Fusible plug in a 5″ gauge copper boiler?

[John Billard 1Participant @johnbillard1]

What are the pros and cons for putting a fusible plug in the copper boiler of my Claud, under construction?

I have never really found it necessary in my 5" Manor and I am minded to omit it unless there are good reasons not to.

Kind regards

John B

[John Billard 1Participant @johnbillard1]

[Philip RoweParticipant @philiprowe13116]

I suppose its like any insurance policy, a waste of time/money until you need to claim! Phil

[Nigel BennettParticipant @nigelbennett69913]

The current UK regs don't insist on it. Can you actually get to it when the loco is fully assembled and running to change it? I haven't come across one yet and I wouldn't fit one myself unless I had to.

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

When my boiler was made by Paul Tompkins (Southern Boiler works LTD) 2 years ago he included a fusible plug as that was the legislation for the CE mark, it was a recently added requirement at the time. Now that we have left the EU perhaps things have changed?

Pete

[Former MemberParticipant @formermember12892]

[This posting has been removed]

[bernard towersParticipant @bernardtowers37738]

I’m with bill why would you not have one?

[Paul KempParticipant @paulkemp46892]

Can anyone reference the "recently added requirement" for CE marking? I can think of a few reasons why there is not much point in a 5" gauge boiler. One being where would you get a plug of known composition / quality in that size that you would be confident of fusing? Do those of you that have them fitted remove them on an annual basis for inspection? If you drop a plug you are supposed to have a re-inspection of the boiler. I suspect the requirement has been added by the certifying body that underpins the CE process rather than any actual change in standards but happy to be proved wrong.

Paul.

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

Posted by bernard towers on 28/06/2021 22:15:36:

I’m with bill why would you not have one?

I think Nigel covered this in stating how hard it would be to replace if it failed. I'm still open minded as to whether I will fit one or not, again as Nigel rightly stated it is not a requirement for the UK. However, I do like the idea of fitting a fuseable plug, after all the boiler is the most expensive part on the model, whether home made or professional. Fuseable plugs in models have been known to fail prematurely and thus it may be prudent to fit a blank?

My own firebox will be fitted with an arch which should reduce the direct heat to the plug, as I say, the jury is still out as to whst I'll actually do when I get there.

We shall see…

Pete

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

Hi Paul, I can only say what I was told, I'm sure that professional boiler makers wouldn't fit one unless forced too to have the then all important CE stamp. As for the plug, you make this yourself, make a blanking plug to fit, drill a suitable hole through the centre and soft solder a copper rivet to it with the head inside the firebox. The idea being that the rivet head will protect the solder for normal running but if too hot the solder will melt thus extinguising the fire. If the plug drops you'll need to remove the boiler anyway and thus it will need testing.

Pete

[Paul LousickParticipant @paullousick59116]

Our model code for boilers in Australia stated that boilers less than 273mm dia should be fitted with at least one fusible plug. (should, not shall. So not mandatory). BUT if you have the option, why would you choose not to have one. It is a safety feature to prevent damage if the crown overheats because of low water level. Could save a lot of money for a new boiler.

Paul.

[Clive Brown 1Participant @clivebrown1]

In the many, many years of model loco operation within the ME hobby, how many instances of boiler damage due to low water level have there been? I've not heard of one.

I find it hard to visualise a small fire, say 0.5kg of coal, releasing around a maximum of 1-2kW or so of heat, significantly damaging a typical silver soldered copper boiler, which will contain at least, some water. When I've soldered a boiler I might use heating of up to 20kW with plenty of firebrick insulation and, even then, it's a stuggle.

[RedsetterParticipant @redsetter]

Posted by Clive Brown 1 on 29/06/2021 08:13:49:

In the many, many years of model loco operation within the ME hobby, how many instances of boiler damage due to low water level have there been? I've not heard of one.

I find it hard to visualise a small fire, say 0.5kg of coal, releasing around a maximum of 1-2kW or so of heat, significantly damaging a typical silver soldered copper boiler, which will contain at least, some water. When I've soldered a boiler I might use heating of up to 20kW with plenty of firebrick insulation and, even then, it's a stuggle.

I agree, and my boiler tester has mentioned this to me more than once.

In normal operation most 5" and smaller boilers have very little water over the firebox crown, this is after all where most of the steam is made. It is quite likely that parts of the crown will be uncovered momentarily while running, as the water must be sloshing around, to use a technical term. In that situation a fusible plug would be quite likely to melt when in fact there is no danger. What usually happens if you let the water get too low is that the steam pressure will drop, and then the blower will not be effective in maintaining the fire, so the system fails safe.

In addition I understand that it is quite difficult to design fusible plugs that will behave consistently as you want them to. So it is not such a simple matter as it seems.

[Dave Smith 14Participant @davesmith14]

Just looked at the current version of AMBSC, para 5.7 which states that a fusible plug SHALL be fitted and makes no concession for diameter, which means it is mandatory on all boilers. It also states the fusible material shall be commercially pure tin, I read this meaning solder with at least 99% tin (eg lead free solder) and leaded solder if you still have some not being allowed.

[BufferParticipant @buffer]

This week I had to repair a small leak around a Bush on my 5 inch gauge boiler. I hit it with a very large propane blowtorch, which sounded like a jet engine, to warm it up and then moved onto oxy acetylene and I was still struggling to get the area hot enough to melt the silver solder. So my question is how much hotter could a fire be? The copper is very good at dispersing heat, would it actually damage anything in the short term if it was dry?

[Dave HalfordParticipant @davehalford22513]

Lets face it a train is unlikely to encounter a hill sufficent to uncover the crown for long enough to do damage or wake up the plug.

Road steam is another matter and over 3" models might be long enough to expose the crown on a hill, no doubt priming like crazy at the same time making the issue even worse.

All that said on a train it's fit and forget.

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

hmm… I'm not sure that I would agree with some of the comments here regarding how difficult it would be for a boiler to fail. Model boilers have failed in the past, IIRC one brand new boiler at my club was ruined due to the crown being exposed. The melting point of copper is just over 1000C, models have recorded higher temps than this in their firebox when measured properly. How often do you look into a well-fired model firebox and see how bright the colour is, this alone will give some idea of how hot it is in there, it can be hotter than the melting point of copper.

There are articles out there where model engineers have measured the heat in their firebox, I recall one where the firebox dimensions weren't that far off the size of my 5" pacific, this was fitted with an arch, temp taken from the arch was 2000-2300F which is easily above the copper melting point.

have a look at this chart next time you look inside a model firebox…it gives a good visual idea of how hot it is in our fireboxes. I'm sure that an 'orange' or even 'lemon' is not that unusual to see…

Pete

[Brian Baker 2Participant @brianbaker2]

With respect Peter, it is not about melting copper, but melting the silver soldered joints the boiler is made with, around 700C should do the trick if held for a little time.

That said, I prefer not to have a fusible plug fitted, preferring to blank off the bush, since I have in the past had several plugs fail for no good reason.

Also in larger locos with fitted ashpans, the boiler has to be removed to replace the plug, bit annoying if the plug fails for no good reason.

Regards

Brian B

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

Sorry Brian, you are of course correct…I was getting a little carried away in trying to show how hot these things can get…

Pete

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

Forgot to add, thanks for sharing your own experience with fuseable plugs, Brian. Paul Tompkins did say to me that he wouldn't advise fitting a blank for the reasons you give.

Regards

Pete

[Peter Seymour-HowellParticipant @peterseymour-howell39349]

Meant to say..'advise fitting a blank'…can't find edit?

[duncan webster 1Participant @duncanwebster1]

This sounds like mission creep! It is known that Winson Brits have no water above the crown if the water is in the bottom of the glass**, but we've got 2 in our club with no bother. If you have a fusible plug in a full size boiler it has to be inspected at regular intervals to make sure it isn't all sealed up with scale. This would be a boiler off job on many models. How many silver soldered boilers have actually failed due to low water? We've had one soft soldered one, but no silver soldered as far as I know.

** correspondence in ME some time ago

[RedsetterParticipant @redsetter]

It would be interesting to know about any documented incidents, as opposed to rumours, of miniature boilers failing due to low water. What actually happened, and was anybody hurt in the process?

The other side of the coin is that there is a recorded incident of a fusible plug failing on a 7 1/4" loco and blowing the fire through the firedoor, badly burning the driver. IlRC this happened on the Beer Heights Railway in Devon.

[Paul KempParticipant @paulkemp46892]

Some interesting views here both for and against. One contributor states you make your own plug with soft solder and a rivet (a practice historically advocated by various ME "guru's). The American boiler code quoted by another requires pure tin – sensible standard depending on the pressure of the boiler. Tin melts at around 232dgrees C. Temperature of saturated steam is around 177 degrees C at 100psi, so on a boiler of WP pressure of 100psi or less you have a reasonable margin although the tin probably will not need to fully melt in order for the plug to fail so it may relieve at a lower temperature than the actual melting point of tin! If you use pure lead with a meting point of around 328 degrees C you have a better margin for service and unwanted false alarms but the strength of copper decreases to around 95% at 100 degrees and further decreases to around 90% at 300 degrees (graphs on interweb I consulted have poor definition so very course values). If you alloy tin and lead the melting temperature can in fact be increased above the constituents which makes it worse! So if you are making your own plugs you need to very sure of the soft solder you are using to end up with a product that delivers the required protection and does not give a false sense of security!

Then we have the maintenance aspect. The plug itself as also mentioned in this thread needs to be removed, inspected and cleaned on at least an annual basis, on full size loco's they are inspected at every wash out (interval between 14 and 30 days steaming). If a layer of scale deposits on the water side of the plug this will reduce the heat conduction away from it and lead to premature melting, the scale can also block the passage left by the melted material and stop the discharge!

It was mentioned an accident resulted from a dropped plug through fire being blown out of the door! To paraphrase a famous film "it's not supposed to blow the bloody doors off". Clearly in that case the bore of the plug was far too large. The purpose of the plug is to give a warning it is not supposed to act as a safety valve to dump the boiler pressure into the fire!

Summary fusible plugs are no substitute for proper boiler management! They are difficult to make and size to be effective. If not regularly maintained they can be unreliable. In anything less than a large 4" scale TE or larger 7 1/4" loco boiler they are unlikely to be of any real practical benefit. A safety device you cannot rely on is not worth having.

I have previously tried to find reliable documented accounts of small boiler failures per se let alone failures due to low water level and found nothing. There is a good ORR report on a narrow gauge loco the driver of which was inexperienced and if I am thinking of the right one managed to continue to run the boiler for a time with a dropped plug and completely wrecked the boiler, plug didn't save anything there! So back to the previous point, manage your boiler competently if you want to prevent damage! Definitely don't rely on something cobbled together in your shed with unknown or unverified materials!

Paul.

[Paul LousickParticipant @paullousick59116]

Hi Dave, Your extract from the model code is not exactly correct.

I checked with my copy of the AMBSC code for copper boilers, Issue 8-2012 which I believe is the latest edition and it states in para 5.7.1 that: "If used, fusible plugs shall be fitted in the highest point of the inner firebox crown that is readilly accessible for plug replacement" and not shall be fitted. So it is optional.

5.7.2 The clear area thru any fusible plug shall be equal to 20% of the area thru the safety valves, with a maximum bore of 4mm diameter. (The pressure valve orifice diameter is taken a Nomogram diagram)

Paul

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