The Steamboating Forum

It is commonly accepted that mono-tube boilers can't explode due to the small amount of water held within. O.K. so far. I wonder, though, does the liniar configuration have anything to do with mitigating explosions should there be a tube failure? Any opinions out there?

Anything containing a fluid under pressure can explode! its just with a small diameter tube the strength is so much greater for a given pressure. The main safety from injury with a monotube system is also however due to the low volume of contents. Assuming a conventional boiler there is two things to cause injury if a failure occured, 1 the steam under pressure has a large amount of energy which it may release explosively and 2 the water content would also largely and almost instantly evaporate into steam due to the sudden release of pressure. In a monotube both these events are minimised.
regards
Jack

In general, if a monotube steam generator will to rupture its tube, it will tend to rupture where there is very high temperature, which greatly weakens the metal tube. If this is the case, then the fluid that escapes from the ruptured tube is well superheated, and the initial release is steam gas, rather than a hot steam-water mixture that can be more dangerous. Of course, there can be ruptures anywhere along the path, depending on the condition of the tubing and the joints.

The flow area for release of steam pressure energy is very small for a monotube rupture, when compared to the rupture of a larger boiler vessel. For example, a steel tube with 0.5 inch OD and 0.065 inch wall thickness, if completely ruptured, has a total flow area of 0.215 square inches. If a 12 inch diameter boiler shell ruptures a seam, say 2 inches wide x 12 inches long, it allows steam/water escape from a 24 square inch flow path. The ruptured monotube has an energy release well below 1% of the ruptured 12 inch shell.

Your Mileage may vary, and as i'm sure you understand, steam boilers can be dangerous in any configuration, however, this fellow describes the failure of the monotube boiler aboard skylark as "It made a pop no louder than opening a can of lager and made me one of the worlds totally converted monotubists."

Hello all

Look at me digging around in the basement!

As far as mono tube boilers exploding goes yes they do and yes they can. All that steam has to go somewhere. I work for a company that uses a number of mono tube boilers and in the 13 years I have worked here we have had two of them com apart at the seams. Both were the result of stuck fire controls and both had burst disk safety devices in place.

These boilers run a ½ tube and are fired with fuel oil. They usually run at 2200 pounds pressure but the second one to go was being used as a hot water source and only under city water pressure.

I wish I had pictures of both but I can tell you they took apart the insulation and sheet metal casing and dropped it all over the street for 200 feet around the trailer they were mounted on. Had someone been standing over them I imagine they would have spent a bit of time in the hospital.

I am working out a way to place a pressure blow off panel on top my boiler so if something does come unglued inside the majority of the steam will go up where I am not.


Cheers
Scott

Cyruscosmo wrote:I am working out a way to place a pressure blow off panel on top my boiler so if something does come unglued inside the majority of the steam will go up where I am not.

That is one of the many good features of the Ofeldt boiler. The steam generator is inside of a casing, if it were to let go, depending on the strength of the casing, it either goes out the stack or if the firebox door is open, there as well.

If you're looking for a fast firing, easy and inexpensive to build boiler, have a look at an Ofeldt. The only obstacle is finding someone to wind the steel coils. But copper coils will work as well, the issue there is connection to the main center drum. Brazing is frowned upon and flared connections are hard to keep from leaking when exposed to fire ( individual components expanding contracting).

I'm in full agreement on the dangers of even monotube boilers. They are the least dangerous, but that does not imply a failure would be inconsequential. Back to selecting a boiler - monotubes will not operate practically without some sort of electronic controls. They are better suited to cars.

-Ron

Hey Ron

I am leaning towards the Thornycroft style boiler like the one Rainer built because I will be using more wood than anything else as a fuel. The fire box in that design in my mind leaves more room for experiments as I can make inserts for other fuel types.

The LaMont style boiler looks interesting too but I am still working out the water circulation part. I am considering building a test boiler simply to toy around with different ideas before I build one for the boat. The test boiler could then be used to heat the shop while I build the boat.

The tube bending and welding are no problem for me I have been pushing steel around since I was 12 in shop class and first learned how to stick weld. And I have held a welding cert for 22 years or so now. The only problem I have welding now is getting the piece far enough away so I can focus on it! LOL... tis a bitch getting old huh...

Cheers
Scott

I think I have read every post on this forum. I don't remember any reports of serious failures. That traction engine blew in the Midwest U.S. a few years ago but that was criminal negligence.

Bart lost a tube due to a design flaw in his gauge piping. Bart seems to be in good health. One of the N.W. Washington boats lost a tube this last year due to failing to put any significant amount of water in his boiler before firing up. An Oldfeldt I believe. I was there and there was a hiss and an embarrassed owner.

We keep on top of things fairly well between peer pressure at events, club inspections here in the N.W. U.S.A and serious concern here on the forum when we hear of anything remotely dangerous. I note that the two failures that Scott observed were due to automation failures!

Almost any size of opening is not catastrophic unless someone is in the way of the steam, machinery gets propelled about, or fragments fly about. I have a large (1.5") blowdown valve and while opening it up at 100 psi and letting the boiler blow down dry is startlingly noisy, I have no fears that there will progressive failure of any sort.

While it is possible that some clueless person might cobble together a bomb, I've not seen one. I think that worrying about the explosion potentialities of different proven designs is an entertaining academic exercise.We are much more likely to get scalded or scorched by piping failure or touching something hot.

I think we are doing quite well on boiler safety. If anyone has any reports that they care to share, I would be most interested and grateful.

This from a person who sits next to a VFT, considered by most to have the largest potential for a big bang.

Mike

If the coil is inside a housing with openings incapable of exhausting high pressure boiler water - instantly expanding ~2000 times it's volume in to steam, it could create an explosion.

If you're looking for a way to fasten copper coils to a steel boiler.... weld steel 'nipples' into the drum that are bored to accept the tubing with .002 or so clearance, and then silver solder coils into place w/ ~50% silver bearing solder... this works like a charm; I did 110 joints on my boiler and only one weeped; it was easily reheated w/ fresh flux and all was well.

Note that silver brazing alloys are suitable for this - bronze is not, since it doesn't flow into gaps very well.

-- Bart