Stainless in the fire

A special section just for steam engines and boilers, as without these you may as well fit a sail.
daysaver1
Steam on Deck
Steam on Deck
Posts: 49
Joined: Thu Dec 10, 2009 11:21 am
Boat Name: No Boat Yet
Location: Livermore, CA
Contact:

Stainless in the fire

Post by daysaver1 » Sat Jul 23, 2011 11:48 pm

I've read often on this forum that stainless steel piping is "illegal" when in the fire, under pressure. I like stainless in my feedwater plumbing as it looks good, doesn't add any rust to the water and it's tough. However I'd like to replace the copper economizer in my smoke box (VFT) boiler with stainless and not sure how the ruling applies there. After dusk, with oil turned up nice and high, I've seen plenty of flame spewing from the funnel of several boats. Comments?
Co-Captain...S.L. Reward
dhic001
Full Steam Ahead
Full Steam Ahead
Posts: 173
Joined: Thu Jun 10, 2010 8:32 am
Boat Name: S.S. Zeltic

Re: Stainless in the fire

Post by dhic001 » Sun Jul 24, 2011 12:58 am

I'm not up on the legalities of such things overseas, but a fried of mine has a couple of monotubes that are all stainless. They've had plenty of use and are showing no ill effects as yet. That said, if something did happen, you'd only notice because the engine stopped, not because the whole thing blew up in front of you, its impossible for it to do so.

In your case i'd use stainless quite happily. Realistically, the worse that can happen is that it blows (which copper will do too), pours a whole lot of water down your tubes and puts your fire out. If you catch it before the fire goes out, you'll be able to shut the water supply to the pump, and run home on your injector or handpump (if you only have a hand pump, and are a long way from home, you'll be buying an injector when you get home as well as a new bit of pipe).

Thats my thoughts anyway.
Daniel
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: Stainless in the fire

Post by fredrosse » Mon Jul 25, 2011 12:06 am

In the ASME Boiler and Pressure Vessel Code, stainless steel is specifically prohibited for the wetted parts of a fired power boiler, and a power boiler is defined as any producing steam above 15 PSI. This is because stainless steel suffers from chloride stress corrosion cracking, and there is virtually no way you can keep your boiler water free of small amounts of chlorides (in the PPB range), unless you use distilled water exclusively, or have a sophisticated demineralization system.

The ASME Code is quite clear in its directive prohibiting its use in the wetted parts of the boiler. I believe the problem is worse where there are high stresses, and, while the pressures we use are generally low, stress can come from bending of tubes due to thermal expansion, tubing hanger attachments (plenty of them in a large coil), welding stresses, or numerous other causes. I think most steamboaters don’t have the technical skills to address these issues, so better to stay with steel (or other permitted materials) for boiler pressure parts. The ASME Code allows copper, if wall thickness is heavy enough, and metal temperature does not exceed 406F/208C.

I have actually seen stainless steel which developed corrosion pits about 1/2 inch deep in 30 days time, on a poorly specified sea water intake system. Carbon steel is much more predictable.

Carbon steel does not have the stress-corrosion issue, and is a very ductile material, and hence forgiving with respect to many stress related issues. Rust is a potential problem, but a few simple layup practices can virtually eliminate the rust issues for many years.

Although some steamboaters have had reasonable service with stainless steel wetted boiler parts, that does not indicate that there are no problems with it. One thing to keep in mind is the potential that the ASME Code will be mandated upon us some time in the future, or that inspectors may wish to follow Code rules even when inspecting a non-Code boiler. In that case, the inspector may not like seeing stainless steel pressure parts when the ASME Code clearly states that it is not permitted.
farmerden
Stirring the Pot
Stirring the Pot
Posts: 447
Joined: Fri Nov 20, 2009 12:14 am
Boat Name: Steam Queen
Location: Shawnigan Lake B.C. Canada

Re: Stainless in the fire

Post by farmerden » Mon Jul 25, 2011 1:51 am

Thanks Fred -now you've got me worried! My superheater which runs the atomizing burner is a stainless "U" shaped pipe,into the firebox and back out to the atomizer.I have a spare on the boat and have never needed it.It rarely sees 30 lb pressure ,is 1/4 in tubing,and as the rest of the boiler [Roberts] is 1/2id copper I don't know why the builder went to stainless in this application. Den
User avatar
DetroiTug
Full Steam Ahead
Full Steam Ahead
Posts: 1863
Joined: Fri Nov 27, 2009 5:56 pm
Boat Name: Iron Chief
Location: Northwest Detroit

Re: Stainless in the fire

Post by DetroiTug » Mon Jul 25, 2011 2:02 am

Fred,

What are the standard layup procedures?

On shutdown, I've been blowing down fully at 10 psi, leaving the blow downs and tri-cocks open until the boiler is cold.

Another question regarding the maximum temperature for copper. Some boilers use copper for Ofeldt type coils, it's obvious that the upper portion of the coil, exposed to flame and absent of water (or is it?) seems to do OK? I've considered building a small Ofeldt for another planned steam project. And then there are superheaters and economizers of copper. I'm a bit confused about the use of copper on the generation side of a boiler.

Also, I'm adding "turbulators" or "retarders" to the firetubes in my tug boiler as they are 1.5" ID. Is it better to retard the heat all the way up the tube, or should there be no twist in the vane above the water level? To allow the gases to escape more quickly in the non-wet portion of the tube.

Thanks, Ron
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: Stainless in the fire

Post by fredrosse » Mon Jul 25, 2011 11:34 am

"My superheater which runs the atomizing burner is a stainless "U" shaped pipe, into the firebox and back out to the atomizer."

The ASME Code does allow stainless steel for a superheater, as this is not a "wetted part", although sometimes it may be exposed to some water carryover from the boiler, this exposure would be a very small fraction of the exposure in the boiler proper.

For really high temperatures, such as a superheater within the firebox, stainless steel is one of the only materials (that we can afford) that can take this temperature. When starting up, there may be no steam flow in the superheater, and metal temperatures can soar, that is why there is often a tube section of the boiler wetted tubes providing a protective gas cooling zone before the hot gasses get to the superheater tubes.

Seems like the stainless steel superheater for your burner is OK according to the rules.
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: Stainless in the fire

Post by fredrosse » Mon Jul 25, 2011 11:57 am

"Adding "turbulators" or "retarders" to the firetubes in my tug boiler as they are 1.5" ID. Is it better to retard the heat all the way up the tube, or should there be no twist in the vane above the water level? To allow the gases to escape more quickly in the non-wet portion of the tube."

ANS: As I am sure you know, in a Vertical Firetube Boiler (VFT), the top section of the tubes will slightly superheat the steam, as this section of the tube is above the water level in the boiler. Most turbulators I have seen are simple "Twisted Tapes" of flat sheet material, and many installations have them through the entire length of the firetube.

However it may be prudent to avoid possibly overheating the top of the tubes by stopping the turbulator at or near the boiler water level. I don't think this is such a problem with metal overheating (although this is a possibility), but more for differential thermal expansion between the boiler shell and the tubes, causing stress on the tube to tubesheet joints. On my sidewheeler VFT boiler I made turbulators of 1/8 inch diameter stainless steel wire, wound on a lathe like a coil spring, with a very loose fit into the firetubes. I stopped the winding in the tube above the water level, this cuts the heat transfer effectiveness on this part of the tube, so it will not run as hot. I am currently propane fired, and the addition of the turbulators cut my fuel consumption in half, quite an improvement.

The attached pictures show how these were arranged.





I will prepare some info on shutdown preferences and the use of copper later, got to get to my day job.
Attachments
WindingTurbulators.jpg
Winding on lathe, coils streched out later.
WindingTurbulators.jpg (25.36 KiB) Viewed 20046 times
WoundTurbulators.jpg
Two Wound Turbulators
WoundTurbulators.jpg (7.22 KiB) Viewed 20046 times
TurbIntoFiretube.jpg
SS Turbulators inserted into tubesheet of boiler
TurbIntoFiretube.jpg (19 KiB) Viewed 20046 times
farmerden
Stirring the Pot
Stirring the Pot
Posts: 447
Joined: Fri Nov 20, 2009 12:14 am
Boat Name: Steam Queen
Location: Shawnigan Lake B.C. Canada

Re: Stainless in the fire

Post by farmerden » Tue Jul 26, 2011 12:57 am

Thanks Fred The more I thought about it, if I am burning wood,then the superheater would not have any steam in it and temperatures would indeed soar! I've never had a pressure build-up in that situation which makes sense because any steam created from left-over water in the super heater would exit thru the atomizer into the firebox.The superheater is on the side of the firebox and not in the direct flow of the fire when burning oil. Details ,details God is in the details they say! :lol: Den
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: Stainless in the fire

Post by fredrosse » Thu Aug 25, 2011 1:37 pm

"If we isolate the economizer in case of a leak,then wouldn't the heat on that empty pipe be too great to use copper? Or is it high enough that the heat will less as the boiler has [hopefully!] absorbed most of the heat before reaching the economizer? Is this a situation where stainless will be allowed? Den"

Sorry I missed this question on the Detroit Tug Thread, but I think it is better to answer on this thread.

The boiler exhaust gas on the outlet side of the boiler will typically run a few hundred degrees F higher than the boiling temperature. For example, with 125 PSIG (8.6 Bar) steam, the boiling temperature is 350F (177C), and the hot gas that the economizer experiences is well over 550F (288C).

ASME Code does not allow copper over 406 F (208C). For our steamboats, even with boiling in the economizer, the metal temperature would not exceed 406F. In normal operation, the water flowing in the economizer keeps the copper metal temperature very close to the water temperature, and it would thus be below the ASME limit of 406F.

If water flow to the economizer is stopped with the boiler fire continuing, then the water within the economizer will boil away, and then begin to overheat, quickly reaching the exhaust gas temperature, which is well above the ASME limit for copper. Depending on several variables, the copper will either be "annealed" by the high temperature, and survive, or if pressure and temperature is high enough, the economizer tube will rupture.

Most small boilers have an enclosure that would protect people from this tube rupture, so it generally would not be a safety issue.

Stainless steel does not have the temperature issues of the copper, so it would almost certainly simply overheat, and survive the temperature excursion. The stainless is however susceptible to the stress corrosion issue, although in the economizer service the chloride concentration would not occur as it does in the boiler proper.

A better choice would be steel for the economizer, cheaper, better, although not quite as beautiful to some of us. I read in another thread that someone wanted stainless for the feed water supply lines, because he didn't like rusty water going into the boiler. Water that contains an amount of oxygen will form rust with steel, and the oxygen is depleted as the rust (iron oxide) is formed by reacting with the steel (iron). In that case, take your pick, either make the rust from your feed water pipe metal, or make the rust from your boiler metal. I would always choose to make the rust with the feed water pipe metal, as replacing that is so much less costly than replacing boiler pressure parts. I have worked on many power boilers, probably over a hundred of them, and industrial boilers all use steel feed water piping and steel economizers, some are still in service after more than 50 years.
dhic001
Full Steam Ahead
Full Steam Ahead
Posts: 173
Joined: Thu Jun 10, 2010 8:32 am
Boat Name: S.S. Zeltic

Re: Stainless in the fire

Post by dhic001 » Fri Aug 26, 2011 10:01 am

fredrosse wrote:" I read in another thread that someone wanted stainless for the feed water supply lines, because he didn't like rusty water going into the boiler. Water that contains an amount of oxygen will form rust with steel, and the oxygen is depleted as the rust (iron oxide) is formed by reacting with the steel (iron). In that case, take your pick, either make the rust from your feed water pipe metal, or make the rust from your boiler metal. I would always choose to make the rust with the feed water pipe metal, as replacing that is so much less costly than replacing boiler pressure parts.
I choose not to make rust in my boiler at all! Rust in my boiler will cost me big money long term. Rust in my pipework will cause issues with my check valves if I'm not careful, and carried through to my boiler will make it look like I don't maintain my boiler properly. As a result, the barest minimum of my feed pipework is steel, the rest being copper (and rubber hoses). As for keeping the rust out of the boiler, thats called using feed water treatment. Long term result should be minimal boiler maintenance bills.

Daniel
Post Reply