Now we come to the most bewildering boiler of them all, the Yarrow - Gresley High Pressure Water Tube Boiler. In theory this has nothing to do with the boilers on the Pacific Engines. In practise Gresley was looking for greater efficiency from his locomotives, and he hoped that the use of a high pressure boiler allied to compound drive, would attain this ideal.
Ships and power stations were the main users of high pressure boilers, and the most eminent builder was Yarrow & Co. of Glasgow. These Boilers were mainly of the Water Tube variety, the best description is that compared to the normal locomotive boiler, they are "inside out", in that water circulates through the tubes instead of hot gasses. In their normal operating environment a constant supply of steam is delivered generally to a turbine, the exhaust steam is passed through a condenser, and then returns to the boiler for re-circulation. The normal configuration for this type of boiler is best described as a "wish bone", consisting of steam drum at the apex , with two sets of multiple water tubes, descending to two set apart water drums, with a fire bed between the water drums.
We have already seen how Gresley had taken note of the Pennsylvania K4s Boiler and used it as the basis for the design of the Pacific boilers, so it is no surprise that the development of high pressure boilers in the U.S.A. drew his attention. In 1924 Alco built the first locomotive with a high pressure water tube boiler. This was a two cylinder compound 2-8-0 for the Delaware & Hudson Railroad, with a working pressure of 350 lbs per sq inch. The boiler design is credited to John E. Muhlfeld, a Consulting Engineer acting for the D & H, with also some design input from Yarrow & Co. Gresley was quickly in touch with Yarrow to see if there would be any advantage in employing a High Pressure Boiler on a Pacific Locomotive, to be comparable in power to the A1 Pacifics already in service. After two years of "discussion", in early 1926 a scheme was prepared by Yarrow showing a 350 lbs / sq. inch Water Tube Boiler mounted on a set of 4-6-2 frames, with proposed 3 Cylinder Compound working. This scheme was unsuitable as the 30 foot length of the steam drum, when shown mounted on the Pacific frames, left virtually no footplate area for the crew. In July another scheme appeared, changes had been made to the Boiler, a "combustion chamber" or "furnace" had been provided, the forward water drums were increased in length and diameter, allowing extra water tubes to be used. The Steam Drum was also lengthened at the smoke box end by 3 feet to make a feed water reservoir, none of these alterations had solved the problem of the lack of space at the footplate end, so the only solution was to lengthen the frames by 1' 9".
Meanwhile in America the trials with the D & H 2-8-0 were proving successful and design work was underway for a second locomotive, but with the boiler pressure, raised to 400 lbs per sq. inch. This loco was delivered in 1927.
By December 1926 Gresley was having second thoughts on the working pressure of the Boiler, and decided that there would probably be a better distribution of the work between the high and low pressure cylinders, by increasing the Boiler pressure to 400lbs per sq inch. More calculations proved in theory, at any rate, that by increasing the pressure to 450lbs per sq inch, it would only be necessary to provide steam for starting at 200lbs per sq inch to the low pressure cylinders. In June 1927 an engine diagram was prepared combining all the forgoing features, with a Pacific wheel arrangement. Steam distribution was via Lentz Rotary Cam Poppet Valves, with divided drive. The Inside Cylinders driving onto the axle of the leading driving wheels , the Outside Cylinders driving onto the middle driving wheels. The trailing wheel set under the cab had been moved back 9" to give a balanced look, due to the increased overhang of the cab footplate, but it also meant that this pair of wheels were carrying the weight of most of the firebox. No weight distribution was shown on this diagram but if a guess were made using later weights as a guide, the trailing wheels could possibly have been loaded to three or four tons more than the driving wheels. Yarrow saw the lengthening of the rear frames as an opportunity to lengthen the fire box, Gresley was more concerned with the likely overloading of the trailing wheels under the cab, and asked for the wheel arrangement to be altered to a 4-6-4 configuration. That he then amended this instruction to ask for two separate sets of wheels under the cab, one set to be carried in Cartazzi slides, and one set to be carried by a Bissel truck, caused the purists some concern and this has been a bone of contention from that day to this, exactly what is the wheel arrangement, a "Baltic" 4-6-4 or a "???" 4-6-2-2. As we are concerned with the boiler we will steer clear of that argument.
Yarrow were eventually allowed to lengthen the fire grate by 7 ½", but then had to shorten the front water tubes to give clearance for the steam reverser, this had a knock on effect in that the feed water reservoir at the front end of the steam drum was now increased to five feet. As the "extra" space gained on the footplate by the lengthening of the frames, had been eroded by the recent lengthening of the firebox, Gresley had second thoughts and asked Yarrow to go back to the original 7'-6"long firebox.
The above is but a short account of the trials and tribulations encountered over the three years that had passed since this project saw the light of day. However in November 1927 Gresley informed Yarrow that, once he received the latest modified drawings, he would give the go ahead for ordering all the necessary materials for the boiler. The more formal contract documents were signed on 31st January 1928, at Kings Cross. One would have thought that with all the design work that had gone into the boiler that things would move along quickly now that the order to proceed had been given, not a bit of it.
Design matters were far enough advanced on the Water Tube Boiler for Yarrow and Gresley to jointly apply for a Patent on 17th January 1928, which was accepted on 3rd January 1929,. An additional patent was applied for in November 1928 to cover the arrangement for the pre heating of the air to the ash pan and Gresley also took out a patent for his design of the valve gear that gave the means to set the cut-off to the high and low pressure cylinders independently.
Gresley visited Yarrow's on 26th February 1928 and asked for the diameter of the small tubes ahead of the combustion chamber to be increased from 1 ½" to 2" so that they would be easier to clean, a perfectly straight forward request. However the number of tubes per row had to be reduced, and also the number of rows were reduced, the front water drums had to be thickened to take the larger tubes, and the tube layout to the front end of the steam drum was also affected. A request was made for the rear water drums to be moved outwards to give extra clearance for the rear coupled wheels. Again simple in its concept but again tube spacing had to be reset and the casing altered. In today's money driven atmosphere this is the moment when the men in suits would do some very creative accounting and then present a rather large bill for all the "Extra Expense" incurred. Yarrow however wryly pointed out that the only thing left from their original drawing was the Title Block. As it was only about a month since the Formal Contract had been signed, Yarrow would not have been far advanced with the fabrication of the boiler, and the alterations would have been mainly to the drawings.
With no more revisions in the offing things settled down with Darlington concentrating on the frames and cylinder blocks and Yarrow and their sub contractors proceeding on the construction of the boiler. Hydraulic and static steam test were carried out in February 1929, no paper record of these tests has been found so it must be assumed that the hydraulic pressure was 675 lbs per sq. in. and the steam pressure 475 lbs per sq. ins. and that the boiler passed with flying colours.
The frames minus the middle set of driving wheels were sent to Glasgow in April 1929, it is not clear if the cylindrical smoke box was sent to Yarrow, along with the Frames, but late in April 1929, the smokebox , front plate, door and also the blast pipe and chimney were fitted by Yarrow. Wind Tunnel tests had been carried out on a wooden model and the result of these tests was that a semi streamlined front end was adopted. This necessitated a modified smoke box, front plate and door , these were sent to Yarrow in late June 1929.
With the Boiler, Smokebox etc. duly mounted on the frames they were ready for what Yarrow described as "Evaporation Trials" in October 1929. As the "locomotive"(I use the term locomotive here loosely to describe the Boiler mounted onto a partially complete under frame) would be stationary there would be no exhaust from the cylinders to create a draught through the Boiler, so steam was passed through a 1 ½" dia pipe which was led up the Blast Pipe. A 3 foot sleeve was fitted inside the Chimney, and the main steam pipe was extended to discharge 3 feet above the chimney extension, so that the operation of the regulator would not effect the draught through the boiler. The safety Valves on the Superheater were set at 465 and 460 lbs. per sq. ins and the valve on the regulating valve chest was set at 440 lbs per sq. in. Also a brick Column 13 ½"sqare was built at the firebox end of the centre flue, to shield the Superheater tube ends from the direct flow of the gases. Yarrow State that the firing was done by a Railway Fireman loaned to them by the London North Eastern Railway Company (no abbreviations here, and all very formal. All inter company communications were by letter, and on looking through the files I only saw one letter confirming a telephone conversation. A far cry from today's instant demands by the chattering Mobile Phone users).
The First Trial was of one hour duration, on 8th October 1929 ( Say equivalent to a 50 mile run),the air supply to the Firebox was through the hot air ducts each side of the boiler, and Welsh Ocean Coal was used. The highest Boiler pressure recorded was 380 lbs per sq. ins and the average Boiler pressure was 361 lbs per sq. ins., coal consumed was 2120 lbs(0.95 ton), water evaporated 10240 lbs. and the average Superheater Temperature was 731 degrees Fahr. The regulator rods became very stiff due to the expansion of the casings, the steam valve to the L.P. injector became very stiff, and the H. P. Injector would not work.
The hollow regulator rods were replaced with solid bright steel rods running on roller bearings. The steam valve to the L.P. Injector which was of stainless steel, was found to be badly scored so it and a similar valve to the H.P. Injector were replaced by Monel Metal Valves. The H.P. Injector was found to have defective internal parts and was returned to Gresham and Craven for repair.
Once the above faults were remedied the Trials recommenced on 14th October 1929, with a 3 ½ hour run (say equivalent to a 175 to 200 mile run). Heated air was supplied to the firebox for the first 1 ½ hours, after that the damper at the front of the ashpan was opened fully, so a mixture of hot and cold air was being supplied. Average Boiler pressure was 410 lbs per sq. ins. Coal consumed 2783 lbs per hr. (Total 4.35 tons). Water evaporated 17333 lbs per hr. (Total 60665 lbs). Average Superheater Temperature 721 degrees Fahr. Which gave a calculated Boiler Efficiency of 57.3%.
The following day 15th October 1929 a second 3 ½ hour trial was carried out,. Baffles were fitted to the side flues at each side of the boiler, the "temporary" blast nozzle was lowered 4 ½" and the diameter increased to 2 ½". Average Boiler pressure was 420lbs per sq. ins. Coal consumed 2542 lbs per hr. (Total 3.97 tons). Water evaporated 16718 lbs (Total 58513 lbs) Average Superheater Temperature 812 degrees Fahr. Which gave a calculated Boiler Efficiency of 62.5%. The draught through the firebox was not as effective as in the previous test, so the "temporary" blast nozzle was altered back to its original 1 ½ diameter. Also six firebars on each side of the firebox for its full length had to be replaced as they were burnt out due to ashes accumulating under the bars, because of the shallow ashpan in these areas.
With the above remedial work completed a further 3 ½ hour trial was run on 16th October 1929. The Baffles previously fitted were retained, and a mixture of hot and cold air was admitted to the firebox throughout. The Average Boiler pressure was 420 lbs per sq. ins. Coal consumed was 3066 lbs per hr. (Total 4.79 tons). Water evaporated 18588 lbs per hr. (Total 65058 lbs) Average Superheater Temperature was 830 degrees Fahr. which gave a calculated Boiler Efficency of 57.95%.
After this third long trial some suggestions were made with regard to the configuration of the Boiler. The baffles as applied were considered too wide and possibly better results would be obtain with Baffles of Half the width. With regard to the side firebars burning out a possible solution would be to build a firebrick wall against the water drum. Alternatively if the firebars had more taper especially those down the sides it may prevent the ashes sticking between the bars. The H. P. Injector had been the source of some concern and had been returned to the manufacturers for testing. Nothing was found amiss and Mr Gresham asked if he could be present at the next boiler trial.
A 3 ½ hour trial was carried out on 29th October 1929. For this trial one third of the superheater elements were removed and the holes in the header plugged, and the baffles were removed, also the fire side of the water tubes were cleaned. Mr Gresham was present at the trial. The Average Boiler pressure was 420 lbs per sq. ins. Coal consumed was 2800 lbs per hr. (Total 4.38 tons). Water evaporated 19205 lbs per hr (Total 67217 lbs). Average Superheater Temperature 625 degrees Fahr. Which gave a calculated Boiler Efficiency of 61% (the maths are slightly out here as on checking I got 60.85%) Towards the end of the trial the steam valve to the H.P. Injector was hammering against the seat and was difficult to operate. The casing at the bottom just forward of the centre flue damper became red hot, possibly due to the damper not being properly shut. During the last half hour the front damper was shut for ¼ of an hour and all the air was taken through the casing. During this period the air temperature in the casing fell by 115 degrees Fahr.
The exhaust gas temperature at the chimney was on occasions considered to be high so it was suggested that the area of the side air ducts be reduced by tapering them at the firebox end to hopefully stop the hot air short circuiting across the tubes to the flue.
The H.P. Injector was tested to find the slowest feed and was
witnessed by Mr. Gresham, two tests were made and the results are as
The opportunity was taken to test the L.P. Injector (believe Davis and Metcalf) one test only with 800 lbs of water fed in 137 seconds which according to the report is equivalent to 19300 lbs per hr. My check calculation makes it equivalent to 21021 lbs per hr.
Everything must have been deemed satisfactory as the assemblage was dispatched to Darlington and the locomotive was completed in late November 1929 and was coupled to a new Corridor Tender built at Doncaster. This pairing shows glaring lack of communication between the two Drawing Offices, the Engine cab side sheet profile was a slight out of vertical curve, whilst the tender side sheets were a vertical straight line. Whilst not too unsightly with a bit more thought a better match could have been made.
Two Test runs were made in December 1929, but no records have been traced, one on 12th South from Darlington , destination unknown (?? Leeds or York ??), and one on 29th, on an Up Express from Newcastle pulling 8 coaches and the Dynamometer Car.
This is a good point to leave No. 10000 and its Boiler, and the Bewilderment will be continued in Part 11 when we take a look at the Boiler in service.
To be Concluded.
Researched and written by Mel Haigh,
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