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Model boiler safety calculations

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duncan webster20/01/2021 18:23:12
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I've got it to run, it is to do with macro security setting in Libre calc.

Fizzy, I'm not trying to argue that there is anything wrong with this or any other definition of acceptable stress. I repeat yet again, none of them (apart from Aussy) say what they are based on.

I'm not even trying to argue that a code should be mandatory, as I've said before if it passes 2* wp test there can't be a lot wrong with it, but how is a boiler inspector supposed to assess the calcs which SFED want without a recognised criterion? if designed up to the Tubal Cain limit, it would fail the spreadsheet limit, if designed up to the spreadsheet limit it would fail the limit suggested by Keith Wilson. The Aussy code tells you how to do all the sums in a small number of pages, with tables to avoid most of the sums. It is not terribly restrictive, I'll put money on it that Fizzy boilers would pass without even having seen one. If we want to use this spreadsheet just let the SFED endorse it.

I've now said my piece repeatedly, so I'll retire gracefully from the field.

Roger Best23/01/2021 16:14:44
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My professional specialism is Nuclear crane design, so you would have thought that I would have something useful to say about design codes and standards.

Not a hope.

They are written by committees, so satisfy no-one. The committee rarely wants to take full responsibility so they leave lots to the designer, and its usually impossible to correlate between a prescribed design feature geometry, a degree of stress and the probability of failure.

At best any design code is a hint of Relative Good Practice for the observance of idiots to reduce risk to the public.

Its great to see people talking about FEA and parallel calculations. That way any anomalies and errors are likely to come out.

One more point, you need a four times safety factor against failure for good safety in a variable load environment. Buildings can make do with less as their self weight is the dominant load. Aircraft fly with this sort of factor to allow for a blustery flight. A 2X hydraulic test should be well away from causing damage if the design is that sound.

Andrew Johnston23/01/2021 17:48:27
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Posted by Roger Best on 23/01/2021 16:14:44:

One more point, you need a four times safety factor against failure for good safety in a variable load environment. .......................Aircraft fly with this sort of factor to allow for a blustery flight.

Not so; if they did they wouldn't fly as they'd be too heavy. For sailplanes limit loads, including gust loads, are calculated and then a factor of 1.5 is applied to get an ultimate load. The structure should not permanently deform at the limit load and not break until the ultimate load is reached. The loads are also speed dependent so each aircraft will have a flight envelope and speed restrictions such as maximum rough airspeed and maximum manoeuvring airspeed. Below maximum rough airspeed a sharp edged gust will stall the wing before the limit load is reached. Above it something may deform or break. Likewise below maximum manoeuvring speed full control deflection can be applied without reaching limit loads. Above it maximum allowable control deflections reduce to about a third of full deflection at never exceed speed. Applying full up elevator at never exceed speed will most likely pull the wings off - it has been done. smile o

Andrew

Andrew Johnston24/01/2021 09:18:53
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That's odd, there was a following post made that simply said "deleted", presumably by the poster. But it now seems to have disappeared. Is it policy to remove posts where the poster has thought better of it and deleted the text?

Andrew

Nick Clarke 324/01/2021 09:35:27
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This bit from Keith Wilson's description of his Bulldog/Dukedog boiler may be of interest (ME 16/5/80)

boiler.jpg

A subsequent letter suggested the value of 5000 be substituted for the 7000 above.

I wonder if the relative safety of model boiler designs is down to the high factor of safety, rather than detailed design.

SillyOldDuffer24/01/2021 14:41:46
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Posted by Andrew Johnston on 24/01/2021 09:18:53:

That's odd, there was a following post made that simply said "deleted", presumably by the poster. But it now seems to have disappeared. Is it policy to remove posts where the poster has thought better of it and deleted the text?

Andrew

Not policy, but I occasionally delete empty posts. I often remove duplicates and rotate photos for the same reason - it makes threads easier to read.

It's a bit risky, so I don't always intervene. No mistakes allowed whilst deleting because it's permanent and rotating photos can mess up the text.

Dave

Phil H124/01/2021 18:43:33
344 forum posts
40 photos

Nick,

I think you have demonstrated my point regarding small model boiler calculations. The author talks about a safety factor of 6 or maybe 10. Then you say that the a letter suggested 5000 rather than 7000 be used in the equation - so why bother at all?

I think that small copper boilers are quite well established regarding plate and tube sizes and that is all based on many years of successful use. There is absolutely nothing wrong with successful historical information being used because it works.

I think we should avoid trying to suggest some kind of design engineering in taking place by throwing some equations about when there s clearly no sound basis.

Phil H

JasonB24/01/2021 18:50:42
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Posted by Andrew Johnston on 24/01/2021 09:18:53:

That's odd, there was a following post made that simply said "deleted", presumably by the poster. But it now seems to have disappeared. Is it policy to remove posts where the poster has thought better of it and deleted the text?

Andrew

I tend to delete them if I see them to keep the thread tidy just as I also delete double posts. Poster could have thought twice, or simply poste din the wrong thread

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