Flying Scotsman and the A4 Boiler

alexl92 · 17 Nov 2015

For many years, 4472/60103 ran with an A4 boiler rather than the correct A3 boiler. From what I've understood, this meant that FS was less reliable/meant it didn't steam quite as well as it should.

Please could someone explain to a relative novice why the A4 boiler in the A3 wasn't a great combination?

RailUK Forums

Zoidberg · 17 Nov 2015

Well, I guess that if the sizing follows stationery, the A4 boiler is half the size of the A3 one, so less steam = less power.

But, maybe not.

sprinterguy · 17 Nov 2015

With regard to Flying Scotsman, the problem was very much the opposite of poor steaming.

The NRM report into the faults with the restoration explains thus:

The A3 boiler (LNER type 94A) and the A4 boiler (LNER type 107) have identical outer dimensions but the A4 type has a larger firebox combustion space, a larger superheater and can work at the higher pressure of 250 psi compared with the A3 boiler pressure of 220 psi. From 1954 a number of new boilers were made for the A4 locomotives and the displaced boilers were used on A3 locomotives in order to replace life-expired A3 boilers. These boilers had their working pressure reduced to the 220 psi of the A3 class as those locomotives were fitted with larger bore cylinders.

At the time of the last overhaul in private ownership, it had been decided to change the boiler pressure to 250 psi, which with the larger A3 cylinders, resulted in the locomotive having a greater power output than either its original form as an A3, or indeed the nominally more powerful A4 class locomotives. Unfortunately this led to excessive demands on the locomotive which was deleterious to its long term condition.

STEVIEBOY1 · 18 Nov 2015

Which type of boiler does it have now following it's recent huge renovations?

sprinterguy · 18 Nov 2015

STEVIEBOY1 said:
Which type of boiler does it have now following it's recent huge renovations?

The locomotive has been refitted with the A3 boiler (thoroughly restored) with which it was fitted at the time it was purchased from BR in 1963.

The A4 boiler was purchased as a spare during the mid-sixties and was subsequently carried by the locomotive during a fair proportion of its' time in preservation to date.

alexl92 · 18 Nov 2015

Thank you!

I believe the boiler may have gone to LNWR as a spare for Bittern... unless I'm getting mixed up with the spare tender, which definitely went to Hosking.

sng7 · 18 Nov 2015

I thought I remembered reading the boiler became a spare for Bittern as well.

MarkyT · 21 Nov 2015

sprinterguy said:
With regard to Flying Scotsman, the problem was very much the opposite of poor steaming.

Indeed. Effectively the A4 boiler design became the standard post war replacement boiler for both the A3 and A4, rather than Doncaster continuing to produce two separate types. The overall dimensions were exactly the same. As The NRM report states, when an A4 boiler was used for an A3, the safety valves were set to the lower pressure the earlier locos were designed for. From a technical perspective an A4 is really just a souped up A3 with a little more power from the higher boiler pressure (although actually with slightly smaller bore cylinders) and beefed up frame plates and other mechanical components.

Boiler diagrams:
A3 - http://www.townend.me/files/drawings - lner pacifics/boiler094.JPG
A4 - http://www.townend.me/files/drawings - lner pacifics/boiler107.JPG

markindurham · 21 Nov 2015

MarkyT said:
Indeed. Effectively the A4 boiler design became the standard post war replacement boiler for both the A3 and A4, rather than Doncaster continuing to produce two separate types. The overall dimensions were exactly the same. As The NRM report states, when an A4 boiler was used for an A3, the safety valves were set to the lower pressure the earlier locos were designed for. From a technical perspective an A4 is really just a souped up A3 with a little more power from the higher boiler pressure (although actually with slightly smaller bore cylinders) and beefed up frame plates and other mechanical components.

Which are the bits that got overlooked or ignored when Scotsman had its last overhaul before the NRM bought her - and we have all seen the consequences...

Willr2094 · 22 Nov 2015

sprinterguy said:
The locomotive has been refitted with the A3 boiler (thoroughly restored) with which it was fitted at the time it was purchased from BR in 1963.

The A4 boiler was purchased as a spare during the mid-sixties and was subsequently carried by the locomotive during a fair proportion of its' time in preservation to date.

Well that's nearly right. The A3 boiler 4472/60103 has recently been fitted with was carried by it during the 1960s but was actually fitted in 1965 during its first heavy repair in preservation (for which it visited Darlington Works). It carried this boiler for the rest of Alan Pegler's ownership, including the disastrous tour of America from 1969 to 1972.

Following the sale of the locomotive to Sir William McAlpine, it ran for another few years with the A3 boiler until the previous A4 boiler was fitted during a general repair in 1978. That is the boiler it carried until 2005, which it took to Australia in 1988/9 and had during the early/mid 90s tour of preserved lines.

During the 1996/9 restoration under Tony Marchington's ownership, it was decided to beef up the A4 boiler to 250psi (the pressure it worked with when it was used with A4s), rather than the 220psi the A3 class was designed for (and the pressure the boiler had been working to until 1995). While that gave Flying Scotsman exceptional efficiency and performance, over time it exhausted the frames and other mechanical components, which meant from 2002 the locomotive became steadily more unreliable (that is why in the first year of NRM ownership it missed a lot of its booked appearances through mechanical problems).

MarkyT · 22 Nov 2015

Willr2094 said:
. . . that gave Flying Scotsman exceptional efficiency and performance.

With the higher pressure of the A4 boiler not limited to 220psi and and the larger cylinders of the A3, Flying Scotsman could theoretically develop even more power than an A4, but didn't have the thicker frame plates the A4s incorporated to be able to cope with that. Relentless competitive pursuit of performance feats on main line excursions, on the S&C in particular, used this output to the full, generating enormous stresses that probably contributed heavily to the extensive frame cracking found during subsequent overhaul.

Willr2094 · 22 Nov 2015

MarkyT said:
With the higher pressure of the A4 boiler not limited to 220psi and and the larger cylinders of the A3, Flying Scotsman could theoretically develop even more power than an A4, but didn't have the thicker frame plates the A4s incorporated to be able to cope with that. Relentless competitive pursuit of performance feats on main line excursions, on the S&C in particular, used this output to the full, generating enormous stresses that probably contributed heavily to the extensive frame cracking found during subsequent overhaul.

Exactly. You also need to factor in the issue that until 1996, Flying Scotsman had never worked with a boiler pressed to anything higher than 220psi - that is the absolute maximum the A1/A3 chassis can tolerate.

You also need to factor in the fact that under Tony Marchington and Flying Scotsman plc ownership it was being used pretty much exclusively on upmarket railtours such as the VSOE - the combination of the extremely intensive work this involved plus the fact its boiler had been upgraded to such an extent that it was too powerful for the rest of the locomotive probably contributed to the extensive cracks found in the A3's chassis.

70014IronDuke · 22 Nov 2015

MarkyT said:
With the higher pressure of the A4 boiler not limited to 220psi and and the larger cylinders of the A3, Flying Scotsman could theoretically develop even more power than an A4, but didn't have the thicker frame plates the A4s incorporated to be able to cope with that. Relentless competitive pursuit of performance feats on main line excursions, on the S&C in particular, used this output to the full, generating enormous stresses that probably contributed heavily to the extensive frame cracking found during subsequent overhaul.

Which, if you think about it, is a tribute to the design team.

That is to say, the original A1/A3 frame (and, presumably, the other stress bearing parts, including the motion) had been designed precisely to match the performance available from the 220 psi A3 boiler.

That, in reality, is good design. If you could just put on an A4 boiler and run it at 250 psi, it would have meant the rest of the locomotive had been, in effect, over-engineered - together with the expense that involved.

LordBerkley · 22 Nov 2015

Can someone explain how exactly a higher pressure bolier would affect the frames?

Was Scotsman able to accelerate faster than it could with an A3 boiler or was it pulling drastically heavier loads, or a combo of both?

I presume the motion also took a beating?

MarkyT · 22 Nov 2015

70014IronDuke said:
If you could just put on an A4 boiler and run it at 250 psi, it would have meant the rest of the locomotive had been, in effect, over-engineered - together with the expense that involved.

Weight was also a concern, potentially limiting route availability. Thicker frames and rods add weight clearly. The A4 (locomotive only) was over 6 tons heavier than the A3 and more than 10 tons heavier than the original A1. As bridges were rebuilt over the years this total weight increase became acceptable. The maximum axle weight was 20 tons on the driving wheels of the original A1s, and about 22 tons on the A3s and A4, with the extra weight of the A4 carried on the unpowered front bogie and rear cartazzi axles.

A variety of locomotive and boiler diagrams here:
http://www.townend.me/files/drawings - lner pacifics/
--- old post above --- --- new post below ---

LordBerkley said:
Can someone explain how exactly a higher pressure bolier would affect the frames?

Higher steam pressure is rather like higher electrical pressure (i.e voltage).
A given volume or flow rate of steam (like an electrical current) at a higher pressure can exert more force than the same quantity at a lower pressure. A4s at 250psi had a cylinder diameter of 18 1/2" yet developed more power than an A3 (as designed) at 220psi with a 19" bore. The original A1 at 180psi had a 20" cylinder bore.

70014IronDuke · 22 Nov 2015

LordBerkley said:
Can someone explain how exactly a higher pressure bolier would affect the frames?

Was Scotsman able to accelerate faster than it could with an A3 boiler or was it pulling drastically heavier loads, or a combo of both?

I presume the motion also took a beating?

Well, trying to apply first principles, if you put steam into the cylinders at 250 psi instead of 220 psi, you get an additional force on the piston of 30/220 = 0.136 times. (or 13.6%, roughly 1/7 more force.)

This is theoretical, as you could not simply feed steam in at either pressure on starting (you'd get wheel slip) and there would be pressure losses through the system, especially as piston speed increases, but the principle still stands, ie, if you had the A4 boiler at full pressure, the forces on the piston would be 1/7 higher than possible with an A3 boiler at its max of 220 psi.

This would mean you could accelerate a train faster - because you have greater force on the piston, feeding through the motion to the driving wheels.

Now, the force from the piston, via the connecting rods, to the driving wheels, mean the driving wheels themselves have to be kept in place - otherwise they would be flung off away from the locomotive. This resisting force is ultimately the job of the frames, (via, i guess, the bearings and horn guides).

So, if you have 1/7 more force on the pistons, it means ultimately that the frames are themselves also subject to an increase in forces of 1/7 - pushing and pulling in synch with the push-pull of the piston as it goes through its cycle.

And yes, the forces on the connecting rod and coupling rods would also all be increased, meaning increased risk of cracking and increased wear on the moving surfaces/bushes.

So, 4472 with an A4 boiler set at 250 psi would have greater theoretical tractive effort and, assuming it had the adhesion, it could accelerate a load faster.

But equally, bearings subject to the increased loads would wear faster and metal parts subject to the same loads (especially if cyclic) could well crack more quickly.

My tuppence worth. Someone will soon come along and point out any errors, I'm sure.

LordBerkley · 22 Nov 2015

Thanks MarkyT and 70014IronDuke

Bit of a dumb moment on my part, there's no way of limiting the pressure that gets exerted inside the cylinders, whether you drive it hard or not.

I had been thinking you could run with an early cut off to compensate for the increased pressure, but even in that scenario there would still be a brief period in which the piston would have the full 250psi exerted on it and hence the frames would still get overstressed.

edwin_m · 22 Nov 2015

Presumably if it was running with 250psi pressure but with the larger bore A3 cylinders rather than the smaller bore A4 ones, this would make the stresses on the motion and (thinner) frames more than on an A4.

MarkyT · 22 Nov 2015

LordBerkley said:
Thanks MarkyT and 70014IronDuke

Bit of a dumb moment on my part, there's no way of limiting the pressure that gets exerted inside the cylinders, whether you drive it hard or not.

I had been thinking you could run with an early cut off to compensate for the increased pressure, but even in that scenario there would still be a brief period in which the piston would have the full 250psi exerted on it and hence the frames would still get overstressed.

With an early cut off the steam trapped in the cylinder continues to expand and exert force on the piston. The pressure reduces as steam expands and the piston moves resulting in a lower 'mean effective pressure' (MEP) over the entire stroke that is less than the inlet pressure, but the effect is proportional to the inlet pressure so a higher supply will increase the MEP at the same cutoff. Theoretically you could use an earlier cutoff to lower the MEP but in reality it would be far too difficult to control effectively, and no help at all at times when you need maximum effort at starting or hauling heavy loads up steep gradients.

As a general rule, engines of any kind and their supporting structures should not be designed or modified to be able to tear themselves apart!

70014IronDuke · 22 Nov 2015

LordBerkley said:
Thanks MarkyT and 70014IronDuke

Bit of a dumb moment on my part, there's no way of limiting the pressure that gets exerted inside the cylinders, whether you drive it hard or not.

I had been thinking you could run with an early cut off to compensate for the increased pressure, but even in that scenario there would still be a brief period in which the piston would have the full 250psi exerted on it and hence the frames would still get overstressed.

I am not a steam cycle engineer or loco driver, but I would assume that locos with higher pressure boilers would (or rather, should be) run with more cut off than those with lower pressure boilers, because they will have more energy in the steam, and for better efficiency, they should be run this way.

But yes, if the driver opens the regulator (which will otherwise throttle the steam entering the cylinders, ie reduce its pressure when running at speed) there will be more presssure in the steam chest and entering the cylinders on a 250 psi boiler than on a 220 psi boiler.

And the first posts talk about the loco being worked very hard, so of course, that would involve the use of higher pressure steam more often, and the pistons etc effectively being worked harder than they were designed to do.
--- old post above --- --- new post below ---

edwin_m said:
Presumably if it was running with 250psi pressure but with the larger bore A3 cylinders rather than the smaller bore A4 ones, this would make the stresses on the motion and (thinner) frames more than on an A4.

Don't mix up force with stress (force per unit area).

The designers of the A4 would know that their pistons would be subject - at full load - to higher forces with the 250 psi boiler. They would therefore design the motion and frames to withstand this additional force. In theory, at least, the stress would not be beyond what the metal can stand.

In practice, it is not as simple, as, eg corners can induce areas of very high stress. But in theory, they would know the forces needed to be carried, and design for that, knowing that the maximum stress that metal in the various components can stand. (of course, they would design with a safety margin too.)

MarkyT · 22 Nov 2015

edwin_m said:
Presumably if it was running with 250psi pressure but with the larger bore A3 cylinders rather than the smaller bore A4 ones, this would make the stresses on the motion and (thinner) frames more than on an A4.

Absolutely, Edwin. The Marchington era Flying Scotsman in that configuration of cylinder size and pressure was theoretically even more powerful than the thicker-framed A4 design. It's probably not quite as clear cut as that as there were some other detail differences in the steam circuit between A3 and A4. The A4 had larger steam supply pipes to the cylinders than the A3 for example, so that may have had an effect, perhaps in Flying Scotsman restricting maximum flow at times of highest demand.

edwin_m · 23 Nov 2015

70014IronDuke said:
Don't mix up force with stress (force per unit area).

I didn't.

The original A3 had lower pressure and a larger cylinder area than the A4. But when fitted with the A4 boiler at 250psi FS had the same pressure but still with a larger cylinder area than an A4. So forces would be higher than in an A4 and stresses proportionately higher still because they were being borne by frames with a smaller cross-sectional area.

broadgage · 24 Nov 2015

LordBerkley said:
Can someone explain how exactly a higher pressure bolier would affect the frames?

Was Scotsman able to accelerate faster than it could with an A3 boiler or was it pulling drastically heavier loads, or a combo of both?

I presume the motion also took a beating?

A higher boiler pressure wont DIRECTLY affect the frames, when the loco is static or being worked only gently, the frames can not "know" what the internal boiler pressure is.
Problems arise when the loco is pushed to the limit, the higher boiler pressure allows more power to be produced, this in turn increases stress on the frames, each revolution of the driving wheels causes a shock or vibration to the frames.
The frames are of course designed to withstand these stresses, but increasing the boiler pressure beyond that intended will increase the stresses to greater than designed for.
There is no immediate danger, remember that the loco ran for some years thus without accident. In the longer term though cracks develop, as has happened.
A higher than designed for boiler pressure would also increase wear on the motion, but this is less of a problem. Wear of the motion is to be expected, and it is regularly inspected and adjusted.
Stress cracking of the frames is more insidious and may not be apparent on routine inspections.

E&W Lucas · 25 Nov 2015

70014IronDuke said:
I am not a steam cycle engineer or loco driver, but I would assume that locos with higher pressure boilers would (or rather, should be) run with more cut off than those with lower pressure boilers, because they will have more energy in the steam, and for better efficiency, they should be run this way.

But yes, if the driver opens the regulator (which will otherwise throttle the steam entering the cylinders, ie reduce its pressure when running at speed) there will be more presssure in the steam chest and entering the cylinders on a 250 psi boiler than on a 220 psi boiler.

And the first posts talk about the loco being worked very hard, so of course, that would involve the use of higher pressure steam more often, and the pistons etc effectively being worked harder than they were designed to do.
--- old post above --- --- new post below ---

No, no, no, no!!

Higher pressure = higher temperature = a higher degree of superheat = greater potential for the expansive use of steam.
For a modern, high boiler pressure loco, with long travel valves, like an A3, high steam chest pressure (full regulator, once it's got moving), shortest possible cut off. In simple terms, a short burst of high pressure steam, given time to expand, and do the maximum possible amount of work, for a given weight of water boiled to steam.
Long cut off driving, with a "throttled" regulator, is for older, lower pressure, saturated or low superheat locos.

A loco is set up for a given boiler pressure. Frame construction, stresses, bearing surfaces, diameter and shape of exhaust passages (getting steam out of the cylinders is as crucial as admitting it, in the interests of efficient operation), lubrication of cylinders/ pistons/ valves, functioning of all steam powered systems, etc.
Simply jacking the working pressure up 30 PSI, shows a total lack of sympathy for and understanding of the loco.

2392 · 26 Nov 2015

Indeed when the "Plant" [Doncaster Works] modified and started fitting the A4 boilers to the A3s, it wasn't specifically mentioned to the various sheds on Eastern/North Eastern/Scottish Regions. The result being when Kings Cross Top Shed needed some superheater elements, for an A3, they ordered a set of A3 elements. When they arrived they didn't fit, no doubt to the consternation of the shed boilersmith. He in turn reported to Peter Townsend the shed master as such and they didn't fit. Mr Townsend contacted Doncaster about this "problem" and was asked; Which A3 do you have the problem with? He replied and the response was along the lines of;.....Ah! That engines fitted with a modified A4 boiler fitted with A3 safety valves. So from now on when you order a set of tubes/elements. Please give us the the engine number and we'll supply the correct tubes/elements.

MarkyT · 29 Nov 2015

2392 said:
Indeed when the "Plant" [Doncaster Works] modified and started fitting the A4 boilers to the A3s, it wasn't specifically mentioned to the various sheds on Eastern/North Eastern/Scottish Regions. The result being when Kings Cross Top Shed needed some superheater elements, for an A3, they ordered a set of A3 elements. When they arrived they didn't fit, no doubt to the consternation of the shed boilersmith. He in turn reported to Peter Townsend the shed master as such and they didn't fit. Mr Townsend contacted Doncaster about this "problem" and was asked; Which A3 do you have the problem with? He replied and the response was along the lines of;.....Ah! That engines fitted with a modified A4 boiler fitted with A3 safety valves. So from now on when you order a set of tubes/elements. Please give us the the engine number and we'll supply the correct tubes/elements.

Nice anecdote there. Is that from one of Peter's books? Just a little correction though: his name is Townend. I should know as I'm his son! Dad always had a good 'back channel' relationship with the Plant, probably more so than other shedmasters, as he had served an engineering apprenticeship there, afterwards working a few years in the drawing office in the late 1940s before changing tack and joining the traffic department, which eventually led to the Top Shed job.

2392 · 29 Nov 2015

MarkyT said:
Nice anecdote there. Is that from one of Peter's books? Just a little correction though: his name is Townend. I should know as I'm his son! Dad always had a good 'back channel' relationship with the Plant, probably more so than other shedmasters, as he had served an engineering apprenticeship there, afterwards working a few years in the drawing office in the late 1940s before changing tack and joining the traffic department, which eventually led to the Top Shed job.

Yes though which one off hand I can't remember. If it wasn't directly from one of his, he certain made a contribution to another with the boiler swapping 'tween the two classes, that had taken place.

70014IronDuke · 3 Dec 2015

edwin_m said:
I didn't.

The original A3 had lower pressure and a larger cylinder area than the A4. But when fitted with the A4 boiler at 250psi FS had the same pressure but still with a larger cylinder area than an A4. So forces would be higher than in an A4 and stresses proportionately higher still because they were being borne by frames with a smaller cross-sectional area.

Indeed, I misread your question (partly because I thought it had already been answered.)

--- old post above --- --- new post below ---

E&W Lucas said:
No, no, no, no!!

Higher pressure = higher temperature = a higher degree of superheat = greater potential for the expansive use of steam.
etc
....

yes. I understand the physics - I just got the nomenclature for cut off a... about face.
--- old post above --- --- new post below ---

MarkyT said:
Nice anecdote there. Is that from one of Peter's books? Just a little correction though: his name is Townend. I should know as I'm his son! Dad always had a good 'back channel' relationship with the Plant, probably more so than other shedmasters, as he had served an engineering apprenticeship there, afterwards working a few years in the drawing office in the late 1940s before changing tack and joining the traffic department, which eventually led to the Top Shed job.

Ah! that explains a lot. I had wondered how come a shedmaster had such influence on locomotive design, or at least modification, policy, eg fitting of Kylchaps and all that.

sidmouth · 9 Dec 2015

2392 said:
Yes though which one off hand I can't remember. If it wasn't directly from one of his, he certain made a contribution to another with the boiler swapping 'tween the two classes, that had taken place.

I will have to have a chat with my dad then! He helped write many of the RCTS LNER books and latterly has created a website dedicated to the Doncaster built boilers.

MarkyT · 10 Dec 2015

70014IronDuke said:
I had wondered how come a shedmaster had such influence on locomotive design, or at least modification, policy, eg fitting of Kylchaps and all that.

There was a group of senior managers involved in the decisions including Gerry Fiennes. They were determined to keep on improving speed and performance throughout the 1950s and not to let main line standards slip during the run up to diesel changeover, a policy that would also ensure they had a capable backup in place during the phase-in of the new traction. The Kylchap conversion was a known quantity with drawings ready and heaps of positive evidence from prewar testing that it improved power output, steaming and fuel economy. Locos could be fitted quickly in the Plant at their next overhaul at a cost of a few hundred pounds, and crucially by the mid fifties the patent had expired so there was no license fee to pay inventors. The business case was justified on coal savings alone but the railwaymen also knew the modification would result in a uniform, reliable fleet of interchangeable pacifics that could help to keep performance standards high right up to the risky changeover to the new diesels. Although diagrams were specifically allocated to A3s or A4s, in practice they were interchangeable, as A3s once modified were found to be able to keep up with the A4 timings easily. After the first few A3s were modified it soon became obvious smoke deflectors would be required with the softer exhaust. That hadn't been a problem with on the A4s as the wedge nose is a very good smoke lifting device at speed. Dad suggested the German style deflector and Doncaster very quickly produced a design. I think rather to his surprise the Eastern Region Board immediately sanctioned their application to all modified locos. The drawing office had done a good job and, love or loathe their appearance, the deflectors performed very well maintaining crews' forward visibility of track and signals.

Flying Scotsman and the A4 Boiler

Established Member

RailUK Forums

Established Member

Established Member

Established Member

Established Member

Established Member

Member

Established Member

Member

Member

Established Member

Member

Established Member

New Member

Established Member

Established Member

New Member

Veteran Member

Established Member

Established Member

Established Member

Veteran Member

Member

Established Member

Member

Established Member

Member

Established Member

Member

Established Member