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Member postings for duncan webster

Here is a list of all the postings duncan webster has made in our forums. Click on a thread name to jump to the thread.

Thread: Model boiler safety calculations
20/01/2021 18:23:12

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.

Thread: Mechanical lubricator ratchet wheels
20/01/2021 16:17:05

One way clutches are cheap enough, but you won't get one that small

Thread: Model boiler safety calculations
20/01/2021 16:11:26

Dave, if girder crown stays are not banned they are allowed, there is no room for inference in standards. You just need to justify them

I can't make the spreadsheet work, it downloads but then I can't enter anything in the boxes. However it still doesn't reference back to any national standard, which is my gripe all along. Note the get out clause on line 4 'All of these are for guidance only and should not be the sole basis for any designs. It appears OK as far it goes but it makes no mention for instance of reinforcement around big holes in the barrel for domes.

BS5500 is based around 0.2% proof stress at the appropriate working temperature and requires a factor of safety of 1.5 between working and proof stress. However it also requires a test pressure of 1.25 times working pressure. This means that at cold pressure test the stress would be proof stress/1.2. According to the Copper Development Association the cold proof stress of annealed copper is 9370 psi, so stress at pressure test should be < 9370/1.2= 7808 psi, and therefore stress at working pressure should be <7808/2=3904 psi, this is less than that suggested in the spreadsheet. Does this matter? Well probably not, the stress at test pressure (8000 psi) would still be comfortably below proof. The proof stress at the temperature corresponding to 100 psi is 8909 psi, so using my 3904 figure the factor of safety at working pressure is 2.3, which is well above the 1.5 required and so there is no need to have any arguments about whether to make design pressure higher than working pressure to allow for safety valve accumulation. The Australian code quotes 3771 psi. In the interests of international harmony I'd plump for this figure, then if you emigrate and take your loco with you it might still be acceptable. And yes I do know that BS5500 is superseded, but it's the only one to which I had access. I haven't checked the spreadsheet values for stay spacing, but have no reason to doubt them.

There are some aspects of the Aussy code which I find restrictive, for instance the ban on end to end longitudinal stays silver soldered both ends, but I can see why, it is very easy to finish up with such stays slack due to differential heating. Don't ask me how I know. They also ban connecting the top and bottom water gauge fittings by anything other than the gauge glass. I do see the argument, but I'm not sure how valid it is.

The current SFED position is illogical, it requires justification in some circumstances but doesn't mention what stress is acceptable. All I'm agitating for is for the SFED to come out of the woodwork and tell us. Complicated calculations are not needed, the tables in the Aussy code or a simple spreadsheet would suffice. It could be produced in both imperial and metric very easily. I would offer to do it, but it would need to be endorsed by SFED, I don't have Professional Indemnity Insurance any more.

This is all aimed at designing boilers which will pass the 2*wp test, you'd be a bit sick if having invested hundreds of £ and a lot of effort it failed. Again I know this doesn't happen often but we had one in our club years ago where the crown came down.

Edited By duncan webster on 20/01/2021 16:14:26

20/01/2021 00:34:54

In my copy of aussy code (1994) para 3.4. 2 is relevant to staying of the backhead and tube plate not the firebox crown.

19/01/2021 23:55:36

Who's trying to change the test procedure? Not me or any other contributors.

Thread: Fast charging anyone?
19/01/2021 17:41:16

watts per mile is a bit of an odd unit, I'm not sure it means anything.

According to the interweb it takes 15-20 kW for an average car to cruise at 60mph, this is 250 to 300 Watt hours per mile, so perhaps that's what pgk's unit should be

converting CO2 and water into hydrocarbon fuel won't get over the NOX problems in inner cities, but it would be a very good start.

Thread: Model boiler safety calculations
19/01/2021 17:19:54

Working to the Aussy code is not difficult, just look up some tables for most things. They are at least based on national standards rather than something written in a book without any justification. Why are people so resistant to it? For PhilH1's example you could get away with a thinner barrel and save some money. I can't find anything banning girder crown stays, they are more difficult to justify but not impossible.

Yes if it passes the pressure test it is OK, but if it fails because of a design error you'll be a bit fed up, it has been known!

Edited By duncan webster on 19/01/2021 17:21:10

Thread: Ethanol
19/01/2021 16:40:34

IPA is no good for firing your Mamod, took me ages to get the soot off

Edited By duncan webster on 19/01/2021 16:41:14

Thread: Motorcycle General Discussion
18/01/2021 22:19:40

Personally I liked the BMW telelever for chasing around, it handled better than the GSX-R1100 up to ~130 ish on 'B' roads.



I'd ask a moderator to delete this if I were you, Mr Plod might be watching!

Thread: Model boiler safety calculations
18/01/2021 17:51:48

+1 for the Aussy code, the SFED document (Brian's link) is not really helpful on how to do calcs, it just says you might have to provide them.

If you have access to a library which keeps such things then PD5500-2003 covers unfired fusion welded pressure vessels in copper, this is probably as near as you will get to a national standard for silver soldered copper boilers. The old standard was BS5500, this didn't cover copper, but it did cover aluminium and some logical adapting can make it cover copper

Thread: Design of boilers
18/01/2021 17:39:58

OP is making the mistake of trying to apply full size design rules to models. If the water spaces around our size fireboxes were too small and restricting release of bubbles then we'd get overheating of the plates, burning and bulging, similarly for gaps twixt tubes. I've not read of any such problems, apart from when spaces are full of scale. Simple way to look at it our firing rates are much smaller than full size, so heat fluxes are smaller. Flow in tubes in our size is laminar, full size is turbulent, so any design rule for full size is unlikely to be helpful. Article by Martin Johnson in ME was most illuminating. Jim Ewins showed many years ago that model boilers are typically 70% efficient, which is comparable to full size. OP seems determined to show that long established practice of both design and construction of model boilers is wrong without any practical or theoretical backup.Let him make a boiler with 2" water spaces around the firebox and a couple of 1" firetubes, not sure how he'll get any coal in!

Thread: bolts
18/01/2021 17:26:32

could it be M2? 9ba is unlikely, it has been a 'non preferred' size for many years

Thread: Motorcycle General Discussion
18/01/2021 00:30:02

Clive's right, it was Roe. I like the look of the Hossack setup referred to by Bill, but as SWMBO won't let me have a bike anymore I'll just have to dream

17/01/2021 18:36:00

Way back in the 1980s I went to a talk by a chap (who I think was called Rose) from Manchester University who had been working on motorcycle front fork problems brought about by some police riders having nasty experiences with big BMW flat twins. He advocated using leading link forks, and Gus Kuhn made some racing Beemers which did quite well. All this resulted in the BMW telelever, which is a sort of cross between teles and the old girder forks. Vincents stuck with Girder type until the end.

This is all based on 30+ year old memory, I suspect that a trawl through the IMechE archive might produce more information

Thread: Digital Height Gauge Recommendations?
16/01/2021 22:55:38
Posted by Pete. on 16/01/2021 17:04:06:

Andrew T, I bought a sylvac digital caliper .........., I read somewhere they have a lifetime warranty and are Swiss made, ...

Anyone know if that's for real? I've got one which is probably 20+ years old which has expired, if it has a lifetime guarantee I might contact them. Of course as it's dead its guarantee might have died with it!

Thread: dual boot Dell laptop
16/01/2021 14:14:26

Thanks to SOD and JAS I think I've sorted it (don't let the laptop hear this, it will find another way to bite me!)

Thread: It Is A Steam-Engine... Using the term loosely
15/01/2021 13:18:41

Nigel, you're getting confused. My reference to 3 kWhr was in reference to the multiplication given by the heat pump, 1kWhr electricity in, 3 kWhr heat out. I know it sounds too good to be true, but it works. 3 kW/hr is a nonsense unit, a kW is 1000 Joules/sec, so kW/hr would be Joules/(sec*hr*1000), which is not very useful. I did not suggest that 3kW is enough to heat my home, I suggested that if I could absorb the solar irradiance I could have 150kWhr/day, which is about 6 kW continuous. If there is also heat coming from below as others suggest, I can have more. Coefficient of performance is a well known term in thermodynamics, not some salesman waffle. I don't think London's suburbs are all that leafy in January, OK irradiance will decline as you go north, that was the only figure I could find

There is no point trying to argue that ground source heat pumps don't work, they do, there are hundreds if not thousands in use. There will no doubt be some situations in which they are not appropriate.

Round where I live are schemes to put small scale hydro on the existing weirs on the Mersey and Weaver, but newspaper reports suggest the one on the Mersey will generate 'enough electricity to power 380 homes', not exactly a game changer.

The only alternative to renewables of some kind is Nuclear, and I don't see any of our current crop of crowd pleasing politicians signing up to that, although I would welcome it. Perhaps 30+ years in the industry has made me biased, but I've still only got one head and I don't glow in the dark (well only my halo (joke))

Edited By duncan webster on 15/01/2021 13:20:37

Thread: Boring bar size ?
15/01/2021 11:38:23
Posted by ega on 15/01/2021 11:11:47:
Posted by duncan webster on 14/01/2021 22:25:06:
Posted by Nigel McBurney 1 on 12/01/2021 11:49:01:

For boring a carb out a couple of mm,I woud make a boring bar from 19 (3/4 inch) mm dia silver steel and use a high speed steel tool bit with a lot of rake .........

Why silver steel? Strength is not likely to be an issue, and silver steel is no stiffer than mild steel.

GHT was asked this question about his similar design; his reply, IIRC, was that silver steel was harder and more likely to be true to nominal size.

So? nominal size of a boring bar doesn't matter (one of mine is black bar), and unless you're looking for very long life in an industrial environment I'm not sure hardness does. It's the little bit of HSS that does the cutting, not the boring bar

Thread: Drawing for a whistle
15/01/2021 00:07:10

What a neat idea! Let us know if it works. I suspect you'll need a pretty good joint twixt divider and tube

Thread: Boring bar size ?
14/01/2021 22:25:06
Posted by Nigel McBurney 1 on 12/01/2021 11:49:01:

For boring a carb out a couple of mm,I woud make a boring bar from 19 (3/4 inch) mm dia silver steel and use a high speed steel tool bit with a lot of rake .........

Why silver steel? Strength is not likely to be an issue, and silver steel is no stiffer than mild steel.

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