In the Trains Minnesota-themed issue there is an article about an old “mountain” line.
IIRC, it says that the line was so steep in places that they would run engines tender-first in order to assure there was water covering the crown sheet. Huh? This was in the 1800s.
I can see where maybe this was a water-pressure issue if pumps were not great back then. ??? But what about when the train got to a downhill stretch …
Basically, I don’t get it.
It should have said “when running downgrade.”
Remember that boilers aren’t run ‘full’ of water, and the crown sheet is appreciably below the shell over its steam space. If you tilt the boiler forward, the water in it tends to run toward the front tubesheet and off the crown.
There are two immediate concerns with this. One is that the vertical distance from the effective water surface to the mouth of the dry pipe decreases; the other is that all or part of the crown becomes exposed. (The first is addressed by tapering the boiler so it is fattest where the steam dome is; the latter by angling the crown so it is lower at the backhead)
On a normal ascending grade, there is comparatively little risk if uncovering the crown; the difficulty is that water glasses/columns and try cocks fail to show the right level of water, particularly in longer modern boilers. Dixie 576, when on display in Nashville long ago, had a small plate next to its water column with a datum line showing correct reading for a “2% grade”. I’m sure other railroads had equivalent approaches.
If you’re wondering why cog-railway engines have that weird hunched appearance, it’s to keep their boilers level when climbing under load (and they aren’t turned, and come down bunker/tender leading, to keep that boiler level even though comparatively little steam is required.
As Mr. Klepper noted, the ‘tender first’ would be on downgrades, if the crown is the concern. Guaranteeing non-breaking flow to the injectors, and enough lifting (suction) for them, which is part of what ‘tenders leading uphill’ (or at least A-tanks uphill) would provide, isn’t the stated reason.
Now, where the real crown issue appears is when the engine is making a heavy pull uphill, with high steam demand, and suddenly crests the grade or encounters a ‘down’ profile. As there
Sorry guys. I appreciate the responses, but I still don’t get it.
In very simple terms, what is accomplished by having the tender lead on descending grades?
Lithonia,
Let me try. This is a very basic explanation.
In a steam locomotive, water in the boiler must cover the top of the firebox because if it doesn’t, the boiler will explode. Water over the top of the firebox helps to keep the firebox metal from getting too hot - that ensures the firebox metal does not expand too much and starts popping rivets and seams of the boiler, because if that happens - kaboom! (Others with more engineering expertise can better explain the physics of a boiler exploding.)
The firebox is usually located towards the rear of the locomotive, just in front of the cab, of course.
Water always runs downhill.
On a very steep grade, if the steam locomotive goes down running forward, pilot first, there is a danger that all the water will rush to the front of the boiler, leaving no water covering the top of the firebox. If that happens, as mentioned above - kaboom!
If instead the locomotive backs down the steep grade, the water in the boiler will rush to the rear of the boiler and so there will always be enough water covering the top of the firebox to keep it at the right temperature and thus, no kaboom!
The tender going first downhill is not to put the tender first - it is to make sure water in the boiler stays over the top of the firebox so the boiler doesn’t explode. The tender just happens to be in the lead position, but the real purpose is to keep water in the boiler covering the top of the firebox.
Here is a link to a picture of what happens when water is not covering the top of the firebox and the boiler explodes:
https://www.reddit.com/r/trains/comments/emiohl/saw_some_posts_about_boiler_explosions_thought_i/
kgbw says it very well, and with an illustration that I think will show you what’s important.
ASSume for a moment that the locomotive has a typical firebox with crown sheet and ordinary circulation, and that it is connected to the tender at the firebox end, so that it runs ‘in reverse’ going tender-first. If designed properly and operated with properly-balanced linkage or a power reverse, a reciptocating locomotive can run in either direction safely.
ASSume further that the ‘grades’ are paired, so the engine starts on the level, climbs steeply to a summit, then descends steeply.
Overall, this clearly favors the tender ‘trailing’, not leading, as the licomotive must produce a much higher mass flow of steam to make it up the grade, and it will accordingly be limiting to use the injector frequently as even with a good North American FWH the feedwater is well below even saturation temperature. So you want the crown end of the boiler pointed ‘downhill’ with a lot of water over it for safety.
Now we go over the summit, and several things happen. The valve gear is wound back toward mid, and the throttle can be closed down or set for drifting, both of which actions greatly reduce steam demand. However, even with oil firing there is substantial heat in the firebox which continues to transfer to the crown-sheet area. As the draft decreases with the lower mass flow, the fire can be reduced but if that is fine too quickly, very bad thermal distortion effects can occur.
Meanwhile the boiler tilts and water runs down into the steam space at the front of the boiler barrel. The visible water in the glass falls, perhaps dramatically, and the fireman may choose to gun and pump at the same time BUT if the engine is expected to be worked uphill again there will be too much
Pikes Peak steam engine on near level ground
Thanks kgbw! I get it now!
The author could have explained better. What was key for me was your explaining:
The point was NOT to have the tender first (as I had thought the point was); the point is just to have the aft end of the loco be on the downhill end, backing down. To make sure there is water over the crown sheet. The tender just happens to lead when you do this. Got it!
OM, thanks for your response also.
I’ve seen the Mt. Washington cog engines on level ground at the base of the mountain and the boiler sits at a pretty sharp downward angle- is there any danger of the crown sheet being exposed? I assume not.
There sure is! You won’t see the engines actively fired on the level for this reason.
I would argue that considerable fire that supplies the convection section can be generated (via induced draft) without raising radiant uptake to levels that endanger the ‘steam cooled’ portion of the crown that happens not to be “water insulated”.
The Southern Pacific cab forward engines blew all those points right out the window. Why the engine was always going uphill backwards as the cab was the leading point along with the firebox and crownsheet for the boiler. I’ve read that it was normal while running up Donner and Techapai both to run with less than a 1/2in of water in the sight glass. A normal level and the locomotive was literally pumping water out of the stack.
That sounds like a lot of hot gas. What would happen to that mere 0.5" when the engine crests? Would it still cover the crown sheet? If one could answer yes, then the low water mark on the AC must have been something like 15" above the crown sheet.
Some engines had two separate sets of gauge glasses/water columns: one for ordinary working, one for severe grades. I have seen at least one picture in which these are dramatically far apart, and my recollection is that the ‘second’ gauge was farther down, which would be what’s expected for a cab-forward. Presumably engine crews would understand how to correct the reading for engine motion, etc. and where to carry water when ‘transitioning to’ or running downgrade.
I am sure this was a design concern even at the time of the first ACs, and would be reflected somewhere either in training materials, recollections of SP steam enginemen, or Baldwin material.
The “tender-first when running down-hill” business was not something encountered in normal railroading, grades to 4%. The grades involved are more like 8% to the 10% limit for adhesion, and then to cog-railroadinig.
He’s talking about a cab-forward, where the water level over the firebox rises when the engine crests the grade.