mono-tube safety facters

[Lopez Mike]

Part of my problem is that I know very little about water tube boilers. Let me think about the problem of enclosure.

A 1/2" pile breaks clean. Big roar and it blows the fire out most probably. Don't most of these type boilers have an air inlet for the fire? I think that there would be a lot of startled people and soot all over the place.

Possible for there to be some sheet metal flying about but I'm doubtful. Something to think about though.

Mike

[barts]

Part of my problem is that I know very little about water tube boilers. Let me think about the problem of enclosure.

A 1/2" pile breaks clean. Big roar and it blows the fire out most probably. Don't most of these type boilers have an air inlet for the fire? I think that there would be a lot of startled people and soot all over the place.

Possible for there to be some sheet metal flying about but I'm doubtful. Something to think about though.

Mike

When I blew a tube (actually, overheated the silver solder and the tube popped out of the boiler) in Otter's boiler back in 1998 due to misread low water it blew the fire out, sprayed me with wet steam/water smelling of diesel & soot through the firebox door and of course large volumes of steam/soot up through the stack.

Everyone was startled, and we got a tow back in. Otter's boiler casing is 16 gage stainless; the rate of steam released through a .430 or so hole is quite insufficient to cause problems given the firefox and stack as egress. The odds of multiple tubes failing at the same time absent gross externally caused mechanical damage are very low, indeed.

When I got it home I pulling the boiler, removed the pressure vessel and inspected the failure; I cleaned the tube & nipple, refluxed everything and silver soldered the tube back into place. I hydro'd at full pressure (400 psi) and called it good. Note that all the copper tubes were annealed after winding and before soldering anyway.

I've known of people being injured from tube failures on larger boilers with open fireboxes; if you can coil steel tubes there are definitely more resistant to low water than copper tubes.

- Bart

[Lopez Mike]

Be glad I wasn't there. After the initial fright I would have teased you unmercifully! Not really but I would have definitely taken a picture of you for the record, so to speak.

So the steam was actually escaping from two holes. One in the main casing and also from the end of the unsoldered tube. But there was no way for it to become a progressive failure.

Contrary to much speculation and mythology, there is no giant rise in internal boiler pressure when a shell or firebox crown sheet fails. The physics of steam formation don't allow it. These stories of 2000 psi blowing everyone nearby to fragments are just excited speculation of ill informed and terrified bystanders.

Mike

[barts]

Be glad I wasn't there. After the initial fright I would have teased you unmercifully! Not really but I would have definitely taken a picture of you for the record, so to speak.

I guess that makes two of us .

So the steam was actually escaping from two holes. One in the main casing and also from the end of the unsoldered tube. But there was no way for it to become a progressive failure.

Yup. It was definitely startling, but by no means terrible. I'm certainly not interested in a more sudden release of the steam, though...

Contrary to much speculation and mythology, there is no giant rise in internal boiler pressure when a shell or firebox crown sheet fails. The physics of steam formation don't allow it. These stories of 2000 psi blowing everyone nearby to fragments are just excited speculation of ill informed and terrified bystanders.
Mike

You perhaps could get noticeable spikes in pressure from water suddenly flashing into steam as it covers an overheated crown sheet.... if your steam loco heads downhill and overheats the crown sheet with too low a water level, and then heads uphill, the extra steam suddenly generated could cause problems. Let's crunch the numbers... the specific heat of steel is .12 BTU/lbm-F.... so if a 500 lb crownsheet heated to 1000F or so and then suddenly cooled to 400F by water splashing on it again, we'd dump an extra (1000 - 400) * 500 * .12 = 36000 BTU into the water... about like burning a quart of diesel. Not enough to worry about... but let's run high water down hill anyway, right?

- Bart

[Lopez Mike]

I think if the crown sheet were that hot it would have collapsed before any water got to it anyway!

I still don't see the pressure rising. Any change in pressure is communicated at the speed of sound throughout the boiler. The boiling action responds equally instantaneously to pressure. It seems to me that any hypothetical 'spike' in pressure would be so brief that it would be absorbed by the elasticity of the boiler shell. This is an interesting subject and not being a steam engineer I could be very wrong.

Funny thing is that due to the relative locations of the crown sheet and the water level gauge and the surprisingly small amount of water normally carried over the crown sheet in locomotive boilers, the false reading problem occurs when climbing a grade rather than when going downhill. The water is higher back where the gauge is. Most trains don't climb very steep grades. 2% is considered a significant hill in most areas of main line service. Back when logging was being done with trains the problem was more severe. Really steep climbs. Like 10 to 12% sometimes! That's why the steam is taken at near the midpoint and up in a steam dome. And on a steep climb the accessories like the whistle and various other things like the injector that got their feed from near the back of the boiler were prone to priming on severe grades.

So far I've been fortunate to have not encountered any steep grades with my launch. Yeow!

The other problem that locomotives had in some areas was foaming leading to false level readings. Fortunately bad water and steep grades seldom came in the same area. Good water in the woods and reasonable grades in the mid-continental areas of the U.S. where horrible water was a problem. Stern wheelers had filthy water issues but probably not foaming. River water might be half mud and crawdads but not likely to be full of calcium.

The damage and deaths from boiler failures seem to be mostly from stuff moving around at high speeds. Less catastrophic failures came produce scalding injuries and death, I'm sure.

What is seldom appreciated is the thrust that can result from a failure. What we have is a really poorly designed rocket engine with chamber pressures at the low end of the rocket engine design range, REALLY crude nozzle shape and an identical exhaust composition to modern liquid fueled rocket engines. Except that in the case of the rocket engines, the exhaust is highly superheated steam.

Our 'rocket' engine has a chamber pressure of 150 psi and a temperature of 350 degrees. A space shuttle main engine has a pressure of 3000 psi and at some places in the combustion chamber the temperatures reach 6000 degrees F. Above the melting point of iron. But what comes out the back is still just steam. (I wouldn't care to sit on top of such a thing!)

With such an engine the thrust equals the weight of one of the larger steam locomotives, 500,000 lbs. And keeps it up for minutes. In the case of locomotive boiler crown sheet failures the thrust has often exceeded the weight of the boiler and has propelled it for some distance. But only for a second or so. The crew was often never found.

With that cheery though, I'll get back to work.

Mike

[Jack Innes]

Mike,
I can confirm the force created from a boiler fracture. While not a mobile boiler, I was closely involved with the aftermath of a low pressure steam heating boiler that blew a section about 10" square out of one lower corner of the cast iron vessel. The escaping water turning instantly to a 10" square jet of steam ripped the boiler free of its 6" iron pipe fittings & propelled it, cartwheel style, about 30' across its boiler room, destroying everything in its path. There was no fire involved but the heat triggered 180 degree sprinkler heads another 20' away. This boiler weighed over 3 tons when scrapped & operated at 1.5 psi. The gauge was jammed at 1.5 psi when things settled & the fracture looked to be mostly quite old but undetectable inside the outer casing. It never leaked. It is still a little unnerving in that I was in the boiler room a few hours before this event checking water level & the automatic water feed & safeties - everything was as it should be . Anyone in the boiler room during the event would not have survived.

[Lopez Mike]

Wow! I had never heard of a cast iron pressure vessel.

Castings, in general have much lower tensile strength than compression strength. I guess that at that low a pressure it they thought it would be fine. But probably it was built before there was any sort of non-destructive testing so hidden flaws would go undetected.

I had a customer who was trying to get an old 305 chevy V-eight to turn over. The rings were rusted to the bore. He came to me asking to make an adapter from the spark plug hole to a scuba tank fitting.

Pause and think about that one for a moment!

I made him an adapter from the spark plug hole to a grease fitting and told him to fill the cylinder as far as possible with water first. I wonder what ever happened to him (snort)?

Mike

[Oilking]

As to which is safer, water or firetube, take look at all the old photos of locies blown off the tracks, steamboats with the house gone, steam donkeys that look like a metal skulpture, and more recent pictures and video of heating plant explosions that blew out the walls ofthe building, they are all firetube boilers.

Aside from the boiler failure on the SS Norway, where the sidewall header failed along both longitudinal welds*, catastrophic failures are virtualy unheard of in modern, 1940+, marine propulsion boilers. In the US Navy's history of high pressure, 600psi to 1200psi, supperheated steam, there has never been a lose of life from a boiler failure. Failed valves or piping maybe, but not from a blown tube or failure of a drum or header.

* stress cracks had been observed over time in this and the other boilers. There is evidence that at some point copper wire or tubbing was driven into the internal crack in an attempt to hide the deffect!

Dave

[barts]

Mike,
I can confirm the force created from a boiler fracture. While not a mobile boiler, I was closely involved with the aftermath of a low pressure steam heating boiler that blew a section about 10" square out of one lower corner of the cast iron vessel. The escaping water turning instantly to a 10" square jet of steam ripped the boiler free of its 6" iron pipe fittings & propelled it, cartwheel style, about 30' across its boiler room, destroying everything in its path. There was no fire involved but the heat triggered 180 degree sprinkler heads another 20' away. This boiler weighed over 3 tons when scrapped & operated at 1.5 psi. The gauge was jammed at 1.5 psi when things settled & the fracture looked to be mostly quite old but undetectable inside the outer casing. It never leaked. It is still a little unnerving in that I was in the boiler room a few hours before this event checking water level & the automatic water feed & safeties - everything was as it should be . Anyone in the boiler room during the event would not have survived.

Hmmm. This sounds like a safety stuck somewhere... the force generated by a 10"x10" opening with 1.5 psi pressure differential is only 150 lbs. To cartwheel 6000 lbs must have been at least 30 psi in that boiler.... note that automobiles typically run 10 psi pressure in their cooling systems; blown radiator hoses aren't this exciting.

- Bart

[DCSmith]

If I remember correctly, during the antebellum period, riverboat boilers made in Pittsburgh had wrought iron side plates and cast iron end plates.