Imaginary Locomotives

[melmerby]

32 minutes ago, rodent279 said:

Aren't tubes extruded?

But the pressure is reversed, it is trying to crush the tube, not expand it.

Edited July 12, 2019 by melmerby

[Northmoor]

29 minutes ago, rodent279 said:

Aren't tubes extruded? Would water tube boilers have extruded pressure vessels, or would they be of conventional boiler plate?

Yes but a boiler is under internal pressure, so the circumferential forces are in tension.  A fire tube inside this boiler is under external pressure, so the forces are compressive, where materials like steel are MUCH stronger (eventually they buckle).

I don't know enough about water tube boilers to explain why they're different.

[RLBH]

1 hour ago, AlfaZagato said:

Are either aluminum or titanium, or alloys thereof, suitable for pressure vessels at any serious push?

Titanium is great for pressure vessels - it's the material of choice for deep diving submarines. It's also incredibly expensive and very difficult to work with. A Aitanium-boilered steam locomotive would be a vanity project of the highest order, and I'm not sure how much practical advantage could be gained.

 

Aluminium would probably be a very poor choice for a boiler, because it has a low melting point and loses strength at fairly low temperatures. The kinds of temperatures you'd expect to find in a boiler.

28 minutes ago, Northmoor said:

I don't know enough about water tube boilers to explain why they're different.

It is, apparently, possible to extrude boiler tubes in such a way that the right structure is created. Metallurgy isn't a strong suit for me so I've no idea how it's done, though!

 

The other issue with an extruded boiler would probably be that it would be difficult to make it tapered. The machinery for extruding a 5' (give or take a foot) boiler would also be fairly expensive and specialised, so you'd need to be confident of building a lot of identical boilers.

[Compound2632]

Apropos of Corbs' Crosti 5MT, the latest Midland Railway Society Newsletter has a short item on the FS 741 Class 2-8-0s of which I was previously ignorant but which I gather contributed to BR's interest in the concept - apologies if this has been discussed before but I don't recall seeing mention. Apart from the usual unusual appearance of the front end, these engines had a single chimney amidships on the starboard side. What I don't understand with this arrangement is how draught is maintained through the boiler tubes?

 

Link to Wikipedia article on the 741 Class.

[Gibbo675]

7 hours ago, Northmoor said:

Yes but a boiler is under internal pressure, so the circumferential forces are in tension.  A fire tube inside this boiler is under external pressure, so the forces are compressive, where materials like steel are MUCH stronger (eventually they buckle).

I don't know enough about water tube boilers to explain why they're different.

Hi Northmoor,

 

In a fire tube boiler the shell, that being the inner and outer fireboxes and the barrel are manufactured in such a way that they become a self supporting structure. The flat sections firebox areas are supported by stay bolts and the tube plates are effectively stayed by the tube bank and also the longitudinal stays, other areas have ring compensators, doubling plates and various types of girder brackets.

 

The result is that no part of the entire structure may move independently to any pother part except by way of minor flexing due to material elasticity. Over time, anything between five and fifteen years, such movement due to vibration will eventually create stress grooving leading to the cracking of the boiler plates.

 

Typically a fire box outer shell will be .625" thick with the inner being .375" if from steel and .625" if from copper. Boiler barrels range from .625" to .875" mainly dependant upon diameter rather than working pressure for the working pressure are generally of an envelope.

 

The outer shell of a boiler may be within reason be any thickness required to hold any pressure that the design brief and weight restrictions may specify, however the inner firebox must transmit the heat from the fire into the water to create the steam. It is this function that dictates plate thickness as the conductivity of the material used has quite a bearing upon the maximum service pressure utilised. As noted above, as new copper fireboxes are .25" thicker than steel boxes, the reason is the relative strength of steel of copper and the conductivity of steel therefore a copper box gives far better conductivity despite being thicker than steel along with a weight penalty.

 

At higher service pressures due to the differential  expansion rates of copper and steel, which is exacerbated with larger fireboxes copper is ruled out mainly for that reason with steel being the preferred materiel. When in service there is quite a temperature gradient between the outer shell and the inner fire box which is further to the previous point. In using steel for the fire boxes of the 280 lb MN and WC pacific boilers, Mr Bulleid was not quite as thick as his locomotives were box shaped !

 

In the case of water tube boilers there were generally a series of drums connected to each other by the water tubes, the combustion gasses passing around the tubes rather than through the tubes as with locomotive type fire tube boilers. As noted in previous posts the smaller diameter of the drums and the tubes allow for higher working pressures due to the fact that the material thickness may be reduced despite higher service pressures.

 

Water tube boilers are also generally more suited to sustained constant outputs for long service periods which reduce greatly cyclic stresses upon the boiler unlike the service conditions of railway work where there are greatly differing service demands to which the fire tube locomotive type boiler is specifically designed to cope with.

 

The drums are supported within a larger containment shell and are free to move for expansion. This arrangement is fine for stationary and marine applications for there is little to no vibration to the structure which would disturb the fixing of the tubes to the drums but is rather unsuitable for locomotive application for the same reason. The containment shell is very often lined with refractory bricks which do not take well to vibration either further complicating locomotive use.

 

This is the very reason that Gresley's 10000 was ultimately a failure, for should any of the tubes leak then the boiler has to be cooled, drained and the access plates of the drums stripped out to allow access to re-seat the tubes into the drums. In a locomotive type boiler leaking tubes may be attended to easily with the boiler full of hot water so long as the fire is out and the pressure released, access to the tubes being through either the fire hole or smoke box door.

 

Below a diagram of a water tube boiler similar in arrangement to the LNER's 10000:

 

 

 

As for the forces to destroy a steel tube, it will take considerably more to burst a tube than it will to collapse one although I cannot recall by what degree the differential is.

 

Should anyone have any further questions then do ask,

 

Gibbo.

[Gibbo675]

19 minutes ago, Compound2632 said:

Apropos of Corbs' Crosti 5MT, the latest Midland Railway Society Newsletter has a short item on the FS 741 Class 2-8-0s of which I was previously ignorant but which I gather contributed to BR's interest in the concept - apologies if this has been discussed before but I don't recall seeing mention. Apart from the usual unusual appearance of the front end, these engines had a single chimney amidships on the starboard side. What I don't understand with this arrangement is how draught is maintained through the boiler tubes?

 

Link to Wikipedia article on the 741 Class.

Hi Compound,

 

The exhaust steam from the cylinders is routed to the chimney on the side of the locomotive and it draws the fire in just the same way as any other locomotive except that the flue gasses pass through the boiler as normal and then through pre-heating drum. The reason the chimney is on the side is that the flue gasses go from the fire box to where the smoke box usually is before then being routed down and into the pre-heating drum under the main boiler. As the end of the pre-heating drum stops short of the firebox that is where the chimney is sited.

 

 

Gibbo.

[Compound2632]

Gibbo, thanks, very clear. I'm grateful for your patience in dealing with the enquires of folk such as I who lack your first-hand expertise.

 

It's a bit like a folded-up version of Webb's combustion chamber boilers for the Greater Britain &c classes, except that in that case the water formed a single volume, so the second set of tubes were rather less effective.

Edited July 13, 2019 by Compound2632

[Gibbo675]

1 hour ago, Compound2632 said:

Gibbo, thanks, very clear. I'm grateful for your patience in dealing with the enquires of folk such as I who lack your first-hand expertise.

 

It's a bit like a folded-up version of Webb's combustion chamber boilers for the Greater Britain &c classes, except that in that case the water formed a single volume, so the second set of tubes were rather less effective.

Hi Compound,

 

Those boilers were a nonsense form the start as any flame that enters a tube will be very soon extinguished for there will not be enough oxygen by volume to support combustion. This is the reason for fitting fireboxes with brick arches to ensure that all of the volatile fractions are fully combusted before the gasses enter the tube bank. Any locomotive that produces smoke at the chimney has either been over fired or has not enough secondary air to support full combustion. Combustion chambers were added to some designs to both extend the path of the flame and also provide a larger crown sheet, the area of which is relative to the amount of steam generated.

 

If you take the discussion back a few post to GWR boilers that do not steam well on Yorkshire coal as opposed to Welsh coal. It is not about whether or not the fuel type is better or not, more that Yorkshire coal, which is bituminous, has a greater degree of volatile fractions known a free carbons rather than the Welsh coal types that are more anthracite having far less volatile fractions. The Welsh coals therefore require comparatively deeper fires with a lot of primary air flow rather than boiler designed to burn bituminous coals that have less reliance on primary air flow and more secondary air flow and generally larger combustion chambers. The chimney and blast arrangements of GWR locomotives are also different to allow for the smoke box to produce sufficient vacuum to pump the air flow through a deep fire rather than through and over a thinner fire.

 

As an aside I would note that the LMS 4-6-4 and 4-8-4 types dispensed with the combustion chamber as the crown sheets would have been sufficiently large in area to provide enough steam raising capacity and also allow a long enough tube length to allow for suitable super-heating elements to give the required degree of super-heat.

 

Gibbo.

[melmerby]

Many Crostis had the preheater alongside the boiler but with the UK width restriction, the under the (smaller than normal) boiler position was where it had to go.

Strictly speaking no smokebox chimney was required but the UK and a few others kept one for lighting up

[rodent279]

Are there any Crosti boilered locomotives operational still? I know there are no Crosti  9F's still extant.

I'd be interested to hear what they sound like.

[Corbs]

Yes, this class 741 in Italy still runs.

 

[RLBH]

Looking at unreasonably large locomotives, the Durrant 4-8-4 and 2-12-2 and the LMS 4-6-4 and 4-8-4 make an interesting comparison.

 

The LMS boiler has a 6'10" diameter and (scaling from an unscaled drawing like a naughty boy) a length of 20'3", fired by a 70 square foot grate with no combustion chamber. Durrant has a 7'3" diameter boiler of 19'6" length, fired from a 75 square foot grate with a combustion chamber. The Durrant boiler has a 7% larger grate, 8% larger capacity, but ought to have 12.5% more gas area. It ought therefore to be able to supply somewhere between 7% and 12.5% more steam - though being shorter, the superheater temperature might be lower.

 

Of course, the LMS locomotives have larger cylinders intended to have a maximum of 65% cutoff to reduce tractive effort, whilst still being able to generate power at higher speeds. The claimed goal was 40% more tractive effort than a 4-6-2 Coronation at 60mph - the larger cylinder diameter and increased boiler pressure gives 35%, the other 5% from smaller drivers. The cylinder volume and grate area on the big LMS locomotives are in similar proportion - accounting for increased pressure - to other British locomotives of modern design.

 

The Durrant locomotives, meanwhile, are fairly small-cylindered machines working at 85% cutoff. That gives his locomotives 5.7% more tractive effort for a given driver size. He cheats, of course, by using 6' drivers on his express passenger 4-8-4, which as a result is pretty comparable to the 5'6"-drivered LMS mixed traffic 4-8-4. But the small cylinders mean that at speed his locomotives become limited by the front end - the express 4-8-4 being only 15% more powerful than a Coronation, considerably less capable than the LMS locomotives.

 

In short, I think they probably take the 'big boiler' school of locomotive design a bit too far, and wind up with more boiler than they can use. They'd be perfectly fine, I think, with a 55 to 60 square foot grate, and a more reasonably sized boiler.

 

To actually use the big Durrant boiler to its' full effect, you need a third cylinder - probably three 21"x28" cylinders, compared to his two 21"x30" cylinders. To keep adhesion within reasonable bounds, use the same 65% cutoff as the LMS locomotives. The result is a good match between boiler and cylinder capacity, and (incidentally) perhaps the most sure-footed British express passenger locomotive for a very long time, which is incidentally 64% more powerful than a Duchess. Oh, and the 2-12-2 version offers about twice the power of a 9F 2-10-0, so that ought to take care of your express freight work.

 

Although.... the fact that we're having to limit cutoff probably indicates that we're a little past the practical limits of British railway infrastructure. Tractive effort isn't increasing at the same rate as power, so the trains are getting faster more than they're getting heavier. It ought to be possible to achieve comparable performances with these locomotives - the LMS or Durrant ones - as with diesel locomotives. Matching early electric performance might just be possible, if the boiler and front end design is really good. But they'd do so at vastly greater cost than just buying diesel locomotives, and are clearly at the limit of their growth - whilst electrification is equally clearly just at the beginning of its potential.

[brack]

It boils down to the same issues we arrived at in the garratt discussion: although there was clearly further development possible for british steam loco design, until you address the problems with unbraked freight stock, 3 link couplings, loop lengths and so on there is not much of a place for high powered, more efficient designs. The express passenger types always seem more plausible because for passenger work some of those issues have been addressed, but for freight work there isn't much point in trying to do better with the existing stock.

[The Johnster]

This is the vital point; a 9F is probably the loco that would have been needed for another 20 years after 1954 had it not been for the diesels, and there's no point in building anything much bigger for freight work.  In fact it would be capable of doing the class 44/5/6/7/52 60mph work even later than that.

 

If you play the 'dieselisation never happened until 1975' game, you can argue for a 4-6-4 or 4-8-4 3 cylinder based on a 'Duke of Gloucester on steroids' and with the biggest diameter drivers that could be got under the boiler capable of the 95mph cruising that featured on the GWML and ECML with type 4 diesels, or the 100mph+ work of the Deltics or double headed 50s north of Crewe on the WCML pre-1974.  This would be doing work well beyond hand firing and oil firing or mechanical stokers would be necessary.  The period 1965-1975 was marked by timetable accelerations everywhere as the railway competed with motorways; the Cardiff-Paddington trains for instance were speeded up by somewhere between 25 and 30% over that decade, with a combination of MAS signalling, load limiting and easing of speed restrictions through junctions, at the same time as the stock went from mk1 to the various 2s, eventually including eth and airco which took about 400hp off the loco; in short, despite the limited loads, more work was constantly being demanded of the locos, and this was the case more or less everywhere; only where 25kv electrification had been installed were these increasing workloads properly coped with.

 

Fast container traffic appeared from 1969 onwards, and had there been no type 4 diesels to pull it might have justified a 2-8-4 capable of sustained running at 75mph with 1,000 ton+ trains, and these would have handled the very heavy sleeper and motorail traffic as well.  

[rodent279]

4 hours ago, Corbs said:

Yes, this class 741 in Italy still runs.

 

Wow, quite the beast! Shame no Crosti 9F's survived. I'd be tempted to contribute to a rebuild if anyone ever got one going.

[PenrithBeacon]

7 minutes ago, rodent279 said:

Wow, quite the beast! Shame no Crosti 9F's survived. I'd be tempted to contribute to a rebuild if anyone ever got one going.

You might not get anyone to drive the damned thing

[Corbs]

Musings...

 

I've no idea what this would be for.

 

Stanier 4-cylinder front end with shortened smokebox, similar to 'Coronation' boiler but with a narrow firebox fed by mechanical stoker. 6'2" driving wheels.

Class 8MT. Shorter than the Pacific, probably not as powerful, but with more driving wheels.

 

More of a stretched 'King' (with smaller wheels) than a 'Duchess'. A 'Jester', maybe?

 

 

or in Maroon if you prefer

 

Edited July 13, 2019 by Corbs

[Budgie]

1 hour ago, The Johnster said:

 

If you play the 'dieselisation never happened until 1975' game, ...

 

I'd prefer to preface that with 'Nationalisation never happened'. It might be interesting to see how the other two of the big four did it. (I am assuming that the LMS would have gone on to use some of what became the BR standards, and the SR electrified everything.)

[Compound2632]

Or, "The Great War Never Happened" - widespread main-line overhead electrification by the 1920s, starting with Derby - Manchester and York - Newcastle.

[RLBH]

8 minutes ago, Compound2632 said:

Or, "The Great War Never Happened" - widespread main-line overhead electrification by the 1920s, starting with Derby - Manchester and York - Newcastle.

That's a very interesting possibility - Raven at the NER was very keen on electrification, of course, and the NER S3 (LNER B16) 4-6-0s were explicitly intended for secondary routes that wouldn't justify electrification.

 

What the Midland would do would be quite interesting - of course, they and the LBSCR were both using 6.6kV 25 Hz AC electrification, so that would presumably have become fairly widespread. What the locomotives would have looked like is anyone's guess, I think.

 

The LYR was also quite interested in electrification - at one point having three completely different systems on the go. I believe that the 1,200V side-contact third rail system was envisaged as their main line electrification approach, with Manchester-Bury seen as the prototype.

 

The LNWR, meanwhile, was seemingly actively hostile to electric railways. It was LNWR influence that killed off LYR and Midland interest post-grouping.

[34theletterbetweenB&D]

 

8 minutes ago, Budgie said:

...I'd prefer to preface that with 'Nationalisation never happened'...

If the railways had been allowed to run as commercial outfits - with the cash owed (by us) paid by the government to make good the wartime deficit - then it is reasonable to expect that they would have 'looked around'. Dieselisation had made great strides in North America, and the rebuilding of the European railways would have informed managements here, and encouraged electrification. 

 

(And more. I would expect that railway owned bustitution and road freight transport for branchlines that could never pay might have moved faster than Beeching, and further airline involvement for journeys that could not be made by rail would have figured too.)

 

So finally imaginary locomotives in consequence. The famous 1949 conference to adopt four existing designs as standard steam locos as the only designs to be constructed, while a combined diesel electric development programme is set underway.  Possibly more of the common multiple working of diesels as seen in North America. Some multi-voltage electrics because earlier electrification meant more of a start on DC systems before high voltage AC became a proven option. A focussed assault on the high speed train to rival the Japanese Shinkansen, tiltingly operational on all 25kV AC trunk routes by the mid-1960s. Development continues aggressively and by 2000 the UK railways boast three hour maximum journey time between any 2 cities with more than a quarter million population...

[Clive Mortimore]

8 minutes ago, RLBH said:

That's a very interesting possibility - Raven at the NER was very keen on electrification, of course, and the NER S3 (LNER B16) 4-6-0s were explicitly intended for secondary routes that wouldn't justify electrification.

 

What the Midland would do would be quite interesting - of course, they and the LBSCR were both using 6.6kV 25 Hz AC electrification, so that would presumably have become fairly widespread. What the locomotives would have looked like is anyone's guess, I think.

 

The LYR was also quite interested in electrification - at one point having three completely different systems on the go. I believe that the 1,200V side-contact third rail system was envisaged as their main line electrification approach, with Manchester-Bury seen as the prototype.

 

The LNWR, meanwhile, was seemingly actively hostile to electric railways. It was LNWR influence that killed off LYR and Midland interest post-grouping.

Except of course when it came to North London and its investment in the Underground Railway. 

[Corbs]

One can dream of the prototype Deltic's design and aesthetics being carried on to a production run, bought by the LNER and painted garter blue or even apple green...

[PhilJ W]

13 minutes ago, Corbs said:

One can dream of the prototype Deltic's design and aesthetics being carried on to a production run, bought by the LNER and painted garter blue or even apple green...

Or silver-grey?

[The Johnster]

One can dream. 

 

There were hints as to where the big 4 would have gone in the 50s.  The Southern was committed to 750v dc and the Next Big Thing was the Kent Coast scheme.  The LNER was heading in the direction of 1,500v dc overhead, and the LMS was investigating diesels with the involvement of EE, as was the Southern for off-electric use.  The GW was playing with gas turbines, but was stuck in a 1920s mindset. 

 

By the late 50s, chances are that diesel loco and dmu manufacturers would have been hardselling to the railway companies, and many of the 1955 scheme locos would have probably appeared anyway, the successful and reliable ones being those that made the cut.  An un-nationalised railway in the 60s and 70s would have looked a lot like BR did.

 

The UIC European standard of 25kv ac overhead electrification would have been highly influential by the early 60s, so, apart from the Southern, again it would likely have played out similarly to actual events.  

 

The biggest ‘what if’ is probably an ECML/KX-Leeds 1,500v dc development, and we know pretty much what it would have looked like from the Manchester-Sheffield and GE schemes.  The GW would probably have stayed with steam until demand for eth and airco precluded it in the 70s, when outsourced diesel electrics would have superceded the Castles; the hydraulics would never have happened!

 

All IMHO and academic, but good fun!