A Coarse Guide to the Steam Locomotive for ‘N’ Gauge Modellers

Started by Train Waiting, December 08, 2023, 09:15:27 AM

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martyn

Quote from: Train Waiting on July 06, 2024, 09:17:43 AMI'm so sorry, chums.

That 'male chicken' nonsense didn't appear when I 'previewed' my reply.

What a shame that No. 2001's name has been changed by our FabulousForum.

My apologies again.

John

Nor my preview.... :)

Martyn

Newportnobby

@Train Waiting Maybe not satisfactory but I've amended the post in the interest of flow (whoever she is)

Bealman

Ok, I'll amend mine.

Christine went to see the C-o-c-k O the North  ;D
Vision over visibility. Bono, U2.

Bealman

Vision over visibility. Bono, U2.

joe cassidy

Quote from: grumbeast on July 06, 2024, 06:35:55 AMThere are lots of references in my books to Worthington, Elesco and Coffin type heaters but I would need to do some research to determine their differences and operations (difficult right now as I'm in France!!, and in a couple of weeks I will fulfill a dream and visit le Citi du Train in Mulhouse!)

Make sure you also visit the motor car museum in Mulhouse, especially if you like Bugattis.

The electricity museum is also worth a visit.

Papyrus

OK, so here is a question I have been wanting the answer to for ages, but now the subject has come up this is the obvious time to ask.

What is a clack valve? What does it do and why is it so called?

Thanks again for such a fascinating thread!

Cheers,

Chris

chrism

Quote from: Papyrus on July 08, 2024, 04:45:43 PMOK, so here is a question I have been wanting the answer to for ages, but now the subject has come up this is the obvious time to ask.

What is a clack valve? What does it do and why is it so called?


It's a simple non-return valve that is opened by the inlet being at a higher pressure than the outlet, closing when the inlet pressure drops.

The way that a steam injector works produces a water flow at a greater pressure than the steam that created that flow, because the volume has been increased by the addition of the water.
 
This increased pressure opens the clack valve against boiler pressure to let the water into the boiler. When the injector is shut off (or stalls) the pressure drops and the clack valve automatically closes.

They will often have a light spring in them to keep the valve closed when the boiler is cold and not under pressure - less important with top-feed than with bottom or mid-position feeds because a top feed cannot leak water, however a leaky or improperly seated clack valve would prevent the boiler making pressure.

I think it's called a clack valve because of the noise that can be made when it closes.


Papyrus

Thanks Chris! I thought it had to be something like that but it is nice to have it so concisely explained.


Cheers,

Chris

Train Waiting

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 33


Hello Chums

We noticed earlier the increase in boiler pressures from 50 psi in Rocket up to 250 psi which became an unofficial standard for the most powerful British steam locomotives.  Of course, if we include the BR 'Standard' classes, it became an official standard as well.  The 'Britannia' 7MT 4-6-2 and '9F' 2-10-0 classes used 250 psi as did the lone '8P' 4-6-2 locomotive Duke of Gloucester, whose true excellence was not discovered until the preservation era.

BR 'Standard' Locomotives in power classes 6, 5 and 4 used 225 psi and those in classes 3 and 2 were pressed to 200 psi.

Although locomotive boilers should be able to cope with a pressure in excess of the design maximum (they are hydraulically tested with pressurised water to about a third more than their working pressure), a safeguard against boilers being subjected to pressures above their limit is required.  This is the safety valves.

The earliest locomotives used a 'steelyard' safety valve in which a weighted lever holds the valve on its seat.  This came from stationary boiler practice but it was found that, on a locomotive, the weights tended to bounce up and down, so the 'Salter's patent balance valves' became normal practice for locomotives.  In these, a spring held the valve on its seat and a pointer on a scale showed the pressure - just like on a spring balance weighing scale.  Unfortunately, these safety valves could be tampered with by the enginemen to increase the boiler pressure and this resulted in some boiler explosions.

In order to prevent tampering, John Ramsbottom of the LNWR invented the famous safety valve named after him in 1856 and most railways adopted it, although spring balance safety valves continued in use for many years.

The disadvantage of Ramsbottom safety valves is that they are quite tall and they were eventually superseded by Ross Pop safety valves, which require less headroom.




[Two Ross Pop safety valves, mounted above the firebox, can be seen on this LMS '2MT' 2-6-0.]


The safety valves are normally fitted towards the rear of the boiler, usually over the firebox.  In some earlier locomotives, in an attempt to avoid tampering by enginemen, the safety valves were mounted further forward, usually at the dome.  NER 'Long Boiler'0-6-0 No. 1275, at the National Railway Museum, is a good example.  As is Gladstone or any of the 'Terrier' 0-6-0T locomotives - Mr Stroudley was enamoured of this practice.

Dugald Drummond, and, later, his younger brother Peter, following in the Stroudley tradition, placed the safety valves on top of the dome on their locomotives for the North British, Caledonian, and London & South Western and Highland (mostly)1, which meant this feature could be seen from Caithness to Cornwall in the Edwardian era.  Dugald Drummond's 4-6-0s for the LSWR and Peter Drummond's locomotives for the Glasgow & South Western had safety valves above the firebox.  This picturingham shows a Dugald Drummond LSWR 'T9' 4-4-0 with the safety valves mounted on the dome2:





On the Great Western, in Mr Dean's time, the safety valves were inside a tamper-proof brass casing above the firebox.  Mr Churchward moved the safety valves to the first ring of the boiler, near the firebox, and retained the brass cover, although it grew shorter over time.




[The later, shorter, safety valve casing can be seen on this 1927-built '45xx' 2-6-2T, No. 5542.]


Apart from tampering, the main hazards associated with safety valves, assuming they are of sufficient area to allow steam to escape at its maximum rate of generation in the boiler - that's why there is normally more than one, is incorrect assembly by fitters or some defect.

A failure of the safety valves can have catastrophic results.

On 21 April 1909, the boiler of Rhymney Railway 0-6-2T No. 97 exploded.  It was torn off the frames and travelled 45 yards before coming to rest.  Both enginemen and a bystander were killed.  The previous day, a fitter had dismantled and then reassembled the safety valves.  Unfortunately, he did this incorrectly (as simple as putting washers in the wrong place) which  meant the safety valves were held firm on their seats and were unable to open.

Something similar occurred at Buxton to LNWR 0-8-0 No. 134 on 11 November 1921.  This time, the boiler burst spectacularly into pieces, some landing 200 yards away.  Both enginemen died.  This was the last locomotive boiler barrel explosion in Britain (let's make sure it remains so); subsequent boiler explosions were of a different type which we'll come to later.

Due to arrears of maintenance caused by the Great War, No. 134 was overhauled by an engineering works in Glasgow and returned to the Railway in July 1921.  The gunmetal safety valves were mounted in cast iron columns and the valves were placed in a gunmetal bush which also provided the valve seat.  Gunmetal has a coefficient of expansion about twice that of cast iron and the morning of 11 November 1921 was especially cold, which restrained the expansion of the cast iron column.

The fitters in Glasgow made the safety valves tight in their bushes with little play.  All this led to the safety valves sticking and failing to relieve the boiler pressure. Unfortunately, it was assumed the boiler pressure gauge was at fault, although it was changed several times. 

After the explosion, a similar boiler was hydraulically tested to ascertain the maximum pressure it could withstand.  The boiler started to give way at 600 psi.  No wonder No. 134's boiler exploded so dramatically.  Its working pressure was 200 psi.

Just in case anyone is interested, gunmetal is an alloy of copper, tin and zinc.

The cause of this explosion ought not to have been a surprise to anyone as a paper, The Construction of Safety Valves, had been read to the Institution of Mechanical Engineers in 1877.  It warned of the dangers of using metals with different rates of expansion in safety valve assemblies.


1 On the Highland, the 'Castle' 4-6-0, normally attributed to Mr Drummond but largely a design of his predecessor, David Jones, had the safety valves above the firebox as was Mr Jones' practice.  The three 'Scrap Tank' 0-6-0T shunting engines also had their safety valves over the firebox because they used the boilers from scrapped locomotives.  The first of Mr Drummond's own designs with firebox-mounted safety valves were the 'New Ben' or 'Big Ben' 4-4-0 class of 1908. 

2  Yes, it's No. 301 again - we saw her before in Part 32 - but this time in a different livery.  No trouble is too much to add variety to these postingtons.  Mr Bulleid's 'malachite' no less.  Which permits me to quote this jolly anecdote, from HAV Bulleid's (OVS Bulleid's son) Bulleid of the Southern3.  After he had taken over as Chief Mechanical Engineer of the Southern, in 1937, Mr Bulleid, characteristically, wanted to make the livery more modern and distinctive.  He had a carriage painted in malachite green and brought to Waterloo for the Directors to inspect:-

'[...] when he (Bulleid) overheard one of the anti-malachites remark with emotion that "it ought to be spelled with an 's' instead of a 'c'" he wittily dismissed this as "rather a gutteral remark."'

I daubed the locomotive's safety valves and whistle with a gold-coloured 'Sharpie' marker - I told you I was a coarse modeller.

3 HAV Bulleid, Bulleid of the Southern, Ian Allan, London, 1977 ISBN 0 7110 0689 X.  Pages 93 and 94.

I am much obliged to the Lakeside & Haverthwaite and Gloucestershire Warwickshire Railways for allowing me to potter around their locomotive sheds.

Details of the boiler explosions are from the late CH Hewison's Locomotive Boiler Explosions, David & Charles, Newton Abbot, 1983 ISBN 0-7153-8305-1.  I believe everyone who goes anywhere near steam locomotives or similar machines ought to read this book... every year.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerie-B

John


Please visit us at www.poppingham.com

'Why does the Disney Castle work so well?  Because it borrows from reality without ever slipping into it.'

(Acknowledgement: John Goodall Esq, Architectural Editor, 'Country Life'.)

The Table-Top Railway is an attempt to create, in British 'N' gauge,  a 'semi-scenic' railway in the old-fashioned style, reminiscent of the layouts of the 1930s to the 1950s.

For the made-up background to the railway and list of characters, please see here: https://www.ngaugeforum.co.uk/SMFN/index.php?topic=38281.msg607991#msg607991

chrism

Quote from: Train Waiting on July 08, 2024, 09:52:11 PMThe disadvantage of Ramsbottom safety valves is that they are quite tall and they were eventually superseded by Ross Pop safety valves, which require less headroom.

Headroom was not the only advantage of Ross Pop valves, the way they work is better too.

Previous safety valves, like Salters or Ramsbottoms, are lifted purely by the steam pressure working against a spring, so they can weep, or "feather", as they are approaching the set pressure and will only gradually open fully. Likewise they will close gradually.

Pop valves have two pistons and seats, a small one that is lifted by the steam pressure in the boiler and a larger one that is initially closed off by the smaller piston. When the pressure reaches the set value, the small piston opens and the escaping steam now acts upon the large piston. Because the lifting force is now increased by the greater area of the large piston, that overcomes the spring closing force immediately and opens the valve fully.

Because the large piston is held up by the force of the escaping steam, that results in the valve closing later and reducing the boiler pressure to a lower pressure at which the valve opened, thereby avoiding hysteresis and the safety valve constantly opening, closing, opening, closing, etc. Often a pop valve will be designed/set to stay open until the pressure is 10-20 psi lower than the opening pressure.

The change to pop valves also caused a change in firing practices. With the older valves, it was considered a sign of firing prowess to have the safety valve feathering as much as possible. With pop valves, the sign of prowess changed to being able to keep the pressure as high as possible without the safety valves lifting.

This change was partially instigated by requests from stationmasters, due to the noise when a pop valve lifted and to the speed at which a trainshed would fill with steam, but also by the firemen quickly realising that the more they got the safety valve open, the more shovelling they'd have to do to restore the lost pressure.


martyn

Thanks again, John.

Not much to add to that. Going back to the ex GER 'Clauds' again, they had four column Ramsbottom valves to make sure excess steam was released. These were later replaced by twin Ross pops. To deal with the then-high pressure of 200psi, the GER Decapod had six Ramsbottom valves, grouped, I think, into a set of four and a set of two, but can't find an immediate reference for that.

Thanks, Chris, for the explanation of the action of Ross pops.

Martyn


Hailstone

I was told by one of the loco maintenance staff at Didcot that the standard pattern safety valve was of the Ramsbotton type, unfortunately I am 1700 miles from my reference library to confirm this. I have a copy of Great Western two cylinder locomotives by E.J (Ernie) Nutty who was one of the Swindon test team that improved the performance of several ex GW locomotves along with S.O Ell which I will look up when I get home. this book was written by Ernie many years ago and is my bible when it comes to Great western locomotives. I was priveliged to meet him when we asked him to inspect 6106 when we were looking to undertake a quick mechanical repair to put it back in service - Ernie's few words disabused the management of the notion and a far more comprehensive repair was undertaken.

Regards,

Alex

lil chris

This is one hell of a read, I up to about page 8 at the present time, a very enjoyable read. No rush I do not want to finish too soon thanks John.
Chris H.
Lil Chris
My new layout  East Lancashire Railway
My old layout was Irwell Valley Railway.
Layout previous was East Lancashire Lines, changed this new one. My new layout here.
https://www.ngaugeforum.co.uk/SMFN/index.php?topic=57193.0

chrism

Quote from: Hailstone on July 09, 2024, 12:07:49 PMI was told by one of the loco maintenance staff at Didcot that the standard pattern safety valve was of the Ramsbotton type, unfortunately I am 1700 miles from my reference library to confirm this. I have a copy of Great Western two cylinder locomotives by E.J (Ernie) Nutty

According to my copy of what I assume is the same book (blue card cover, no publication date but with a preface dated 1977), the safety valves are two independent direct loaded valves fitted into a common casting - which also included the mounting faces for the clack valves if the boiler was top fed.

Each valve has its own spring, unlike the Ramsbottom type where the two valves are linked by a lever at the top and a central spring on that lever to close both valves.


Train Waiting

Thank you very much, chums, for your helpful posts on GWR safety valves.

The late Professor Tuplin, who was a close observer of Great Western locomotive practice, stated that the direct-loaded safety valves in their brass casing made 'blowing off' noise less noticeable to people standing near the engine on a station platform or suchlike.

I understand the 'direct-loaded' type of safety valve was a derivative of the early 'steelyard' safety valves where a spring replaced the weight.  Use of these safety valves went back to Timothy Hackworth's time.  Unfortunately, the spring had an adjusting nut which could be tampered with to increase boiler pressure.  'Direct-loaded' safety valves tended to be placed in a brass casing to give some protection from tampering. 

The tamper-proof quality of Ramsbottom safety valves made these a popular choice and their use became widespread, but not universal, from the eighteen-seventies.

*

Quote from: lil chris on July 10, 2024, 09:56:11 AMThis is one hell of a read, I up to about page 8 at the present time, a very enjoyable read. No rush I do not want to finish too soon thanks John.
Chris H.

Many thanks for your kind comments, Chris.  The series has particularly valuable input from expert contributors, to whom I'm especially grateful.

*

Thanks again and all good wishes.

Pip-pip

John
Please visit us at www.poppingham.com

'Why does the Disney Castle work so well?  Because it borrows from reality without ever slipping into it.'

(Acknowledgement: John Goodall Esq, Architectural Editor, 'Country Life'.)

The Table-Top Railway is an attempt to create, in British 'N' gauge,  a 'semi-scenic' railway in the old-fashioned style, reminiscent of the layouts of the 1930s to the 1950s.

For the made-up background to the railway and list of characters, please see here: https://www.ngaugeforum.co.uk/SMFN/index.php?topic=38281.msg607991#msg607991

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