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Post by ejparrott on Nov 29, 2014 12:05:50 GMT
Failure to specify or purchase correct grades of stainless fastenings is an issue I occasionally have, when nuts and bolts ring together and will not ever come apart, especially after a few years at sea...
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Post by Deleted on Nov 29, 2014 12:15:19 GMT
Exposure to the "Briny", eh??........... now that's an entirely different kettle of fish ------- "I say Jeeves, I just cracked a joke!"......................."Very nearly, Sir"..
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Post by joanlluch on Nov 29, 2014 12:36:28 GMT
You obviously haven't looked at my build thread, or registered the fact that I have said I am a CNC turner by trade. Prep and programming don't take very long at all. The expensive bit is the length of time it takes to machine, which you've made worse by deciding to make everything in stainless. Ed, this is what I have been told. I do know that 6 wheels (to say) is proportionally (significantly) cheaper than 1 wheel, so where the difference can be? Only obviously to something that is done once on either case, preparation?, programming?, fixed fee?. I do not know how to call it, but this does not change my whole point. Another thing that makes a particular part to be more expensive is the need for that part to be retro-fitted to the CNC mill to process several faces, so I attempt to avoid that if possible. Again this is what I have been told. Maybe you can instruct me to get a cheaper price by allowing me to understand the cost structure of a CNC milling facility. Just in case. But please do not go again to the cost and trouble of S.S machining because this is out of my equation.
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Post by joanlluch on Nov 29, 2014 13:09:44 GMT
I would have thought a person who claims to be an engineer would have chosen the best materials fit for each function and purpose. Stainless steel in various grades has its uses but there are other materials and alloys that do a better job in many circumstances, why not use these. Persons who have worked with stainless steels will know that as a generic material there are many places where its application is most ill advised. Perhaps my over 40 years professional experience is littered with seeing a more than average numbers of instances of stainless steel failure. Ian, I am a chemical engineer. I have been using S.S heat exchangers and chemical reactors for 2 decades with no problems whatsoever, this includes pressurised vessels. I have already said that my choice for that material, where it CAN be used, is not based on the best/cheaper but on other reasons. I do not think that it is productive to comment further on that any longer. To my experience S.S fails at places where only very particular materials will succeed. For example, do not use it where corrosion due to sulphuric acid can be present. This includes outlet pipes of furnaces where sulphur containing combustible is burned. I learned that the hard way.
On a miniature locomotive, if you burn sulphur containing coal, do not use S.S. in the boiler or at the super heaters because SO2 present in the flue gases will produce sulphuric acid by combination with moisture as the whole thing cools down. Next time you use the loco you may get a hole o crack in the boiler or super heater.
S.S. is also attacked by Chlorhidric acid
S.S. is not suitable on sliding mechanical parts, but it is perfectly fine if one of such parts is a much softer material such as plastic. S.S. is not suitable for heavily machined parts because of its tendency to grip and break milling tools. Now, if you know about any applications I am not aware of where it would not be suitable (regardless of whether there is a better material) please post them, but I guess they will be the already mentioned or derivates of them. I am not talking about which is the best material on each scenario, just about suitability.
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Post by joanlluch on Nov 29, 2014 13:16:22 GMT
Failure to specify or purchase correct grades of stainless fastenings is an issue I occasionally have, when nuts and bolts ring together and will not ever come apart, especially after a few years at sea... This happens. Sometimes this can be minimised by adding a drop of oil before fitting the nuts. There are specific products for this as well. But possibly they will not help in your environment or after some years.
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Nov 30, 2014 9:25:38 GMT
Joan
Good to hear from you and see your appreciation of the strengths and weaknesses of SS.
As a Chemical Engineer you are no doubt aware of Duplex steel typically grade 2205, this is the only "stainless" type steel approved for boilers in Australia. 2205 has a high tolerance to chlorine/chlorides and many other nasties.
My problem is the generic use of the term "stainless" for any steel containing varying amounts of chromium and nickel and then the presumption that no particular care is required with its handling, machining and treatment of water when it is used for a boiler.
Unfortunately SS is just a matrix of galvanic couples waiting to be exposed so corrosion can start.
One of the best containers of concentrated sulphuric acid is mild steel.
Regards Ian
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Post by joanlluch on Nov 30, 2014 14:54:26 GMT
Hi Ian. I haven't used 2205 in my past. I found it to be 318LN but as said I haven't used it. It is good to know that it is approved in Australia for miniature boilers. I will definitely investigate this grade.
I had once a corrosion problem on some exhaust pipes originally made out of 316 S.S. After one year or so several tiny cracks started to appear that caused leakages. The pipes were replaced by 316L and then by 316LTi but the problem appeared again after one and a half year or so. So not that much improvement. The final solution was to redesign the exaust pipes to make possible the use of enforced polyester. 17 years latter they are still in place and working.
In this case S.S was a failure. I wonder if 2205 would have worked, as it was not tried. Corrosion was caused by the formation of sulphuric acid in the pipes.
Joan
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uuu
Elder Statesman
your message here...
Posts: 2,808
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Post by uuu on Nov 30, 2014 15:13:09 GMT
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Post by joanlluch on Nov 30, 2014 20:24:17 GMT
Hi Wilf, that document is a gem. Thank you very much for posting it. Joan
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fang
Seasoned Member
Posts: 100
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Post by fang on Dec 6, 2014 10:26:40 GMT
Well I've been looking about on the net a bit more and still haven't found any useful Model Engineering data to go on
Ideally I'd like to know if you have a steam engine, of known bore and stroke, and valve cut off, is there a way to work out (roughly!!) how much steam that engine uses per stroke, and therefore per minute, etc?
Having calculated this, there must be a sq in of heating area needed to produce this quantity of steam?
And knowing the area needing to be heated, there must be a preferable depth of coal on the grate, which when burnt releases a certain amount of heat, from which the grate area can be calculated?
I realise that there is a whole host of other factors to consider, but my main concern is that the boiler will not produce enough steam, or far too much!!! A boiler that will only produce 1 lb of steam when the engine uses 3 lb would obviously not be a lot of use!!!
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Dec 6, 2014 11:23:01 GMT
hi fang,
you have almost written the formula in your post above!
the missing bits are indeed published.
you need to get a copy of the volumes of 'Locomotive Panorama' by E.S. Cox, or have a look at the BR test reports from the early 1950s. also have a look at chapter 9 of martin evans' book 'model locomotive boilers' describing the tests carried out by jim ewins on his famous 5"g 0-6-2T loco.
there are unfortunately lots of variables that do not make this subject an exact science. being very good at firing a miniature loco 'little and often' in the right place on the grate and at the right time and on the run makes a huge difference, as does the mechanical condition of the loco and how well the valve gear is set. the size and quality of the coal also can make a huge difference. and then there is the never ending subject of superheaters. and the smokebox draughting and adequate space between the firebars and adequate depth and openings in the ashpan can also have a big effect on the steaming rate of a boiler.
there are lots of examples in fullsize of when the boiler didnt match the needs of the cylinders. the LBSCR Marsh 'C' class 0-6-0 locos and his 'I1', 'I2', and 'I4' all had boilers and fireboxes that were far too small. there is the famous story of one of the 'I1s' hauling the royal train from victoria to epsom races in june 1907 with Marsh and Lawson Billinton on the footplate and the train struggled to get there and back and lost time - which was unthinkable for a royal train hauling the monarch!
cheers, julian
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