Steam & Excursion > Carry-over from 'throttle questions"..... drifting valves...

In my opinion, so called "drifting valves"  are not effective.

First, the associated fittings and pipes/ports are incapable of handling the volume of air needed to fill the cylinder cavities at speeds above 15 to 20 MPH.   Air does not have the expansion capacity of steam.  Sucking air through the drifting ports results in a phenomenon known as 'wire-drawing' the air stream.

Wire-drawing refers to the similar action while pulling taffy, or bubble gum: as you yank the gobs apart they get vey thin in the center, or represent seriously lowered pressures.  Coupled with the inability of air to sufficiently expand to fill the cavity, no 'drifting relief ever occurs. 

A second shortcoming is the effect of the absence of steam-flow bringing cylinder lubricant to the cylinders, when the throttle is closed.   This lack of lubrication is not as much of a problem with hydrostatic lubricators account of the the fact that live steam is always flowing, and carrying lubricant to the steam passages continuously, re gardless of throttle position.

Mechanical lubricators pump tiny amounts of oil on to a 'spoon' mounted in the steam delivery pipes, right above the spool-valve housing.  On superheated engines, the high steam temperature makes the tallow-based valve oil evaporate as a blue,
grease-laden smoke.  You see a similar greas distribution effect in cooking hoods over flat-top cooking griddles in diners, etc.

On steamers, closing the throttle while rolling stops the steam flow to the cylinders, and the flow of new grease-smoke into the flashing pistons in the cylinders.

With a closed throttle, as the piston rapidly recedes in the cylinder when the admission valve is open, but the only thing that happens ( in the absence of steam flow) is that a powerful vacuum develops in the closed cavity.  When the valve opens the exhaust rings to the smokestack's blast pipe, the higher pressure in the smokebox rapidly fills the suction of that immense vacuum cavity ( the 'vacated side' of the piston).

Along with bringing in cinders and smoke from the smoke box, there is virtually NO lubrication ferried from the steam pipes, there being a closed throttle. This is aggravated by the newly introduced smokebox cinders & grit into the cylinder cavity, eroding the piston rings ( and valve rings), as well as the cylinder liner bore and wearing the rod packing with the grit that sticks to the piston rod.  

At cruising speeds, the speeds which pistions travel mean that piping cannot handle the amount of air flow needed to prevent vacuum formation.

Wagner valves are just as bad, being advertised as shuttling the steam/air gasses from one side of the piston to the others...through way too-small piping and restricting shuttle valves. At low driver speeds, that may work, but and speed faster than a fast-walk, the piping cannot carry the the amount of gasses needed to prevent a vacuum creating event.

Its hard to conceive of the speed that a 27" piston travels in the cylinder, and the volume of air that instantly needed to prevent vacuum effects. 

So, the best way to combat the vacuum effects, is simply to carry the throttle open sufficiently to ensure steam flowing into the cylinders.  Opening the cylinder cocks while drifting, allows you to see that steam is beneficially flowing into the cylinders.
Obviously, you don't want to use enough power to yank the train, especially in a downhill situation, but you can simply use enough 
steam to hiss loudly at the cocks, but still not power the train.

So, wise use of the steam and throttle, will do wonders for lubrication , keeping the dirt out of the cylinders and makes continuous pressuring of the cavity very easy.

You must run the engine and the train with intention, be aware of all that's going, and protect and prolong the life of the engine.

Your thoughts?

​W.

Not proofed, yet...

Wes,

The Santa Fe Railway had the longest, continuous down grades in the U.S. (at lest 75 miles in one location), and had their more modern locomotives equipped with the Wagner Drifting Valve system/design. It seems to have worked very well for their particular railroad and high speed main line operating conditions.

Please comment on this interesting system, and the benefits that only the Santa Fe seemed willing to use.

Wagner valves are just as bad, being advertised as
> shuttling the steam/air gasses from one side of
> the piston to the others...through way too-small
> piping and restricting shuttle valves. At low
> driver speeds, that may work, but and speed faster
> than a fast-walk, the piping cannot carry the the
> amount of gasses needed to prevent a vacuum
> creating event.....

Just the fact that the connecting passages are vastly too small to handle the massive amount of alternating, volumetric capacity, end-to-end ( and back) at speeds above 30 MPH.

A cylinder-full of hot gasses stuffed to the other end, in milliseconds, hundreds of times per minute.
I suspect that the Wagner system sales befitted the ATSF, so they sold a system they had a stake in, back to themselves...
I suspect that their good engineers ran thier locos properly, regardless of the presence of the Wagner valves.

Just my suspicion .... it's also my opinion about how loco appliances were sold to various customers , by various vendors ....
Salesmen, Booze, money and hookers had a lot to do with some of the odd-ball equipments  that engines were stuck with...

I

W.

I always subscribed to the cracked throttle with steam blowing at the cylinder cocks.  It would also register on the steam chest pressure gauge.

We had a couple of instances where someone slammed the throttle shut hard and it caused the relief valve to open so hard that it flipped the "lock" over which held the valve open.

We had a label on our steam chest gauge before we restored 1522 and I never could understand what it said as none of the in cab photos showed it well enoug to read.    I ran across a company shot from one of the cabs recently (don't know what class) and can finally read what it said.  It gives the steam chest pressure at cutoff positions to "cushion" the engine.  With our light trains, this wouldn't have worked too well as we would have accelerated unless going upgrade.

Wish my Dad had still been around.  I could have learned so much from him about running steam.    I didn't rely on the back pressure gauge all that much, I tweaked it to find the "G" spot.  You could always tell because the engine would ride like a pullman, the exhaust had a nice little rhythm to it and all was right in the world.  Glancing at the steam chest/back pressure gauge would tell me that the back pressure needle was usually right around halfway between the pin and the pressure gauge.

---

Wes wrote: Salesmen, booze, money and hookers." I see I followed the wrong career path. Also, there's a country song in there somewhere. d

I have a couple of questions for the folks on here with in-cab experience:

1) As long as the back pressure gauge shows a small positive pressure (1-2 psi), aren't you assured that you have the throttle opened enough to prevent drawing smokebox gases into the cylinders?

2) Why would it be desirable to use a device like a Wagner valve instead of just leaving the throttle sufficiently open during drifting?  Is it all about fuel economy and avoiding wasted steam?

Scott Griggs
Louisville, KY



Edited 1 time(s). Last edit at 01/21/17 07:16 by sgriggs.

To wcamp1472,

This thread couldn't come at a better time as recently I wondered about cylinder vacuum when steam was "shut off"or the throttle closed.  In mentioning the vacuum formed with the valve system mentioned, were there other systems that prevented a vacuum from forming?, or was it just common practice to open the drains and apply steam?  I keep all of the threads like this in the followed section as some day I'd love the chance to operate a steamer, but have something of a clue as to what's going on and needed when I finally get to.  I know about the Nevada Northern camps, I just don't have the time (for the most part) to get out there.

If you look at the theoretical, that nasty gas law again and consider that since your closed throttle gives
you alternating compression and vacuum in the cylinder you'll see that at some point you heat the air up
to the point where you vacuum actually becomes a slight pressure. Same principle is why the Wagner works,
yes as Wes says the crossover line is too small but that's the throttling device that gives you back pressure.
Driving that air back and forth heats it to the point where it does pass through the crossover point but at an
elevated temperature. Thing is and as stated this is only one side of the problem, lubrication is the other.
You're better off at a notch or two in reverse. The short cutoff chokes the expanded steam and gives you
enough back pressure to restrain the engine.

I, along with every other engineer in Southern Colorado and Northern New Mexico, were taught to do this wrong.  We were taught to slam the throttle closed, put her in the corner and let her drift, sucking air in through the snifters.  I had read about the "proper"way to drift an engine, but no one seemed to want to listen to this.  In our case and the speeds we were running (less than 25mph with 44" drivers), the snifters seemed to be able to draw enough air into the cylinders to push the air up the stack and create noticeable draft.  In fact, when it was necessary to pull through a sag, with the reverse hooked up high and the throttle cracked enough to keep up speed, there was less draft than when drifting as above.  I do believe in our case, there was enough air drawn into the valve chest/cylinders to keep a vacuum from being formed the the cylinders.  The fact that the locomotives drifted nice and quiet seems to support that fact.  If you neglected to put the valve gear in the corner, cutting off the admission of air, a vacuum would quickly form, and you'd get a good pound going instantly.

Of course, there was still the lubrication issue, as sucking the air in didn't mix with the valve oil.  But, the engines ran for YEARS without burning up their valve and piston rings.  When the valves were pulled for new rings, they were of course covered with carbon and burned on valve oil.

This method of operation goes back many many years.  I specifically asked old D&RGW heads how they drifted then locomotives down the hills, the answer was the same - "throttle closed, in the  corner".  The Grande heads did this all over, mainline from Alamosa to Durango, Silverton Branch.  Everywhere.  The guys run this way in Durango to this day, so it is not a C&TS situation.

Curiously there is what has always been what is called a "drifting throttle" on the locomotives.  This is a 2" or so pipe from the turret (saturated steam) connected into the branch pipes or the top of the steam chests (below the point where the oil is inserted).  Its primary function was to be used when descending long steep grades with light engines.  You put the engine a few notches in reverse motion opened the Drifting Throttle and ran on compression.  I found the drifting throttle was also real handy to get through sags without having to open the main throttle, but that is another story. 

After leaving the Narrow Gauge I realized how terribly we abused the locomotives, but they seem to keep going without dire consequences.  When I returned to the NG for a short time a few years later, I tried to teach the guys my "new-found" knowledge.  I was mostly ignored.  I did use the drifting throttle on down grades.  There was just enough volume in that 2" pipe seat the snifters with the valve gear in the corner.  It did reduce that nice drifting draft the engines have when "sucking wind", which my firemen didn't care for.

Re:callum

What you say might hold water, except for the fact that the spool valves open and close the ports, 
The inner rings connect to the steam delivery pipes, & with the throttle closed, that pipe is under vacuum....
The outer rings communicate with & open to the stack, and the atmosphere. 

The vacuum still contains a such small amount of air, that the heat of compression never really occurs. The huge volumes of the cylinders and the rapidly receding pistons, at cruising speeds, the piston velocities are way too-rapid to allow the supposed compression/heating cycles.

Study also the Le Chatelier Water Brake, that changes valve events , and uses limited water injection for cooling.
The best example that I observed, was used on their steam engines---- descending from the mountaintop, all the way down,
In that use, the Water Brake worked wonderfully, but the valving events are altered from conventional operations.

​W.

 

Wes, true but you've got the whole pipe volume to the throttle to consider but in practice that
volume would disappate over a period of time due to piston ring leakage and as you say would
approach a vacuum condition. Thing is, that vacuum would also "suck" some free air from the
exhaust side, I doubt you operate in a continuous vacuum. We never had a long enough hill to
play with to fully analyze what was happening! Good conversation though, thanks.

Out

callum...

​There's just no way that valve ring leakage can replenish the cylinder volumes, with 4 "opened volumes", per driver revolution.
Especially at medium or high driver revs,

There is a 'giant sucking sound', regardless.

​I like the idea, above, of a separate live steam pipe, modest diameter, to mitigate the vacuum problem. 
Steam has a wonderful expansive capacity...like 16,000 times (compressed volume) when released to atmosphere.
Air has NOWHERE near that expansive capacity, and cannot fill the cylinder volumes, simply through the short period of time, (and the restricted passages) that the exhaust ports are open.
​So, yes theres good reason to drop her down in the corner......longer valve travel does help a little with a wider exhaust port opening, but its still woefully inadquate compared to the immese volumes of the vacuum capacity, multiplied by 4 cylinder's full, per revolution.  Say a 70" driver spinning at 250 RPMs, a modest pace.
Give her a little steam...problem easily solved.

​Also mentioned above, is monitoring the Back Pressure gauge ------- if you will note, maintaining that modest back pressure may take a considerable throttle opening ---- at high driver RPMs, and the throttle must be closed down commensuate with reducued driver RPMs.. Failure to reduce the throttle sufficiently, will soon add tractive effort while you intend to slow down!   So monitoring the back pressure gauge, together with tell-tale eveidence of the opened cylinder cocks, and the long valve travels
​(of the down-in-the-corner reverse lever) all will combine to aid in smoothe train control.

​While all that is going on, the fireman is struggling to keep the boiler pressure up & strong ----- since he's fighting the fact that there is NO draft up the stack, the long valve travel AND the closed-down throttle.
​His only draft comes from the smokebox blower pipe & cab mounted valve  ------ a very weak draft source, compared to the full exhaust nozzle.
 
He has to keep a bright fire, maintain a good water level, manage the injector, and keep the boiler near the safety valve setting, -- so that he supplies the engineer with snappy air compressors, to be sure that the train brakes can be quickly released,
when the engineer operates the brake valve..
​Going down-hill is FAR MORE tricky than an uphill fight.

​You've really got to know your stuff to drive a train, successfully, down-hill.
​You got to know the track profile (grades, curves, crossings, etc), the dispatcher's orders, the wayside signals, the timetable details, the train characteristics, the weights ----  you've got to know the characteristics of various locomotive airbrake systems.  The list goes on and on...

​That's why i avoided taking the engineer's seat, as a career-move.
 No, thank you.

W.



Edited 4 time(s). Last edit at 01/21/17 19:15 by wcamp1472.

Like many other things, this is getting more complicated than it needs to be.

Many roads had specific instructions about how they wanted drifting done.
And not all were the same.

Some large, modern locomotives were built with drifting throttles (see photos
in Kratville's books on the 4-8-8-4 and 4-6-6-4 UP locomotives, for example). 
These didn't last long because after some wear, they were blamed for causing
engines to wander away while unattended, usually into the turntable pit.

It ain't rocket science.  Large, high-speed locomotives are drifted by leaving the
throttle barely cracked and the reverse gear about half way forward.  Open the
cylinder cocks momentarily to make sure there is steam coming out (not just air),
then close them and pay attention to the train behind and the track ahead.

Your mileage may vary.

I practiced what Realist just mentioned except that I was a little farther into the corner to carry lubrication.  Always worked for me.  I would never slam a throttle shut as I thought it was bad practice.
Wes had the operative statement.   Know the railroad and where you're running.   Makes all the difference in the world.  When on trips, I have had very good pilots that would keep you advised in plenty of time of what is coming and pilots who were like a clam and I had to find out on my own.  I still laugh when I think of the one I had in TX when I was doing three trips on the BNSF EAS from 2001.   All hill & dale.  First trip he kept me ahead of the profile.  Grab a little air and skin 'er back  were his operative terms.  Second and third trips were like I was on home rails.  Fun day.