Bulleid Pacific boiler pressure

Apologies in advance as I am sure this will have been discussed at some point but I'm wondering why Bulleid's pacifics all had their boiler pressure reduced from 280 psi to 250 psi during the 1950s. Was this purely to save on coal consumption or did it help to reduce wheelslip problems? As an aside, the Bulleids I am familiar with in preservation have all been downrated again to 225psi. Is this common across all preserved examples? If so was there some kind of engineering council that agreed to standardise on a lower figure or does this come from the railway inspectorate? Thanks!

 

The Bulleids that have been on the MHR over the years, and indeed those visiting examples that I've driven have all been rated at 250psi.

 

34081 is set to 250 psi. The reduction from 280 psi in BR service was afaik to save on boiler maintenance costs.

 

For various reasons, usually to do with the steam reverser's tendency to make its own decision as to what the cut-off should be, drivers drove on the regulator and rarely, if ever opened it fully to give 280 p.s.i. at the steam chest. The high pressure therefore wasn't used but did react on maintenance costs, so was reduced to 250 p.s.i. to bring these down, without making any significant difference to the steam chest pressure.

 

Interesting reply from Chessie. I fired Bodmin many times for my sins back in the 80s and the pressure gauge was clearly marked to blow at 225psi which kept us firemen on our toes that little bit extra. . I wonder if Andy Netherwood looks at these pages? He will know the history if this has changed back up to 250.

 

I'm sorry to disagree but Bodmin was 250psi. The St5 was 225psi but as I say the Bulleids, 07, 16 and 105 were 250psi. I'll leave it there now.

 

An S15 red-lined at 225 psi? Interesting. I thought they were either 180 or 200 psi.

Seems I can't read......

 

Standard 5. Yes 250 lb saved on maintenance and has been said only rarely was more than 200 lb used then or now as you will know if you read my column. Wayne used all the steam he had climbing Honiton with Clan Line on 23rd September but that was because the coal used mean a short spell with pressure down to 180 lb until Steve Rodenhurst got it back to nearly 250 lb. On my footplate rides in the 1960s some of the drivers used all the steam available when chasing high speed, eg Dave Parsons on 34026 in January 1966 on the 530 pm down with 410 tons, recovering from a 19 mph tsr at Steventon to reach 95 mph at Winchester Jct he used 210 lb of the 210 available ie full regulator (with 22% cut off) Gordon Porter of course only ever used full regulator and expected the fireman to keep up! Poor old Tommy Moult! Oh dear nostalgia again but those were the days.

 

And I was even wearing my glasses when I read it! And I've just been to Specsavers.

 

I don't have it to hand at this instant, but D.W. Winkworth's book on the Bulleid Pacifics states that the de-rating allowed BR to use a cheaper grade of steel for the stays, replacing one which was hard to obtain, rather than being due to any particular issues with the boilers at the higher pressure. Never seen it claimed anywhere else, but that doesn't mean there isn't some truth to it.

 

An S15 pressed to 225 ok yes i know it did say Std5 actually 506 did run with the line set at 200 when it had the ex 825 boiler i'm not sure if when it recieved the urie boiler the pressure was dropped back to 180 or not.

 

Actually 220 lb available. This was a very good figure and showed very little loss of steam from boiler to steam chest

 

This semms to be very plausible. Pressure vessel design codes permit lower design stresses as temperatures are increased. The saturation temperature at 280 psi is of course higher than at 250 psi so lowering the pressure wouldcould well have increased the maximum allowable stress. Of course lowering the pressure also reduced the actual stress.

 

The three that I've fired (21c123 / 34028 / 34059) were all rated at 250psi.

My understanding, as others have said, was that the reduction in boiler pressure was to save maintenance costs while having in practical (rather than theoretical) terms a fairly negligible impact on performance.

It would be interesting to know whether a further reduction to 225psi would prolong firebox life; if so, it may well be worth doing for those running on preserved lines.

Tom

 

On further reflection, my guess is that if there is anything in the stays story then the background is standardisation. The ideal for the railway is surely to keep stock of no more than one grade of copper, one grade of steel and one grade of Monel for stays. Having to keep a special grade of more expensive, possibly difficult to obtain steel on hand for two classes, even if there were a good number of them, is undesirable. If they have more than ample power, a little de-rating is tolerable.

The maintenance costs angle is interesting, because I'm sure @Jamessquared has previously quoted costs from the mid '50s showing that the West Countries, despite high overall maintenance costs, had some of the lowest boiler repair costs across a variety of classes (like Halls, for instance). Now of course they were all still fairly new at that point, but the de-rating only started around 1952 and was not applied to all engines immediately, I think, so a lot of the data would have been when they were running around at 280psi. Chapelon's argument was always that higher boiler pressure was not necessarily a significant factor in boiler repair costs, provided good water treatment was used. Because of the steel boxes, the Bulleids needed and got good water treatment; I've occasionally wondered if some of the issues experienced in preservation are precisely due to the lack of this in the early days on preserved lines.

I suspect it's something that just has to be accepted that Bulleids need new inner fireboxes more often than engines with copper boxes, but just think of the difference in cost for a comparable size firebox in copper. At least with the SDR all tooled up they can be done on a batch basis.

 

Steam loco boilers weren't built to design codes. They were designed to what the drawing office thought was OK from many years of experience.

 

When I worked on 34016 Bodmin (a JB engine) it was redlined on the gauge at 225psi and in similar fashion JB's three Maunsell moguls were downrated from their BR pressure of 225 down to 200. This was during the early 1980s and it may have been a condition of the boiler inspector back then or it may have been JB's own policy, but with the shorter train lengths on preserved lines it did not impact performance. It has occurred to me however that keeping sufficient steam without lifting the safety valves waiting at at stations is more difficult when the valve lifts at 225 rather than 250 and it needs more skill and timing from the fireman. Perhaps uprating again to 250 gives more leeway for a less experienced fireman to keep the engine from blowing off. Although to us this is merely an embarrassment and a waste of coal it does also scare the young customers , especially if the engine has a face on it at the time!

 

Slightly confused by that - AFAIK the Maunsell moguls were always 200psi from building.

Tom