For a brief explanation of the boiler see this page.
The Clan Class 6 Pacific locomotive was originally built with a riveted boiler, designed for 250 psi with a copper inner firebox. This reflected the traditional approach at the time. Oliver Bulleid on the Southern Railway had taken a different approach in the 1940s when he specified an all welded, all steel boiler and firebox for his Merchant Navy and West Country Pacific locomotives. We have decided to conduct a thorough review of options available to us, recorded in a "Component Design Review – Boiler" document. Below is a summary of our conclusions from our initial design review.
The recommendations from the Engineering Committee have now been approved by the Council of Management. This allows us to start the detailed design phase and selection of supplier.
All Welded Boiler and Firebox
Virtually all modern boilers are manufactured by welding plates together. Welding has a number of major advantages
- Welding can be automated (in places)
- No overlapping plates are required, butt joints are used - this saves weight
- No rivets are required, thus saving time and cost in drilling and reaming, also a minor saving weight.
The image shows 70000 Britannia's boiler at Crewe in 2006 - with thanks to the copyright owner Nigel Fraser Ker and his web site http://www.fraserker.com/britannia/ for allowing us to use this photo. Below the dome you can see a horizontal plate riveted to the side of the boiler. This covers the seam on the boiler barrel; there's an even bigger one on the inside. A similar plate can be seen on the top of the front first barrel ring. Again a larger plate is on the inside. All of these plates are not required in a welded boiler leading to a significant weight reduction.
X-ray examination will be used to prove the integrity of the welds.
Some weight saving will useful. In order to comply with modern main line running standards, we will acquire additional equipment, e.g. air braking, electronic recording equipment which will add weight. Hopefully this will be balanced by weight saving in the boiler. Our aim is to keep within the 19 ton weight limit of the original Clans. however this is less important these days; even heritage lines are able to take large Pacific engines.
Steel Inner Firebox
All of the BR Standards were fitted with copper inner fireboxes, although this was not an easy decision. E. S. Cox makes the following comments in his book British Railways Standard Steam Locomotives :
The six narrow firebox boilers based upon L.M. and G.W. practice already had copper inner fireboxes, and the same material was chosen for the four wide firebox boilers. With the improvements in design and manufacture made by Stanier upon former G.W. practice, and with the continued development under Ivatt, copper fireboxes had given excellent service on the L.M.S., particularly in the case of the "Pacifics" on that line which ran 750,000 miles and more between inner firebox renewals. Moreover although expensive in first cost, there was a high scrap value amounting to from 70 to 75 per cent of the cost of new plate for the material reclaimed at the end of the day. The steel firebox on the other hand, in universal use in America, and in wide use elsewhere had been introduced by Bulleid to his Pacific locomotives for the S.R. and these were also giving good service so long as adequate water treatment was assured. Lighter and cheaper in first cost than copper, steel fireboxes were also able, by virtue of their weldability, to avoid lapped joints, frequent source of deterioration, and to eliminate tube leakage at the firebox end even under conditions of sustained high output. It was also easy to apply circulation improving fitments such as the thermic syphon. The choice was nicely balanced, but the decision came down on the side of copper mainly because, with the indifferent and mixed qualities of water still forthcoming over the greater part of British Railways, it was clear that steel was likely to prove a source of anxiety in this particular application due to corrosion and cracking.
The key words are that the "choice was nicely balanced" in 1951.
If water is used direct from the water main in some parts of the country, then you are going to get problems. The London, Tilbury and Southend Railway was notorious for boiler problems (because of the hard water); whereas in Scotland, the better water quality meant that boiler reliability was much higher. A steel firebox is more prone to corrosion problems than a copper boiler. After the introduction of the Merchant Navy class, fireboxes were lasting about 5 years before being scrapped with major corrosion issues. This led to the introduction of the T.I.A. water treatment developed by M. Armand, the Director-General of the French Railways; this significantly reduced the corrosion problem.
Heritage railways and main line steam operators have found that the correct water treatment gives a major improvement in boiler availability and a number of heritage railways have introduced water treatment plants.
Therefore one of the major disadvantages of a steel inner firebox is reduced to a low level.
Copper is a better heat conductor than steel so a redesign for a steel inner firebox will need changes to increase the space between the sides of the inner and outer fireboxes. This will reduce stay stress. The best way to achieve this is under review.
A steel firebox will weigh less than a copper firebox. O.V.S Bulleid states in his 1946 paper to the Institute of Mechanical Engineers "Some Notes on the Merchant Navy Class Locomotives" that for the firebox - "The use of steel would effect a large reduction in weight as compared with copper, which in the case under consideration would be at least 1.5 tons."
After some consideration, the project has decided to design a boiler with a steel inner firebox.
Boiler Pressure at 250psi
Whereas the Class 7 "Britannia" locomotives had boilers pressed to 250psi, the Class 6 "Clan" locomotives had boilers designed to 225psi. To build a boiler in the 21st century we have to follow the latest boiler codes and regulations. When we calculated the plate size for the boiler, it became obvious that the difference between a 225psi and a 250psi boiler would be minimal. For example
- For the front ring which is a parallel sided cylinder, the calculated plate size was 10.06mm for a 225 psi boiler and 11.1mm for a 250psi boiler. The nearest, rounded up boiler plate thickness is 12mm. Therefore there is no difference in plate sizes for the different pressure boilers
- For the rear ring which is a tapered cylinder with a large hole for the dome (this weakens the structure), the calculated plate size was 12.3mm for a 225 psi boiler and 13.5mm for a 250psi boiler. The nearest, rounded up boiler plate thickness is 15mm. Therefore, again, there is no difference in plate sizes for the different pressure boilers
There are some firebox plates which will need to be thicker for 250 psi but these will be in the minority.
The increased boiler pressure plus the Lempor exhaust has been calculated to allow the locomotive to pull an additional 2 coaches over Shap summit between Lancaster and Carlisle. This provides a useful increase in revenue for the operator; this making our locomotive more attractive to hire.