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Member postings for Paul Horth

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

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Thread: Girder crown stays - are they overstressed?
23/12/2011 17:08:27
On the question of cutouts in the girders, I have checked through my 1990s magazines for the more recent boiler designs by Martin Evans.
 
In the six designs below, he used girder stays without ANY cutouts.
 
Roedeer 1994
Eastern Belle 1996
Caledonia 1996 (7 1/4 g)
Adams T6 Flyer 1997
Harringay 1999
Greene Queen 1999 (7 1/4 g)
 
I no longer have magazines for the earlier designs so I can't check those, but it would seem that at some point after the Princess of Wales(1971), Martin Evans decided that these cutouts were not worth making for improved circulation, and/or that they might weaken the stays.
 
In the Durham boiler, I did what has been suggested and simply drilled 1/4 inch holes in the vertical webs. This was less work than cutting the crescent shaped holes on the drawing. Nigel has confirmed that these cutouts do not affect the stress in the girders. I can't judge whether these holes have much effect in improving circulation or reducing scale, but perhaps someone who has built one of these recent designs could have an opinion.
 
Paul
 
 
20/12/2011 20:43:57
I greatly appreciate Nigel's work in carrying out the FEA analysis on the firebox girder stays. I think he has done the hobby a service.
It is now clear that the stresses in these girder stays designed by John Haining, and, by similarity, also the Martin Evans designs, are within the safe working limits used for other parts of the boiler. In my view we can conclude that the calculation of the girder stress using a simple beam model, (the point from which I started this thread) gives results that are quite unrealistic. The prediction of firebox collapse has not happened, and now we know why.
 
Nigel's work is a vindication of the design talent of the two eminent model engineers, Evans and Haining, who had to work without FEA, but using their judgment and experience, backed up by experiment.
 
Paul
07/12/2011 19:41:02
And yet, my boiler still stands, despite your prediction from conservative structural analysis that it should have collapsed.
How so?
My calculations stand ready for your checking.
Consider another example. An aircraft is a pressure vessel, with 1 bar internal pressure and close to zero external pressure. If the aircraft fuselage was designed according to industrial pressure vessel design codes, it would be far too heavy to fly. So it is designed differently, and yet the skies are not full of exploding aircraft. This experience is good enough for thousands of people to trust their lives to them every day. They don't even get hydrostatically tested!
Conclusion: we should be ready to accept that the evidence of reality has priority over predictions from a simplified analysis. When he introduced girder stays, Martin Evans carried out a test at 3 x working pressure on a boiler he designed,, to prove their validity, whereas the structural analysis would have led to the mistaken conclusion of failure.
 
Paul
06/12/2011 10:49:13
Thank you, Jason and Geoff. These answers help my understanding.
I readily admit my lack of knowledge in structural engineering, and I started this thread in the hope that others could offer an explanation of this seeming contradiction between the calculated stress in a simple beam, and actual experience.
The firebox crown is arched, not flat, and this must confer some strength (resistance to bending) which is in addition to the girders and acts to relieve them from some of the load. I can see that the whole assembly of girders, cross bracing and arched crown, rigidly joined together and rigidly joined to the flanged end plates (these are not knife edges) acts as a kind of box which is stronger than two loose beams with empty space beneath. However, calculation of such a structure would be completely beyond me, I have not learned the basics sufficiently to use Roark..
 
We may have to conclude that the simple analysis of Mr Forsyth and the AMBSC is indeed too conservative, something I am not qualified to judge except by the evidence of my non-collapsed boiler.
 
Paul

05/12/2011 23:54:51
ERROR ALERT...
 
"equal to the pressure divided by the area of the firebox crown."
 
That obviously should have been MULTIPLIED.
My apologies!
Paul
 
 
 
05/12/2011 23:48:07
Eddie,
 
Yes I agree with you that either my calculations are wrong, or the prediction of the behaviour from the assumptions used in structural design and materials science are not as good as we think. I would dearly like to be wrong about the stress, so I can feel a little more relaxed when running the engine. But if I am wrong then so is Mr Ross Forsyth, because my calculation follows his and gives results in agreement with his , and he is a structural engineer and speaks for the Australian code.
I will gladly send to you my calculation spreadsheet but I have not found a way to send in a spreadsheet to this forum, if you wish i can email it to you directly if I have your address.
Basically, Ross Forsyth considers the girder stay to be a beam simply suported at the ends which carries the load on the firebox crown as a uniformly distributed load. Two beams share the total load, equal to the pressure divided by the area of the firebox crown.
I have calculated the section modulus of the girder according to published equations for a tee section beam.My calculation yielded the same result for the section modulus of Caledonia and Roedeer stays (Martin Evans), as from Mr. Forsyth in his article.
 
I suggest that perhaps the girder stays do not act as simply supported beams, that the evidence shows that the beam and firebox assembly is somehow stronger than the simple model can predict.
 
I am reassured by your observation that the structural model predicts collapse, but that has not happened, not just in one lucky case but across many examples. if it has not happened yet then this model is probably not accurate and I am probably not going to face a boiler failure in the future..
 
 
regards
Paul
05/12/2011 10:34:52
Eddie,
 
Thanks for your reply - but Jason asks the right question, which is really what I was asking. You must be wrong.
As well as the numerous boilers to this design owned by others, I have two, one made by me and the other made professionally. Neither shows any bending in the firebox crown whatsoever, despite being tested to 2 x working pressure several times and each boiler having been run in steam over several seasons.
Therefore, either my calculations of stress are wrong, or your conclusion is wrong and the higher stress can in fact be tolerated and accepted? This is what I am asking.
 
I am not from Australia, all I know of their code is what I have read in Ross Forsyth's article. I undestand the limit of 3770 psi to refer to working pressure not test pressure ( that would be really low). Martin Evans may quote a design stress of 3125 psi but in fact his designs would have a much higher stress in the girder stays. This was Mr Forsyth's point and I have now confirmed that. Neither has John Haining stuck to your quoted figure of 2187 psi, the Durham design exceeds that by a factor of about nearly 8!
Unless my calculations and Mr Forsyth's are not correct.
 
I repeat, I am not talking about a new design which I can modify after taking good advice, but about an existing boiler already completed and in service, similar to many others which are likewise tested and proven. Likewise for many of Martin Evans' loco designs, there must be hundreds of these in service.
 
Paul
 

 
 
04/12/2011 13:53:26
Thanks for the replies, I look forward to more comments.
 
I have now written a spreadsheet to calculate the stress for different crown stay designs, which makes it easy to check a few other cases. Some of the recent designs by Martin Evans for copper boilers come out as follows:
 
Roedeer 7.25 gauge 7600 psi
Harringay 5 gauge 5270
Tennant 5 gauge 14000
Greene Queen 7.25 gauge 14780

Some of these stresses are pretty high, it seems to me.

If i can figure out how to attach a spreadsheet, I will send it in, so you can try this check for yourselves. I no longer have the earlier John Haining boiler designs, apart from the Durham, so I haven't checked those.

Neville Evans has used rod stays in his recent boiler designs.

The design stress quoted by the UK Copper Board for annealed copper tube at 170 C is 26 MPa, or 3770 psi. This is the same as the limiting figure used in the AMBSC.
 
There are a large number of Maritn Evans boilers and John Haining Durham traction engines currently in steam. As Jason says, there have been no boiler failures...yet. I would like to think that this evidence of large numbers operating at higher stress without a prroblem, means that the higher stress is in fact safe. Do others agree??
 
Paul
 
03/12/2011 21:18:07
Many boiler designs use girder-type crown stays for the firebox crown (rather than stays connecting the crown to the boiler shell). There must be hundreds of engines in steam with such crown stays. I have made a 2 inch Durham Traction engine, designed by John Haining, which uses this type of crown stay.
I was going through some old copies of M.E when I read an article (Jan 1999) by Ross Forsyth, from Australia, where he showed that a typical copper girder crownstay would have stress which greatly exceeded the limit set in the Australian model boiler design code (AMBSC). Consequently, many boiler designs now used in the UK would not be accepted in Australia.
 
Their maximum stess allowed for copper is 3770 psi for any location in the boiler. When I checked the stress in the crown stays on my traction engne, I found that it came to 16100 psi, more than four times higher. This compares to the ultimate stress limit for annealed copper of 25000 psi, as advised by Martin Evans.
 
I would be interested in the opinions of others on this subject. Has this argument been settled over the last ten years, or should I be concerned about the high stress in the girder stays in my boiler? The same concern, if valid, would apply to many others.
 
Mr Forsyth says that the AMBSC does not prohibit girder stays, but it is clear that to meet the AMBSC stress limit, such stays would have to be really massive, so in effect they would be not practical, and stays to the outer wrapper would have to be used.
 
Please let me hear your comments.....
Paul
 
 

Edited By Paul Horth on 03/12/2011 21:19:08

Thread: boiler water window
29/04/2011 15:30:49
Michael,
OK, I'm glad that your idea worked out. I certainly did not intend to strike a negative or critical note. Good luck and thanks for sharing a clever idea.
Best wishes
Paul
25/04/2011 21:24:49
Michael,
 
Are you confident that you will be able to see a water level in the window? It's dark inside the boiler, and there will also be droplets on it. The conventional tubular gauge glass allows light from behind which makes the water level visible. Also, are you sure that 10 mm will give you the range of level that you need to check?
 
Good luck
 
Paul
Thread: steam turbine and generator
21/01/2011 13:27:24
Richard,
I agree with most of what you say. I would just note that Herr Jeggli has produced 4.5 watts from a rotor 30 mm in diameter running at 40000 rpm, and a boiler which fits inside a Gauge 1 loco and runs at 4 bar. So, it is possible to produce some watts without a huge amount of steam.
I would like, as you say, to make a small simple machine, but would hope to improve on the bucket-type impeller and thereby get a little more power, though I am sure that I cannot compete with the Jeggli turbine. For this I would expect to need a generator which would run in the range 20000 - 40000 rpm. I would not try to use a divergent DeLaval nozzle to get supersonic steam speed because I don't expect this velocity can be efficiently used in the simple wheel.
Paul
21/01/2011 10:46:57
OK Ian, sorry for being dumb! Not for the first time...
The thing is, some people are ambitious and talented enough to make what you suggested. Not me though.
Paul
20/01/2011 17:29:34
Ian,
What attracts me about a turbine is the inherent simplicity, i.e. one moving part. The idea of making an axial piston swashplate engine, with three cylinders, valves, funny crank joints etc etc is rather more complex with umpteen smallmoving parts and does make me shiver. I can think of many problems with this, starting with the design.I don't actually need to make the electricity, you see. The turbine would just be a neat gadget like the one that Brian has shown. I'd still like to try an impulse wheel with through-flow passages, though. It would be parasitical on the traction engine boiler and the engine-driven pump.
One major problem with a stand-alone steam turbinewould beproviding an effectivewater pump, which would have to be driven through a massively reducing gear train. Even Prof Chaddock had problems, having to cut his own gears for this. He was trying to pump into a flash steam boiler at 500 psi though which is pretty damn ambitious.
Paul
19/01/2011 17:38:56
Gents,
Thank you all very much - i now have several references for high speed electric motors which I need to look into, to be driven by a turbine,
It will be no surprise to anyone that I do not plan to start making a turbine tomorrow! I will need to think a bit more about the design (thinking is cheap) then decide if I am ready to commit time and effort to actually cutting metal. If and when I do, I will be sure to report back to this forum. I have had a lot of help from here, just in a short time.
Les - I think that Dyson motor would have too much power for me. The impeller in Sir James's right hand looks to be the wrong shape for a steam turbine but I guess it coul be changed. One for bigger bys than me I think.
Paul
17/01/2011 16:48:43
Hansrudolf,
 
Thank you. That certainly is a fast speed. Do you know if this type of motor can be run as a generator?
This site seems to be in the USA, but I will look around for a UK supplier.
 
Paul
16/01/2011 19:27:44
Better late  than never?
 I have now re-read some of the turbine articles discussed above. The year 2008 Model Engineer had no fewer than three articles on miiniature steam turbines. Plenty for me to digest.
In several model turbine designs that I have seen, the blade or impeller cavity is a semicircular cutout formed by an endmill in the periphery. This is relatively easy to form and gives a kind of impulse action but is nothing like the impulse blades on a full size turbine. I have called these impellers "Stumpf" but I am not sure if this is the correct name. For these wheels, the steam jet is tangential. In some designs the steam jet is on the wheel cente line, so that the steam impulse is directed straight at the cavity, and the steam dissipates in all directions after hitting the wheel.This is not very efficient.  In others, the jet is offset to one side so that the steam flows around the semicircle and is reversed. This is a better way to capture the steam energy.
For a more efficient impulse blade, the steam is fed in from the side of the wheel, flows through the blade and out the other side, reversing its direction but keeping a high velocity. It is more difficult to cut blades suitable for this action.
 
I will briefly summarise the turbines in the 2008 articles, as they represent these different types.
 
(1) Raymond McMahon, No. 4332 and 4334 15-8-08 and 12-9-08.
This turbine has the semicircular cutouts with the jet on the centre line. Diameter is 1.5 inch, 31 mm. It drives a dynamo from a rechargeable torch. The power and rpm are not stated, but probably in the tens of milliwatts, to light some LEDs. The housing has been made to look very realistic.
(2) Malcolm Stride, No. 4328, 20-6-08
These are for driving a boat not a generator. Two wheels are described.
Wheel 1: Blade shape is hard to see in the photo. Dia 50 mm.Speed 105000 rpm, down to 30000 rpm on load. Performance - drives a 3.5 foot boat.
Wheel 2: Blade shape semicircular cutout, with jett offset to the side. Dia 40 mm. Speed 90000 rpm to 40000 rpm on load. Performance - drives a 4.5 ft boat.
This indicates to me that the blade shape of the second wheel gave an improved performance. Note the high speeds though.
(3) Werner Jeggli, no.4318 and 4320.
Herr Jeggli's wheel is closer to a true impulse wheel, with side inlet and outlet and narrow curved passages in the periphery. Diameter 30 mm.Speed 53800 rpm no load, 34000 rpm at load. Power 4.4 watts. Smaller than the other wheels but I suspect more powerful because of the blade design.
 
When or if I make an attempt, I would aspire to make this kind of wheel, though i agree that the othr wheel do work and are thus good enough. It will also depend on what kind of motor I can use as a generator.
 
Paul
 
15/01/2011 11:16:47
James,
 
Thanks. I have looked up this motor and found this data, and a lower price!:
 

Mtroniks Vision 500 Marine >>

Mtroniks Vision 500 is a 540 size Johnson motor
This 540 sized Johnson high speed motor is ideal for marine use. Unit comes complete with motor suppresion capacitors.

Dimensions :
L57.0mm
D36.0mm
Shaft diameter 3.0mm
Voltage, 12V
Off load current, 1.18A
Max current, 54A
RPM, 16500RPM
Part No: V500M

Price: £4.99 (Including VAT at 20%)


This could be what I am looking for. From the data above I can calculate the performance as a generator and see if it could match the turbine. The speed is one of the highest I have found. I would then have to design a robust coupling and frame to hold the turbine and motor together.
Thanks for the help

Paul
14/01/2011 23:15:28
Werner,
 
Thank you for your message. I have the magazines somewhere in the spare room, I will retrieve them and read about your turbine once again. I hope that your amazing train is running well, I consider this to be a heroic achievement of true model engineering. You have designed and built several complicated interacting systems and made everything work. I know how difficult that can be.
I would be interested in the details of the Swiss motor that you used as a generator, I expect that you have described that in the magazine.
Best regards
Paul
14/01/2011 10:37:52
James,
 
Thanks for the photo. Your model certainly shows that it's not necessary to use blades with difficult shapes to get a working result. If your turbine produced several watts then that is an impressive performance.
Could you tell us please how the steam enters and leaves your rotor? I can't tell from the photo.
And, could you please give some details on the motor? The power of 36 Watt would seem to imply quite a large motor.
 
Thank you
Paul
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