The early LMS Stanier locomotives and high superheat [ex 92214 Topic]

From the 92214 topic... http://www.national-preservation.com/threads/br-9f-92214.443723/page-7#post-1132577

The early LMS Stanier locomotives and high superheat... Its something I don't understand.

Its uncontroversial that high superheat brings increased thermal efficiency at the price of increased lubrication challenges and other increases in maintenance overheads. That was known before WW1, and its documented that Churchward used moderate superheat because he considered the cost/benefit analysis (in modern language) to be better at that time.

Its also, I hope, uncontroversial that the GWR Stars, Saints and Castles steamed well and were more than adequate performers with moderate superheat.

So why did the Jubilees and the other Stanier locomotives on the LMS need high superheat in order to steam adequately?

I can well understand that the high superheat should and did bring them superior thermal efficiency at the price of increased overheads.
What I don't understand is why they were poor steamers with moderate superheat when their cousins demonstrably were not.

 

Different coal. Which is why the GW locos needed higher superheat later. If you look at the difference in performance between the 28xx on Welsh coal and on standard coal in the 1948 exchange trials you'll be on the right track. A loco set up to steam on hard coal will steam OK on Welsh coal if fired correctly, but the reverse is not so easy.

It wasn't just the superheat, either, there were blast-pipe issues as well. Stanier had no info on three cylinder engines, and indeed after the troubles with the Jubilees he built no more.

In the 50's under Cook various Swindon-derived blastpipe proportions were tried on LNER 3-cylinder engines and they didn't work terribly well there either, though apparently they did make a tremendous noise.

 

and they looked nice as well (or at least one did, for a brief period) A superlative example of how the unquestionably excellent standard of Swindon engineering led to a superciliousness towards the designs of others,
71000's chimney being another...

 

The reasons for the early LMS tapered boilers' problems are numerous and complex, coal is the start point. Despite rumour, the GWR did not use best Welsh steam coal exclusively, but it did use good quality coal - higher than that supplied to LMS locos. Secondly, GWR enginemen were trained to a higher standard than LMS men, who merely got the 'pick it up as you go along' system. Thirdly, low superheat depended on high boiler pressure and a GWR fireman would keep the boiler within 5psi of blowiong off. If it dropped 10psi the driver would be giving him hard looks, while at 15psi, words would be said. An LMS engine with high superheat was less sensitive to pressure falls unless a big effort was needed, and an LMS fireman with, say, a 225psi boiler, would not be unduly worried if pressure fell to about 180psi. I've done it myself!

Realistically, the early LMS boilers had a lot of problems with ratios: tube length to diameter and tube cross-sectional area to grate area. Low superheat did not inhibit steaming - there are many good-steaming saturated boilers - but it did exaggerate poor steaming. The two cylinder engines, even with the poor ratios, performed adequately; it was the three cylinder 5XPs (Jubilees) which had the serious problems. These were compounded by incorrect blastpipe/chimney dimensions. And although Stanier had no prior experience of three cylinder locos, the drawing office did - witness the Royal Scots and Baby Scots. And I doubt that the CME was so involved with the detail design as to specify the draughting proportions. They were, however, a long way out. The attempts at curing these engines' poor steaming revealed the poor boiler ratios, and so while a problem had not been perceived in the two cylinder engines, the fresh knowledge was applied to these also.

See 'Raising Steam on the LMS' by Arthur F. Cook 9199) RCTS ISBN 0 901115 85 1. You won't get a better explanation!

 

The LNER had a similar approach to boiler pressure to the LMS, having locos generally with a much larger boiler capacity relative to power than the GW. The Ivatt approach as opposed to the Churchward. The Stephenson design steam engine is an amazingly flexible beast, so flexible that when some unfortunate set of circumstances causes a design to 'fail', it is difficult to sort out which of the numerous empirical interactions in the design is the critical one.

The Royal Scots of course were not designed 'in house', and the mechanism of the Baby Scots was just a copy. Very good engines though.