Steam & Excursion > Steam flow vs. Steam pressure

As discussed earlier, the power
Regulation of the steam loco is not so much “pressure regulation” and throttle opening as it is steam temperature optimization.

With super-power era locomotives,
It is the steam temperature that propels the train down the rails.

The upper limit of the operable
steam temperatures is the burn-point of the cylinder wall lubrication
scheme. For now, the upper limit of the lubrication temperatures is in the region of total steam temps of
750 deg. F.

So, you will find that steam is a poor conductor of heat. Water has way more heat absorptive capacity, than steam.

We recognize steam increase in temperature by increased molecular
Vibration of the steam ( gaseous State of water) particles —
the vibration of the steam molecules increases both in frequency and in amplitude.

Thus, more ‘steam’ leaves the units, than enters the units...the greater the excitation & separation of the steam molecules, the less weight of steam is used on each stroke of the pistons...
( less weight of steam exits —- fewer numbers of highly excited molecules—-..but it’s a hotter gas)..

The longer the steam spends inside the hot superheater elements, the greater will be the vibration activity.


The throttle valves are down-stream from the superheater units...
As you close the throttle, the steam gets trapped in the units... the idea is to use the throttle to control the velocity of the steam through the units—- you want to increase the
‘Dwell-time’ of the steam — so that the longer it spends at the hot-end of the units, the hotter the steam will be..

Doyle’s job is to trap the steam flow in the units long enough to raise the steam temps towards 700 to 750F.

The saturated steam pressure remains virtually flat... at around
300 PSI.. ideally, you’d want to
add 300 more degrees to the saturated steam temp of 400F.

By routing the heat-saturated steam through the superheater tubes,
into the flame path coming from the firebox, we heat the ‘units’ piping to near
the flame tip temperatures...

The flame tips cool off very rapidly inside the boiler tubes surrounded by the much cooler water... Flame tip temperatures are ‘cooled’ down to the surrounding boiler water... in about 10 feet from the firebox..


Flame tips can reach 3,000 deg. F.
But, boiler water is 400 F, and colder at the belly of the boiler ...

Steam molecules/particles passing through the hot superheater units get further separated and vibrate more quickly than when at 400 deg.

But, remember, the superheated steam pressure is never greater than the boiler pressure..

So let’s put all this together...
as HotWater described, he and Doyle will operate the boiler to maintain pressures nearly at 300
PSI, gauge.

When cruising, Doyle will maintain a steam flow that promotes smooth
steam cycles in the cylinders...
that may mean a longer admission period, per stroke ( the engine will tell you when that ‘smooth point’ is reached ... its very subjective).

Doyle will balance the steam flow
by closing-down on the throttle, so that the pressure at the steam spool valves ( cylinders) is about 50% of the boiler pressure...

That slows the speed at which the steam flows through the units....

A benefit of closing down the throttle is that the exhaust draft
(Velocity) up the stack is much reduced...

The reduced draft through the firebox allows HotWater to
“lean-out” the air/fuel mixture .. and get flame tip temps up near 3000 F!

Now, with Doyle having the throttle
Partially closed, and the steam flow slowed way-down... his superheaters deliver white hot steam, at about 150 psi... at the piston ... with 700 deg. Steam...

The engine just sips a little of the steam at a time, while cruising
close to 70-per...

Sipping steam means less cold water, less fuel oil and a white-hot fire...

So, that’s why we love to see a loco
Matched to its train, and allowed to stretch her legs...

BigBoy does that with a mile of freight behind the tender...
but, we’re likely never gonna see that scale of operations on a 4000..

It’s a credit to the UP designers and Alco that the 4000s did that every day—- during WW2, and after ..

It takes a big train to get a 4000 to superheat... and a handful of coaches is no match for that boiler/ cylinder combination ..

W.

Posted from iPhone



Edited 10 time(s). Last edit at 05/28/20 15:40 by wcamp1472.

Interesting, and way above my pay grade.

Wes, how does that jive with the often stated concept that operating with the throttle wide open and controlling the power of the locomotive by regulating the cutoff (except when the shortest practical cutoff has been attained, and the locomotive is still producing too much power for the load) is the most efficient way to run?

I have read this time and again, from experts such as Paul Kiefer of the New York Central, Alfred Bruce of ALCo and Ralph P. Johnson of the Baldwin Locomotive Works, just to name a few.

 

yankingeorgia Wrote:
-------------------------------------------------------
> Wes, how does that jive with the often stated
> concept that operating with the throttle wide open
> and controlling the power of the locomotive by
> regulating the cutoff (except when the shortest
> practical cutoff has been attained, and the
> locomotive is still producing too much power for
> the load) is the most efficient way to run?
>
> I have read this time and again, from experts such
> as Paul Kiefer of the New York Central, Alfred
> Bruce of ALCo and Ralph P. Johnson of the Baldwin
> Locomotive Works, just to name a few.
>
>  

That seems "correct" to me, too.  The PRR Altoona tests were nearly all run with wide open throttle and using the reverse lever to regulate steam usage and I think the NYC road dynamometer tests were run with open throttle too.  

Scott Griggs
Louisville, KY

A throttle is a flow regulating device, it has an indirect relationship to pressure. The cylinder valves are a directional control device and
as such throttle only by their time of opening vs cylinder travel. So if you want maximum power you want minimum pressure drop which
would relate to the throttle valve being wide open. You now have almost boiler bp at the cylinder valve, a device designed to introduce
steam in one end of the cylinder and exit it at the other, and vice versa. It was not intended to be a throttling device. But since the amount
of cutoff is variable it tends to act as such. Obviously either way works, in the real world we'd sc the throtle and go to a proportional
directional valve.

Out 

Re: "yank..." & sgriggs...

It all depends on the trailing load and max allowable speeds...

On flat level track, passenger trains have minimal rolling resistance ....
Once you've accelerated the train to the allowable track speed--- if still running
at full power, you will continue to accelerate...but you'd soon be exceeding the speed limit..
What do you do?

Scenario 2..
You're descending a slight downgrade ....5 miles, or so, long...the train rolls & accelerates by itself...
to max allowed speed ....
How do you set the throttle & valve gear?

You're the engineer on a NKP Berk, doing 60-per, and trailing 80 cars of freight ...
over gently rolling, mid-west countryside...
NOW, you can run it wide-open...and the engine will tell you how to set it up
for full-power operation ...

Even under full-load conditions and a shorter cut-off...you'll find that the engine's
power accelerates train too fast..... your only choice is to reduce the pressure at the
steam chest...to maintain allowable track speed ....
Since the front-end throttle is down stream from the superheater units ...the rate of flow
through the units gets reduced,increasing the dwell-time of the steam traversing the units...
superheat continues to raise.

With a suitable-weight train holding her back...almost all your "adjustments" increase 
the temperature of the superheat reaching the pistons .... the power combination MUST 
be modulated to suit track characteristics.  With the power cut-off properly set, your only choice 
in maintaining allowable track speeds is to reduce the throttle opening....

The UP's sixty mile climb up the Continental Divide provides a rare opportunity for
sustained power input to the trailing load...  
But that's not often the case in the rest of the country.

The authors you cite are correct...but real-life conditions intervene in daily practice.
( in the PRR's Test Plant, you could adjust the water-brakes to have the maximum restraining
load applied for hours on-end... which is one reason the plant was constructed ...)

In daily railroading, the sustained  wide-open throttle conditions are short lived, in actual train operations.
Accelerations from a stop are the most common max-loading conditions...once you're 
cruising at allowable speeds....the steam flow to the cylinders must be properly 
modulated to avoid exceeding timetable-allowed track speeds..

With maximum train weights, You can tell as you adjust the flow-regulating devices
while finding the "right combination" of balanced settings --- steam flow & cut-off timing ....
the engine will "talk" to you.... and you can feel when the best cruising combination is reached.
( The firemyn's job is to keep the fire lively, yet refrain from wasting too much steam
out the pop-safety valves...if the engine is "adequately  loaded",  a stoker-fired engine
will practically 'fire-itself'  --- with lighter trains the firemyn woirks continually  to keep-up
a lively fire..yet refrain from wasting steam out the pops.)

Now, fully-loaded, THAT'S FUN !!!

W.

 



Edited 6 time(s). Last edit at 05/30/20 08:22 by wcamp1472.

Wes,

It surprises me that during the steam era, engineers were frequently forced to run at partial throttle to avoid exceeding speed limits.  I can understand why speed restrictions for curves or bridges would create situations where it was necessary to run with the throttle less than wide open.  However, if this was common I think I would have to ask a few questions.  If a locomotive can't even run with wide open throttle and the shortest practical cutoff without exceeding speed limits, that tells me the road didn't give the locomotive a proper tonnage rating - or- they routinely put consists that were far too light behind it.  That seems like operational malpractice.  That locomotive has huge reserves of excess power and is operating in a terribly inefficient manner.  It makes one question why there was such a big push toward ever larger, superpower locomotives.  I mean, if I have to run my new Berkshire locomotives at part throttle and 30% cutoff to avoid exceeding the speed limit, why did I need Berkshires in the first place?  Mikados or Mountains would have been perfectly sufficient.  Or, why aren't my new Berkshires paying their way by hauling heavier trains at the speed limit?  That was the promise of superpower.

I'm a bit confused by your post.

Scott Griggs
Louisville, KY

From my experiences, there is really no need for "wide open throttle" when cruising/top speed is reached on flat level track. Once 60 or 65 MPH, say, is reached on level track, the throttle is reduced a bit, and the cut-off is increased, i.e. reverse gear moved forward until the "crack" sound comes back at the stack. That way, the engine is running VERY efficiently on the heat of the seperheated steam, and fuel & water are reduced, while still maintaining 60 65 MPH. Can't tell you how many times I've seen this work perfectly on SP 4449 and UP 844, with a good sized train.

HotWater Wrote:
-------------------------------------------------------
> From my experiences, there is really no need for
> "wide open throttle" when cruising/top speed is
> reached on flat level track. Once 60 or 65 MPH,
> say, is reached on level track, the throttle is
> reduced a bit, and the cut-off is increased, i.e.
> reverse gear moved forward until the "crack" sound
> comes back at the stack. That way, the engine is
> running VERY efficiently on the heat of the
> seperheated steam, and fuel & water are reduced,
> while still maintaining 60 65 MPH. Can't tell you
> how many times I've seen this work perfectly on SP
> 4449 and UP 844, with a good sized train.

I wonder if there is a difference between operation in the 1930's-1940's steam era and the excursion era, such that generally far less is being asked of locomotives in the excursion era in terms of speed or tonnage?  The locomotives you mention are capable of hauling 1000 ton trains at speeds near the century mark.  If I'm hauling an 800 ton train (maybe 12 cars) at 65 mph, that engine is barely breathing hard (especially when you consider that it is closer to its natural peak DB Horsepower capability at 60 mph than it is at 90 mph, when air resistance and machine friction are higher).  It seems like there is latitude to run the throttle/reverse lever in a variety of ways and keep the train moving at that speed without much difficulty.  However, if I'm hauling the 1000 ton Los Angeles Limited at 90 mph, and the train resistance is 50% higher, the power requirements go up immensely and I'm not going to be able to make the schedule unless I operate the locomotive with an open throttle for maximum power.

Scott Griggs
Louisville, KY

Scott...

Rarely did they operate at full power, all the time.

Most track is either flat or downhill.
Uphill sections are least common.
It takes little or no power applied to the loco draw bars , once up to speed...

The Berks were derived from Mikados with small grates that were
commonly over-fired, which built-up the depths of the firebed..

The larger grate areas of the Berks allowed the draft rates to be decreased... the fires stayed hotter and thinner... the advantage was that most of the fly-ash was expelled up the stack, instead of
increasing the depth of the firebed..

The early Berks had drivers only slightly larger than the Mikes of the era...

The challenge with Consols, Mikes
& Berks is that all the main drivers are near the COG of the locomotive. Whatever shakes the center of gravity, shakes the whole machine...

The solution was two-fold: use bigger diameter drivers (reduce the RPMs), move the main driver to axle no. 2. That moves the most reciprocating mass further from
the COG... reduces pounding the whole ‘structure’...

Moving the main driver to the no. 2 position forces the cylinders further forward ( to avoid a too-short main rod..). That cylinder move forced the adoption of the the 4-wheel pilot truck’s.

The resulting 4-8-4s, were faster, more powerful, better balanced and
lower COG— by virtue of a longer
chassis & boiler...

So, operating a loco at reduced power demands, is s GOOD THING! Even today, diesels do not often run wide open, except on uphill pulls... again, once rolling, trains take a much-reduced “power input”... if you could observe the governor lay-shaft on the prime mover, you’d see that the injectors’ fuel racks are barely open ...

But you’d see that the intake manifold ( turbocharged) pressures were near 30-psi ( unless reduced to reduce
NOx gasses)..

You would also note, by observing the load Amp-meter, the the traction motor current would be very modest, and not near it maximum,
full-time rating.

Today’s RRs benefit from newer power by having fewer locos needed to move a specific freight train..

All locomotives don’t need run at full power all the time—- it’s a terrific waste of fuel, always has been.

Marine engines DO run wide-open
when on the high-seas... diesels love it, on account of the steady operating temperatures ...

Posted from iPhone



Edited 1 time(s). Last edit at 06/02/20 08:25 by wcamp1472.

sgriggs Wrote:
-------------------------------------------------------
> HotWater Wrote:
> --------------------------------------------------
> -----
> > From my experiences, there is really no need
> for
> > "wide open throttle" when cruising/top speed is
> > reached on flat level track. Once 60 or 65 MPH,
> > say, is reached on level track, the throttle is
> > reduced a bit, and the cut-off is increased,
> i.e.
> > reverse gear moved forward until the "crack"
> sound
> > comes back at the stack. That way, the engine
> is
> > running VERY efficiently on the heat of the
> > seperheated steam, and fuel & water are
> reduced,
> > while still maintaining 60 65 MPH. Can't tell
> you
> > how many times I've seen this work perfectly on
> SP
> > 4449 and UP 844, with a good sized train.
>
> I wonder if there is a difference between
> operation in the 1930's-1940's steam era and the
> excursion era, such that generally far less is
> being asked of locomotives in the excursion era in
> terms of speed or tonnage? 

Don't forget the terrain being operated over. I can remember many cases in 1984 on the New Orleans Worlds Fair Daylight when 4449 was cruising at 70 MPH on the SP Sunset Route, where she was taxed pretty well. The same thing occurred with 844 on Westbound moves, which is slightly up-grade most of the way west.

The locomotives you
> mention are capable of hauling 1000 ton trains at
> speeds near the century mark.  If I'm hauling an
> 800 ton train (maybe 12 cars) at 65 mph, that
> engine is barely breathing hard (especially when
> you consider that it is closer to its natural peak
> DB Horsepower capability at 60 mph than it is at
> 90 mph, when air resistance and machine friction
> are higher).  It seems like there is latitude to
> run the throttle/reverse lever in a variety of
> ways and keep the train moving at that speed
> without much difficulty.  However, if I'm hauling
> the 1000 ton Los Angeles Limited at 90 mph, and
> the train resistance is 50% higher, the power
> requirements go up immensely and I'm not going to
> be able to make the schedule unless I operate the
> locomotive with an open throttle for maximum
> power.
>
> Scott Griggs
> Louisville, KY

Scott...

Rarely did they operate at full power, all the time.

Most track is either flat or downhill.
Uphill sections are least common.
It takes little or no power applied to the loco draw bars , once up to speed...

The Berks were derived from Mikados with small grates that were
commonly over-fired, which built-up the depths of the firebed..

The larger grate areas of the Berks allowed the draft rates to be decreased... the fires stayed hotter and thinner... the advantage was that most of the fly-ash was expelled up the stack, instead of
increasing the depth of the firebed..

The early Berks had drivers only slightly larger than the Mikes of the era...

The challenge with Consols, Mikes
& Berks is that all the main drivers are near the COG of the locomotive. Whatever shakes the center of gravity, shakes the whole machine...

The solution was two-fold: use bigger diameter drivers (reduce the RPMs), move the main driver to axle no. 2. That moves the most reciprocating mass further from
the COG... reduces pounding the whole ‘structure’...

Moving the main driver to the no. 2 position forces the cylinders further forward ( to avoid a too-short main rod..). That cylinder move forced the adoption of the the 4-wheel pilot truck’s.

The resulting 4-8-4s, were faster, more powerful, better balanced and
lower COG— by virtue of a longer
chassis & boiler...

So, operating a loco at reduced power demands, is s GOOD THING! Even today, diesels do not often run wide open, except on uphill pulls... again, once rolling, trains take a much-reduced “power input”... if you could observe the governor lay-shaft on the prime mover, you’d see that the injectors’ fuel racks are barely open ...

But you’d see that the intake manifold pressures were near
30-psi ( unless reduced to reduce
NOx gasses)..

You would also note, by observing the load Amp-meter, the the traction motor current would be very modest, and not near it maximum,
full-time rating.

Today’s RRs benefit from newer power by having fewer locos needed to move a specific freight train..

All locomotives don’t need run at full power all the time—- it’s a terrific waste of fuel, always has been.

Marine engines DO run wide-open
when on the high-seas... diesels love it, on account of the steady operating temperatures ...

I hope this has been helpful..

W.

(Yet to be proofed..)

Posted from iPhone

Don’t know why this essay duplicated itself...

I’ll get into it later..

W.

Posted from iPhone

Scott,

I think what you’re missing is the distinction between pressure-operated engines and heat operated engines...

Piston operated Internal combustion engines are
Definitely dependent on
Cylinder pressures..

Modern, jet powered airplanes are
Powered by HEAT, not by a pressure-based scheme...

Yes, jets have compressor stages, but that’s not the source of the power... it is the whirling blades in the path of the the heat flow that powers the turbo-fan drive-shaft.

Military jets continue to use the ‘reaction-propulsion’ — a less efficient use of jet propulsion,
Than the turbo-fan engines....

So, when I’m on a commercial jet passenger plane, I’m aware of the the immense power and thrust on take-off and climb to cruising altitude... BUT, when at cruising altitude, I LOVE IT when the pilot reduces the power, and leans-out
the fuel flow..

You can notice a clear change in the engines’ sound as both the fuel and the fan-blade pitch is changed,
From power ( fuel-hungry) take-off
Power to cruising power...

The same efficiencies are characteristic when running
a steam loco on superheat conditions...

A closed down throttle, reduces the speed of the steam through the units..

Thus increasing the ‘dwell-time’ of the lower-energy, heat-saturated steam, to the higher energy, superheated steam. You don’t need full boiler pressure at higher RPMs... but you need the benefit of
Full-stroke power... all the way to the opening of the exhaust rings of the spool valves.

A good engineer, let’s the heat of the steam do the work, and not the boiler pressure.. he traps more of the heat with lower steam flow, than by old-school thinking..

I’ve seen too many old-school engineers pound engines mercilessly, instead of taking advantage of superheating...

They’re not ‘pressure machines’,
they’re HEAT machines..

It’s what led Parsons to invent and perfect the early steam turbines...
He recognized the need to extract more heat out of the steam, than was wasted up the stack...

Parsons’ perfection of the steam turbine led others to try raw fuel and flames to spin the turbine blades...
Thus, the modern direct-fuel jet engines were perfected ...

You ought to visit the Udvar-Hazy
Smithsonian Institute at Dulles airport, here in Northern Virginia.
It has a marvelous display of recip aircraft engines, from the Wright brother’ aluminum-block, 4-cylinder engine of 1903... all the way to the
36-cylinder engine of 1947... (the sole extant version).

There is also an early jet-aircraft engine about 1/3 the size of the big recips— with twice the power capacity... and nothing but spinning blades... wind whistles right through it... no pistons, no cylinder valves...
A pure heat engine..

Parsons was way ahead of his time in the late 1800s..

W.

Not proofed, yet

Posted from iPhone