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Boilers with flue tubes but no superheater elements.

JimC

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From 1925 the GWR fitted a pair of 5 1/8 diameter flue tubes in the upper corners of untapered boilers that had belpaire fireboxes, pressure 165psi and above and no superheater. This is reckoned to have reduced cracking in the corners of the firebox. Tapered boilers like the 94xx never had this feature, but it was seen on all post 1934 designs and also on replacement boilers on smaller pre group pannier tanks and side tanks like 850s, 2021s and I think 517s. Did any other lines use this design feature?

 

I would imagine, BTW, it was a pragmatic innovation, based on experience, rather than theory. Plenty of P class boilers on pannier tanks had been superheated before 1925, with a single row of flue tubes, and by this date the superheater elements were being removed. If those boilers were seeing a significant reduction in problems then it was an obvious thing to try. It wasn't done on the taper boilers.

 

It seems to have been first used on replacement boilers for small pre group pannier tanks, 850s and 2021s, in 1925. The Std 11 boiler of 1924, which was basically a variation on the Metro boiler didn't have them, although the Std 21 on the 54s 64s and 74s, which was a Std 11 with a drum head smokebox, did. 

 

Worth noting that although P class (and other) boilers were standard and interchangeable on the outside there were any number of different tube arrangements tried on the inside during the Churchward era. Tube layout was clearly a preoccupation in the drawing office. 
 

Edited by JimC

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Jeremy Cumberland
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It wasn't done on the taper boilers that had more room to lay out the tubes at the firebox end.

Aren't the tubes in taper boilers parallel, so the tube spacing in the firebox tube plate is the same as the smokebox tube plate?

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JimC

I would have thought that too, but apparently not. I've got a scan of the  drawing "Arrangement of tubes Standard boiler No.7" in front of me, (59502, June 1920) and whilst the arrangement is basically the same, they are definitely spaced differently at the firebox end.

 

I've just spent some time trying to understand the drawing - in the past I've given up, frustrated!
As well as the position of every tube at 3in to the foot scale it has insets at full size showing arrangement of small tubes at each end around the centre. At the smokebox end it shows hexagonally packed with horizontal spacing of small tubes 2 3/8in and vertical spacing of 1 3/8 in. At the firebox end it shows 2.33 in horizontally and 1.45in vertically. And yes, it does have that mix of decimal and vulgar fractions! First time I can recall seeing decimals on a GWR drawing. As I'm sure you all know without me telling you the vulgars work out at 2.375 and 1.375.  Which means that the firebox end tubes are spaced narrower horizontally and wider vertically compared to the smokebox end! When you go over the whole drawing it gets even more complex. The top couple of rows of tubes are horizontal at the smokebox and arched slightly, following the inner firebox profile at the other end. The bottom rows at the firebox end are stretched down towards the grate. The whole arrangement is immensely complex and very difficult to understand in words or even on the drawing! It seems to me that immense effort must have been put into optimising the layout.

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