Stainless steel tubes

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Stainless steel tubes

Post by lostintime » Fri Oct 08, 2021 2:03 am

I understand that stainless steel is not a material that is to be used in the wetted area of a boiler, but I'm trying to find all the reasonings. I've found some information on stress cracking from chloride contamination in the water, and have seen some bizarre cracking in stainless steel shafts first hand that should have never happened given the low level of stress they were under, But curiosity gets the best of me so I ask, if "fresh" "hard" water (untreated well water with 20 grains of hardness) were used to fill a mostly closed system with stainless steel fire tubes would it be prone to fail, and if so why?.
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Re: Stainless steel tubes

Post by barts » Fri Oct 08, 2021 4:57 am

Stainless steels are particularly susceptible to pitting corrosion and chloride induced stress cracking, which is quite undesirable in pressure vessels. Welding is tricky as well; getting an excellent match in alloy is important as well. Many of the problems with 1970s nuclear power plants involved leaks in stainless steel heat exchangers, for example.

On the other hand, if boiler water is kept mildly alkaline, mild steel corrodes in a predictable manner. Welding is straightforward - 6010 and 7018 welding rod is well known. Mild steel works well - why not use it?

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Re: Stainless steel tubes

Post by Kelly Anderson » Sat Oct 09, 2021 12:16 am

Also, stainless steel is very poor at transferring heat from fire to the water, the opposite of what is wanted.

With the added labor in fabrication and the added cost of the material, one might be able to buy two or three steel boilers for the cost of a stainless steel boiler, and there are there are steel boilers 100 years old and still in good condition. They only require a little common sense care to avoid corrosion.
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Re: Stainless steel tubes

Post by RNoe » Sat Oct 09, 2021 2:59 am

Here's the Australian Miniature Boiler Code for building with "Duplex Steel, a type of stainless steel.

https://nanopdf.com/download/1-trevor-heath_pdf

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Re: Stainless steel tubes

Post by lostintime » Sat Oct 09, 2021 4:35 pm

I will build with conventional materials. The only reason I ask is I have access to 4 vertical tube pressure vessels, 48" tall, 164- 1.25" tubes each for a little over scrap price. BUT the tube sheets and tubes are stainless steel (ammonia refrigeration evaporator but asme stamped to 250psi/400f). Being able to use one would save a lot of time and money, enough that it's hard to listen to the rational side of my brain when it says run away from the idea.
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Re: Stainless steel tubes

Post by fredrosse » Sun Oct 10, 2021 1:40 am

"....... if "fresh" "hard" water (untreated well water with 20 grains of hardness) were used to fill a mostly closed system with stainless steel fire tubes would it be prone to fail, and if so why?."

The ASME Boiler Code addresses the use of hundreds of materials for boiler construction, prohibiting many for use in boilers, and does allow various stainless steels for the wetted parts of a boiler, but not austenitic stainless unless rigorous water quality is assured, at insanely pure conditions. The stainless steels that we typically encounter (austenitic types 304 & 316, among others) are prohibited unless water quality is always maintained well below 1 part per million of "hardness", a very tiny fraction of the "20 grains of hardness" of your question.

AS mentioned above, the stainless steel is susceptible to chloride stress corrosion, and this detrimental condition is accelerated as temperature rises. Anyone who is remotely familiar with the boiler code would immediately reject a stainless steel boiler. While inspection criteria at steamboat meets is often very relaxed, who knows what standards might be applied at some future steamboat meet? I know the local scrap yard I frequent has some kind if a portable hand held device that can immediately identify if a piece of metal is stainless or not. It even registers the % of elements in the alloy. Why mess with something that is an attractive bargain, when you know that the authorities having jurisdiction in these matters have clearly made it known that stainless is prohibited for power boilers.
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Re: Stainless steel tubes

Post by fredrosse » Sun Oct 10, 2021 1:58 am

With respect to the thermal conductivity of boiler tube materials, the following is from a previous post:

"The copper vs steel subject rears it's head again and I still question the consensus. "

I can say with great certainty there is virtually no difference in heat transfer if Copper vs. Steel tubes are used in a steam boiler. I have worked as a professional engineer in the heat transfer, fluid mechanics, and thermodynamics arena for nearly 45 years, and, at least in these realms, I know the real technology.

A very reasonable analogy can often make this more clear. Let us say we are having a traveling race, having three separate segments, for 9 miles total distance.

The first 3 miles of the race is done with brisk walking, on foot only,
The second segment is using any motor vehicle you like, and is also 3 miles distant.
The third segment for the remainder of the race, again three miles, and you use a good horse.

In this analogy, the first segment of the race, on foot, represents the travel of heat from the flue gas to the surface of the tubes, and this process is by far the greatest resistance to travel, at 3 MPH, taking about 60 minutes time.

The second segment of the race, with any motor vehicle, say a Ford Pinto, or a new Corvette. This represents the travel of heat through the tube metal, Steel tubes (Ford Pinto) or Copper Tubes (Corvette). This process, takes 3 minutes for the Ford, at 60 MPH, and only 1 minute with the Corvette at 180 MPH.

The third segment of the race, with a thoroughbred horse, represents the travel of heat from the tube surface into the boiling water, a traveling method far faster than walking, but much slower than the automobile, say 30 MPH taking 6 minutes to complete.

So in this travel race, the entire three segment trip with the Ford Pinto takes 69 minutes, and with the Corvette 67 minutes. While the Corvette is wildly faster than the Ford Pinto, the overall trip time is virtually no different. Yes, the Copper tubes do make better heat transfer, but not in any way significantly.

As to the question of using a tube material with even lower thermal conductivity than ordinary steel, for example, stainless steel, which has a thermal conductivity about 40% of ordinary low carbon steel, the answer remains the same: The tube material has virtually no significant impact on boiler steaming capacity, be it copper, or carbon steel, or stainless steel. The conductivity of tube material is not the relevant resistance to heat flow from the fire to the boiling water. The heat transfer coefficient getting fire heat to the surface of the tube is the real "bottleneck" of the process, while the metal tube conductivity issue is one or two orders of magnitude less important here.
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Re: Stainless steel tubes

Post by Lopez Mike » Sun Oct 10, 2021 3:01 pm

Thank you Fred!!! I've been preaching this forever and not getting far with my copper tube advocate friends.

There are some aspects of copper tubes like ease of replacement that might be considered. Myself, I roll in mild steel and do a sealing TiG weld and have yet to have a boiler repair problem. To be honest, I roll the tubes in for convenience. It creates a stable structure for assembly.

The beauty of a monolithic assembly with the tubes welded in is that you can pretty much ignore thermal shock issues. A structure assembled with small parts of it at the melting point of iron is unlikely to be bothered by dumping cold water in it or heating it rapidly.

Store your boiler either bone dry (my choice) or full of water with a reasonable pH and your grandchildren will enjoy that boiler.

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Re: Stainless steel tubes

Post by lostintime » Tue Oct 12, 2021 9:34 pm

The race analagy has me wondering.. I the closest place I have been able to locate that sells boiler tubing is a three hour drive, but I can get schedule pipe fairly locally. How bad of a hit would using 1" schedule 40 pipe be on heat transfer in the overall scheme of things. I intend to tig the tubes inplace regardless.
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Re: Stainless steel tubes

Post by Lopez Mike » Tue Oct 12, 2021 11:54 pm

Hmm. They certainly would have the wall thickness to last forever! And strong enough. You might have to clean off any paint or such. Whether the extra wall thickness would be a thermal issue don't know.

Fred and others with more technical experience and regulatory knowledge will chime in here for sure.

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