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Post by joanlluch on Feb 15, 2015 18:30:05 GMT
Given that Kalrez (FFPM) is so expensive I registered to the Dupond site to figure out what to use. Performed a chemical suitability check for "Steam" and got this shocking results:  Dupond by joan.lluch, on Flickr  Dupond2 by joan.lluch, on Flickr So acording to this only "Kalrez" and "PTFE" is suitable for steam above 150 C. But " Viton" is only suitable in steam for temperatures not over 150 C? So now what ?
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Post by Roger on Feb 15, 2015 23:14:09 GMT
Well, considering how completely unsuitable Silicone is supposed to be, Doug seems to be able to use that without it failing immediately. Although we may exceed the parameters given by the manufacturers, those are intended for serious long lasting industrial applications. The number of cycles we are likely to put them through is relatively low, and we probably won't reach the temperatures we think we might. In short, it's worth trying materials that don't look ideal on paper but may well do the job in practice. I'm going to use my own experimental PTFE design of rod seals because I think they have the best chance of sealing and lasting a long time.
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Post by Deleted on Feb 16, 2015 0:05:40 GMT
Many use Viton O rings without any problems so perhaps the conditions in cylinders etc. are not that extreme on most designs. I think it's only when you start using very high levels of superheat that the problems start to appear. I did try measuring the cylinder temperature on my Helen Long when I first ran her on steam and the digital thermometer I used went off the scale of 160°C. That's probably why the original silicon O rings I used n the pistons didn't last very long. The PTFE rings seem to have lasted ok though.
John
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robmort
Hi-poster
3.5" Duchess, finishing 2.5" gauge A3 and building 3.5" King
Posts: 174
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Post by robmort on Feb 17, 2015 17:43:24 GMT
Don't know what you mean by "silicon": butyl or silicone?. Silicon is a semi-metal used in electronics.
Silicone rubber has a high temperature resistance to 250degC (at least) which is why it is used in ovens etc. But it is soft and is not suitable for o-rings subject to much movement (e.g. pistons).
EPDM is usually recommended for o-rings for steam (but maybe not high superheat).
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Post by timjenkins31 on Feb 17, 2015 18:08:27 GMT
EPDM is only rated to 125degC, I've used FKM viton successfully which is rated to over 200degC
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Post by ejparrott on Feb 21, 2015 9:00:27 GMT
This is the point where I very un-helpfully say stick to iron cylinders and piston rings, then you don't have a problem.....
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Post by Deleted on Feb 21, 2015 9:40:32 GMT
Wilson tried hydraulic cylinder lip seals on an engine . Claimed they sealed perfectly and had long life but who knows . Worth a fresh look at though . Needed a two part piston to assemble .
Two part pistons give many more options for seal configurations than solid ones . Maybe a PTFE turning same shape as lip seal would be worth an experiment . The hydraulic lip seal shape picks up pressure and seals against the cylinder walls in a very predictable way so that is a bonus anyway . Also they have long contact with cylinder compared with O rings which is another bonus .
Personally on a practical running engine as opposed to an experimental design I would be with Ed and use iron cylinders , solid pistons and rings .
Cylinders cast in a quality iron machine easily and last forever .
I think some peoples bad experiences with iron cylinders come from the days when at least a few model engineering suppliers regularly sold castings made from sweepings off the foundry floor . Blow holes , hard spots and instant rust guaranteed !
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Post by Doug on Feb 21, 2015 11:59:48 GMT
Just a quick but IMO important fact
Everyone keeps saying 150deg etc for superheated steam etc arround all over the place BUT the temperature in the loco is not constant yes the steam may be 150deg at superheat but by the time it gets to piston rod seals it's probl only 100 deg or less
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Post by Roger on Feb 21, 2015 12:54:09 GMT
Just a quick but IMO important fact Everyone keeps saying 150deg etc for superheated steam etc arround all over the place BUT the temperature in the loco is not constant yes the steam may be 150deg at superheat but by the time it gets to piston rod seals it's probl only 100 deg or less I think you're quite right in pointing out that the extremes aren't likely to be seen all of time or in every place. I too was coming to the conclusion that Viton 'O' rings are almost certainly adequate at least on the piston rod seals if those are fitted. I'm not quite so sure about the piston rod ones, but certainly that average temperature are not going to be constantly high. I'm still inclined to use a purpose designed seal there as much for my own interest as anything. |
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Post by Deleted on Feb 21, 2015 13:20:26 GMT
Just to add to Doug's comments which I agree with...when I was researching whether I would use any gasket between cylinder and frames I noted that cylinder temperature isn't constant and to put it simply has hot/cold fluctuations. This was why I decided against insulation, preferring to let the smokebox temp generated through the frames help to keep the temperature more constant.
Pete
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Post by Doug on Feb 21, 2015 14:11:21 GMT
This constant worrying about temperature and expansion kinda reminds me a lot of work, I work for a machine tool business and when I worked as a service engineer I had constant run ins with the design team as they would spec pumps seal etc. etc.
what looks good on paper does not always play out in real life other considerations get missed something as small as a cold draft on a machine can alter its performance significantly, even if it did work well in a lab.
all good info tho so I am not decrying it :-)
just don't worry, experiment it's more fun!!
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Post by gwr14xx on Feb 21, 2015 14:43:24 GMT
Just a quick but IMO important fact Everyone keeps saying 150deg etc for superheated steam etc arround all over the place BUT the temperature in the loco is not constant yes the steam may be 150deg at superheat but by the time it gets to piston rod seals it's probl only 100 deg or less Surely, if it is only at 100 degrees, and still under pressure, the steam will already be condensing back to water! |
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Post by Deleted on Feb 21, 2015 14:44:07 GMT
Temperature of cylinder walls is usually a good bit less than incoming steam temperature .
Highest cylinder temperature that Ewins recorded was (from memory) only about 295 F (145 C) - barely hot enough to make a good cup of tea .
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Post by Doug on Feb 21, 2015 17:29:41 GMT
Just a quick but IMO important fact Everyone keeps saying 150deg etc for superheated steam etc arround all over the place BUT the temperature in the loco is not constant yes the steam may be 150deg at superheat but by the time it gets to piston rod seals it's probl only 100 deg or less Surely, if it is only at 100 degrees, and still under pressure, the steam will already be condensing back to water! Yes it does I did get dribbles out of the rods with the last set of seals. Horses for courses really if I got 147 deg on my cylinders the I would bet my seals would go bye bye but it does just not get that hot |
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Post by joanlluch on Feb 21, 2015 17:40:40 GMT
Temperature of cylinder walls is usually a good bit less than incoming steam temperature . Highest cylinder temperature that Ewins recorded was (from memory) only about 295 F (145 C) - barely hot enough to make a good cup of tea . Michael, I suppose this must be the outside temperature, actual temperature in the inner bore surfaces should be higher, but difficult to measure in practice. I think inner temperature should be about the median of inlet and outlet steam temperatures. Does it have sense in your opinion. |
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jma1009
Elder Statesman
Posts: 5,917
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Post by jma1009 on Feb 21, 2015 22:19:27 GMT
michael is as always completely correct re cylinder temperature, and jim ewin's highest figure was as michael states (the lowest figure being 121 degrees C). jim had radiant superheaters fitted. once again this supports the theory of the use of superheaters otherwise in miniature the temperature creates condensate which is the devil of steam locomotive cylinder efficiency.
cheers,
julian
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Post by ejparrott on Feb 22, 2015 8:22:48 GMT
All this talk is kind of tempting me to build the cylinders for my MW with drillings for thermocouples in....
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Post by joanlluch on Feb 22, 2015 8:30:59 GMT
Julian, My question starts from the fact that there is a difference of temperature between the cylinder bore and outer surfaces of the block. Steam heats the cylinder block from the inside and this heat is dissipated by the external surfaces of it, so this transfer of heat is manifested by the said difference of temperature. Both temperatures would be equal if the cylinder block was perfectly and absolutely isolated (no heat transfer), but this is not the case. Since inner temperature should be difficult to measure, I was asking whether the actual figures correspond to the outer surfaces of the cylinder. I am sure that Michael did already understand the question.
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Post by Doug on Feb 22, 2015 9:07:11 GMT
michael is as always completely correct re cylinder temperature, and jim ewin's highest figure was as michael states (the lowest figure being 121 degrees C). jim had radiant superheaters fitted. once again this supports the theory of the use of superheaters otherwise in miniature the temperature creates condensate which is the devil of steam locomotive cylinder efficiency. cheers, julian Hi Julian a question on superheat is it better to have radiant super heaters on my speedy? I guess this would give better power although I may have to rethink my seals due to the extra temp. My butch super heaters are not radiant and don't work particularly well the steam seems "wet" most of the time. |
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Post by ejparrott on Feb 22, 2015 9:16:41 GMT
I'd always have full length radiant superheaters myself.
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