Steam & Excursion > Expert question: Throttle psi starting from a stop

Have a question about applying throttle from a stop or at very slow speeds.  At a stop, even if the throttle is barely cracked open, won't the pressure in the cylinder(s) eventually rise to that of full boiler pressure? Seems to me for a slow controlled start, even small throttle openings must result in some movement over time as the pressure builds in the cylinders.  Is this an effect that has to be compensated for with experience?

Thanks,
...bill

Illbay Wrote:
-------------------------------------------------------
> Have a question about applying throttle from a
> stop or at very slow speeds.  At a stop, even if
> the throttle is barely cracked open, won't the
> pressure in the cylinder(s) eventually rise to
> that of full boiler pressure?

Yes, if no piston movement takes place.

Seems to me for a
> slow controlled start, even small throttle
> openings must result in some movement over time as
> the pressure builds in the cylinders. 

Yes.

Is this an
> effect that has to be compensated for with
> experience?

Yes. That is why an experienced Engineer "pumps" the throttle during starting in order to prevent wheel slip.


> Thanks,
> ...bill

When stopped, keep the cylinder cocks open to prevent undesired cylinder pressure buildup. Starting, leave them open and it will both give you better control and prevent condensate from damaging pistons, heads and rods due to hydraulic locking, until cylinders warm up.

-LD

And why you pay attention to the reverser when you put the "little lady" to bed at night with steam still in the boiler.
"Chains, chains, my babies got me locked up in chains", too early for singing? 

Out 

Remember how PRR's 2-10-0s originally had
maximum cutoff just over 50%? So if the left
piston was just past full-forward and the right
piston was just past the 50% cutoff, the only 
way the engine could pull at all was from the
auxiliary-port steam in the right cylinder--
the port being a slot 1/8 inch by 1-1/2 inches.

So how long did it take for the cylinder to reach
250 psi? How well did the port avoid getting blocked
by gunk? Was that why limited cutoff went out
of fashion?

Baron Vuillet's book mentions GN 4-8-4s starting
passenger trains on upgrades-- he says they
cracked the throttle and released the brakes,
allowing the engine to roll backward until
it was stopped by the pressure in the cylinder,
then pulled the throttle wide open and
started the train smoothly.

timz Wrote:
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> Baron Vuillet's book mentions GN 4-8-4s starting
> passenger trains on upgrades-- he says they
> cracked the throttle and released the brakes,
> allowing the engine to roll backward until
> it was stopped by the pressure in the cylinder,
> then pulled the throttle wide open and
> started the train smoothly.

I used a variation of this to start full tonnage trains on the 4% grades of the Cumbres & Toltec.  Worked well, and seldom got stuck on "center".

Some thoughts ...

Limited cutoff....
Rotative torque applied to the driver crankpin is maximum mechanical advantage with the crank a little ahead-of the 90-degree, vertical line —(- axle center, crank center, rim at the rail—— that imaginary vertical line...)

[edit: even though boiler pressure is “cut-off” at the 50 % point , the trapped boiler pressure in the cylinder continues to exert rotative effort to the piston & crank pin, at a declining rate... but, the piston/cylinder on other side is approaching it’s maximum “90-degree point”, thus, continuing the tractive effort ..]

In all cases, “Rotative power” declines to ZERO
towards the end of the piston travel..

So, a cutoff at about the 90-degree mark has minimal impact on the total Rotative “mechanical moment”.

With small-ish drivers, and a longer rotative piston-stroke, you’d not
find any measurable drop-off in hauling power...

Every degree of rotation beyond 90, means that there is less and less benefit. from declining steam pressure on the piston face...

[Note: typically you’d find that the “exhaust” rings of the spool valve open the passage up the stack several degrees BEFORE the 180-degree mark —- likewise, the admission rings would open the steam port a little ahead-of the actual “dead-center” of the piston stroke... called the “Lead” ( say: “LEED”..) which, if excessive, can have a negative impact on starting..]

The decline in “beneficial torque” reaches zero ( at a sinusoidal rate) as the main rod gets to horizontal... any pressure at that point simply has the piston pushing, laterally, against the journal box and the engine frame—- no angular effort is applied to the rail..


Starting trains:
Large drivers, with crank pins
closer the axle, are more susceptible to stalling at starting...

Freight engines tend to have the crank pins closer to the wheel rim.

Piston speeds ( wear-rates) are higher for freight locos...

Passenger engines typically have shorter piston stokes, and larger driver diameters, thus piston speeds are lower, compared to freight’s — at comparable speeds..

“Slack...”

The secret to starting is: “Slack”.
With knuckle couplers, there is about an inch and a half of “free slack” between each coupled pair...

Later improvements in passenger cars’ coupler design reduced the “uncontrolled” slack to zero..

For steamers, The first few degrees of rotation are crucial to continuous rotative torque .... once you get several revolutions, you’re well underway..

You’ll note in the engineer’s technique, described above,
what’s actually happening is that he’s ‘taking slack’...to get the needed rotative advantage in getting started.

Starting Technique varies depending on the grade and
standing slope of the track at starting.... if level, slack is your friend... if down hill, you’ll roll
simply by releasing the brakes...
Starting a train, with a steamer, on an up-grade slope can be a tough challenge for an engineer..

Freight trains have lots of slack, today’s passenger trains have virtually NO “slack”... older passenger cars tend to have simpler coupler designs and more slack between each car-pairs.

So... I hope this expands your horizons..

W.

Posted from iPhone



Edited 3 time(s). Last edit at 05/28/20 06:34 by wcamp1472.

Thanks for all of the thoughtful replies.   
The answer about opening the drain cocks makes a lot of sense to me. By allowing a constant flow (albeit small) from the boiler, cylinder pressure is mostly regulated by throttle position.
It must be quite a handful getting a big engine moving without slipping in a safe manner.

Was there ever a concept of a pressure regulator applied to a steam throttle? 

Thanks,
...bill

Illbay Wrote:
-------------------------------------------------------
> Thanks for all of the thoughtful replies.   
> The answer about opening the drain cocks makes a
> lot of sense to me. By allowing a constant flow
> (albeit small) from the boiler, cylinder pressure
> is mostly regulated by throttle position.

A lot depends on the skill of the Engineer. For example, rarely does Doyle McCormack open the cylinder cocks on SP 4449, unless we've been stopped for some time. If there has been any amount of condensed water in the cylinders, the steam operated type of cylinder cock will "pop" as the hydraulic pressure of the water exceeds the steam pressure holding the cylinder cock closed. Then the cylinder cocks are opened to expel the condensed water from the cylinders. As information, when running at speed, the cylinders get so hot, that during a brief stop, there generally is no chance of condensed water to accrue within the cylinders (it is simply too hot, well in excess of 350 degrees F). 

> It must be quite a handful getting a big engine
> moving without slipping in a safe manner.

Again, this all relates to the skill & experience of the Engineer. It really isn't all that difficult, once one learns the particular characteristics of one's locomotive.


> Was there ever a concept of a pressure regulator
> applied to a steam throttle? 

No. Why would such a device be necessary? Then, what would happen if/when it failed?


> Thanks,
> ...bill

Illbay Wrote:
-------------------------------------------------------

>
> Was there ever a concept of a pressure regulator
> applied to a steam throttle? 
>

Yes, it's spelled "E-n-g-i-n-e-e-r" ;-)

Other than that, the throttle IS the regulator, an I believe it is called that overseas.

-LD

Pressure regulator for steam throttle?

Yes!

It called the “Engineer..”
( and his firemyn)...


With superheated operation,
the consistent load on the boiler is
regulated by balancing many factors: the draft through the firebox, up the stack, the expansive
setting of the spool valves’ travel,
the ‘mean effective pressure’ on the piston faces ( cylinder pressures as maintained by the throttle setting),
the fire’s intensity, the ambient air temperature, the mass behind the tender, the speed, the grade and
the track configuration, the allowed
Speeds over the railroad portion being navigated, traffic controlling signals, drawbridges, timetable rules and specs, as well as approaching junction points and crossings...

As well as train airbrakes control, operation and skills... and the constant radio ‘channel chatter’,
occasional dispatcher communications, train crew instructions, etc..

Put all that together in a computer program, mount it in the cab of the streamer and enjoy the show...

You could always add the recently mandated Positive Train Separation
rules..

W.

Posted from iPhone



Edited 2 time(s). Last edit at 05/28/20 07:51 by wcamp1472.

A "regulator" in the correct sense is a normally open device that restricts "closes" as the downstream pressure rises to the regulator setting. A throttle
is a manually operated flow control device which restricts downstream flow. Do not confuse pressure with flow, pressure is a reaction to flow. A 300 osi
boiler open to atmosphere will not maintain 300 psi, the 300 psi comes as a reaction to steam flow against a load, combination of throttle/valve setting
and trailing load and obove all the limiting factor of the safety valves.

Out 

I never opened the cylinder cocks at a brief stop.  Our other two did, thinking it would make for a smoother start.  If there were a lot of people around, they would get a hot steam and steam oil fogging if they were too close.   I was lucky too that the 1522 was not a slippery engine.  Lay down a little sand as you're coming to a stop, leave the train stretched and use the booster if necessary.   Lot less jolting around the folks on the train.
May be counter to most opinion but always worked for me and made for super smooth starts.  1522 was an awesome engine and would work down to a stall and not slip.  In very extreme cases, I have used the release the brakes and let the engine ease back into the train with the throttle open and some pressure in the cylinders method. I just never needed to do that more than a couple times.
You get used to an engine's traits, strengths and personality and it helps.

Illbay Wrote:
-------------------------------------------------------
> Thanks for all of the thoughtful replies.   
> The answer about opening the drain cocks makes a
> lot of sense to me. By allowing a constant flow
> (albeit small) from the boiler, cylinder pressure
> is mostly regulated by throttle position.
> It must be quite a handful getting a big engine
> moving without slipping in a safe manner.
>
> Was there ever a concept of a pressure regulator
> applied to a steam throttle? 
>
> Thanks,
> ...bill

If I remember correctly, the Pennsylvania Q2 Duplex (4-4-6-4) locomotive had an anti-slip device that was designed to reduce throttle of the slipping drivers/cylinders automatically if wheelslip was detected.   There is an account In Don Ball, Jr's book, The Pennsylvania Railroad 1940s-1950s, in which another railroad borrowed a Q2 for testing (believe it was against a Norfolk & Western A) and the automatic slip control system was not working.  Like many of the newer design features on the PRR duplex locomotives, it wasn't fully sorted out before steam locomotion was sidelined by internal combustion power.  

Scott Griggs
Louisville, KY

sgriggs Wrote:
-------------------------------------------------------

> If I remember correctly, the Pennsylvania Q2
> Duplex (4-4-6-4) locomotive had an anti-slip
> device that was designed to reduce throttle of the
> slipping drivers/cylinders automatically if
> wheelslip was detected.   There is an account In
> Don Ball, Jr's book, The Pennsylvania Railroad
> 1940s-1950s, in which another railroad borrowed a
> Q2 for testing (believe it was against a Norfolk &
> Western A) and the automatic slip control system
> was not working.  Like many of the newer design
> features on the PRR duplex locomotives, it wasn't
> fully sorted out before steam locomotion was
> sidelined by internal combustion power.  
>
> Scott Griggs
> Louisville, KY

That test of the PRR Q2, on the N&W, is well documented in the book "The A, Norfolk & Western Mercedes of Steam" Revised & expanded edition, by Ed King.

Yes!  I have that book, but I had forgotten it made mention of the Q2 testing.  I need to read up on that part.  The A was, in many ways, so far ahead of its time.