Discussion in 'Steam Traction' started by Foxhunter, Jan 30, 2018.
Is this leading towards "British Exit" as a name?
Quite apposite, given the general chicken-related naming theme seen thus far!
After the bill I've just had for the car, that's entirely appropriate...
Perhaps Pegasus could lead to a 'My Little Pony ' Tie in with some Rainbow smoke generators and strategically placed sparkly stars...
...and the Queen blinks at the light!
The sadly lost "Silver Link" would be an interesting candidate.
My favourite A4, especially in its “silver” livery.
Not quite what I had in mind. There was a version of the A4 that was close to being authored prior to WW2 which was to have a boiler pressure of 275psi. Not quite the 20bar/300psi to be found elsewhere but getting there you might say.
But almost the 280 found elsewhere...
I think it is easy to fetishise the pressure number without thinking it through. Higher pressure theoretically gives more TE, but only if you can use it - a factor that was realised when the Bulleid pacifics were downgraded to 250psi, after it was noted that drivers rarely used above about 200psi on the steam chest gauge. The thermodynamic benefit of higher pressure is fairly small when seen in terms of the overall thermodynamic efficiency of the loco. On the other other hand, boiler construction and maintenance costs go up. Its notable in this country that AFAIK, all the main locos built with conventional, but high pressure, boilers (i.e. > 250psi) were subsequently modified to reduce the working pressure.
Engineering isn't about a top trumps style quest for the biggest number: it is about making rational compromises of competing factors to come up with the best overall design. In that light, generally maintenance costs proved more significant than absolute thermodynamic efficiency, and the restrictions on loading gauge and axle weight limited the gains that could be achieved by increasing power.
All fair points, although I do wonder if the Bulleids would have been driven on part regulator quite so much if their original reversers had been more amenable to short cut-off working. Whenever I've seen actual costs quoted for Bulleid boiler repairs in BR service (pre-downrating) they were cheaper than the equally new LNER A1 boilers, but at least once source claims the steel grade needed for the stays when worked at 280psi was difficult to obtain (which isn't necessarily the same thing as expensive, but if it means your valuable asset is out of service, it soon becomes it).
The one clear advantage of higher pressure in the UK was to allow power with relatively small cylinders. With the Gresley three-cylinder configuration, the inside valve chest trades off against the inside cylinder; if you can make the cylinder smaller (say by increasing boiler pressure) the valves can be bigger. This happened from the A3 to the A4, and compared to the Bulleids the A4 still has quite small valves, despite having bigger cylinders. It's possible that Gresley was looking again at the power-at-speed question and wondering how it could be improved, to allow heavier trains to be worked to the existing high-speed express schedules.
Copper more expensive than steel?
Quite possibly a significant part of it. Also, I'd have to think that by then the significant use of welding in the firebox instead of lap seams would have saved a lot of time in repairs. My impression is that while the 280psi Bulleid boiler was technically successful, even if it was decided the economic case didn't stack up, the 280psi County boiler with a traditional copper firebox was less so.
On the international scene, the adherence of British practice to copper fireboxes right to the end was a bit of a peculiarity. Steel fireboxes were perfectly successful in the UK where the water was good, either naturally or by treatment, and when the fleet was big enough that this was understood (I believe a number of steel firebox Black Fives were built and promptly sent to Scotland). 3402, as an orphan, may have been more troublesome.
Almost brought it back round to V4s there...
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