So the basic operation of a steam train is fairly straightforward- add fuel to the fire, the fire heats water, hot water boils into steam, then the steam drives some pistons. Most of the stuff I’ve read surrounding boiler operation involves keeping the fire hot enough, but what happens if I’m a dummy and I add too much fuel, allow too much air in, or otherwise allow the fire to grow hotter than is generally recommended? Did train crews have a way to cool the fire, or would they just handle this by venting excess steam to keep the boiler pressure within safe limits and wait for the fire to consume its excess fuel naturally?
At a minimum they can release steam. Pressure relief valves should do that before you can get the boiler too hot.
Ever heard the term banking the fire?
I am definitely not an expert on steam engines, but here’s my (limited) understanding of it.
It’s important to keep the water level in the boiler properly maintained. If you don’t, then the boiler can overheat, and possibly explode. The water level is maintained by adding water from the tender car through a water injector (a check valve of sorts) in the boiler. You really want to keep a close eye on the water level and basically just add a little at a time. If you add a whole bunch of water at once, the cooler water will put a lot of thermal stress on the boiler.
If the boiler did overheat, adding a bunch of cold water at that point was a huge explosion risk, so instead they would reduce the fire and wait for the boiler to cool down. Once the boiler cooled, then they could safely add more water to it to get the water level back where it belonged. Of course, if you get to the point where you need to do this, then your train operators have royally screwed up, especially the guy whose job it was to watch the water level.
The impression I have is that as long as the water level in the boiler is properly maintained, you don’t have much of a risk of overheating.
ETA: Mr Downtown’s link has a much better description of how they controlled the size of the fire than anything I could have written.
As a fan of steam locomotives in general, my reading the accounts of firemen (whose job it was to see to the boiler in general) showed that the good ones would know the line they were on and how much steam would be required at a given point. They would build up pressure and heat in anticipation of a long grade, then ease off stoking the fire as the end of the grade neared and the load leveled out along with the track. They would add water “little and often” as one old hand put it, to keep the boiler fairly full and yet not waste water. The water injected into the boiler was usually preheated in more “modern” steam engines to avoid dropping the temperature of the boiler water and losing steam pressure. Learning the ways of these old engines was quite an art.
If you are steaming along under a full load and add too much coal to a boiler it will not over heat. The boiler pressure will determine the temperature. Adding too much fuel will cause the fire to cool because the of incomplete combustion, and the engine will begin pour black smoke (blacking out). You can not get too much air. Steam engines are induced draft and under heavy loads normally do not get enough air. A boiler has 3 end points. 1. firing rate, 2. Steam flow rate, 3.circulation rate (water flow inside the boiler), . In a properly designed boiler the end points are reached 1,2,3.
If the fireman is still stoking the engine after the load begins to decrease then the pressure can increase above the normal operating pressure. The safety valve can open and relieve excess pressure.
The things the fireman can do is bank the fires. In this case the coal would be pulled from the center of the fire box to the outside edges. This will cool the fire, the fireman has to be sure the fire does not get too cool and have coals begin to go completely out. Another thing the fireman can do if increase the water level, but if the engineer has the throttle part way open he has to work with the engineer. If the engineer opens the throttle water can carry over into the steam line. Or he can ask the engineer to open the drains on the engine.
I never thought about it before, but the water being added into the boiler must have been under pressure somehow. Seems like there must have been a pump.[sup]*[/sup] Was it powered by the engine, driven by the wheels on the tender, or something else entirely?
- I suppose you could have an airtight cap and pressurize the whole water tank somehow, but another pressure vessel seems like the last thing you’d want under the circumstances.
Never mind, I looked it up. Interesting stuff.
As a last resort in an emergency - maybe a stuck valve - the fireman could dump the contents of the firebox onto the rails.
The early London Underground trains were hauled by steam engines and, as you can imagine, smoke was a problem. Sir John Fowler (who went on to co-design the Forth Bridge) came up with a solution that failed spectacularly. It was hushed up at the time and the engine became known as Fowler’s Ghost. The idea was that heat to create the steam was stored in firebricks.
I am sure there was a damper to “dampen” the fire. This denies oxygen to the fire and therefore the fire cools down some.
Hopefully this is obvious to everyone, but as long as the boiler is filled with water, the boiler cannot get hotter than the boiling point of water. More specifically, the surfaces of the boiler heated by the fire need to be in contact with the water inside - which it should be if the boiler is properly designed and the water level is appropriate. This is the same reason why it’s possible to boil water in a paper cup. The fire may burn off the paper above the water surface, but not below.
The boiling point of water depends on pressure, but if the pressure gets too high, it’s the steam pressure itself that will rupture the boiler, not the elevated temperature.
Do steam train crews have any way to artificially cool the boiler?
As bob++ said, in extremis, the fire can be dumped from a coal-fired locomotive or the burner shut off in an oil-fired locomotive, but the bricks in the firebox will still be hot. It will take a while for things to wind down and the boiler to stop generating steam. And, yes, davida03801, there is a damper that will make a coal fire slow down, but rather slower than the above emergency procedures.
The best way to stop the steam generation is to add water to the boiler. As anyone who’s made spaghetti knows, if you add even a cup of cold water to a six-quart pot at a roiling boil, it will be a couple minutes before you get that roiling boil back; in a closed boiler, the pressure drops if an amount of (relatively) cold water is added, even if the fire is still burning. Of course, if you’re moving you’re consuming steam, which is why under ordinary circumstances you’re constantly tending the fire and adding water anyway.
This assumes, of course, you have water to add to the boiler. If you’ve got a roaring firebox, a low boiler, and no water to add, you’re pretty well screwed. The best thing to do would be to dump the fire, let the safety valves to their job and, if the water disappears in the sight glass, open the blowdown valves before the crown sheet gets dry. You will likely be asked to find another career but at least you’ve avoided a boiler explosion, which can be quite spectacular (to say nothing of fatal).
Crown sheet? In a fire tube boiler the crown sheet is the part of the boiler above the fire box. In the illo, the fire box is the white, square on the right and the crown sheet is the flat area on top with the white on one side and the green (water) on the other. As you can see, as the water drops, the crown sheet will be the first thing to go dry. No longer having water on one side of it to carry off the heat, it will get hotter than it is designed for. If you really screw up, it will get red – or even orange – hot, weaken, and despite the stays, drop into the fire; explosion ensues.
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The more common scenario is for someone to go, “Oops!” and in a panic, open an injector to get water into the boiler. If the crown sheet is glowing, the thermal stress of the cold water makes it fail, and explosion ensues.
Now, all this would be a Very Bad Day for an engine crew. On a more ordinary day, as engineer_comp_geek and Snnipe 70E have said, the engineer and fireman are working as a team, anticipating what’s coming up and adjusting the fire and boiler level so you’re neither starting up the grade with a cool, empty boiler, nor coming to a stop with an over-full, hot boiler. It’s an art. Banking a fire, as Snnipe 70E and Mr Downtown alluded to, is mostly done when you’re finished with a locomotive for a while, keeping it simmering so to speak, so you’re not starting from cold iron when you want to use it again. If you’re moving, banking the fire is not needed.
If the crown plate is that high up, what’s all the rest of that water doing? Why isn’t the plate much lower?
The rest of the water is being heated by the many fire tubes that run through the boiler. These tubes provide most of the surface area for transferring heat from the hot fire to the water. See this photo - the boiler is filled with those tubes.
It’s that high up because it’s the top of the firebox. The firebox is the size it is to ensure combustion of the fuel (coal or oil). In the illo of my preceeding post, you’ll notice the fire arch, the flat thing angling up at about a 20-dgree angle from the left side of the fire box. This forces the flame and hot gasses to take a longer route before diving into the fire tubes, the horizontal white lines to the left (labeled “smoke tubes” for some dumb reason).
Now, most of the heat exchange to boil the water takes place in the firetubes so I suppose you could design a firebox so it dangled below the boiler and only the gasses rose up and into the firetubes to do the work, but you can see how that would be very awkward for a horizontally-oriented locomotive. Also you would have to
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[li]Have the tubes bend from horizontal to vertical (making a boiler designer blanch)[/li][li]Have the tubes vertical all the way up (which makes for a dinky boiler)[/li][li]Have a space behind the boiler for the gasses to rise into before entering the fire tubes (which would still have a crown sheet, just smaller)[/li][/ul]
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If you think about it, the pressure in a boiler is trying to force it into a sphere. Most of the boiler is a cylinder which is reasonably strong, but then you’ve got this firebox intruding into it at one end. It’s the weakest part of the boiler, which is why it has all those stays holding it in place. If locomotive designers could think of a way to just keep the boiler a simple cylinder without that pesky firebox intruding into it, they’d have gleefully embraced it. Locomotive boilers and their fireboxes (including an easy to uncover crown sheet) are designed the way they are for several good reasons.
It’s kind of like that old joke.
“Doc, it hurts when I let the crown sheet go dry,”
“Then don’t do that.”
According to the old movies, as well as discarding the fire, you could lock the whistle on, to discard steam,
This thread got me thinking all day yesterday on things my father told me growing up I had not thought about in years and years.
When my father was a boy he was quite close to his fathers 2nd cousin, Caleb, who lived in the same house as a boarder for the entire time my Dad was growing up. This guy was a fireman/stoker on the railroad for some 35+ years.
Per Caleb it was a bit of a trick to keep the fire and steam just right for the journey. You needed to build up a head of steam to climb the hills, not so much for the flat runs in the valleys, etc. Again per him if you knew what your were doing it was not much work to keep things just right. On the other hand if you did not know as you were doing you had to work like hell cause you were too much loaded one way or the other.
Well, if we’re going by old movies, I’d say the most common practice was to dynamite the bridge over the gorge, plunging the engine into the river below.
Which seems drastic, but I’m sure the various parts of the boiler would cool quite quickly.
It’s a lot more difficult than that. First, two guys need to engage in an epic battle on top of the train cars, then detach some cars to save a comely young lady in distress, with the hero on top of a detached car. Then that car has roll downhill faster than the out of control locomotive and re-attach itself so the hero can try desperately to slow the locomotive, then failing that run back to the re-attached car, grab the young woman, then grab hold of an over hanging tree branch an swing both of them to safety just before the train plunges into the gorge. But clearly that must be a lot easier than releasing steam or dumping the coals.
Someone mentioned closing an air damper above, that might raise the temperature of the fire initially.