A question about domes

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muggins
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A question about domes

Postby muggins » Tue May 05, 2020 2:58 pm

What exactly is the function of the bit of plumbing on pre-grouping locos which runs horizontally back from the top of the dome for a short distance then descends vertically into the top of the boiler? I'm on about the arrangement on Terriers, Pecketts and suchlike.

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Mountain
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Re: A question about domes

Postby Mountain » Tue May 05, 2020 4:34 pm

I know what they were used for in regards to a Mamod model and that is to make sure water did not go down to the cylinders (So it was steam and not water that fed the cylinders). I believe rhe same idea is used with the prototype locomotives.

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flying scotsman123
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Re: A question about domes

Postby flying scotsman123 » Tue May 05, 2020 5:31 pm

They're actually the safety valves, Salter Spring safety valves, and early type that fell out of favour after about 1880 or so. Hopefully this should explain.
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muggins
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Re: A question about domes

Postby muggins » Tue May 05, 2020 5:39 pm

Thanks fs123. Safeties is all I could think of, but being only familiar with the pop type, I couldn't think why these would need to be the shapes they are! Now I understand ...

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Bufferstop
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Re: A question about domes

Postby Bufferstop » Tue May 05, 2020 5:53 pm

Yes it is the same "Salter" who make kitchen and bathroom scales who when I was living in the area, were still making spring balances in West Bromwich. Old Hawkeye George who was our metalwork teacher used to reckon that his engineering group, if they were any good, ended up working for Avery who made scales for chemistry labs right up to railway weighbridges, the thick end of the group ended up down the road at Salters.
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Mountain
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Re: A question about domes

Postby Mountain » Tue May 05, 2020 6:23 pm

Safety valves and domes were not always in the same place. I have not looked at the locos mentioned as with many designs they put safety valves on top of the domes.
An example of a loco that had the saftey valves in seperate places were the GWR pannier tanks like the 57xx.

muggins
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Re: A question about domes

Postby muggins » Wed May 06, 2020 7:07 am

So OK ... some of the Victorian locos that have what I now know to be the Salter spring safety valve also have a brass(?) wossname on top of the boiler, just forward of the cab, which is very much like the housing round the pop valves on GWR designs. The MR Kirtley 2-4-0s have it, for example. What's that in aid of?

Bigmet
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Re: A question about domes

Postby Bigmet » Wed May 06, 2020 9:57 am

Might be a cover over another safety valve, or could be the cover over the 'steam fountain' which supplied the crew with steam for the various subsystems and so was positioned near the cab: whistle, injectors, brake ejector, slacker pipe, lubricators, feedwater and brake pumps, cataract, sanders, etc. (Not necessarily all of them, a selection according to what equipment the designer decided his loco should be equipped with.) Not really that familiar with the minutiae of Derby's designs.

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Re: A question about domes

Postby muggins » Wed May 06, 2020 10:01 am

Cheers Bigmet. Steam fountain definitely sounds feasible to me, but I'm now wondering why there might have been a need for a separate safety valve. There's definitely some queerthings on those old locos!

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Re: A question about domes

Postby Bigmet » Wed May 06, 2020 1:12 pm

Ah well, various early safety valves could be fiddled with to increase pressure before they released, an expedient used by crews to increase available force in the cylinders. So another valve that couldn't be 'adjusted', that blew off if the design pressure was somewhat exceeded but before danger threatened, was a good plan; until tamper proof safety valves became available.

muggins
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Re: A question about domes

Postby muggins » Wed May 06, 2020 1:15 pm

Yup, that makes sense to me.

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Re: A question about domes

Postby muggins » Thu May 07, 2020 6:55 am

Found a couple of references last night which confirm that the item in question on the Kirtley locos is indeed a decorative housing around a safety valve that's set at a slightly higher pressure than the one(s) alongside the dome.

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Mountain
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Re: A question about domes

Postby Mountain » Thu May 07, 2020 2:53 pm

It was almost an unofficial common practice for crews to wedge something in the valves so they could get that little bit extra from the locomotives performance! No wonder in the early years they obtained such unusually high speeds! :D

I never forget the local councils dumper which had a diesel engine with a guvornor and the workmates would dissable the governor when going along the road from site to site and the thing would zoom along until one day the negine gave up on them. It was sent to the council mechanics for repair. The engine block had almost split in two with a large crack all the way along... The works mechanic looking at it puzzled said "I've never seen that before". The workmates kept quiet!

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Re: A question about domes

Postby Flashbang » Thu May 07, 2020 3:45 pm

muggins wrote:What exactly is the function of the bit of plumbing on pre-grouping locos which runs horizontally back from the top of the dome for a short distance then descends vertically into the top of the boiler? I'm on about the arrangement on Terriers, Pecketts and suchlike.

Does this explain the dome and its piping? The main steam pipe is shown inside of the casing but could equally be on the outside https://www.lakesiderailway.co.uk/how-a-loco-works/
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