Bulleid Pacifics - Past or Present

I like it! Cue the music every time she drifts into the station….

 

Apparently the winner is "Elizabeth II" which is somewhat dull and predictable to my mind, but probably the right choice for the occasion.

 

What were you expecting, Maximilien Robespierre?

 

I suspect this is the one that will stick ;-)

 

Its only ever going to be be Barney the Dinosaur to me.

 

As far as evaporation rates are concerned it should be remembered that the A4 only had a grate area of 41.25 square feet, the 'Duchess' 50, and the MN 48.5 - figures from memory, so stand to be corrected. On this basis, the yet to appear Peppercorn A1 would probably produce results very comparable with the 'Duchess' and MN. Interestingly, BR found during testing at Rugby that removal of the Thermic Syphons fitted to the MN made little or no difference to its steam generation capabilities.

 

Worth remembering that the purpose of fitting thermic syphons to the Bulleid Pacifics was as a measure to ensure that in low water situations, there would continue to be a flow of water over the firebox crown. They weren’t fitted with the objective of increasing evaporation rate.

Tom

 

On the topic of thermal efficiency: Bulleid was of the view that passengers would notice a train that was delayed (for example due to being short of steam for the job) or didn’t run at all, but wouldn’t care much about a few percent change in coal consumption. In other words, steam raising was prioritised over efficiency. In the same way, from a passenger’s point of view, a loco too big for the job is less of a crime than one struggling to keep to schedule because it is on the limit of its capacity.

It’s possible to get too hung up on efficiency and ignore more important factors - from a century earlier, you might consider the remarkable claims Beattie made for the thermal efficiency of his locos, while, glossed over, the availability must have been poor by contemporary standards.

Tom

 

As this implies the risk of a collapsed firebox crown sheet, I'm afraid that I can't buy this explanation.

 

Bulleid to the Institute of Mechanical Engineers, Dec 1945

QUOTE “The successful introduction of the Nicholson thermic syphon had provided a reliable means of
improving the boiler circulation whilst at the same time, giving added security against the overheating
of the crown plate” END QUOTE

Taken from an advance copy of the paper.

Michael Rowe

 

See Bulleid’s quote given below by @Maunsell907 - whether you buy it or not, it was the explicit reason stated by Bulleid for fitting syphons.

Tom

 

Thermic syphons didn't prevent the boiler explosion of the C & O No. 1642 in 1953.

 

An excellent point Tom. I would also imagine that a bigger engine working well within its capabilities is going to wear out far slower than a smaller engine frequently being pushed hard.

 

Sounds a bit WIBN to me

 

I think this was more of an issue on larger boilers, hence greater usage of thermic syphons in the US.

 

I refer you to exhibit A - imagine being the poor boiler smith trying to keep that lot in order ready for the morning's service while working overnight in a dimly-lit running shed



Tom

 

The way I interpret this is that he is saying that with the water level surrounding the firebox being below that of the crown sheet, that the syphons will still eject water over the higher level of the crown sheet itself and cool it down, thus preventing collapse. This I find difficult to accept. In this situation whether the cause is pump or injector failure, faulty water gauges, mis-handing, etc., the end result is the same .

 

Isn't all mentioned there the case with or without thermic syphons?

 

That is the mechanism, and it is the stated reason that Bulleid fitted them to his pacifics. I've also seen it stated as the reason they were fitted to certain American locomotives.

You may find it difficult to accept, but the point being made is that that was the reason stated by Bulleid as to why they were used.

Tom