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Le Châtelier Water Brake
Le Châtelier's Principle (1888)
If a system at equilibrium is disturbed by a change in temperature, pressure,
or the concentration of one of the components, the system will shift its
equilibrium position so as to counteract the effect of the disturbance.
The following are a couple of replys to a post on The Narrow Gauge Railroad Discussion Forum. The question was "What are Water Brakes and how do they work"??
Posted By: Earl the road foreman.
The "water brake" is sometimes also called the "drifting throttle", and although there are differences, to use is the same.
With the water brake, a small amount of water is tapped off the boiler and fed into the valve chests of the locomotive. As this water is about 350 degrees (when boiled at 200 psi) it flashes into wet steam at a lower pressure as it goes down the pipe to the valve chests. The locomotive is placed in reverse motion. The steam acts as a compression brake against the pistons. The idea is the wet steam and low pressure doesn't destroy the lubrication and damage the locomotive when it is running in reverse.
The more modern superheated locomotives used drifting throttles. This is saturated steam pulled off the turret ahead of the cab through a valve and piped into the valve chest. As before the locomotive is placed in back motion and the steam is used as a compression brake. Care must be taken as to not overheat the cylinders as the mechanical lubricator that pumps oil into the valves and cylinders cannot pump against the high pressure produced in the cylinders.
Which brings up several points in LeMessena's article which I had trouble with. He stated that the the water brake was used to bring trains downhill instead of airbrakes, which couldn't have been done. The valve and piston rings would have been eaten alive. If 3 3600's hefted a train up Tennessee Pass how is one 3600 going to hold it back on the downgrade with saturated steam? Yes, the water brake/drifting throttle is a big safety net when the air starts getting thin and the train is handling poorly and would certainly be useful in holding the train back while you tried to pump up the auxiliary reservoirs (done this many times), but to use it as the primary braking force won't work.
Another problem is very heavy buff forces on the head end when all the braking power is on the front. This would tend to shove cars out of the head end of a long heavy train the same way using too much dynamic brake will do today.
Posted By: William L. Petitjean, P.E.
The water brake seems to be a device of unthinkable complexity because it is a steam engine working backwards -- in other words it uses steam as a compression fluid to absorb energy and produce a braking effect through the drive wheels. And yes, it is much like a Jacobs compression brake on a four stroke diesel engine.
In a locomotive water brake a small amount of hot water from the boiler is admitted to one side of the cylinder when the piston is about to begin its return stroke (normally the exhaust stroke). The hot water is admitted directly into the cylinder through a separate valve arrangement. The reverse lever is set into reverse so the valve events are backwards relative to the direction of travel. This insures that the valve does not open the exhaust port as normally occurs when pulling power from the locomotive. The farther into reverse the lever is pulled, the greater the compression braking effect.
Since the hot water from the boiler is at, say 200 psi and the pressure in the cylinder at dead center is, say 10 psi the hot water immediately flashes into a very wet steam after it is admitted into the lower cylinder pressure. Now, as the piston begins its return stroke it starts reducing the volume in that side of the cylinder, thus compressing the wet steam. Since the force of the train going down the hill is doing the work on the piston it is absorbing power or is braking the train as it compresses the wet steam to a higher pressure and temperature. If you look at an adiabatic expansion curve for steam you can see that the compression brake is just the flip-flop version -- adiabatic compression that follows right back up the expansion curve.
When the piston gets close to the end of the return stroke the valve opens the exhaust port and the compressed steam is released out the stack. The piston goes on through its power stroke (no power though because the steam is shut off and the reversed action valve is open to exhaust), begins its return stroke and does the whole thing all over again. When our side of the piston is moving on its power stroke the other side is acting as the compression brake so you get four braking pulses per revolution from two cylinders just the same as you get four power pulses when you are pulling the train up the next hill.
The principle is very simple and elegant. However, the practical application is severly prostituted by the volumetric realities of the actual engine. First, if you go back to the adiabatic expansion curve you will see that the pressure begins to climb very fast if you reverse things and start compressing the steam. Once you get past about 15 or 20 psi the pressure increase creates a curve that almost goes up vertically while the cylinder volume changes very little. Now, if we think a little further we recognize that when you are pulling power from the engine you are usually operating at say, 50% cutoff. This means you are only expanding the steam adiabatically for half the power stroke. However, if you pull the reverse lever back far enough when braking you are compressing your wet braking steam for almost the entire return stroke. The compression ratio is huge and you can easily compress the wet steam way above the boiler pressure and create scorching high compression temperatures far above the superheat temperature of the biggest engines. Furthermore, if your wet steam valve is somewhat crude (which they were) you might admit slightly too much hot water into the cylinder (it doesn't take much) and it will flash into steam at some higher starting pressure. Now your piston is compressing an already high steam pressure and you will get even higher cylinder pressures before the exhaust valve lets the steam out.
This is why old timers remember D&RGW engines coming into terminals with the paint burning off cylinder saddles. And this is why Al blew out a cylinder head with his water brake. You had to handle them with kid gloves or you could quickly wreck a lot of machinery. You could easily build so much pressure in the cylinders that it was almost like a hydraulic lock. The moving force of the whole train had no trouble bending iron and breaking castings.
I think the D&RGW shopmen just stopped fixing the things after a while because they knew if the water brakes didn't work they wouldn't have to keep repairing the damage wild engineman would cause with improper, careless operation.
The C&TS engines have a crude version of the water brake that is not used for train control. It is used to hold a light locomotive on the steep Cumbres downgrade so they don't overheat the driver tires with the engine brake. The old D&RG was undoubtably hoping the Feds didn't really know much about water brakes so they could use them as a smoke screen and postpone spending money on driver brakes. Anybody who knows much about the history of water brakes realizes they are no replacement for a good driver brake.
Actually modern, precision hot water injector valves and cylinder head relief valves could make the old water brake a valuable "dynamic brake" on the D&SNG and on the C&TS. I asked Steve Jackson about it once and he replied he would rather buy more cheap brake shoes for the cars than wear out hard to fix locomotive running gear with a water brake. Dang! I thought I was on to something.
I think the old water brake is just one of those pipe dreams that sounded real good but just made everyone's life miserable on the railroad. I hope this puts the water brake to bed for awhile.
Posted By: Kelly.
The Le Châtelier water brake is something that I have always been fascinated with, because of its operational concept, and its mystery. Why was the Rio Grande the only US road to use it, and why was it removed from the surviving Rio Grande 2-8-2's on the C&TS, the engines most able to employ it of today's US steam operations?
Installation was simplicity itself. A ½" angle valve was plumbed into the backhead below the waterline of the boiler. This ½" line ran forward to the saddle, where it split into two branches that connected with the exhaust passages in the saddle (not the steam passages). When this valve was opened, boiler water would run into the exhaust passages where it would it would flash into a mixture of very wet steam and water. Depending on how much the engineer opened the valve, he could get a fair amount of this mixture to come out of the stack. If done while the engine was stopped, this would have absolutely no tendency to cause the engine to try to move, since the exhaust passages are always open to the atmosphere.
Boiler water, (388 degrees at 200 PSI) when released into the atmosphere, partially flashes to 212 degree steam by using the energy stored in the water due to its being 176 degrees hotter than the 212 degree boiling point at atmospheric pressure. The water does not have enough energy in it to entirely flash to steam, most remains as liquid water (reduced in temp. to 212), albeit as a very fine mist.
In operation, the engineer would close the throttle, open the valve, open the cylinder cocks, and move the reverse lever a few notches into reverse, regulating the reverse lever for the amount of braking desired, and regulating the valve to supply just enough water to keep a wisp of steam coming out of the stack. The cylinder cocks must be left open at all times to allow the water a place to escape, otherwise severe damage to the engine could result. This makes for a very noisy operation.
The purpose behind adding the boiler water to the exhaust is twofold. First, its volume breaks the vacuum that would form in the exhaust passages so that smokebox gasses don't go into the cylinders. Even on a perfectly clean oil burner, these gasses would be at least 400 or 500 degrees, combined with the added heat of compression, would make short work of the oil coating in the cylinders. Second, by compressing the mist of liquid water, the heat of compression is absorbed by the water, at most turning it back into steam, which is then blown out of the cylinder cocks, leaving the cylinders considerably cooler.
The Rio Grande #683 (a slide valve engine) at the Colorado Railroad Museum is the only engine I know of that has an intact water brake. The valve is at the 3 o'clock position on the knuckle of the backhead, and the pipe is intact into the saddle. I have seen other Rio Grande engines that are not lagged in that area of the back head that have a plug installed in that same 3 o'clock position, and the K-36's on the C&TS have the two ½" pipe nipples sticking out of the saddle, indicating to me that they had water brakes installed in the past. Why were they removed?
The Rio Grande K class engines running today are fitted with a "drifting valve" which supplies steam from the turret to the steam chests (not the exhaust passages) and are commonly used for compression braking. Use of these valves for this has never made much sense to me, since they do nothing to prevent smokebox gasses from entering the cylinders, unless it is assumed that the valve and piston rings leak through to exhaust enough that this steam gets past them to break the vacuum in the exhaust passages after all, a poor situation to begin with.
Last Update 05/30/03
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