Finescale - of a sort?!

[simond]

I can see merits in both suggestions, though I’m at a loss to understand why not-very-hot steam should be any more deleterious to the bourdon tube than very-hot water.

I wonder if ensuring that the tube was full of water made it a bit heavier, thus reduced any tendency to vibrate? It would still vibrate, I guess, just more slowly, which might be preferable.

however I found this, which reiterates the point about temperature

Why Do I Need a Steam Siphon and How Does It Work?

A steam siphon can be a very useful device. If you’re wondering how it works, this article will teach you.

blog.ashcroft.com

 

[Peter Insole]

That's a relief Simon, I was rather afraid that I might have been talking nonsense - as well as being frustrated by not being able to immediately lay my hands on any historic "proof"!

Pete.

 

[Giles]

Entirely right! The water in the loop acts as a 'buffer' protecting the gauge from the temperature and latent heat of the live steam!
One sometimes gets hammering from the injectors, but I've never notice it affecting the pressure gauge over-much.

 

[Peter Insole]

I do hope you are not embarrassed by all this Dog Star... but I fear you are not the only one that has, nor will be misled by some "old boys", who we would all much prefer to show respect, telling tall tales!!

Pete.

 

[Richard Gawler]

A similar principle is (should be) applied to wiring electrical panels, well that was what I was always taught, you always ran the cables the long way around the cubicle before terminating in the fuse/breaker. Most panel failures are due to heat at the fuse/breaker and subsequent cable failure, having slack in the panel allowed for a quick chop back and repair, saved my butt on many occasions. Same for any sensors at remote locations, always leave a loop or two of cable before it enters the device.

Also, the installation drawings and cable run-out sheets can be wrong. If you take a wire to its first destination via the longest possible route, you have a good chance it will be long enough to reach its correct destination.

 

[Peter Insole]

OK, Mick, but I'm sorry to maintain that evidentially, (so far) it is NOT the prime reason for the loop - which remains for the purpose of creating a liquid trap.

As I understand it; hot water is not expansive in the same way live steam is, (highly so)... and the latter apparently is what needs to be avoided in a Bourdon tube gauge!

I am prepared to accept however that the extra length does provide an additional benefit of some flexibility - and maybe even a degree of vibration damping!

Thinking about it further; expansion, contraction and vibration on a working locomotive can cause precisely the same fatigue problem at ANY piped steam joint, (especially ones that are effectively closed between fittings, and not open ended) yet long loops to aid repairs are rarely provided for in those situations?!

If some old fitters claimed that was the reason for the loops, they may not have been telling porkies... but I still think they were being misleading - which is pretty much what I suggested in my post anyway!

Pete

 

[mickoo]

Yes, but Dogstar is still right

In addition, two important points I tried to be very clear on, I didn't say it was the primary reason (although I've deleted the post now because it clearly offended) nor did I say the other suggestion was incorrect.

Pretty much all the gauges I fitted had loops of pipework and if they didn't, we added them as good practice; we never fitted any to vacuum gauges as they were deemed 'safe' if the pipe fractured. We were trained and instructed to fit loops as a stress relieving method for the pipework into the gland, no other reasons were given and none sought.

Why loops were added there and no where else I'm not really sure, I did hear that it was (perhaps) for safety (in as much as trying to reduce the chance of failure) as gauges tend to be something people poke there faces up to, so having a joint fail then would not be good.

Thinking back we did have some York refrigeration sets that had extra perspex shields in front of the high pressure gauges and pipework, they were running 300-350 psi (liquid) refrigerant which is not something you want squirted in your face, the same sets had suction gauges running at -50 psi (gas) with no loops or perspex screens.

So apologies if my dialogue is misleading, confrontational or porkies, but my experience leads me to believe that looped pipework is an aid to stress relieving.

 

[simond]

This is more intriguing than I expected.

I hope we can all agree that;

• a gauge tube burst would be a bad thing, as it could spit the gauge window, probably the scale and maybe some bits of mechanism at the crew at face height, along with any water in the pipe, followed by live saturated steam.
• a vacuum gauge tube would not burst, but could crack - such a crack would be well within the capability of the ejector or vac pump, it would be inconvenient, but not a job stopper, I guess.
• a water trap could be used to maintain a slug of air in the gauge, which, coupled with the water, would insulate the bourdon tube from the heat of the steam - as a guide saturated steam at 10 bar / 150 psig is ~184C.
• any lengths of thin pipe will act as a flow restrictor, in the case of a burst
• water is incompressible, but much denser than steam or air - a slug of water in the pipe has inertia and will damp any sudden surges or vibrations in the steam space, presumably reduces vibration in the gauge, and wear in the mechanism.
• the pipe is connected between the gauge (fixed rigidly) and the manifold (also fixed) and is therefore subject to mechanical stress as it warms and cools, and due to changes in steam pressure (which have the effect of trying to straighten the pipe). These are repeated, hence fatigue is a key risk. In some cases the gauge is fixed to a cab sheet, others to a bracket on the boiler. This too is likely to impose mechanical loads on the pipe. In general, the longer the pipe, the less concentrated the stresses.

have I missed anything?

which is the “main” reason?

I think I’m going for fatigue of the pipe, but protection of the bourdon tube from thermal effects might be a close second.

 

[Osgood]

“I seek to disagree with the learned bloke from Kent”.

A quick scan of the www with reference to material put out there on gauge operation and installation suggests the prime reason is to provide a condensation trap.
Under installation I didn’t find one reference to pipe vibration / fatigue, although this could be considered a beneficial byproduct of the trap.

At what point does the trap loop itself become a potential source of vibration?
“I implore you to put a clamp on that loop Sir, or move your wife and children to a place of safety with great vamoose”.

 

[simond]

hi Tony,

the brief googling I did suggests the same thing, but I don’t understand why.

The bourdon tube is a partially -flattened piece of some copper alloy tube, curved into a “C” shape, blocked at one end, and connected to the pressure source at the other. Copper alloys are not going to have their properties changed at a couple of hundred degrees C, so it’s difficult to see why there should be some concern about steam in the bourdon tube.

this link again says “to prevent steam entering…”, but annoyingly doesn’t say why you would seek to do that: https://www.ashcroft-gauges.com/gauges/manuals/manual-pressuregauges-installation.pdf

however, a little earlier, under Temperature, it suggests that the maximum service temperature for a soft-soldered gauge is 121C.

Hypothesis; hard soldering affects the temper of the bourdon tube, so whilst feasible, is difficult to use, and may lead to complexities of design, calibration or whatever. Soft soldering doesn’t, so is easy, and therefore cheaper. Hence put siphon tube between gauge and steam to ensure gauge does not get too hot. Plausible?

apart from the obvious fact that the railways were consummate engineers, and could certainly have developed a process for hard soldered pressure gauge manufacture 100 years ago had they wanted to…

atb
Simon

 

[Osgood]

Yes, I found that Ashcroft material too. An American outfit - always thought they were a trad. British concern.
You could be right on the soldering thing.
The other thing I looked for but didn’t find was a gauge which had an inbuilt condensate loop - easy to install in those huge old cases if it was considered so important.

 

[Giles]

Hypothetically - is it possible that the bending/straightening characteristics of the tube may change with temperature, and having the water-trap to keep the temperature at the gauge down ensures accurate reading?

 

[Peter Insole]

Please don't worry Mick, I was not offended in any way whatsoever - and you really didn't need to delete your reply either!

I did wake up this morning in a cold sweat, fretting about my own "outburst" last night... and have just opened up the thread with every intention of deleting my own!!!

The debate arising might be mystifying to an outsider, (and you never know, might even further confuse AI bots) but is absolutely fascinating - and a delight... as per usual!

Pete.

 

[mickoo]

Please don't worry Mick, I was not offended in any way whatsoever - and you really didn't need to delete your reply either!

I did wake up this morning in a cold sweat, fretting about my own "outburst" last night... and have just opened up the thread with every intention of deleting my own!!!

The debate arising might be mystifying to an outsider, (and you never know, might even further confuse AI bots) but is absolutely fascinating - and a delight... as per usual!

Pete.

It was impressive, four exclamation marks to boot

Whilst looped pipe work on a steam gauge will form a condensate trap, I’m not sure how effective it would be due to changes in temperature and condensate as at some point it will have near zero water in there during its working span.

That aside I’m struggling to see how one would achieve a condensate trap in compressed air, refrigeration, vacuum or hydraulics all of which I’ve seen looped pipe work on or fitted.

Having said that, I never in 13 years witnessed a single pipe or joint failure on any gauge pipework looped or not. Dozens where pipe work entered or left a compressor despite multi looped pipe work, but never a gauge.

 

[Peter Insole]

One thing I would add, if nobody minds, is that the original question was about the long loop of pipe going right round the gauge - and is something that one time appeared to be a common practice.

Further searching seems to suggest that the modern preference is for a "U-bend" or "pigtail" - set below the gauge. Both solutions do take up more space, but I assume they are now deemed to be a more efficient means of achieving the same end result?

What should be remembered though is that the early steam engineers were pioneers, and were frequently discovering unexpected problems. Naturally, some of their solutions may not have been entirely successful, while others proved to have additional benefits.

One thing that can be certain though; They did learn - and boy, they really knew how to burn coal and make steam work for them!

I fear, (and sadly there is more than enough evidence from insiders within the heritage railway movement) that we tend to forget too much of that hard won knowledge and skills.

Pete.

 

[simond]

Pete,

I’d agree, it is easy to assume we’re smarter now than our forebears, but I’ll bet they knew stuff we haven’t a clue about.

The current debate about the altar stone at Stonehenge provides an excellent example.

best
Simon

 

[Osgood]

So Pete, tell us how you made the drawgear......

 

[SimonT]

Hydaulic pipes and loops. Never seen a loop on an aircraft hydraulic system and, trust me, aircraft are full of vibrations at loads of different frequncies. My suggestion is that it has to do with the range of temperatures and thus expansion that the system experiences.

 

[Peter Insole]

Who could have guessed that attempting to answer such a straightforward question would turn out to be opening something of a can of worms, (!) and even in the process end up skating perilously close to the "play nicely" rule?! I am now rather keen to steer away from any more dreadful metaphors and move on - or at least further forward from the backhead anyway!

Once again, I am daftly setting myself a bit of a fresh challenge:

Without having been able to gain access to the smokeboxes of either "Wren" or "Dot" - on account of their doors both being bolted firmly shut, it is difficult to determine the material that the flattened and tapered blast pipes were made of. The only potential clues that I can find are some online images of a sectioned Isle of Man loco at Manchester, and there it appears that Beyer Peacock (assuming a degree of originality of course) built them up and brazed them from fairly heavy copper sheet.

Bearing in mind that for our model; (a) the pipe is a decorative, non functioning representation, and (b) it will hardly ever be seen anyway, (!) there is absolutely no good reason why I shouldn't simply construct or carve it from wood, and then paint it to look like burnt copper - in time honoured fashion!

Nah!

Being me, and thinking that I would perhaps like to able to fit some sort of smoke generator at a future date, I really want it to be hollow throughout, and fancy trying to beat it out the real, metal stuff! At least our one rises vertically from the steam chest in a nice, straight line, so just how hard could that be?

I have started by making a basic, internally dimensioned core:



I was then able to wrap paper around it and mark out the precise geometry required for cutting some copper sheet.

Annealing the copper should make it a bit more workable, but I have already been warned not quench flat sheet, as it can become seriously mis-shapen?!

I do anticipate some tears before teatime though!

Pete.

 

[oldravendale]

Only you would consider doing this Pete, only you! And I don't mean just the blast pipe. It's wonderful, bonkers stuff. Please keep at it. I need my daily fix.

Brian