Steam & Excursion > Steam Throttle Questions

I have limited knowledge of steam locomotive mechanics so please bear with me on what could be viewed as "Stupid Questions". However, I have a good need to know the answers.

First. If the throttle in the locomotive cab is opened, does the actual throttle valve want to stay in the open position, or does steam pressure work against it to close the valve?
I would assume that the notches on a throttle quadrant are there to lock the throttle in place in order to counteract the natural tendency for steam pressure to want to close the valve.

Second. If an engine is working full throttle, and working very hard, would a sudden and rapid closure of the throttle cause damage to the engine?
I know this was done for commuter operations and that the fireman was anticipating the rapid throttle changes and ready to make any necessary changes. The question in this case refers to what would result if the throttle was closed abruptly and unexpectedly under heavy load.

Thanks!
R.R. Conway

 

Are there 2 valves; steam enters from above and below, equalizing pressure so throttle holds in any position? see pic.

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I think its to help open the larger valve as without it throttle pressure would make it hard to start opeining and then also holding it open. Notice the diffrence in size and where the steam acts upon it.
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> Are there 2 valves; steam enters from above and
> below, equalizing pressure so throttle holds in
> any position? see pic.

On a mutiple valve throttle, the valves are opened by a cam which opens various valves as the throttle is opened and disengage when the throttle is closed.

RRBadTrack Wrote:
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> I have limited knowledge of steam locomotive
> mechanics so please bear with me on what could be
> viewed as "Stupid Questions". However, I have a
> good need to know the answers.
>
> First. If the throttle in the locomotive cab is
> opened, does the actual throttle valve want to
> stay in the open position, or does steam pressure
> work against it to close the valve?
> I would assume that the notches on a throttle
> quadrant are there to lock the throttle in place
> in order to counteract the natural tendency for
> steam pressure to want to close the valve.

Dealing with an American Multiple Valve Frontend Throttle, you are pretty much correct, i.e. release the latch and the throttle will quickly close pulling the unsuspecting "guest Engineer" right up out of the seat.

> Second. If an engine is working full throttle, and
> working very hard, would a sudden and rapid
> closure of the throttle cause damage to the
> engine?
> I know this was done for commuter operations and
> that the fireman was anticipating the rapid
> throttle changes and ready to make any necessary
> changes. The question in this case refers to what
> would result if the throttle was closed abruptly
> and unexpectedly under heavy load.

Well, the first thing would be a massive slack run-in! Also, closing the throttle all the way on a steam locomotive that does NOT have drifting valves, Will cause the cylinders to draw a vacuum on the smokebox, drawing all sorts of soot, etc. into the valves & cylinders (definitely NOT a good thing).

> Thanks!
> R.R. Conway
>
>  

"Dealing with an American Multiple Valve Frontend Throttle, you are pretty much correct, i.e. release the latch and the throttle will quickly
close pulling the unsuspecting "guest Engineer" right up out of the seat."

Do most mainline steam locomotives operating (or soon to be operating) today have this type of throttle? Or do most of them use a throttle that pretty much stays open unless manually closed?

"Well, the first thing would be a massive slack run-in! Also, closing the throttle all the way on a steam locomotive that does NOT have drifting
valves, Will cause the cylinders to draw a vacuum on the smokebox, drawing all sorts of soot, etc.into the valves & cylinders (definitely NOT a good
thing."

Oh course this isn't something good from a train handling standpoint.
Do most mainline steam locomotives operating (or soon to be operating) today have these types of drifting valves?

Thanks again.
R.R. Conway
 

RRBadTrack Wrote:
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> "Dealing with an American Multiple Valve Frontend
> Throttle, you are pretty much correct, i.e.
> release the latch and the throttle will quickly
> close pulling the unsuspecting "guest Engineer"
> right up out of the seat."
>
> Do most mainline steam locomotives operating (or
> soon to be operating) today have this type of
> throttle? Or do most of them use a throttle that
> pretty much stays open unless manually closed?

Yes. I don't recall, offhand, many, if any, "main line" steam locomotives operating that do NOT have frontend throttles, except for Southern Rwy #4510 and #630, on the Tennessee Valley Railway Museum trackage (they both have dome throttles).


> "Well, the first thing would be a massive slack
> run-in! Also, closing the throttle all the way on
> a steam locomotive that does NOT have drifting
> valves, Will cause the cylinders to draw a vacuum
> on the smokebox, drawing all sorts of soot,
> etc.into the valves & cylinders (definitely NOT a
> good
> thing."
>
> Oh course this isn't something good from a train
> handling standpoint.
> Do most mainline steam locomotives operating (or
> soon to be operating) today have these types of
> drifting valves?

Surprisingly no. I'm not aware of any of the big modern steam locomotives that had "drifting valves" to break that suction (except maybe N&W J #611). The Santa Fe 4-8-4 locomotives(#3751 and soon to be operational #2926) have the Wagner Drifting Throttles, which allow steam flow through the valves & cylinders, without drawing a vacuum on the smokebox, for VERY long down-grade operation.

> Thanks again.
> R.R. Conway
>  

Drifting valves are nice to have, but not essential.

A good engineer knows never to completely close the
throttle when drifting.  Leaving it cracked serves the
same purpose.

Thanks for the information.

To be sure I understand, most if not all throttles on a currently mainline capable steam locomotive will close unless acted upon to either open it or keep it open. Correct?

Regarding the scenario I described about rapidly closing the throttle during heavy load, would the potential damage be the same if the situation lasted a minute to 90 seconds before the locomotive came to a stop?

Thanks again.
R.R. Conway

611 does indeed have drifting valves.



Edited 1 time(s). Last edit at 01/20/17 18:24 by nsrlink.

Rapid closure of the throttle to react to a slip won't cause any damage. It will cause a small pressure spike in the boiler as the flow of steam stops momentarily, but that's all. I recall a New River trip with the 765 where my fireman, Tom Stephens, was firing her so close to the pops that every time we slipped and I slammed the throttle shut to stop the slip, the pop would lift for a few seconds. It would stay lifted for 3-5 seconds and reseat. Only the FIRST pop (there are 3) would lift, which meant that the pressure spike was only a couple of pounds.

Closing the throttle for a long period (your 60-90 second scenario) would do three things, all of them bad.

• The slack woud run in, causing possible passenger injuries back in the train.
• The flow of lube to the cylinders stops. The valve oil is carried into the cylinders in an atomized mist carried within the steam. Shut off the steam and you've also effectively shut off the lube.
• On a locomotive without drifting valves, the negative pressures developed in the cylinders would cause them to suck cinders and air (read oxygen) into the cylinders from the smoke box. If the cylinders are really hot, the oxygen admitted will cause the lube on the cylinder walls to instantly flash and burn. Combine that with the very abrasive nature of the cinders and you can do a lot of damage in a very short time.

This is why we ALWAYS stretch brake the trains we pull beihind the 765. It makes for smoother train handling and never sets up a scenario where we could damage the cylinders. Our "Prime Directive" is this: If the 765 is moving, the throttle had better be open, even if only a little.



Edited 1 time(s). Last edit at 01/19/17 12:57 by Rich_Melvin.

nsrlink Wrote:
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> 611 does indeed have driving valves.

Maybe the 611 has "drifting valves"?  Never heard of "driving valves".

Snifters?

Posted from Android

Correct me if I'm wrong Rich but installation of drifting valves on modern piston valve locomotives was not a universal application. Some locomotives, including the NKP Berkshires, were never equipped with them. 765 did not have them.
As with some other railroads, NKP’s operating practice dictated open throttle running that is, using the brake and cutoff to operate a train or locomotive under heavier draft conditions. Of course the throttle was opened and closed shuttling around the yards or coming off a turntable. Under those conditions, of course, creating a vacuum in the cylinders was not a problem. 
 

Drifting valves were usually applied only to steam locomotives used in mountainous territory where they would have to drift downhill for many miles.

Corrected to drifting.  I'll blame auto-correct & my inability to proofread.

HotWater Wrote:
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> nsrlink Wrote:
> --------------------------------------------------
> -----
> > 611 does indeed have driving valves.
>
> Maybe the 611 has "drifting valves"?  Never heard
> of "driving valves".