Steam & Excursion > Boiler pressure

Ok, I need another learning lesson from the experts. This may seem rudimentary but I’m still learning. What exactly does boiler pressure do for a steam loco? I noticed the trend for pressures as the technology “matured” but I’ve never understood what it mattered? Thanks in advance!

Posted from iPhone

In basic terms, higher the pressure = more ton-miles locomotive at the expense addtional water and fuel (coal, oil, etc) and maintenace.

Follow on Q: Generally, what was involved in raising a given loco's pressure? Are working pressures set lower for longer boiler life, so 25-50 psi increase can be made without undue remanufacturing?

Thanks,

* JB *

JP86 Wrote:
-------------------------------------------------------
> Ok, I need another learning lesson from the
> experts. This may seem rudimentary but I’m still
> learning. What exactly does boiler pressure do for
> a steam loco? I noticed the trend for pressures as
> the technology “matured” but I’ve never
> understood what it mattered? Thanks in advance!
>
> Posted from iPhone

Lets all remember that it is the HEAT of the steam that does the "work" in a railroad steam locomotive (the engine is basically a 'heat machine'). Thus, the higher the boiler pressure, the hotter the steam temperature. Even on suterheated locomotives, the high boiler pressure means hotter steam temp into the superheater units, resulting in hotter stem temps out of the superheater and into the valves.

The early understanding of steam’s capabilities were related to pistons
Operating in closed cylinders.

Early, stationary ( fixed) , steam engines used in mines and factories
Typically turned large ‘fly-wheels’
Powering machinery, pumps,
Etc.

In early marine use, paddles
could be either ‘stern-wheelers’
( one cylinder for each end of the
Paddle shaft), or ‘side wheelers’
mounted a-midships, with a central
Cranked shaft attached to a
‘Diamond-shaped, oscillating,
Beam. The engines used a
‘Walking-beam’ arrangement
account the machinery to build
long, accurate parallel guides had yet to be invented.

The ‘walking beam’ converted
the piston’s Back& Forth motion
Into rotary motion of the paddle shaft & it’s crank.

The early uses of the boiler provided the ‘push’ to turn the
Cranks and to turn machinery
In mills and factories...

Early coal mines rapidly filled with ground water— steam powered pumps solved those problems.

All by using shear steam pressure..

To be continued..
W.

Posted from iPhone

HotWater Wrote:
-------------------------------------------------------
> Lets all remember that it is the HEAT of the steam
> that does the "work" in a railroad steam
> locomotive (the engine is basically a 'heat
> machine').

But it's the pressure acting on the piston that does the work. The throttle regulates the pressure admitted into the cylinder - not the heat.

Yes, I know the two are inter related, the same way voltage and current are related to electrical power.

To the OP, I propose an experiment:

1) Get a straw, and put a wad of paper on a table in front of you.

2) Gently blow at the wad of paper through the straw to push the wad of paper away from you. Note the speed of the wad of paper.

3) Retreive the wad of paper, and repeat the experiment, this time blowing as hard as you can.

You've just demonstrated to yourself the effect of higher (boiler) pressure. 

At first, I was surprised by your question. How could someone who's taken even a basic grade-school science class not understand the effect of boiler pressure? Well, a lot has changed in the decades since I went to school.

I also teach reliabilty to engineers and others who develop equipment maintenance plans and schedules. In my class, I used a coffee pot as an example of how to look at functions and functional failures (Failure Mode Effect and Criticality Analysis). Everyone knows how a coffee pot works, right?

Imagine my surprise when I started encountering degreed journeyman-level engineers, many with Masters degrees, who didn't know how a coffee pot worked? I'm not talking about "spark chasing" Electrical Engineers, either. Mechanical, structural, and aerospace engineers who don't have a clue how a Mr. Coffee coffeepot works. To them, making coffee means going through the Starbucks drive through. Evidently the two year lower division"Physics for Engineers and Scientists" and Freshman "Thermodynamics 101" classes don't have time for practical demonstrations like they used to when I was a student.

But I digress... Point is (I guess) "Common Knowledge" has become a lot like "Common Sense"  - an oxymoron.

I hope my experiment proves helpful.

Arved Grass
Fleming Island, FL

Arved Wrote:
-------------------------------------------------------
> HotWater Wrote:
> --------------------------------------------------
> -----
> > Lets all remember that it is the HEAT of the
> steam
> > that does the "work" in a railroad steam
> > locomotive (the engine is basically a 'heat
> > machine').
>
> But it's the pressure acting on the piston that
> does the work. The throttle regulates the pressure
> admitted into the cylinder - not the heat.

Sorry, but I totally disagree! Starting out, yes the pressure does move the pistons, but as the superheating effect on the steam flow increases the temp. of the steam, the increasing heat of expansion increases the "work", i.e. horsepower. For example, when accelerating, as the stroke of the valve is reduced ('hooking up' the reverse gear) the speed of the steam flow through the superheater units is reduced, thus increasing the steam temp. even more. On SP 4449, at 298/300psi boiler pressure, and about 3/4 throttle with the reverse gear "hooked up" properly, the steam chest pressure is about 200 to 225 psi.. Yet the steam temp. is over 600 degrees F, and she is STILL accelerating with the increasing HP resulting from the very high stem temp.


> Yes, I know the two are inter related, the same
> way voltage and current are related to electrical
> power.
>
> To the OP, I propose an experiment:
>
> 1) Get a straw, and put a wad of paper on a table
> in front of you.
>
> 2) Gently blow at the wad of paper through the
> straw to push the wad of paper away from you. Note
> the speed of the wad of paper.
>
> 3) Retreive the wad of paper, and repeat the
> experiment, this time blowing as hard as you can.
>
> You've just demonstrated to yourself the effect of
> higher (boiler) pressure. 
>
> At first, I was surprised by your question. How
> could someone who's taken even a basic
> grade-school science class not understand the
> effect of boiler pressure? Well, a lot has changed
> in the decades since I went to school.
>
> I also teach reliabilty to engineers and others
> who develop equipment maintenance plans and
> schedules. In my class, I used a coffee pot as an
> example of how to look at functions and functional
> failures (Failure Mode Effect and Criticality
> Analysis). Everyone knows how a coffee pot works,
> right?
>
> Imagine my surprise when I started encountering
> degreed journeyman-level engineers, many with
> Masters degrees, who didn't know how a coffee pot
> worked? I'm not talking about "spark chasing"
> Electrical Engineers, either. Mechanical,
> structural, and aerospace engineers who don't have
> a clue how a Mr. Coffee coffeepot works. To them,
> making coffee means going through the Starbucks
> drive through. Evidently the two year lower
> division"Physics for Engineers and Scientists" and
> Freshman "Thermodynamics 101" classes don't have
> time for practical demonstrations like they used
> to when I was a student.
>
> But I digress... Point is (I guess) "Common
> Knowledge" has become a lot like "Common Sense" 
> - an oxymoron.
>
> I hope my experiment proves helpful.

Force = Pressure times Area, pure and simple, no mention of heat. Heat is not doing any work it's the pressure exerted that's doing the work.
Superheating, expansion, all that, provides the thermal energy to allow enhanced expansion of the operating steam. It allows expanion beyond
what saturated steam would allow BUT it's still the pressure in the cylinder that's exerting the force (X area).

Out 

callum_out Wrote:
-------------------------------------------------------
> Force = Pressure times Area, pure and simple, no
> mention of heat. Heat is not doing any work it's
> the pressure exerted that's doing the work.
> Superheating, expansion, all that, provides the
> thermal energy to allow enhanced expansion of the
> operating steam. It allows expanion beyond
> what saturated steam would allow BUT it's still
> the pressure in the cylinder that's exerting the
> force (X area).
>

There are many equations with force in them... Force = mass x acceleration, there's no pressure in there... The one you've picked proves nothing, so the first half of your post is wrong.
The last thing you say about pressure exerting a force is correct.

This is all about energy conversion. Chemical potential energy (carbon bonds) are converted into thermal/heat energy when coal/oil burns. That in turn is absorbed by the water which evaporates. The increase in volume causes an increase in pressure which allows work to be done. It is definitely more correct to say that the heat energy is doing the work, because the heat is the fundamental energy source that is converted to kinetic energy of the locomotives moving parts.
However, in order for that to happen it has to be contained in a fixed volume in order for the pressure to build and to facilitate the work to be done.


 



Edited 1 time(s). Last edit at 06/25/20 14:31 by exhaustED.

Gentlemen,
I first want to thank you for your willingness to educate the Physics neophytes on this Forum  which I am one.
My background is in Electrical Engineering so I try to relate that to the thermodynamics of steam.

It seems to me to call a steam locomotive a heat engine is misleading.  If that were true why are there pressure and not temperature gauges in the cab?
We can move a steam engine with air pressure and very little heat.  Removing the cylinder head and heating the piston accomplishes nothing.
Now don't get me wrong here as heat if certainly required for steam.  The more heat you have (super heat) the more work can be done.
So heat is our energy source.  More heat = more work.  So a given volume of super heated steam will hold a given pressure for a longer period of time.

Please be gentle as I am still learning at a young 68 years of age!

Rgds,
Ike

dad4077 Wrote:
-------------------------------------------------------
> Gentlemen,
> I first want to thank you for your willingness to
> educate the Physics neophytes on this Forum 
> which I am one.
> My background is in Electrical Engineering so I
> try to relate that to the thermodynamics of
> steam.
>
> It seems to me to call a steam locomotive a heat
> engine is misleading.  If that were true why are
> there pressure and not temperature gauges in the
> cab?

For what it's worth, there were quite a number of railroads that equipped their more modern steam locomotives with temperature "gauges" (pyrometers) with thermocouples mounted in the steam flow to a valve. The SP had their GS-4/GS-5 Daylights equipped with Pyrometers mounted in the center of the cab, so that the Engineer could adjust the power reverse gear in order to obtain maximum temperature at the valve/valves.

> We can move a steam engine with air pressure and
> very little heat. 

But you can't move it very fast with any sort of load attached.

Removing the cylinder head and
> heating the piston accomplishes nothing.

Of course not, as with the cylinder head off, there would be no pressure nor heat retention.

> Now don't get me wrong here as heat if certainly
> required for steam.  The more heat you have
> (super heat) the more work can be done.
> So heat is our energy source.  More heat = more
> work.

And there you have it!

  So a given volume of super heated steam
> will hold a given pressure for a longer period of
> time.
>
> Please be gentle as I am still learning at a young
> 68 years of age!
>
> Rgds,
> Ike