Bulleid Pacifics - Past or Present

But the quote from OVSB says that the reason was to improve water circulation and give added security against overheating of the crown plate. I can understand that there will be a convection current in a thermic syphon causing cooler, lower water to go into the syphon, be heated and rise up to the top and over the firebox crown. But would that happen if the water level was below the exit from the syphon ie below the firebox crown, the convection current "circuit" would be broken?
....should this discussion be in the Bulleid Pacific thread?

 

Presumably the water in the thermic syphons would be boiling so, even if the water level is just about down to the crown, a mixture of water and steam would come out of the top. That is certainly not a situation that you want to get into, but if you do the syphons could give you just a little longer to sort things out than without them.

 

Any boiler will have maximum and minimum water levels for safe operation. If the limits are too close to one another it would make the boiler difficult to manage, and if this is the case I think thermic syphons would make it safer. However I don't know if this is the case for the Bulleid. I suspect it was just a case of "opinion engineering" at the time.

 

On the Locomotive Performance & Timing thread

One reason for the rebuilding was to achieve more consistent performance of the valve gear, but what else changed at the front end? Did the conventional Walschaerts give better events than the chain-driven gear when the latter was working correctly?

 

Hi MellishR
I believe that with the original Bulleid design, there were two chains and a scaled down Walschaerts - with a step up 3:8 lever, to give the correct valve travel. There is obviously more opportunity for "Lost Motion" even when new, due to the running clearances in all the various pins and links. At higher speeds this gave rise to the more erratic valve motion - not helped by the steam reverser "creeping". Thus these valve events would reduce the cylinder efficiency. I do not think there were any internal changes to the pistons and valves (apart from the external drive rods to the valves) on the rebuilds.

 

On rebuilding the middle cylinder was replaced with a new cylinder with inside admission valve, the outside cylinders with outside admission were retained.

 

On GSN we're planning to replace the middle cylinder with the "original" design (technically it's the series 3 middle cylinder we'll be using, though the differences are subtle). The coal consumption of the original design is something we'll have to factor in financially, though if we can keep the reverser and valve gear in good condition to prevent creep, that should help with the efficiency.

 

What are you doing about the oil bath? I know the originals were welded to the frames and the natural movement of the latter caused the oil bath welds to fail and consequently leak. I’m not an engineer but would riveting provide a little bit of movement to avoid this?

 

Would advocate a suspended tub with a combination of fixed and flexible attachment points and 'Brush' seals where the rods come in... but then I can never quite figure out the extent of the thing or how the thing was built / attached from the drawings/ pic.

 

It's TBD at the moment

 

I believe there were continued developments regarding oil bath sealing in the 1960's on BR, and obviously, there are the experiences of the "original" WC/BBs in preservation times. I'm sure the recent knowledge is being shared within the Bulleid locomotive fraternity.

 

It is being; we aim to learn from our light Pacific brethren instead of simply replicating the design from the 1950s.

 

Fuel consumption is tricky; if you look at the bulletins, adjusting for the calorific value of the coal used, gives somewhere between 5 and 10% difference in the amount of coal needed to raised the same amount of steam.

 

Do I recall specific mention of problems associated with welding steel of different section as much as geometry?

Have any significant mods be made to the oil bath on remaining light pacifics in original condition?

One slightly off-piste. AIUI, Bulleid's original preference was for MN valve gear to be driven by Cardean shafts, with the idea put paid to by wartime exigencies. Does anyone know whether any design work was undertaken, before the chaim drive was settled upon?

 

Not off-piste at all in a thread called Bulleid Pacifics - Past or Present
I have muttered before that I consider it a shame that the project aims to reproduce (more or less) the troublesome arrangement that Bulleid had to adopt because of wartime conditions rather than what he wished to use.

 

I take your point over the valve gear and whilst the 21C11 project will be expensive enough, I wonder what the costs, as well as technical challenges, would be of adopting a totally new way of driving the valve gear.

 

Maybe the P2 Locomotive Company would be of help there? They may be using poppet valves rather than piston valves, but they will still need to design and make the cardan shaft arrangement, which will be a good part of the valvegear.

Richard.

 

It's interesting to note that while Bulleid was trying out a new valve gear type, with the commendable aim of reducing man hours and manpower on shed, Thompson's A2/2 Pacifics were being built in almost the same setup as the rebuild Merchant Navies and BoB/WC would be around 8-10 years later (the main difference being divided drive versus driving onto a single axle).

I've been looking at some new primary evidence for the original Bulleids and I am trying to work out in my head whether I think it shows them in a better light. The interesting thing is the emphasis on reduction in time on shed requiring maintenance/repairs. Did this play out in reality? I think it did actually.

I had always questioned the oil bath/chain driven valve gear in wartime prior to writing the Thompson book; during the development of that book I saw it in a much better light. I am now getting to grips with understanding the design better and frankly it's gone back onto the side of "genius - but" as opposed "why bother trying that?"

Valve gear choices ultimately seem to have been derived from real world necessities on the one hand and an innate design to innovate on the other. Where choices of valve gear look more conservative it normally comes from understanding what valve gear is meant to be doing on a daily basis rather than striving for one off performances.

Bulleid's choice of chain driven valve gear crosses those boundaries, clearly. The desire to innovate came from a real understanding of the issues encountered in wartime, and the needs of the railway for future traffic (there's no doubting that the light Pacifics really came into their own in BR days, for instance). The objectives of the design's genesis I would say were met. Should they have been rebuilt? In hindsight the rebuilt Bulleid Pacifics (Jarvis Pacifics?) show what could have been done ten years before very easily and there's no doubting of their excellence.

I can't shake the feeling that in rebuilding them, the advantages of the originals derived from good intentions was lost. I don't think major alterations to the setup would have been necessary, just a slight change in design and material spec required.

(Steam locomotive design feels like an exact science at times for certain things and then at other levels it's about minimising exacting details and deliberately being less exact to make sure there's an element of play and give in something which changes size and shape with differing levels of temperature. Something of a black art and also not...!)

 

Its betwixt and between isn't it? Miniaturising the valve gear and running it in an oil bath with tight tolerances and safe from water and dirt/abrasive is clearly a good idea *if your detail engineering is up to the challenge*. Sufficiently well designed the drive chain would tend to isolate the valve gear from suspension movement which is also helpful. But the detail design wasn't good enough and the result was frankly a failure. As I've said before (and to a number of managers in my career) its the detail where you fail.