Steam & Excursion > Couple of steam questions

I'm re-reading my copy of "One Man's Locomotives" by Vernon Smith. In that book he mentions cross-compound locomotives (Soo Line) - meaning high pressure (smaller diameter piston cylinder) on one side, low pressure (larger diameter piston cylinder) on the other. It was not something I'd heard of before. I was aware of Mallet's with high-pressure cylinders up front, low-pressure behind. So was the steam used twice, going from the high side to the low before exhausting?

Secondly, regarding some previous posts (I believe by Mary McPherson) mentioning the rebuilding of 2-8-0 engines to 0-8-0 switch engines; what's the (switching/operational) benefit to removing the two-wheel leading truck?
 

You’ve got a couple of confusing sentences.

So, here’s the basics.
The first use of the steam exhausts
a great amount of, what could be beneficial heat, if re-used..

In compounds, to get the equivalent piston thrusts,
the pistons for the re-used steam are
a proportionately larger diameter.

However, it takes consistent pressure to power the LP pistons—
- so, the pipe that connects the two sets of cylinders is
called the receiver…after several revolutions, the pressure builds in the receiver,
and also in the LP pistons.

Mallets have the HP pistons, the smaller diameters, feeding the larger diameter, 
LP pistons.  The torque delivered to two the wheel sets is roughly the same,
delivered by different sized pistons.....compounding!

it's important to remember that the pressure ( lbs per sq. Inch) sent to the
LP cylinders, also is acting on the exhaust-side of a HP piston, while its working.
The pressure on the side that's exhausting steam to the LP pistons, 
acts against the boiler pressure powering the HP side of the piston.

You can have 200 psi boiler pressure, powering the HP pistons, and it's
 trying to fight the 100 psi going to the LP pistons..  So, the effective ( useful)
pressure in the HP piston is only 100 psi!  It's 'fighting' the exhaust pressure
of 100 psi --- that's powering the LP pistons..
Compounding is not 'free'.

Because it takes many driver revolutions to sufficiently pressurize the
'receiver pipe', compounds are fitted with an ( optional) live-steam,
pressure regulating system that allows the engineer to use live, boiler
steam to supply the LP pistons, during starting, or very low speed
'booster purposes'.

[ Exam Question, for 'extra credit':   What is the pressure acting on the
   exhaust side of the LP pistons?]

BUT, boiler pressure steam, if fed unrestricted, to the immense LP pistons,
the boiler pressure on the large pistons would spin the LP drivers out of control..
So, they limit the live steam pressure admitted into the 'receiver pipe'
to control the steam pressure powering the LP pistons.

 The pressure-limiting is a function of the self-regulating pressure valve ....
The engineer does not directly control the pressure being sent to the
LP pistons.  

Such direct use of steam is called "operating in simple", as opposed to
operating in "compound".   Many tourist trainloads are comparatively light,
so many engineers simply always operate in 'compound', the LP pressure
will eventually fully pressurize the receiver pipe.  

Remember, that the starting pressure in the HP pistons  can be full boiler
pressure, since there is not yet any 'negative' pressure ( against the HP pistons)
building in the receiver pipe. The HP drivers will be developing greater tractive effort,
 at the rail--until the 'negative' pressure in the receiver pipe increases.

You'll see the similar size-relationships in turbines, steam or fuel, ---- the turbine's
 'propeller shaft' is fitted with a series of discs, of increasing diameters, the discs
have hundreds of short, twisted fins around their circumference.  
Between the discs are fixed fins to re-direct the heat/gasses to the next,
larger diameter stage.

So, you'll see the lower pressure, lower heat, fins at the far end---the discs are
much larger diameters In order that each 'stage' delivers the same torque to the shaft,
even though being moved by lower pressure, lower heat gas flows..

Visit technical museums, and you'll see jet turbine engines opened --- so you can
see the increasing fin-arrangements ---- note the 'compounding effect' of the larger
and larger stages..at the 'power-end' of the jet.  

The first, inlet, stages are used to compress the intake-air prior to adding the fuel --
- comparable to 'supercharging' in piston-powered engines...

​W.

Posted from iPhone



Edited 8 time(s). Last edit at 02/06/23 14:45 by wcamp1472.

2-10-2 Wrote:
-------------------------------------------------------
> I'm re-reading my copy of "One Man's Locomotives"
> by Vernon Smith. In that book he mentions
> cross-compound locomotives (Soo Line) - meaning
> high pressure (smaller diameter piston cylinder)
> on one side, low pressure (larger diameter piston
> cylinder) on the other. It was not something I'd
> heard of before.

You are correct,

> I was aware of Mallet's with
> high-pressure cylinders up front, low-pressure
> behind.

The other way around - a Mallet is high pressure at center of locomotive exhausting to low pressure at front.

> So was the steam used twice, going from
> the high side to the low before exhausting?

Correct.

> Secondly, regarding some previous posts (I believe
> by Mary McPherson) mentioning the rebuilding of
> 2-8-0 engines to 0-8-0 switch engines; what's the
> (switching/operational) benefit to removing the
> two-wheel leading truck?

One less wheel to derail.  Negotiate sharper curves and switches.  Don't need the guidance at switching speeds.  Fewer parts to maintain.

-LD



Edited 1 time(s). Last edit at 02/06/23 09:00 by LarryDoyle.

> Secondly, regarding some previous posts (I believe
> by Mary McPherson) mentioning the rebuilding of
> 2-8-0 engines to 0-8-0 switch engines; what's the
> (switching/operational) benefit to removing the
> two-wheel leading truck?

One less wheel to derail.  Negotiate sharper curves and switches.  Don't need the guidance at switching speeds.  Fewer parts to maintain.

-LD

And more weight on drivers where it is needed for more tractive effort.

Adding weight on drivers does not increase tractive effort, by itself. Tractive effort is a purely theoretical figure that does not include a variable for the engines weight or its distribution.


-LD

GOOD CATCH...
" ....a purely theoretical figure..."

I wish I had learned that, decades ago...
In my early years, I wasted a lot of time, flipping pages back and forth
in RR books by railfans... I was seeking to make unrealistic comparisons..

A lot of my railfan-authored books I've marked up with 
comments correcting their unsupported speculations...

W.

( Also, you'll not find calculations regarding the 'force' comparisons between
   sanded & dry rail, steel-on-steel..... Since earliest times, folks have
   used sand to increase locos' traction ---- but, you'll not find any calculations,
    or records regarding the actual comparison numbers...

   What's the greatest power an N&W Y6b, [ in 'simple'] could produce on sanded,
      dry rail?  It's gonna' be way higher than the theoretical number.
      It probably 'pegged the meter' on the dynamometer cars!)...



 



Edited 3 time(s). Last edit at 02/08/23 12:46 by wcamp1472.

Thanks everyone for the replies. I obviously don't have anywhere near the depth of knowledge on steam, and I especially had not come across the "receiver" to balance out high/low cylinders.

Yes, the receiver is a chamber that takes the exhsust pulses from the
HP cylinders ---- smooths them out---  into a steady, average pressure,
like about 125 psi, to supply the Low Pressure pistons.

When starting in "simple" the HP cylinders' exhaust path is 
sent to an exhaust-ring around the top of the conventional 
'blast' nozzle, below the base of the smoke stack.
( The stack's lower end is flared out, curved, funnel-like base which
   is called the 'petitcoat pipe')

The additional exhaust ring only sees internal steam flow when the engine
is operating in 'simple'.  You can hear two engines exhausting, when In 'simple'.

While the exhaust from the HP pistons is diverted to the stack, live steam,
at a reduced pressure (replicating approximately the same pressure as
in "true compound" operation.   And, for emphasis, the larger pistons would
exert too much force ---if they were operated at full boiler pressure.

So, a pressure-limiting valve for the receiver pipe, reduces the Boiler Pressure,
(live steam) that pressurizes the 'receiver' pipe feeding the LP pistons...

The HP cylinders deliver more power to the rear drive-wheel set, when in
'simple', since there is NO back pressure acting against the HP  pistons,
as in compound use.  

As explained, if the HP cylinders are sending 125 psi to the LP 
pistons, that same 125psi is 'acting against' the live steam from the boiler,
and against the HP pistons  ... so, in compound, the power to the rear set of drivers
is lower, than running in "simple".

So, that's why some engineers, in excursions, simply stay in 'compound',
since they have plenty of HP , tractive-power to get started ..... after traveling a couple
of hundred feet, the --- once used steam-- pressure in the receiver pipe increases,
and the front engine (LP) adds its power to hauling the train  ---- once the
receiver pipe reaches its "operating pressure"..
~
It's a key concept.. and easily misunderstood.

Wes

( It would be 'instructional" if a medium-pressure Steam Gauge, 
  reading the receiver pipe pressure,  was in the cab for the engineer's use..
  It would be an eye-opening opportunity...)

Wes & All,
I find this very interesting.  In looking at some compound locos I came across the PL&E NU-1b 0-8-8-0 #9090.
This engine had 26" HP rear cylinders and 40" LP fronts with a stated 200psi boiler pressure and 52" drivers.
I do not recall ever seeing pistons this large but my experiences are limited!
My questions are:
1.  How did the designers decide what size piston(s) they needed in these compound engines?
2.  Why did they use a spool valve on the HP cylinders and a slide valve on the LP cylinders?
3.  Were there other engines that used pistons this size or larger?
Thanks again for sharing your wisdom with all.
Ike

2-10-2 Wrote:
-------------------------------------------------------
> Thanks everyone for the replies. I obviously don't
> have anywhere near the depth of knowledge on
> steam, and I especially had not come across the
> "receiver" to balance out high/low cylinders.

Due to the distance between the high pressure cylinders and the low pressure cylinders and the need to put the plumbing to make the engine flexible, Mallet's all have receiver pipes between the high pressure exhaust and the low pressure engine.

I can't think of any other compound engine in locomotive service that has a receiver. Cross compound, balanced compound, Vauclain compound, tandem compound, three cylinder compound - none of 'em have receivers. But, they do have starting valves to enable starting simple, then switching to compound once moving.

-LD

A person could lay awake think of this -

How many starting valves were on the Matt Shay?

-LD

Re NU-1b….

About what year was this loco
designed & built?

That would be a clue to ascertain if
it was superheater equipped.
My suspicion is that it was built as a saturated engine…. So, once-used steam had given up a lot of it’s energy by the time it left the
HP cylinders.

Manufacturing of Superheated locos
involved the designers getting
Up-to-speed in their skills, and the
factory crews getting familiar with
new processes.

The Schmidt superheater was patented about 1905, yet production
off assembly lines didn’t get popular
much later, like after 1912…

So, my guess is that the loco was designed about 1906, and built by
about 1908, as saturated, leaving the factory.

I suspect that slide valves were less
expensive, and at 200psi, spool valves were far superior in handling
the boiler pressure, and the lower pressure allowed use of cheaper
slide valves, on the LP cylinder castings

Another factor is whether the valve gear was operated by an early form
of “power-reverse” cylinder, or not.
If built without a power reverse cylinder, it would have had the same
disadvantages for the engineer - and the reluctance to unlatch the reverse
lever —— to hook-her-up, and fight.the task of re-latching the lever while underway.

Spool valves are balanced, so shortening the valve travel was easier for the engineer,
on saturated engines.  The designers, working with a 'saturated' engine, would have seen
the advantages of using easier-ro-operate piston valves on the HP cylinders,
 so that could be one reason for the difference. 

I suspect that air-operated power reverse cylinders didn’t become
manufactured by suppliers until after 1916. Or there about.

W.

Posted from iPhone



Edited 1 time(s). Last edit at 02/12/23 07:25 by wcamp1472.

Wes,
From Richard Leonard's site.....delivered by Alco in 1916 and yes it was superheated.  Description below:
One of two members of class NU-1b, delivered by ALCo in 1916, P&LE compound 0-8-8-0 No. 9090 poses in or near Pittsburgh at an unknown date in the late 1930s. Articulated locomotives were rare on the "water Level Route" New York Central System, and these lumbering Mallets were probably restricted to heavy switching duties on its coal-and-ore-hauling subsidiary. With a low driver diameter of 52 inches and a boiler pressure of 200 p.s.i., they had high pressure (rear) cylinder dimensions of 26x28 inches, and low pressure (front) dimensions of 40x28 inches. They weighed 468,000 pounds and exerted 87,020 pounds of tractive effort. Specifications for the grate area of the NU-1 class are given as 80 square feet, with 5290 square feet of evaporative heating surface and 1235 square feet of superheater surface. No. 9090 was retired and scrapped in 1951. Wayne Koch contributed the image, from an unknown photographer.
Thanks,
Ike