lighter stronger tubing

Hi again,

which steel tubing do we use for boilers?
saw this one, 7% stronger, 30% lighter. will this work well? [www.bainitesteel.com]
would love to hear about experiments with differently shaped pipes, like with inner grooves or small fins

Thanks,
Rahul

October 25, 2014 Finned tubing has been around for a long time. I am looking at p. 475 of "Heat Exchangers: design and theory sourcebook" by Afgan and Schlunder of a tube with inner integral spiral fins. P. 477 has schematic drawings of 22 inner fin tubes that were tested. The issue when boiling water is that external fins are more relevant. There is something like a 25 to one ratio of heat exchange between a gas and a liquid such as water therefore the air side and not the water side is where the fins should be. The big problem with fins has to do with water pumping failure. At the level of sophistication that most of us are working we are always having water pumping issues and it is for a different reason each time. Any time a tube runs dry the fins melt off. That is why everyone is looking at LaMont style steam generators or multi-path ones. Anyhow, there are books on the subject and it is a good one to pursue. Tom Kimmel

Rahul,

There is also an interesting twisted tube, where the tube is compressed with several sets of rollers then twisted. This leaves both fins on the outside and a minor spiraling on the inside. I had an interest in this because it appeared that it was possible to set it up in a small shop and twist to ones own specifications.

If the fins are too deep then they are wasting space, if too shallow they are not effective enough. If the fins are too narrow they won't conduct enough heat and could even burn. As with all trade offs in engineering, the ideal fin would be a tapered one that is thicker at the base for the more heat being conducted. That one is too hard to make.

Multiple paths using smaller diameter tubes is also an effective approach, but require a very well balanced water distribution system. Cyclone did that using six tubes in their generator.

The calculations for heat exchange are generally only within 50% -30% of reality. That is actually pretty close considering having to estimate the various parameters. Heat exchangers have to be built. The design and calculations will only get you in the "ball park".

Tom has good advice.

Best,

Bill G.

PS. The Bainite steel looks like a new process of tempering. I would be leary of that tempering remaining strong in a boiler. Remember that the tubing needs to be bent for the boiler also.



Edited 1 time(s). Last edit at 10/25/2014 11:06AM by Bill Gatlin.

Are you talking boilers or steam generators? Boilers are commonly built from A-178 tube or also A-106 pipe.
Attached is a sheet of ASME approved alloys.
All seamless pipe and tube.
Wall thickness depends on pressure and heat.
A superheater may have much thicker wall.
Rolly

steam generators; thank you for the insights. will keep it simple for the prototype, whenever I get to it. guess that steel may lighten up the heat exchangers at least, if high heat makes it difficult to work with as easily

always been on my mind to reach out to Tom and Dan; they are a huge compendium of experience and information. Everyone on the forum too. just that I've been holding off until am in timezone nearby. ~12tz away right now


Thanks,
Rahul

The problem, as I see it, is that bainite is not a steel alloy in and of itself. An alloy is a metal that has had other elements added to it in order to give it specific properties; in this case steel is an alloy of iron.

When you heat a steel alloy to high temperatures, various types of grain structures will form in the steel as it cools. These include ferrite, cementite,austenite, martensite, pearlite and bainite. Depending how long the metal is held at various temperatures or how rapidly it is cooled, different grain structures can form in the same alloy. Careful application of this phenomenon is how we achieve the various heat treating processes.

You could have a piece of metal with a very favorable grain structure but composed of an alloy that does not best suit your needs. Likewise, reheating the alloy beyond its transition temperature and recooling it can cause the grain structure to change to something you didn't necessarily desire.

So before picking something like a bainite tube, you would need to know what the temperatures of the tube wall will be (the surface of which may exceed that of the steam inside) and the transition temperature and characteristics of that given alloy.

To sum it up succinctly, I don't think there is enough information given for most of us to make a rational evaluation of the suitability of this material.

Ken



Edited 1 time(s). Last edit at 10/26/2014 11:21AM by frustrated.

The failure of steam generator tubes is not at the normal working temperature but when the tube momentarily runs dry, becoming red hot and going beyond the critical temperature of alloy steels, any of which would thereby lose what ever quality of hardness it may have been manufactured to achieve. The sudden ingress of cold water to a red hot bit of alloy steel tube, even if it had not yet burst under steam pressure would cause immediate embrittlement and rapid failure by cracking. I presume this is why low carbon steels are used in monotube generators - the steel may not be hard or particularly strong but is little changed in structure by overheating/fast cooling and therefore more reliable.

Mike

Mike a very good tube for superheater sections of a steam generator is SA-213 T91

[www.sunnysteel.com]

Rolly

Found this note on boiler R&D quite fascinating, on the myth of boiler heating square footage. It's about Navy fire tube boilers, very different things. Yet, the statement about a slower heat release rate being better, points to something interesting with a basis am yet to learn about.

[www.cleaver-brooks.com]

Rahul

I was surprised to see the Bainite Steel website crop up here, since the target market for the tubing is not boilers. I know the inventor of the process, used to be a neighbor of mine in Washington Township outside Detroit. Ken is right about bainite not being a grade, it's a steel microstructure made by specific cooling rates. Not quenched and tempered, actually remarkable stuff for properties. It's more appropriate for armor or car weight reduction than high pressure tubing.

I make lots of pressure vessel quality plate for the energy industry, and the base design need is not strength, but resistance to final failure. Standard PVQ plate is actually pretty weak, with a minimum yield strength of 36 ksi. The key is the minimum tensile strength, when the plate stops stretching and forms localized necks where it can fail. That varies by the grade from 55 to 70 ksi. You want the pipe or shell to start swelling, so you can see it begin to fail, not split catastrophically. Old boilers that have been overfired look like elephant hide.

Mike's also right, high strength is fine until you get up around 1550F or so, depending on the chemistry. The atoms rearrange in a different structure, called austenite, which is a lot weaker. That's the reason the Trade Towers fell, not the planes hitting them.

Tom



Edited 1 time(s). Last edit at 12/14/2014 10:27PM by TH.

Thanks Tom! Yes, I read the trade towers analysis too. This was a typical materials newbie question.

The other application was the roll cage of a light aircraft thundergull.com which uses chromoly. The conversation there too seemed to stall, at the point of the strength of the welds, as the process seemed to be quite sensitive to temperature and timing. The last thought there was whether to apply the process after welding normal steel or before, the technical paper on the site dint shed much light on that aspect. Wonder if you know something about that too

Happy 2015,
Rahul

All files from this thread

File Name	File Size		Posted by	Date
CommonBoilerTubeSpecifications.pdf	12.8 KB	open | download	Rolly	10/25/2014	Read message