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Post by ejparrott on Mar 23, 2013 9:44:04 GMT
I've been through the code again and again and can't find an answer on stainless.
The code states stainless boilers are outside the scope of the code. Fine, what about stainless as a construction material for items fitted to the boiler? DAG suggests making clacks with a stainless portion I think, between fitting face and ball chamber. Lots of use use stainless balls in check valve's or stainless spindles in steam shut off valves, lots of stainless 90deg ball vales are now in use as regulators......
What about a flat dome top? I'm thinking of using a 316 stainless top to my dome, about 3/4" thick, drilled and tapped for the safety valves, passages drilled in it for the injector valves, blower valves, ejector valve etc. I know of one Romulus as well which has a stainless plate sandwiched between boiler flange and dome to support the ball valve regulator.
Its not forbidden in the code, is it allowed?
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uuu
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Post by uuu on Mar 23, 2013 12:14:08 GMT
Can I quote from an article in issue 141 of 7 1/4"gauge news, by Dave Giles?
There are many different grades of stainless steel, which can resist corrosion or grades that can resist variations of pressure however time has proven they are not suitable to operate under both conditions at the same time. The expansion rate of the 300 series grade of Stainless Steel is almost twice that of carbon steel. Over the years time has proven that the cyclic heating and cooling of the 300 grade of stainless steel causes hairline cracks to develop beside or along the welded seams. For this reason stainless steel is not recommended as a suitable material from which to manufacture miniature steam locomotive boilers.
The author goes on to descibe the expermental building of a boiler in duplex saf 2205 alloy steel. This steel was specially devloped to resist cyclic stress cracking at welds.
It may be that your intended use of the materal is not welded. Have you any soldered joints, or is your part solid. what is the differential expansion between stainless steel and copper?
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Post by ejparrott on Mar 23, 2013 16:57:09 GMT
My part is a flat stainless dome 'lid', bolted to the top of the bronze pad silver soldered to the copper boiler. No welding or soldering to the stainless.
I am not currently a member of the 7 1/4" society, so I haven't seen the article. I have however seen an article about a duplex boiler.
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johnthepump
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Post by johnthepump on Mar 23, 2013 18:03:30 GMT
A few years ago one of our club engines had a regulator failure. This turned out to be a stainless steel shaft (of unknown grade) failing inside the boiler, the end had split into many shards looking like a paint brush.
JtP
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pault
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Post by pault on Mar 24, 2013 11:34:45 GMT
Hi All
As has been stated the main issue with stainless boilers are problems relating to the welds and the heat affected zones around them. There is no real issue with using stainless in the application you describe. There is a slight difference in the coefficient of expansion between 316/304 and bronze, but the difference is no worse than that between copper and bronze which we accept. As far as galvanic corrosion goes stainless and PB are at about the same level in the galvanic series which is far better than the difference between copper and PB which again we accept.
I can see no engineering problems with what you are proposing
Regards
Paul
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Post by peterseager on Mar 24, 2013 11:59:42 GMT
I think the S/S dome lid would not be permitted just because it crosses the line from being a boiler fitting to being a part of the pressure vessel. Ironically we happily accept S/S screws to hold the dome on.
Peter
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Post by ejparrott on Mar 24, 2013 14:41:55 GMT
hmm..valid point...I wonder if there's any PB1 at work....
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Post by marshall5 on Mar 24, 2013 17:54:08 GMT
My understanding is that s/s is vulnerable to chloride cracking from the chlorides in the boiler water. As the dome cover is not normally in contact with the water maybe it's not a problem in reality. However as the use of s/s, as part of the pressure vessel, is not permitted under our 'club' schemes I would go for bronze. Ray.
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pault
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Post by pault on Mar 24, 2013 19:32:22 GMT
Peter raises an interesting point, the dome cover is clearly part of the pressure vessel, no argument there. Since failure of the screws holding said cover in place would result in failure of the pressure vessel and the uncontrolled release of the contents it could be argued that the screws are part of the pressure vessel. It is no different to the silver solder or rivets that hold a conventional boiler together.
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Post by steambuff2 on Mar 24, 2013 20:22:45 GMT
True, but the screws are not exposed to Water or Steam so should not suffer any ill effects.
Dave
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pault
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Post by pault on Mar 24, 2013 21:16:14 GMT
No argument with what you say Dave, just playing devil’s advocate for the fun of it. I have come across a number of boilers where things like manifold bolts and others break through into the steam or water space. Besides the regs just say no SS as part of the pressure vessel not you can have it above the water line.
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