Steam & Excursion > Wide-open Throttles

There are, basically, two variants of flow control schemes, so called ‘throttles’, for supplying steam to the main cylinders.

1. A dome mounted, fitted with a single, balanced , spool valve.

The spool has two major diameters, the lower, smaller diameter operates in a cylindrical chamber—- that gets pressurized
from a small, centered, cylindrical valve with about a 1” ‘lift’...(called the pilot valve ).
The pilot valve pressurizes
the lower, balancing chamber.

Without the balancing feature, the dry-pipe to the cylinders has atmospheric pressure on the lower
side, and boiler pressure on the upper side...

[ formula: Area = pi X R squared... for a 10” dia throttle
valve that would be 25 X 3.1415 =
78.5375 sq. inches...
Times a BP of ( say) 200 psi...Xs 78.5375 = 15,707.5 Pounds of pressure holding the throttle valve closed... about 7.8 tons..

Thus, the need for the balancing chambers! What engineer could manually operate a throttle fighting that much weight? ]

Without the balancing chamber the
Throttle valve would be virtually impossible to lift —— account of the
Immense surface area — with boiler pressures over 200PSI..it would be virtually impossible to
Lift a ‘single element’ valve.

However, after about 20% of the valve opening, the flow is virtually at it’s maximum—- almost an all-or-nothing
operating range..

2. The multiple poppet valve, “front-end”, throttles have a series of progressively sequenced &
progressively larger port openings.

There is a long shaft, running crosswise of the smokebox
( controlled from the engineer’s position), that opens the individual poppet valves ( covering the port openings)
in a sequential Manner. Typically the throttles are made with either 5 poppets or 7 poppets.

They are all constructed with a small diameter blalancing piston
.... like the older throttles, a small
Pilot valve is the first valve in the sequence to open... pressurizing
the lower ‘balancing chamber..
the chamber extends to supply all the poppet valves with balancing pressure..

The cam shaft holds the first poppets open during the opening sequence.. so that all are open
with a “wide-open throttle”...

Since, like the older style throttles,
a ‘little opening’ becomes a full opening very quickly...

The smaller, individual diameters of each of the small poppet’s ports limits the flow past
an induvidual Poppet—the throttle opening is limited according to the number of poppets lifted..

The series of smaller openings, operating sequentially, gives a MUCH better ‘flow-control’ to the
steam going to the cylinders..,

The sum of the flow areas of the valve & ports exceeds the areas of the supply piping and the
delivery piping to the cylinders by about 20%... you don’t want the throttle casting/housing to limit
the designed steam flow,.. of the system.

‘Modern’, ‘front end’ throttles are placed down-stream from the series of tubes making up the superheater
units..thus, reducing the amount of time it takes to fill the units with boiler steam...

With older, dome-mounted throttles,
( up-stream of the units) It takes time to fill the units with steam, and in case of badly slipping drivers,
the quantity of steam in the units must be depleted... before the spinning stops... ( once spinning,
the friction surface becomes molten steel... a liquid , with very low
‘coefficient of friction’...

Once spinning, even with the old style throttle closed, the residual steam in the units and delivery pipes
aggravates the slipping duration..

With modern, front end throttles, the limited quantity of steam in the delivery pipes— to the cylinders—
is much smaller.

Since the closed throttle keeps the steam backed into the superheater units..there is much greater control
over the flow of steam to the cylinders.

It is the limited open area of each lifted  poppet that controls the allowable flow-rate to the cylinders.
More poppets opened, more steam flows and precision control rules the day.

You can understand that the flow rate of steam is entirely dependent on the piston speeds, in feet per minute.
Thus, the greater ease of limiting the flow, by limiting the sequentially opened poppets and their related port sizes.

With the throttle lever against the roof, all the poppets are open..
BUT... with a short cutoff, the steam flow to the cylinders can be very small ... thus, a half-open throttle can
deliver steam at full boiler pressure—- account of the low steam demand of the cylinders...

Thus, do not become confused by the apparent steam flow past the throttle... it can be delivering maximum BP,
depending on the width of the spool valve’ s port-opening at each piston.., the Cut-off at the valve gear..the driver rpms, etc..

It’s an “old-wive’s tale” about wanting a wide open throttle all the time... you can have full BP at the spool valve
( with a reduced throttle opening).... depending on the cutoff and flow rates—- as determined by driver RPMs &
valve cutoff setting...

Appearances can be deceptive... It takes a lot of experience to run a locomotive well & efficiently..

It’s an art of balancing ALL the factors from the fire’s intensity, to
the slope of the R-O-W ( positive or negative), to the allowable speeds controlled by wayside signals,
as well as existing Timetable rules and speeds...& special orders/instructions

It’s a complex environment..

W.

Not proofed, yet..

Posted from iPhone



Edited 14 time(s). Last edit at 06/04/20 11:35 by wcamp1472.

Thank you for this post and your other posts of this type. 

The technical information and "hands on" operational detail is just fascinating. 

Brian E 

 

Fascinating stuff to be sure. Although technically trained; I do admit to getting lost when it comes to the inner workings of a steam locomotive! Which leads to my next question:

What kind of training were the engineers and firemen given before being allowed to operate a steam locomotive? Were they all trained to a level where they could give this kind of detailed description of throttles? We are spoiled today with the use of simulator's which can reproduce all the intricasies of train control and diesel functions; but back in the day it was all learned at the throttle! Was there theory tests as well as operating tests? Seems to me these crews had to retain a lot of knowledge before ever cracking the throttle on a locomotive.

Thanks for your insight into the almost forgotten craft of operating a steam locomotive.

Brian N.

I agree with the other posters, Wes, thank you for another lesson on how all of this marvelous engineering design stuff works.

It’s a wonderful question..

I’ve got to construct a valid reply from what I was able to glean from back in the day...

I didn’t become involved as a volunteer on steam until after I’d spent 4 years in the USAF, mostly
Great Falls, Montana... )1960-1964.

So, I came to steam LATE, but did get to be student of some of the best in the East..

So, I’ll give you what I’ve been able to figure out..., but hopefully, there’s
more experienced contributors that have a different background and more relevant circumstances ...

I’ll get back to you on what I’ve found...

Wes

Posted from iPhone

Trained? Best training was working on the nuts and bolts of one before taking the throttle. There's a lot of feel to running a
steamer and that comes with experience but it's nice to know what does what before pulling the throttle.

Out 

Student trips.  Written tests. Learning to fire coal, oil good steamers, how to deal with job problems.  Time on the job, learning what you could from good engineers.  Written tests.  Verbal tests. Rules test. Experience.  You just don't hire out and start pulling throttles. 

Frisco1522 Wrote:
-------------------------------------------------------
> Student trips.  Written tests. Learning to fire
> coal, oil good steamers, how to deal with job
> problems.  Time on the job, learning what you
> could from good engineers.  Written tests. 
> Verbal tests. Rules test. Experience.  You just
> don't hire out and start pulling throttles. 

And even then, you learn something new everytime you either run or fire. Education is ongoing and continuous.

Txhighballer Wrote:
-------------------------------------------------------
> Frisco1522 Wrote:
> --------------------------------------------------
> -----
> > Student trips.  Written tests. Learning to
> fire
> > coal, oil good steamers, how to deal with job
> > problems.  Time on the job, learning what you
> > could from good engineers.  Written tests. 
> > Verbal tests. Rules test. Experience.  You
> just
> > don't hire out and start pulling throttles. 
>
> And even then, you learn something new everytime
> you either run or fire. Education is ongoing and
> continuous.

Absolutely! Especially when something doesn't work, like suddenly the feedwater system no longer puts water into the boiler. How long does it take before the Fireman realizes it isn't working, and then what to do about it?  Fun and games.

Interesting topic. The veteran engineer who taught me the basics of running ( Mr. Charlie Strunk- CRRofNJ) amongst other things made me practice determining speed by referencing the passing landscape and not the speed recorder. He would cover the speed recorder with a rag and check my figures ay various speeds up and down the curve. To pass I had to be within 1-2 mph of actual. I took a while of OJT but I finally got respectfully good at it. Charlie was fond of saying " if you don't know your speed you don't belong in that seat".

   I found over the years that each locomotive had their own individual "quirks" and that it took a few trips to learn how best to get the maximum performance out of them and that a LOT came down to running by " feel" especially when running track speed over undulating terrain with a long passenger train. 

   Another interesting truth I witnessed over and over. Some veteran engineers were excellent steam runners and some were awful. You could tell in a couple minutes after starting out if he was one or the other. Bottom line is....some guys are naturals and some guys will never be any good.

   Thanks, Ross Rowland 

I have an old copy of the fireman's manual on the Union Pacific that was basically a question and answer book. As I recall engineer's had the same and then there were the regular tests they were given as a refresher. And then that which they learned out on the road from the regular crews.

Bob Krieger
Cheyenne, WY

Another fine piece...thank you again, Wes. For the first time I finally have some operational insight into the differences between dome and front-end throttles. It was all mechanicals before...now I am starting to see them from the actual operational perspective.

Yes...

There were several evolutionary steps... ideas that were tried, found
partially successful and then perfected..

These developments,
from concept, to trial models, to casting patterns & designs, to the completed, marketable products, often took several years before final mass-produced (?) versions were perfected.

One rare evolutionary step was a single, pilot valve-assisted front end throttle, mounted ( visible) in front of the smokestacks ( of mid-1920s products ).

Elesco’s later versions combined the superheater ‘header’ (manifold) that handled the two ends of each superheater unit..together with the multiple-poppet steam-flow, sequential, control valves.

So, the combined casting was one of the more sophisticated, hard to manufacture, appliances on the loco.

The latter variants are very easy to regulate and responsive to subtle
Throttle adjustments...

Steve Wickersham, a loco restoration supervisor for Ross, who has operated both N&W 611, and Lima-built C&O 4-8-4, 614, said that he much preferred the 614’s
7-poppet, front-end throttle over the 611’s 5-poppet version... He liked that the 614’s throttle was more sensitive, or responsive..

Engineer’s preferences, no doubt..
A lot of opinions are influenced by what folks are most familiar with.
😀

( Edit: A huge improvement in steam flow-control was the repositioning of the throttle valve, AFTER the superheater header...

Earlier adoptions of the superheater were down-stream ( after) of the throttle,
with the result that the engineer had to, first, fill the units with steam before it even reached the cylinders.. leading to very sluggish
Throttle response times & slow sensitivity..to throttle changes).

W.

Posted from iPhone



Edited 3 time(s). Last edit at 06/04/20 15:57 by wcamp1472.

Ross, what you said about speed is right on. I had an engineer tell me there were nly 2 or 3 things that always worked a split rail serail,and looking at the ground was another. he was correct. I had many a butt chewing by a RFE for not checking the speedometer by my watch. They would ask if I clocked a mile, my answer was yep. I didn't see you. I looked at the ground. I was always within 1-3 mph. The only thing I have been on that I couldn't do that was the TGV in France, but I never got to run180 here.

I learned several ways to chck my speed from different hog heads. One was to count the rail joints, another was the poles in the pole line ( on most lines, can't be done now ). Aa mentioned earlier, looking at the ground works. One method not mentioned was counting the exhaust beats from the stack, which of course got way more difficult the faster you ran.