[Paul KempParticipant @paulkemp46892]

Magic numbers, my thoughts;

We want to build a boiler.  We know it’s physical dimensions and the pressure we want to operate.  The absolute minimum theoretical shell thickness considering pressure alone will be that thickness that produces a circumferential stress in the cylinder that is equal to the UTS of the material we want to use (ignoring distortion as it has to distort before failure if it is a ductile material).  Using that thickness to build the boiler will result in certain failure when it exceeds operating pressure.  But wait, we are required for the first pressure test on the shell to pressurise it to twice working pressure, we can’t do that, it will fail, so actually to guarantee a pass we need to use a thickness of our material that will resist a pressure of 2xWP +1.  I suggest +1 because we want the absolute minimum thickness at which we can pass the 2xWP test with certainty.  We already have 2 magic numbers, my arbitrary +1 because I wanted to be certain it wouldn’t fail at 2WP and in fact the multiplyer of 2 for the test pressure, who decided 2 was the optimum number?  Whatever, we now have a minimum strength thickness for our boiler that will resist a 2xWP test.  It is already twice as strong as it needs to be based on material strength alone to be operated once at its design working pressure.

We now need to consider the other properties of the material we wish to use outside its basic UTS (or YP), it’s resistance to corrosion, creep, ductile properties, susceptibility to fatigue, behaviour at elevated temperature at the same time as considering how long we want it to last, how many pressure cycles will it go through in its design service life, what external stresses or other stress raisers is it subjected to? For each of these considerations we can assign a correction factor (magic number) or limiting factor through modelling or calculation to define the minimum operating thickness.  We don’t have the luxury of being able to ask Mr Greenly to explain how he arrived at his magic number of 8 but as it provides a result that is close to the alternative proposal I am assuming there was some science behind it, suggested by the results of the two methods being close to each other.

Personally I like magic numbers, I wouldn’t be happy with a plane, train, boat, bridge etc having a magic number of 1 for design strength and with systems that have a poor FMEA, ie single point of failure.

The sole advantage proposed for the YPM appears to me to be ensuring there is no distortion when the 2xWP test is applied and the material remains in its annealed state.  But as the results in terms of thickness correlate quite closely between the UTS and YPM is there a fundamental issue?

I can’t comment on the Aussie code, I don’t have it and thus haven’t read it.

On Lez’s point re RIDDOR, I am very familiar with it and Duncan is correct the definition of workplace is a very grey area and as various individuals have found to their cost the fact they were operating in a volunteer environment for no financial reward is no defence and Courts have ruled in favour of HSE in some instances even where the public were not present.  I mentioned RIDDOR as one example of why an incident would be reportable.  That is a separate discussion though and a distraction from the YPM debate.

As for holding the hobby back, I have no issue with modern manufacturing methods or using available technology to assist design.  The EU accept stainless steel for boilers, the UK should review this.  My criteria for change is it needs to bring clear and tangible benefit over what we had before.  With the small difference in results even ignoring Luker’s points I am on the fence.  If a notified body (that is after all who will approve our design) accepts this is the future direction then I will happily comply.

Paul.

[lezsmithParticipant @lezsmith]

Hi Luker,
With reference to your post, the YP method presented, clearly states that it was for use with “Copper” therefore your point about not using it for other materials is clearly as stated in the proposal.

You go on to say “Design a shell to the minimum thickness” is NOT what was stated in the proposal, it clearly states the calculated value is the minimum thickness required to resist permanent deformation during the 2x working pressure testing, the proposal goes on to state an appropriate safety value MUST be added to account for operational conditions the boiler will encounter, rather than an arbitrary 8x used in the current UTS method.

You state “Practically, this example brings the YP design method much closer to failure than the UT method. And that’s not what we want…”
How can this be true if the points in previous posts above and in the examples given, the minimum copper thickness is generally thicker?

As you said, I also don’t want to get into a debate here on the correct application of numerical methods, therefore I will refer this question to my old professor of Engineering at MIT, hopefully some undergraduate can take this on as research for his/her degree.

 

 

[lezsmithParticipant @lezsmith]

Hi Paul,

First off, thank you for a very good post, no insults just interesting commentary 🙂

Taking your last comment first, fair point on RIDOR, it clearly states workplace, but as you point out “how to define a workplace is debatable”, if I employ a gardener to cut my grass is my personal home now a workplace?

However, an interesting fact was raised “where do I find the RIDOR reports” just doing some research I’m trying to find all the reported “Model Boiler failures” but I’m not able to find any, clearly their have been some, at least 2 as mentioned in the above posts, so where are the records?

I’m not going to comment on the YP vs UTS anymore, we put the presentation and methodology together not as you must abide by this, it was meant to be here is an alternative that does NOT use magic numbers and follows other standards such as the AMBSC that have used the Yield point of annealed copper as the reference rather than the UTS. Our hope is it would provide an alternative by using well defined values that can be referenced in easily obtained literature.

If you are ok with your boiler deforming during the 2xWP test no problem as the builder and perhaps the designer that is your prerogative, just because we provided another tool for the workshop it does not mean you have to use it, but at least it is available and like most tools covers a specific function (defines a minimum copper thickness that will not deform under the 2xWP test) the reality is 99% of boilers will use copper that is significantly thicker than this minimum value, and so they should, they need to increase the thickness by adding a safety margin based on the environmental situation the boiler will be used, then the normal next step is to find what thickness is commercially available, this could be slightly thinner than the calculated value, however with the YP method it is very easy to work backwards to find the calculated safety value for the thinner copper, allowing the builder/designer to consider if the decrease in the safety value is ok or not.

 

 

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