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Post by springcrocus on Aug 6, 2021 8:53:40 GMT
I am about to make the twenty-two screws that hold the cylinder on top of the boiler on my Allchin traction engine. These screws protrude into the water space and cannot, therefore, be mild steel. For those not familiar with this type of construction, there is a doubling plate below the cylinder which provides 1/4" wall thickness for the screws to fix to.  Hughes recommends making screws from phosphor bronze but my preference is to make them from stainless steel. Before I commit to making them, can anyone offer a valid reason why it would be better to stick with the recommended phosphor bronze? Regards, Steve |
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Post by suctionhose on Aug 6, 2021 9:30:34 GMT
Steve,
I don't think it matters. The Allchin on our sideboard - bought it / never steamed - has stainless studs by the look of it.
With my 3" scale TE's, 10mm steel shells, I tap a blind hole for my studs so they don't impinge on the steam space at all. With your copper being doubled to 1/4 thick is that a practical proposition?
Can you buy studding, screw it into a blind hole (say 3/16 deep) and cut it off with a Dremel?
If you do use screws suggest turning the head to look like a stud and nut. Nice for the detail.
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Post by springcrocus on Aug 6, 2021 9:54:18 GMT
Ross,
Too late, the holes are drilled and tapped. I was planning on making a length of 5BA studding, cutting to length and loctiting nuts on with a section of the thread protruding. I would also put a dab of red hermatite on the lower part before screwing into the shell. Studs cannot be fixed first, of course, because then I wouldn't be able to fit the cylinder over them.
One possible problem would be differential expansion: anyone have an opinion on this?
Regards, Steve
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Post by suctionhose on Aug 6, 2021 10:31:50 GMT
Wouldn't worry in that size
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Post by ettingtonliam on Aug 6, 2021 10:46:01 GMT
Just curious how threading bolts directly into the copper shell (OK I know theres a doubling plate) sits with boiler inspectors these days? Yes, I know thats how it used to be done, I've done it that way myself years ago.
My Locomotion boiler, designed about 1995 by David Piddington, with input from Alec Farmer, has multiple items shown screwed into the shell and then caulked with Comsol. I didn't like this so I've replaced them with bronze blind bushes, tapped for the appropriate size. Admitedly the inside of the shell looks like a porcupine,(theres over 40 of them) but my boiler inspecter is happy with these.
I suppose its too late for you to fit bronze bushes?
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Post by suctionhose on Aug 6, 2021 10:49:36 GMT
Consider loctiting the studs into the tapped copper with high temp Loctite?
I've used 620 which is good for 230 deg C. 100 psi saturated steam is under 180 deg C. Fair margin on it. Used it to save a blunder and has been ok for 10 years.
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Post by springcrocus on Aug 7, 2021 7:42:15 GMT
Just curious how threading bolts directly into the copper shell (OK I know theres a doubling plate) sits with boiler inspectors these days? Yes, I know thats how it used to be done, I've done it that way myself years ago. My Locomotion boiler, designed about 1995 by David Piddington, with input from Alec Farmer, has multiple items shown screwed into the shell and then caulked with Comsol. I didn't like this so I've replaced them with bronze blind bushes, tapped for the appropriate size. Admitedly the inside of the shell looks like a porcupine,(theres over 40 of them) but my boiler inspecter is happy with these. I suppose its too late for you to fit bronze bushes? Ideal world and all that, Richard. If this was my own start-to-finish project, I think your suggestion would be the best way forward. However, I'm reluctant to bring this boiler back up to silver-soldering temperatures in case it all falls apart.
The boiler inspector currently has a project of his own underway that uses the same technique so I'm reasonably confident that he will accept this, as long as it's well-made.
No-one else appears to have an opinion about the original question so I will make the fixings from stainless steel.
Regards, Steve
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Post by 92220 on Aug 7, 2021 8:08:21 GMT
Use bronze screws, or modify the design to make the tapped holes blind to the inside. You should not use stainless steel screws in a copper boiler because of galvanic degradation of the steel. Also, stainless steel (any type)can rust inside a boiler for the same reason. Some model engineers have found they have had to replace all their stainless steel screws because they have started to degrade after quite a short time.
I've just done a check on google and found a section on the Atlas Steel site showing the various qualities of stainless steel including their resistances to corrosion under varying circumstances. Use in boilers is basically not recommended. One point they make is that chlorides in boiler water, will cause corrosion of the stainless steel unless you use a special grade. That is making it quite clear that stainless steel screws, in a model boiler, are going to corrode in service. Buying the special stainless and making the screws is the only safe way to use stainless screws. Phosphor bronze screws are easy to make and don't cost the earth, and will do the job without the risk of corrosion due to chlorides, or galvanic action.
Bob.
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Post by springcrocus on Aug 7, 2021 8:31:51 GMT
Thank you, Bob. I will follow up your references and probably now change to phosphor bronze.
As Ross said, his Allchin has not been steamed and is primarily a display model. I would like mine to be a working model and don't really want to keep coming back and fixing / modifying things.
Regards, Steve
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Post by suctionhose on Aug 7, 2021 9:20:24 GMT
If it is of any interest, full size practice for cylinder bolts was to use a steeply csk head bolt from the inside. They were called carrot bolts and the hole was taper reamed from inside first. Just a piece of trivia...
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Gary L
Elder Statesman
Posts: 1,208
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Post by Gary L on Aug 8, 2021 1:17:56 GMT
Use bronze screws, or modify the design to make the tapped holes blind to the inside. You should not use stainless steel screws in a copper boiler because of galvanic degradation of the steel. Also, stainless steel (any type)can rust inside a boiler for the same reason. Some model engineers have found they have had to replace all their stainless steel screws because they have started to degrade after quite a short time. I've just done a check on google and found a section on the Atlas Steel site showing the various qualities of stainless steel including their resistances to corrosion under varying circumstances. Use in boilers is basically not recommended. One point they make is that chlorides in boiler water, will cause corrosion of the stainless steel unless you use a special grade. That is making it quite clear that stainless steel screws, in a model boiler, are going to corrode in service. Buying the special stainless and making the screws is the only safe way to use stainless screws. Phosphor bronze screws are easy to make and don't cost the earth, and will do the job without the risk of corrosion due to chlorides, or galvanic action. Bob. Hi Bob Everything you say is true, but in marine practice a bronze-stainless steel combination is considered acceptable from the galvanic POV (copper being seldom used but pretty close in the series). The chloride issue is a serious one in a boiler water space, but in the steam space I'm not so sure. The big weakness with stainless (even 18/8) in marine applications is its susceptibility to crevice corrosion when used underwater (or even in damp places on deck). Any exposed surface will remain bright and look completely sound, while the portion of a fastening that is buried and thus shielded from oxygen will rust away almost as fast as mild steel if the damp can get at it. This often happens at the interface, where a buried stainless fastening emerges into the open, or at a joint where there is leakage. Corrosion starts to form a crevice, and oxygen still can't get in to form the protective skin, so eventually the fastening snaps off at the joint. Many a mast has been lost this way! Whether it would happen in a boiler with fresh water is an interesting question. I haven't seen a problem with regulator spindles for example, but they generally live in the steam space. Gary |
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uuu
Elder Statesman
your message here...
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Post by uuu on Aug 8, 2021 8:08:37 GMT
Talking of regulator spindles, I remember one at the PumpHouse that was crumbling away. Very strange to see.
Wilf
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Post by 92220 on Aug 8, 2021 8:19:28 GMT
Hi Gary.
Good points. As you point out, stainless needs air/oxygen to for the protective layer. There is no air, in a working boiler, to protect the stainless, and depending on quality of the water supply, there can be chlorides present. However, I have been doing a bit more digging on the 'net and have come up with a couple of points that are worth considering:-
Found this on the Model Engineer website under a heading "Copper boilers and stainless bolts:
(Quote 1) "I don't know what your boiler is made of but Loctite told me you cant use stainless in copper because of the different rate of expansion.". It just goes to show that even experts make mistakes!! The ceficcient of expansion for copper is the same as for austinetic stasinless, so Loctite chemists, have boo-booed there!!
(Quote 2) " I am a chemical engineer having worked for 2 decades in the chemical industry. I know this is a one year old thread but I wanted to clarify something about Stainless Steel, which is a very used material in the industry. I mean it is not only for cooking stuff. Stainless steel main weakness is that it is attacked by hydrochloric acid and sulphuric acid. Other that that it will resist virtually any other chemical including very strong acids such as nitric acid. Crevice corrosion may be caused by the lack of oxygen and can happen in very rare conditions, but not on a boiler. It is not definitely caused by lack or 'air' contact. This kind or corrosion also occurs in other alloys such as aluminium, titanium and copper. My take is that what Russell describes was caused by chlorides. Many modern industrial steam boilers are made out of stainless steel and virtually all heat exchangers are made entirely on stainless steel."
I would guess we are still not clear on this one!!!
In my industrial experience with stainless has only been using it in an air environment, not airless, so I only go on what I can find on the 'net, so I am a bit surprised there is so much controversy between the experts. Because of this I do think that playing it safe, and making phos bronze screws is the way to go.
Bob.
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Post by ettingtonliam on Aug 8, 2021 11:11:12 GMT
Over 30 years ago I was installing pressure vessels and pipework in a nuclear waste treatment plant. It was new build so there wasn't any of the nasty stuff about. Obviously everything was stainless steel (can't remember what grade now) and one lesson I've always remembered is'Don't clean off stainless steel welds with a carbon steel wire brush' because if you do, after the work has stood outside for a few weeks, you end up with stainless vessels and pipes covered in rust, from fragments of the wire brush, and the client is an extremely unhappy bunny.
Anyway, the other thing I remember is getting a technical note pointing out that in certain combinations of temperature and pressure and in the presence of chlorides, stainless can corrode, and so we shouldn't use felt tip markers or insulating tape to mark or identify stainless components, because felt tips and masking tape both contain chlorides (or did then).
Oh yes, I presume PB is OK strengthwise for use as small screws holding the cylinder to the boiler?
I have a personal interest in this subject because Locomotion's cylinders are vertical with the bottom half buried in the boiler, and the bottom cover is below the water line. I was going to use commercial 2.5mm csk screws to hold the bottom end cylinder cover in place. Oddly, there is very little load on these screws, because boiler pressure forces the cover onto, rather than off, the end of the cylinder. It probably won't matter much, because even at 7 1/4" gauge, its not going to be a serious passenger hauler, so will only be steamed infrequently.
The fixings holding the cylinder to the boiler are blind tapped holes into a heavy gunmetal bush, so stainless should be OK here, because the studs are never exposed to steam.
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Gary L
Elder Statesman
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Post by Gary L on Aug 8, 2021 20:31:58 GMT
Hi Gary. Good points. As you point out, stainless needs air/oxygen to for the protective layer. There is no air, in a working boiler, to protect the stainless, and depending on quality of the water supply, there can be chlorides present. However, I have been doing a bit more digging on the 'net and have come up with a couple of points that are worth considering:- Found this on the Model Engineer website under a heading "Copper boilers and stainless bolts: (Quote 1) "I don't know what your boiler is made of but Loctite told me you cant use stainless in copper because of the different rate of expansion.". It just goes to show that even experts make mistakes!! The ceficcient of expansion for copper is the same as for austinetic stasinless, so Loctite chemists, have boo-booed there!! (Quote 2) " I am a chemical engineer having worked for 2 decades in the chemical industry. I know this is a one year old thread but I wanted to clarify something about Stainless Steel, which is a very used material in the industry. I mean it is not only for cooking stuff. Stainless steel main weakness is that it is attacked by hydrochloric acid and sulphuric acid. Other that that it will resist virtually any other chemical including very strong acids such as nitric acid. Crevice corrosion may be caused by the lack of oxygen and can happen in very rare conditions, but not on a boiler. It is not definitely caused by lack or 'air' contact. This kind or corrosion also occurs in other alloys such as aluminium, titanium and copper. My take is that what Russell describes was caused by chlorides. Many modern industrial steam boilers are made out of stainless steel and virtually all heat exchangers are made entirely on stainless steel." I would guess we are still not clear on this one!!! In my industrial experience with stainless has only been using it in an air environment, not airless, so I only go on what I can find on the 'net, so I am a bit surprised there is so much controversy between the experts. Because of this I do think that playing it safe, and making phos bronze screws is the way to go. Bob. Hi Bob Very interesting, and as you say, the experts don't help a lot! My 'take' on this, is that the first consideration should be what you are expecting from your stainless bolts/studs. If it is pure corrosion resistance, then there are better materials available. If it is ultimate strength, then stainless is your man, providing you can design out the environments it doesn't like. By ultimate strength, I mean that both stainless and bronze will be more than strong enough for the purpose in tension; but after a few years will you be confidently able to unscrew the nuts without shearing the slender studs? I think I would choose stainless myself, in this sort of scenario. All the mounting studs on my Paddington boiler are stainless, and none of them penetrates into the steam/water space, so I have no qualms about them. As regards environment, what sort of duty will the boiler be performing? If it will be in steam 24/7 then a build-up of chlorides in an oxygen-free environment is inevitable, with all the predictable bad effects on stainless steel. But our models generally spend the huge majority of their lives cold, and often drained, and filled with clean fresh water anyway. The small number of total hours in steam is not likely to have any noticeable adverse effects. The experience of colleagues in Europe where all-stainless boilers are very common, seems to bear this out. In marine work, bronze is pretty much universal for the raw-water side of heat-exchangers, for the reasons already given. But this is a very severe environment, and my feeling is that model boilers have an easy life in comparison. Feel free to disagree! Gary |
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Post by chris vine on Aug 8, 2021 20:47:13 GMT
I am not sure what grade of stainless steel the model engineering suppliers sell, but I think it might be 303??
If so, then the tensile strength of 303 and phosphor bronze PB102 is very similar!!
thinking about the suggestion to attach the studs into the boiler with Loctite: I am not so sure this would be a good idea as you will have to remove the studs if you want to remove the cylinder as they are at an angle. There will be precious little to grip them by with the cylinder still in situ.
Chris.
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Post by springcrocus on Aug 8, 2021 21:52:39 GMT
Some interesting views being aired here, and all with valid reasons for choosing one or other material.
To recap, they have to be studs or bolts because they are mounted around the diameter of the boiler shell but, if studs, have to be removeable. To look authentic, they need to replicate studs with a nut on the outside and this is what I had planned to do. If stainless, then standard mild steel nuts loctited onto the top of the stud with a small section protruding would both function correctly and look the part. If phosphor bronze studs were used then I would really need to source phosphor bronze hexagon material to make matching nuts but I cannot find any of the correct size and there would be a colour mismatch if I used mild steel nuts. I could, of course, paint them black the same as the cylinder will be.
One other important point is that the thread is 5BA and I'm not prepared to even consider alternatives. The holes are already drilled through and tappped to size. I have both 1/8" dia phosphor bronze rod and 1/8" dia S316 stainless steel rod in stock so material for the studs is not an issue. Only the very end of the thread will be exposed to the inside of the boiler and will be in the steam space, not in the water space (as I incorrectly stated earlier) and I would coat the thread with red hermatite (well, the modern equivalent, anyway) prior to assembly.
Stainless steel would be my favoured choice but, based on the additional information above, does anyone still feel that this is the wrong choice?
Regards, Steve
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Post by ettingtonliam on Aug 8, 2021 23:36:14 GMT
On the whole I think I'd use stainless steel. Theres large numbers of traction engines out there with stainless studs and I've yet to hear of one failing because of using them.
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Post by 92220 on Aug 9, 2021 8:21:01 GMT
Hi Gary. Good points. As you point out, stainless needs air/oxygen to for the protective layer. There is no air, in a working boiler, to protect the stainless, and depending on quality of the water supply, there can be chlorides present. However, I have been doing a bit more digging on the 'net and have come up with a couple of points that are worth considering:- Found this on the Model Engineer website under a heading "Copper boilers and stainless bolts: (Quote 1) "I don't know what your boiler is made of but Loctite told me you cant use stainless in copper because of the different rate of expansion.". It just goes to show that even experts make mistakes!! The ceficcient of expansion for copper is the same as for austinetic stasinless, so Loctite chemists, have boo-booed there!! (Quote 2) " I am a chemical engineer having worked for 2 decades in the chemical industry. I know this is a one year old thread but I wanted to clarify something about Stainless Steel, which is a very used material in the industry. I mean it is not only for cooking stuff. Stainless steel main weakness is that it is attacked by hydrochloric acid and sulphuric acid. Other that that it will resist virtually any other chemical including very strong acids such as nitric acid. Crevice corrosion may be caused by the lack of oxygen and can happen in very rare conditions, but not on a boiler. It is not definitely caused by lack or 'air' contact. This kind or corrosion also occurs in other alloys such as aluminium, titanium and copper. My take is that what Russell describes was caused by chlorides. Many modern industrial steam boilers are made out of stainless steel and virtually all heat exchangers are made entirely on stainless steel." I would guess we are still not clear on this one!!! In my industrial experience with stainless has only been using it in an air environment, not airless, so I only go on what I can find on the 'net, so I am a bit surprised there is so much controversy between the experts. Because of this I do think that playing it safe, and making phos bronze screws is the way to go. Bob. Hi Bob Very interesting, and as you say, the experts don't help a lot! My 'take' on this, is that the first consideration should be what you are expecting from your stainless bolts/studs. If it is pure corrosion resistance, then there are better materials available. If it is ultimate strength, then stainless is your man, providing you can design out the environments it doesn't like. By ultimate strength, I mean that both stainless and bronze will be more than strong enough for the purpose in tension; but after a few years will you be confidently able to unscrew the nuts without shearing the slender studs? I think I would choose stainless myself, in this sort of scenario. All the mounting studs on my Paddington boiler are stainless, and none of them penetrates into the steam/water space, so I have no qualms about them. As regards environment, what sort of duty will the boiler be performing? If it will be in steam 24/7 then a build-up of chlorides in an oxygen-free environment is inevitable, with all the predictable bad effects on stainless steel. But our models generally spend the huge majority of their lives cold, and often drained, and filled with clean fresh water anyway. The small number of total hours in steam is not likely to have any noticeable adverse effects. The experience of colleagues in Europe where all-stainless boilers are very common, seems to bear this out. In marine work, bronze is pretty much universal for the raw-water side of heat-exchangers, for the reasons already given. But this is a very severe environment, and my feeling is that model boilers have an easy life in comparison. Feel free to disagree! Gary Hi Gary. No I don't disagree. In fact I would agree with everything there. Bob. |
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