Fusible plug in a 5" gauge copper boiler?

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

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.

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.

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

I believe all the Polly range of locos have fusible plugs fitted and have done right from when the pollys were made by JGS Clarke.

Edited By Baz on 30/06/2021 16:42:27

Thank you for all your comments and advice!

It seems pretty clear to me that there is little advantage in fitting a fusible plug into a 5" gauge copper boiler. It was a good discussion.

Kind regards

John B

Just to be pedantic (who me) a mix of lead and tin will always melt below 328 and can be as low as 183 (eutectic)

Paul I am very confused extract from my copy of issue 8 - 2012? This copy came from a fellow club member so I am not sure where he got it from.

Hi Dave,

I am equally confused. This a copy of my code. Also Issue 8 - 2012 for copper boilers and paragraph 5.7.1 is different ??????

I will contact my boiler inspector to see if I can clarify the matter.

Paul

This may be a case of issue8A and issue8B. there should be a list of changes somewhere.

Hi Dave H.

There are no changes listed. The revisions are controlled by the "Issue" number and I have checked with my boiler inspector who confirms that Issue 8-2012 is the latest copy.

Dave S.

My boiler inspector told me that fusible plugs are optional on copper boilers with a barrel OD of 50 - 203mm. Steel boilers require at least one fusible plug.

He is at a loss as to why your copy is different but queried the page that you posted and asked if the Issue 8-2012 that is shown on page 37 is also at the bottom of page 38.

Paul

Paul

Copy of the whole page. We will go with your version as it makes more sense at our scale. Also It does not matter to me as I only use it as a guide, although a very good one.

I'm stil at a loss as to why there are differences and will search further for a reason. As you say, our code is only a guide and as long as your boiler complies with your own code, everything will be OK.

Good luck with your build and have fun.

Paul

Hi Dave,

I have stirred up a bit of a hornets nest here after I brought the matter to the attention of the boiler inspectors at our club who passed the question to the committee members responsible for the code.

They cannot explain exactly why there are 2 different copies of the code for fusible plugs in copper boilers but suspect that a preliminary copy "escaped" while preparing the final document for publishing.

They have confirmed that the use of fusible plugs in a copper boiler is optional and either way will comply with the Australian code. (It would have been a different matter if the other way around)

Paul

Paul..

Always happy to oblige with hornets nests. Thanks for the update.