mono-tube safety facters

[Lopez Mike]

If I remember correctly, they blew up a lot! Of course that may well have been from lax or no regulations and irresponsible operation.

As an example of what you can get away with, I recently was brought a small steam plant for boiler replacement. The engine was a single cylinder oscillating type of perhaps 1/2" bore and 3/4" stroke. It was built 80 years ago by the owner's father out of odds and ends including a small sewing machine flywheel. It turned very freely and ran easily on low pressure air. The throttle was a length of 1/4" surgical rubber tubing pinched between two wooden levers by a bolt with a wing nut. The pressure gauge was the oil pressure gauge from a Maxwell automobile.

The heat source was an old electric iron inverted. The boiler was a one quart rectangular oil can lying on its side on the iron. There was a home made safety valve. No feed water system. Just shut it off and refill when low on water. I have no idea how one was to know when it was low on water! Perhaps when the solder began to drip.

The vintage oil can was rusted through. I found a replacement can and installed all the fittings from the old one. I cautioned him about 'over firing'.

He called to say that it runs well. I hadn't the guts to charge him for the job. As yet, nothing on the evening news.

Mike

[DetroiTug]

Some of the boilers at Greenfield Village here in Dearborn have cast iron ends.

This boiler safety discussion made me think of this video I seen a while back. I'd be out of that building so fast. Then he comments "It was just to test the issues"



-Ron

[Lopez Mike]

I'm impressed. Mostly by the burning paint, the escaping steam from the poor fittings and the fact that the plastic wheels didn't melt or catch fire!

I wonder if the whole panjandrum would be able to spin an automotive alternator and charge a battery?

The battery chargers for clandestine radio sets in WW2 used a boiler almost as simple. It was made of a double height pressure cooker and was batch loaded with water. Again, I have no idea how they knew when to stop and add water. Maybe there was a sight gauge.

On YouTube there seems to be a negative correlation between the quality of the camera work and how interesting the subject is. Wonderful, stable shots of someone's garden. Work like this where I really want to see the last few seconds before the final blast.

Mike

[Mike Rometer]

My concern is more that there is a child so obviously present throughout. Agghhhhhhhhhhhh!

[Lopez Mike]

Looking at it carefully, the pressure was maybe 40-45 psi. If the fool didn't let the water level get too low, and, and, and . . . . .

As a child I was present at a number of inadvertently exciting events caused by my father (a motorcycle racer among other things) and his brother-in-laws (loggers) and seemed have survived reasonably intact.

My dad crashed a burning board wall on a motorcycle at night on two successive years to raise money for the March of Dimes back in 49 and 50. The first year, the wall was stout and hadn't burned long before he hit it at maybe 35-40. He exited at slow speed with a bent front wheel and not connected to the bike. The second year the wall was flimsy and the fire was allowed to mature. He exited at such speed that he nearly had an accident stopping in the limited area of the baseball field. Flaming boards landed around me on both occasions. I was 6 and 7 years old. I suspect that it has affected my attitude towards risk.

A lot of things look risky but are really not. Other things like driving after drinking or driving while using a phone look under control but are wildly dangerous!

I think that air compressor steam plant is somewhere in between. I wouldn't do it without a sight gauge and a hydro test.

I grew up with 'stumpy', 'one eye', and 'three finger', and those were the girls!

Mike

[Oilking]

The Video brings to mind an event where the air receive on a 70ft, wooden hulled dragger, let go. Fortunatley the engine room was not occupied, and no one was hurt, and the boat stayed afloat. From the outside the planking was cracked, and bulged on the side where the tank had been. With several broken frames and deck beams plus its age, it was hauled out and munched up by an excavator.

The point: any gas under pressure has the potential for a catistrophic pressure release, ie. BOOM.
At the same temp and pressure, the bigger the container the bigger the boom.
A steaming boiler also has the hazard of the water flashing to steam during such ab event.

Again all things equal the monotube with less water volume, and small tubes would appear to be safer, but if it's operated at 400psi or more an several degrees of superheat that changs the picture.

MSA developed ecape masks for the Navy that had a 3000psi or more air reservoir made of a coil of 3/8in SS tubing banded together in a compact bundle. The thought was that unused masks left in a fire posed a threat to the fire fighters, and that the failure of the small tubing was much less of a hazard than a small cylinder that could fragment in such conditions.

With all the ramblings above I guess it all boils down to this: use a design appropriate for the application, properly built of the right materials, properly maintained, and properly operated, and the world will be alright. Oh yes, that big brass cased antique gauge you picked up at the garage sale, have it tested before you trust your safety, and that of others to it.

Dave

[DCSmith]

Actually, the cast iron ends were mentioned in the chapter on boiler explosions. I'd like to reference the book, but many of my volumes are still in boxes from moving. As far as determining the water level in a pressure cooker boiler, I believe when they were used for steam-boxes to bend wood, it was common to place marbles or ball bearings in the boiler. When the water got low, they would start to rattle; that's when it's time to shut down and refill.

[Lopez Mike]

For some reason, I find that very alarming. Maybe even terrifying! I think there may be some loose marbles all right! Just not in the boiler.

Now that I think of it, I believe that when generating steam for bending wood, the boiler is at atmospheric pressure. Might melt the bugger but no bang.

I'm trying to think out the succession of events when a pressure cooker type boiler runs low on water. How low does it get before the heated surfaces start to exceed the temperature of the steam? Enough to weaken them, I mean. Depends on the design of the heat source, I suppose.

I have always assumed that the upper (dry) parts of my VFT flues got a little warmer than the water but how much I have only a poor notion. Like as hot as the stack temperature? Is there really much superheat happening up there?

Mike

[Albert]

Mike,

Some years back, when reading an article on the VFT boiler of the Stanley steam cars, I was very impressed by a sentence like ",,, as the boiler contains no more water when the lower tube plate starts getting overheated, this type of boiler is intrinsically safe". When it was my turn to design the boiler for my boat, this was one of the reasons why I decided to select a dry fire box type (no water leg). Nevertheless, I certainly try to keep the boiler water level well above the mid-height indication.

I agree with you regarding the interesting question: What happens really, what are the physical processes taking place short before the tube plate starts getting dry? How big is the potential danger at this stage?

Albert

[Oilking]

Mike,
A friend of mine used a 15ft lenght of 5in aluminiun irrigation pipe for a steam box. One end had a cap welded on it, and the other had a hinged and gasketed lid that was far from a perfect seal. He would dump in about a gallon or two of water, and the wood to be steamed, close the lid, and then position the pipe with the capped end down, and over a fire. When steam had was evidenced escaping from the lid end for an appropriate time the latch was releases and the wood removed, more water added, and more wood for the next cycle. This all worked well until for some reason it was allowed to boil dry resulting in melting the end that was in the fire. Disappointing, but no catastrophy. A pressure cooker that has a constant draw of steam from it will do much the same, run out if water stop making steam, and then then melt if its aluminium. Not unlike a kettle on the stove boiling dry.

A much different event is a sealed cooker of about .5cuft volume containing a quart and allow to boil dry. In theory the pressure could be around 1600psi , well above what the cooker could handle. Hopefully the safety relief systems would function well enough to vent pressure before the failure point could be reached.

Dave

Dave