Steam locomotive explosions?

OK, this is probably a stupid question:

What makes a steam locomotive explode? If the boiler runs out of water, wouldn't it just be hot inside? Or does remaining steam just get to too high a pressure?

I am having trouble visualizing this...

If the pressure saftey valve is malfunctioning, and there is no other means for the steam pressure to be released, then it will find its own way!

KA BOOM! Boiler explosion!

An old photo of a boiler explosion on a steam locomotive is found on the following site via Google:

florida photographic collection; then type in NO38697; and NO38698 for a second view of the accident.

for two views of the exploded boiler, attributed to low water pressure and a defective safety valve.

ACLfan

Let's consider the thermodynamics of this.... Water in a pressurevessel (fancy name for a boiler or sealed tank) boils at a higher temp than 212 deg F. For example, at 200 pounds of pressure, water boils at 386 deg. So, for example, a steam loco boiler contains 2500 gallons of water, under 200 psi (pounds per square inch) pressure, with all that water being at 385 degrees. Now, the way the firebox area is constructed, there is a firebox formed of 3/8" to 1/2" steel plates, supported by 1" dia threaded "staybolts"bewteen the inner wall (firebox) and outer wall (boiler wrapper sheet). Water circulates all around the sides, both ends, and the top of the firebox, aka the "crown sheet" (there are some good diagrams of this on the net, and in books). The fire is burning on grates in the bottom of the firebox, and is at about 2200 degrees. The water on the other side of the firebox sheets rapidly conducts the heat away, and makes steam in the process. Temp at waterside of sheet is about 600 degrees. Steel gets "plastic" and loses its tensile strength at about 1200 degrees. The firebox steel is usually rated at 50,000 psi tensile strength....see where I'm going with this? The vast majority of boiler explosions (actually firebox failures) are caused by low water. If the crew is inattentive to the water level in the boiler, or can't get water in (due to mechnaical component failure or false reading on water gage), then the crown sheet is the first to get uncovered and goes "plastic" as the full strength and heat of the fire takes the steel temp up to and over 1200-1500 degrees. The steel now has NO strength to resist the boiler pressure (remember, it's still at 200 psi), and the crown sheet is forced by the boiler pressure off the staybolts. As this happens (in tenths of a second), the 386 degree water under 200 psi all turns to steam-- the water is way above boiling point, and now there is no more pressure. And, water turning to steam expands 1600 times in volume-- so 2500 gallongs of water turns instantly to 4,000,000 gallons of steam!!!!

The force created is so great that the back end of the boiler (where the cab is) is usually forced off the frame, and the entire boiler is launched into a trackside field, sometimes hundreds of feet. Needless to say, the crew usually doesn't make out too well...

There was a good reason why that in the steam era, the water glass (boiler water indicator in loco cab) was called "the most important window on a train". A crew ignored low water at their peril.

See the US DOT-FRA Accident reports for the 20s through the early 50s for some detailed descriptions of loco boiler "incidents". Really gives a feeling of respect for the power of steam, and for the oldtimers who handled it thousands of times daily for 150 years.

A boiler can still explode even if it's below "working pressure" -- the setting of the safety valve. The safety valves are there to relive excess pressure, not to prevent crown sheet failures.

H.F. Malone's post below is an excellent description of the thermodynamics and physics of most boiler explosions.

Note particularly that a boiler can explode at internal pressures far below the safety valve setting. Once the crown sheet is uncovered boiler strength is lost in a matter of seconds and the inevitable results, whatever the internal pressure.

However boilers can also explode should the safety valve malfunction (i.e. fails to open at set pressure) or is tied down and rendered inoperative by the engine crew. In the early days of railroading safety valves were sometimes tied down to achieve a little more power out of the engine, or by a crewman annoyed by the loud noise from the valve blowing off while standing.

These explosions often split the side of the boiler rather then blowing out firebox/crown sheet. The end result to the engine (also to the crew and anyone unfortunate enough to be nearby) however was pretty much the same.

The first thing I taught my sons about steam is that if they are ever invited into the cab of an engine under steam, the first thing they should look at is the waterglass. Not that the assigned crew is likely to screw up, but just a good habit to get into, in case they ever have to fire steam.

So, this summer, my youngest and I were in the engineroom of the Liberty ship John W. Brown, and it was under steam. Well, he forgot, so I asked the fireman/watertender where the glass was. He used his flashlight as a pointer, and there it was, about ten feet up off the deck! Yes, it was lit up, and showed half of a glass. Big pair of boilers, thats for sure-

Seam-splitting boiler failures became somewhat rare by the teens and twenties, due to the loco builders moving away from the use of simple "lap-seam" boiler construction, and going to reinforced butt-seam joints. That increased the safety of the boiler shell (the big cylinder part) tremendously. There are many places (state juristictions) where old lap-seam boilers (many "traction" or thresher engines have them) are only able to operate a very low pressure, if at all.

The use of steel plates and good metallurgy instead of the old "wrotten iron" (yes, that's a "joke") on the 1800s locos helped considerable, as did the art & science of good water treatment in the teens to the fifties.

The most well-known case of "tying down a safety valve" is the 1830-era "Best Friend of Charleston" (S.C), which blew up on it's first trip due to that occuring. Modern (from the 1890s on) locomotive safety valves were difficult to "disable", and remember that at a minimum, each loco was required to have two safeties, one set at boiler pressure, and the second set at 2 lbs higher. Big engines of the later era had three valves, the third opens at 3 lbs above working pressure.

99.9 times out of 100, "boiler explosions" were/are caused by low water crown sheet failures. Any photos you see of a boiler pulled loose or launched off the frame is one of those events.

As a follow-up bit of information to my earlier post on this thread, as mentioned, the explosion was caused by a combination of low water and a faulty saftey valve (as determined by follow-up inspections). The less-than-adequate amount of water was superheated by the fire in the firebox, and created an enormous amount of steam pressure that had no place to go due to the faulty safety valve, except......

The explosion destroyed the cab of the locomotive, and the locomotive's bell was discovered over a mile away, according to published reports.

Fortunately, the locomotive crew had taken a break at the lumber company's commissary, and were not on the locomotive at the time of the explosion.

The two referenced photographs adequately captured the destructive power of the explosion on the locomotive.

ACLfan

Wow, what an explaination! Unfortunately, though, it is similar to the one I got from my boyfriend, who works in HVAC in a Boston building complex. Meaning I am still a bit confused!

I think what I need is to understand just where the water all goes. How does it get from the tender to whereever it goes next? Is it water or steam that goes through those pipes within the boiler?

I think I need a good visual aid... just like in school!

JD

Nells, what you need is for the BF to "gift" you with a "Your Hand on the Throttle" at Valley RR in Essex, CT. You'll get a class and instruction about what's inside the thing, and then get to run it. And there will be people there to make sure you don't blow it up!! Is your b-day coming soon? How bout Xmas? Drop some hints!!!!

Boiler explosions also tended to be more common near a summit ... three well-documented incidents at Cobleskill, NY (D&H), Hinton, WV(C&O), and Lehigh Summit, PA (DL&W) in the later years of steam come immediately to mind.

The cause here is the exposure and overheating of the crown sheet due to low water, coupled with a surge of unevaporated water within the boiler when the summit is reached and the underlying grade changes.

H.F.Malone wrote:There was a good reason why that in the steam era, the water glass (boiler water indicator in loco cab) was called "the most important window on a train". A crew ignored low water at their peril.

Steam locos were equipped with a low water alarm system. When the alarm went off, crews were encouraged to 1) dump the fire immediately, and 2) get as far from the locomotive as soon as possible.

Peter @ Trains.com wrote:There were two boiler explosions on NYC Hudsons.

On 26 December 1944, The boiler of 5219 exploded due to low water over the crown sheet, while standing at a signal in a passenger station at Elkhart.

On 7 September 1943, a streamlined J3a No 5450 exploded for the same reason while travelling at 70 mph near Canastota NY. The control handle to the feedwater pump had become disconnected. This locomotive was fitted with a "Barco low water alarm" which must also have been defective. The first ten cars were derailed. Three loco crew were killed and twenty five train crew badly injured.

Both these accidents suggest that a serious lack of experienced operators and maintainers existed at that time, presumably due to such people joining the armed forces.

Not sure but the explosion mentioned on the D&H at Cobleskill is in Jim Shaughnessy's D&H book. Jonelle, if you can, look at some of the pictured of thewreck sites of some of the explosions. The utter force of one of these explosions is really hard to comprehend. Friend of mine, Kurt Ferris, was killed by a boiler explosion in the 90's out west someplace.

Work safe

Derail

Fairmont M-19, track speeder, Boston and Maine #617, original condition!!!

Boiler explosions also tended to be more common near a summit ... three well-documented incidents at Cobleskill, NY (D&H),....

The D&H explosion was quite far from the summit of Richmondville hill [approx 4 miles] and was still in a 'nose-up' ,water to the rear, attitude as far as the boiler was concerned. So if being at the end of a very long haul was a factor for low boiler water I can agree. But if the point was that the water ran forward in the boiler, suddenly exposing the crown sheet, I don't think that was a factor in the D&H tragedy.

I don't mean to dis LCJ here, but low water alarms were something that came along relatively late in the steam era. They were an option, not a standard item; IIRC the ICC required two water glasses, with lights at night, or a lit water glass and tricocks on the backhead. Maybe two glasses and tricocks were required. I've fired a handful of engines in my youth, on tourist lines, all built in the 1920s, 1930s and 1940s, and none of them had low water alarms.

The other thing that is interesting is that the ICC (and now the FRA) required two means of getting water into a boiler. This can be two injectors (typically one on each side of the locomotive), or a feedwater heating system and an injector. Usually with the latter setup, the fireman had the feedwater system controls on his side, the engineer had an injector. Also, either system, alone, has to be sized to keep up with the boiler's demand for water at maximum load.

In the case of NYC 5450, a check of both the feedwater system and the injectors before leaving the terminal should have prevented this tragedy, unless the linkage to the FW pump came adrift on the road. In that case the engineer's injector should have been used. Counting on the low water alarm to work, if properly maintained, can lead to disaster. (I would agree that a wartime shortage of experienced maintenance and operating personnel most likely contributed to this accident).

As stated before by Mr. Malone, the water glass is indeed "the most important window on a train".