Steam & Excursion > Boiler pressure.

A friend of mine asked me what determines a locomotive’s boiler pressure and how does it effect how the locomotive itself works. I didn’t know how to answer it because I don't know. So here’s where I turn to the experts of TO!

Posted from iPhone

What determines it ... you mean, how does
the designer decide what pressure to use?

Higher pressure means smaller cylinders for
a given tractive effort, which is an advantage.
Question is, is it enough of an advantage.
None of us knows. Fans figure higher pressure
is more advanced -- maybe it is, but we
don't pay the bills, and we don't know what
works out best overall.

When P&LE got its first 2-8-4s around 1946,
they had 230 psi. Us fans can't explain that--
apparently the RR figured the low pressure
would save them money.

High boiler pressure advantage:  More power, higher temperature and therefore greater efficiency. 

High boiler pressure disadvantage: locomotive more likely to blow-up; heavier equipment, higher working temperature resulting in greater wear. 

Narniaman,

Boiler  pressure has nothing to do with likelyhood of blow-up, severity maybe, but probability is not increased.
The safety factor of 4:1 increases the design structural strength, commensurate with the increase in BP.

Blow-ups of boilers built after 1900, due to “structural deficiencies” are non existent.  
Older, pre-1900,  styles of fabrication ( lap joints, etc) can cause structural failures, and blow-ups.

Virtually every boiler explosion in the 20th century is the result of crews letting the water get so low over the crown sheet that it softens , sags, strips-off of the crown stays, and causes sudden release of steam and water into the firebox...is solely the result 
carelessness of the attendant crews.

Steel at 1000 deg F, has a barely visible, deep red color ( in a darkened setting),  at that temperature its streangth is 8-times weaker than when covered with boiler water.   What happened to that 4:1 safety factor?

Please try to stay with the facts of boiler engineering and boiler design.

Wes Camp

 



Edited 1 time(s). Last edit at 04/14/19 18:31 by wcamp1472.

wcamp1472 Wrote:
-------------------------------------------------------
> Narniaman,
>
> Boiler  pressure has nothing to do with
> likelyhood of blow-up, severity maybe, but
> probability is not increased.
> The safety factor of 4:1 increases the design
> structural strength,p commensurate with the
> increase in BP.
>
> Blow-ups of boilers built after 1900, due to
> “structural deficiencies” are non existent.
>  
> Older, pre-1900,  styles of fabrication ( lap
> joints, etc) can cause structural failures, and
> blow-ups.
>
> Virtually every boiler explosion in the 20th
> century is the result of crews letting the water
> get so low over the crown sheet that it softens ,
> sags, strips-off of the crown stays, and causes
> sudden release of steam and water into the
> firebox...is solely the result 
> carelessness of the attendant crews.
>
> Steel at 1000 deg F, has a barely visible, deep
> red color ( in a darkened setting),  at that
> temperature its streangth is 8-times weaker than
> when covered with boiler water.   What happened
> to that 4:1 safety factor?
>
> Please try to stay with the facts of boiler
> engineering and boiler design.
>
> Wes Camp

==========================
You are, of course, correct. 

Perhaps I should have said that the increased pressure requires more sturdy and therefore heavier construction to maintain an adequate designed safety margin. 

I was under the impression that structural steel steel retained about 60% of it's strength at 1000 deg F, and it wasn't until it reached a temperature of 1400 degrees or so that it's strength dropped eight-fold. Is the steel used in locomotive more sensitive to heat? 



 

Do you care to write one of your extensive responses Wes? I enjoy learning from them!

Posted from iPhone

The 1,000 deg figure is illustrative purposes, only.

With 3000 deg fires on the grates, a dry crown will have no cooling —— uncooled, the crown will go to 3,000 deg, but will probably strip off of the threads & blow-up well before that.

It ain’t about the text book charts—— boilers are confined to, & respond to the laws of physics.
In the 20th century, Low Water is solely the cause of boiler explosions, NOT structural, or design deficiencies.
Low water is the result of a series of bad actions committed by humans.  Unfed, fires tend to go out, fireboxes cool down.
Continuing to feed fuel to a boiler that is dry above the crown, is the ultimate act of stupidity.

For a recent account of a loco boiler explosion, look up ( on the wwweb) the NTSB mid-1990s report on the boiler explosion on the Gettysbutg RR.  In the report’s record of the testimony, by the employees of the RR, when asked to describe the proper way to test a boiler sight glass, NONE of the staff of the Gettysburg RR, could describe the proper procedure, when giving testimony to the investigators...

That type of ‘training failure’  must be eliminated, going forward...

Wes

to be proofed , yet

 



Edited 2 time(s). Last edit at 04/14/19 18:25 by wcamp1472.

Question 1 - If the locomotive is hot and the bottom try cock is dry, are you in trouble?
Answer - Not for long.

Out

Ha!💥

That’s SICK!😆

Wes

Posted from iPhone

It is my understanding that, over the life cycle of the steam locomotive design, better steel alloys also had higher Yield strengths.  These boilers could run at higher pressures without being excessively thick and heavy.

I have also heard that when certain steels became "War-strategic" in WW2,  some locos had to go back to lower pressures becuase the good stuff was not available.  This might explain  the 1946 loco?

Sentence 2, above...
What about clarifying the:  “might explain the 1946 loco..”?

W.



Edited 2 time(s). Last edit at 04/15/19 12:50 by wcamp1472.

Wes:  Second Poster TimZ had this comment:     When P&LE got its first 2-8-4s around 1946,  they had 230 psi. Us fans can't explain that--apparently the RR figured the low pressure  would save them money.



Edited 1 time(s). Last edit at 04/15/19 12:50 by Kimball.

Kimball Wrote:
-------------------------------------------------------
> It is my understanding that, over the life cycle
> of the steam locomotive design, better steel
> alloys also had higher Yield strengths.  These
> boilers could run at higher pressures without
> being excessively thick and heavy.

I don't recall reading about any War Production Board restrictions on steel for steam locomotive boilers built during WWII. There were however, restrictions on the material used in light weight rods needed for those locomotives built with roller bearing side rods (such as the Santa Fe 2900 class 4-8-4s).

> I have also heard that when certain steels became
> "War-strategic" in WW2,  some locos had to go
> back to lower pressures becuase the good stuff was
> not available. 

Again, I've not read anything about such restrictions involving boiler steel. As I mentioned, above, there was restrictions by the WPB on light weight materials needed for roller bearing equipped rods.

This might explain  the 1946
> loco?

 

As I recall the SFe 3765 class (maybe 3775 class too)
had nickel steel boiler shells -- the 2900s were built
with straight carbon steel, which means the shells
had to be thicker/heavier with the same 300 psi.
No idea whether WPB prohibited nickel steel --
maybe SFe decided nickel steel was a mistake
in any case.

One point to add to this discussion; one of the key factors involved in limiting boiler pressure on steam locomotives is essentially based upon the design of the machinery, i.e. the cylinders & running gear. Higher boiler pressure means higher heat which means higher horsepower at the cylinders. If the running gear machinery is NOT properly designed for such high HP, then problems will develop sooner with rod bearings and other components within the machinery. I prime example being the famous KCS 2-10-4 locomotives that had 310 psi working boiler pressure, except the running gear machinery was apparently not robust enough to withstand such high HP. The working boiler pressure was subsequently lowered to 300 psi.

Jack, You don’t SERIOUSLY believe that BS, do you?
In my opinion, what you stated is absolutely nonsense.

HP refers to high rotative speeds, BP concerns affect low-power, torque-related issues.
And a mere 3% change has NO realistic, proportional relationship to a serious reduction of strain on the driving parts.
A greater strain is applied by sanding the rail——during starting.
(Safety valve settings are used to limit the wheel slippage, has nothing to do with ‘safety factor’ of the boiler design.)

Don't encourage perpetuating non-logical, non factual thinking.
Maybe, if they were truly concerned with over-stressing the machinery, they’d go for more than a 10% BP, reduction..in order to lower the HP.

But again, that has more to do with excessive starting forces ( bending and breaking stuff), than with high speed running, where POWER comes into play.
If the running gear’s integrity was a concern, you’d lower the maximum allowable speed, by a more considerable amount than 3%.

Wes

Well, I guess I was taught incorrectly by some of the "old heads".

And starting forces are only limited by the trailing tonnage, a caboose hop on a 310 psi locomotive isn't any more damaging than
one on a 230 psi locomotive. You'll do a lot more damage when you reach the adhesion level and go into slip.

Out



Edited 1 time(s). Last edit at 04/15/19 14:10 by callum_out.

Maybe KCS just didnt see any benefit to that extra 10 psi. How much more work can just that small percentage increase do anyway ? They might have put the pencil to it and found out just how much more coal it took to get that extra 10lbs. And it probably pulled the same weight train just as well at 300, maybe with just a little less wear & tear.

Posted from Android

What reading I have done indicates the KCS engines would tear up the track starting. Too much engine for their run down tracks:)

Posted from iPhone