I have long wondered how is it possible for a steam locomotive with hundreds of attached cars, all loaded to the brim, to pull them against such weight resistance?
The superheated steam enters the cylinder and presses against the piston, but the resistnnce from the immense weight of all the attached cars and the locomotive itself, why doesn’t the cylinder ends blow out rather forcing the piston to move? I’m fascinated.
I simply don’t understand how a locomotive can move an inch pulling against such weight?
Also, with the extreme pressure in the boiler, how is water pulled or sucked from the coal or tank car into the boiler?
I did a Google search - “How a steam locomotive works” and in a second I found this online article on the exact subject in the Trains magazine website:
The pressure of saturated steam on each square inch of the piston head is greater than the friction of all the movable parts down the line…believe it or not,…else it wouldn’t work. So, the cylinder wall, but also piping and the boiler plating, and the flues in the boiler, etc., all have to be able to withstand high pressures, but the highest would be between the steam dome and the cylinders. The cylinder walls are capable of withstanding considerably higher pressures than they face, and the same goes for the piston head. Remember, too, that the packing around the piston and valve rods must also withstand that pressure, as must the seals. The only thing that will break the cylinders, if all else is in spec, is water. That is why the first thing the engineer does before he touches anything else to get underway, is to kick the cylinder **** lever at his feet, thus opening them (two per cylinder, one aft and one forward due to the bi-directional working thrust of the piston) to allow condensation to escape. Water will not compress, so if he failed to open the cylinder cocks, the piston would turn into a hydraulic one, not a steam one, and nasty things would happen. Like him being late for supper that night, maybe, oh, and getting fired, too…or at least demoted to rag washer. [B)]
believe me the engines are carefully designed to take the strains. A huge articulated is actually designed down to the thousandths of an inch. And the mechanisms run like clockwork.
A standard boiler is welded in the firebox and the flues as well. Mostly however, the flues in the smokebox are not welded but force expanded in place, this allows for expansion/contraction of the flues. nope, no steam gets by.
you could have a water pump to get water into the boiler from the tender but most steamers have whats called an injector.
Its a special valve thats worked by 2 cranks, one for water, the other for steam.
The injector has a blowdown pipe for excess water/steam to escape, however when its working properly, nothing escapes. The Injector works on the principles of heat, the loss of, and suction, a jet of steam is forced into a chamber where water is introduced and there is a one way valve. Start the steam first, you get the blowdown exiting. Introduce the water you may start getting a spray of water exiting but then you may hear this sing songy sound from the injector and the blowdown stops and its working. You have to play with the steam/water controls, but with the right turns, it hits.
Where the valves sit to make it work is variable because changes in boiler pressure and water level will affect how well it operates so its always playtime to get it working.
Standard practice, at least in cars coupled with knuckle couplers, is to back into the train to bunch the slack. Then, when the locomotive starts forward, it moves an inch or so before starting the first car, another inch or two before starting the second car… This was much more important during the steam era, since most cars did not have roller bearings.
The drivers would spin before the cylinder head would blow out, unless there was some defect in the cylinder casting or solid water got between the piston and the cylinder head. Steam is compressible, water isn’t
Water is fed into the boiler through a check valve, either at the top or halfway down the side. Look for the heaviest piece of pipe that terminates at a fitting in the second boiler course. That’s the feed line, and there are usually two, one on each side.
No, that is not correct. Pressure is equal throughout the boiler itself. Pressure downstream of the throttle is less than that in the boiler, due to both throttling and expansion of the steam. Pressure in the valve chests and cylinders is usually taken to be 85% of boiler pressure at starting, and 60 to 65% of BP when working at maximum power. When calculating tractive force or effort, this figure is known as the “Mean Pressure”. It will vary with boiler pressure, valve chest pressure, cut-off, and the speed of the engine.
Sorry, you’re wrong. Pressure is equal throughout the boiler, wherever it is measured. Why would you think otherwise? And since when is there an “exit of the feedwater pump” at the bottom of the boiler?
Really? I’d love to see a copy of whatever boiler code you’re working to.[}:)]
Tubes/flues are expanded and then flared or beaded into the front tubeplate/fluesheet to stop them moving due to expansion/contraction. If they could move, they’d leak water, not steam, since the tube nest is always covered by water in a conventional locomotive boiler. The top of the tubes are below the crown, remember? The tubes/flues also have a secondary role as stays, which they could not fulfil if they were free to move relative to the tubeplate…
Locomotives with feedwater heaters have pumps, as do/did very early locomotives that predate the development of injectors.
In US practice the cylinder head was secured by nuts on studs. The head also had part of the casting that was a driving fit inside the bore of the cylinder.
Many modern locomotives with GSC cast engine beds had the back cylinder head cast integral with the cylinders, making a very strong component that required no additional maintenance.
Which is why most modern steam locomotives in the US and elsewhere in the world had water relief valves. The driver/engineer may well neglect to open the drain cocks before starting, but a bigger problem is water carry-over due to priming. In that case the amount of water finding it’s way in to the cylinders will be far greater than that caused by condensation before starting, and it will happen too quickly for the driver/engineer to react in time. In the US, the Pennsy had their own design of water relief valve, others were proprietary items made by the likes of Nathan, Alco, Hancock and Okadee.
as fireman on steamlocos (admiddetly little tram engines) there were always injectors I had to struggle with, and others going easy: click, chack, SWOOOOSHHH if i remember correctly. O Go***he stress, at a station stop, of seeing a low water level, a not-to-write-home-about pressure and knowing the engineer will have to chug away in a couple of moments…
That’s what I admire about the “super power” like the modern Berkshires, it appears they steamed so easily they could run full throttle, full cut-off for ages while keeping up pressure AND water level! A fireman’s dream…
Sorry yourself Mark, you are wrong. Be careful when you speak in absolutes, as there are not too many about. But, water is not weightless except in outer space, therefore the steam pressure plus the height of the water equals the pressure at the bottom of the boiler, and because the pump was usually mounted lower than the boiler the pressure would be even higher, and because of flow losses in any piping system where the fluid is flowing, the pressure at the starting point is always higher than the pressure further on. This is what makes the fluid move.
On contemporary power generation steam boilers in fact the pressure is significantly higher at the bottom of the boiler/exit of the feedwater pump, because the boilers are hundreds of feet high.
Also, at starting, there is technically no (or very little) flow, so there are no flow losses, therefore maximum cylinder pressure is essentially equal to boiler pressure. Thats why starting tractive effort was the highest.
I doubt many here would be interested in the ASME Boiler Code, or the Piping Code, or all the nuclear industry junk.
Just to quantify the forces here…
I found these details for a PRR K4s - I can’t swear to their accuracy, but they are in the ballpark for many large steam engines.
With a boiler pressure of 205PSI and assuming no losses in the whole regulator system/main steam pipe, and a cylinder bore of 24" the load on the cylinder cover is…
12 x 12 x pi (the area of the cylinder end cover) x 205.
This gives a figure somewhere around 92,700lb on the end cover, or 41.4 tons. A big force, but that’s why the cylinder end cover studs are so large!