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Fusible plugs for model loco boilers

Fusible Plugs

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Dave Wrenn15/11/2019 20:18:14
4 forum posts

It is recommended that I fit a fusible plug into the appropriate bush in the top of the firebox of my 5 inch gauge Britannia locomotive. Does anyone know where I can purchase fusible plugs 5/16 - 32 tpi threads. Any help would be appreciated. Many thanks Dave w

Nigel Bennett15/11/2019 21:17:19
306 forum posts
11 photos

Who recommended that? Particularly on a Britannia, accessing it every year or so to change it would be a real pain. It's not usually a mandatory requirement that you fit one; if a bush has been provided, I'd just fit a plain solid bronze bush and forget about it.

Dave Wrenn15/11/2019 22:12:37
4 forum posts

Hi Nigel thank you for replying, My Britannia (pictured) has a Swindon Copper Boiler made by Trevor Tremlen which I bought 9 years ago and the documentation that he gave me for it recommended that a fusible plug be fitted in the firebox crown bush, however I fitted a phosphor bronze plug and have never had a problem keeping the water level up the top of the gauge glass. Unfortunately this summer whilst a frien was driving it he lost sight of the water level and thought it was above the top gauge glass nut but in fact the water level was below the bottom gauge glass nut which resulted in a leak in the front boiler tube plate around one of the super heater tubes. This has now been repaired and the boiler tested by the club boiler testers and now we have no leaks but it was noticed the recommendation of the fusible plug, however I do not want to fit a fusible plug of my manufacture and I would rather fit a commercial plug if available off the shelf but this so far does not appear to be so. The other issue is exactly as you say to replace the plug if it went whilst running the loco would be a major issue to replace so I am tending to think I will replace the solid plug that I have been using and ensure anyone who does drive it can keep the water level up the top of the boiler. Sometimes I think that these statements in documentation are there just to cover thmselve, the boiler makers I mean. Anyway I will let you know how I get on and my final decision.

not done it yet15/11/2019 23:23:51
3554 forum posts
15 photos

What is wrong with making one? Hole drilled, counterbored, fluxed, tinned and filled with appropriate solder. Make several while at it.

Paul Lousick16/11/2019 00:50:36
1212 forum posts
502 photos

Our code in Australia for model boilers over 4" diameter with a design pressure over 250kps (36psi) states that it should be fitted with a fusible plug. The plug should be filled with tin, not lead or solder as it has a lower melting point. A fusible plug is not only a safety feature to prevent a possible explosion but will help to prevent a costly repair in case the water gets too low.

Paul.

JasonB16/11/2019 07:00:43
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16528 forum posts
1759 photos
1 articles

3/8 x 32 were the smallest I could see, if you don't want to make (fill) your own you could turn down that size and rethread.

Dave Wrenn16/11/2019 07:32:30
4 forum posts

3/8 x 32 was the only ones I could find (Polly Models) and I will have a look today to see if it can be turned down to correct size and re-threaded

JasonB16/11/2019 07:39:07
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Moderator
16528 forum posts
1759 photos
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Steamfittings also have that size

Russell Eberhardt16/11/2019 10:32:00
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2497 forum posts
85 photos

That does seem to be rather small. Here in France fusible plugs are recommended for any boiler used for public running and the smallest size in the recommendation is 1/4 Gas, that is just over 1/2 inch OD.

Russell

norm norton16/11/2019 23:21:21
98 forum posts
6 photos

I understand that they are not recommended for 5" gauge in the UK. Speak to an informed club boiler inspector and refer to the boiler code documentation. Sorry I cannot be specific on where to find this.

Paul Lousick17/11/2019 05:38:24
1212 forum posts
502 photos

Similar in Australia. Fusible plug body should not be less than 1/4" BSP.

Paul.

Robin King17/11/2019 10:15:57
85 forum posts

At the boiler inspectors seminar in Cardiff last month one of the speakers was David Vere and his view was that fusible plugs were problematic for a number of reasons not least being scale build up on the water side, combustion product deposits on fire side, and the difficulty of obtaining an alloy for the core which is suitable for the working pressure/temperature of the boiler. I don't know if David is a member of this forum and can contribute direct, but if not I'd suggest reading his two pages of seminar notes on the subject before making any decision as to plug or not. I've no doubt that Southern Fed would provide a copy of asked.

Russell Eberhardt17/11/2019 11:04:32
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2497 forum posts
85 photos
Posted by Robin King on 17/11/2019 10:15:57:

At the boiler inspectors seminar in Cardiff last month one of the speakers was David Vere and his view was that fusible plugs were problematic for a number of reasons not least being scale build up on the water side, combustion product deposits on fire side, and the difficulty of obtaining an alloy for the core which is suitable for the working pressure/temperature of the boiler.

I guess that's why the minimum diameter is specified - less likely to get blocked. However dismissing them as problematic is a bit strange. They can't be any worse than not having one at all even if they don't work!

Russell

3404617/11/2019 14:07:28
795 forum posts
6 photos

Spoken to a member on MECH who has a detailed Brittania build thread going at present, 48 pages so far, and is at boiler stage.

He says drawing does not call for one and he will not be fitting one. General opinion amongst the 5 inch chaps is that they are not needed.

Bill

Robin King17/11/2019 14:26:36
85 forum posts

As I said earlier it's worth reading the whole of David's seminar notes to fully understand all of the points that he made and which are based on his long experience of building and testing boilers.

Tim Stevens17/11/2019 14:58:32
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1101 forum posts

Paul Lousick - the melting point of tin is lower than lead, but both are higher than the usual recipes of soft (lead + tin) solder (but not by very much). So it is difficult to justify the advice you were given on melting temperature alone. Perhaps it is better to use tin which melts all at the same temperature, rather than solder which stays pasty over a range of temperatures? The actual temperatures for pure metals are: Pb = 327.4 C [621 F], Sn = 231.9 C [449 F]. Soft solder softening temperatures listed in Machinery's Handbook range from 361 to 518 F.

Regards, Tim

Dave Wrenn21/11/2019 07:00:20
4 forum posts

Gentlemen I have now purchased a 3/8 x 32 tpi fusible plug and have turned it down to 5/16 x 32 tpi to fit in the top firebox bush of my 5 inch Brit. This appears to have sufficient material around the core (tin) and is sufficient length to protrude above the top of boiler by about 3/16 inch minimum. However I am still not 100% convinced about fitting it as the loco is only used for passenger hauling and I have never had any problems in the past 9 years keeping the water level at the top of the gauge glass with the solid plug fitted and secondly as you quite rightly pointed out it would be a real pain to remove each year to clean or if it failed. Thanks to you all for your advice and comments I will let you know what I decide to do.

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