Which silver solder for live steam boilers ?

[brian777999]

1. I will soon be building my own live steam boilers for stationary engines which will operate at about 15 - 20 PSI. My (very large) hardware store has 5% and 15% hard solder in the plumbing section but I have read that only 45% hard solder is suitable for boilers. Can anybody enlighten me on this subject ?

 

2. Should I use a high temperature solder for the ends and then a slightly lower temperature solder for the fittings : steam pipe, safety valve, pressure gauge ?

 

3. I intend to use a propane torch for the heavy soldering on the boiler ends but I am wondering if a small butane pencil torch will be hot enough to solder the fittings and other pipe work ?

 

http://www.ebay.com.au/itm/Gas-Butane-Pencil-Torch-Welding-Soldering-Solder-Tool-Refillable-PencilTorch-New-/360839410410?pt=AU_Hand_Tools&hash=item5403b472ea

[pointstaken]

Brian,

.

I would suggest that a search of www.model-engineer.co.uk would more than answer your question. There is a great deal of information on there regarding soldering and what to use

 

Dennis

[Beatty 139]

The old favorite for silver soldering of model boilers was Easyflow No2, how ever apparently because of the cadmium it has been withdrawn because of the EU safety police!

 

I would suggest that you avoid the DIY suppliers and look to one of the many model engineering specialist suppliers as from the top of my head I cannot recall what the current favorites are.

 

What I would suggest you do is go find some good books on Model boiler construction, further to this is the boiler you intend building to a published design or your own as if it is to your own you should study design theory carefully, although our small boilers in G1 and the 16mm/G scale works do not hold much water you would not want to have your hands next to one if it failed. Further to this if you ever want to take your loco and run it at a public exhibition or meeting you will need a boiler test and certificate.

 

Don't worry about different temperatures of silver solders for different parts each time the solder is melted its next melt temperature is increased slightly so you do not normally have an issue, but silver soldering is a skill like all others that has to be practiced and learned.

 

Simple tips are

Heat.....

 

You will need a lot of it but not to much as to boil the solder so a good hearth of fire bricks and the works surrounded by bricks to reflect the heat back into the work is important, heat the who job evenly to just before melting point then use the hottest part of the flame to run the solder in and let if flash around the joint, we want a neat solder ring that fully penetrates the joint, we are not aiming to have the finished boiler hallmarked! you will know if you have too little heat as the solder will melt and just sit on the metal as small balls, the heat needs to go into the metal of the parts you are soldering evenly not just the part with the smaller mass.

 

Preparation.......

 

Silver Solder will not gap fill to any great degree so parts should be close fitting but not an interference fit as the solder will no be able to capillary into the joint a tip here is a few light center punch marks will give just a few thou clearance to let this happen, parts can be held in position while soldering by a few copper rivets.

 

Cleanliness.......

 

Everything must be scrupulously clean, emery tape and wire wool are good here, clear all oxide back and a good pickle in acid, no need to use sulfuric or anything nasty dissolved citric acid crystals from a home brew shop will work just as well, they just take longer to work but at the end of the day can be flushed down the drain safely, and after pickling a good scrub in good old Jiff scouring powder.

 

So at the end of all that well made parts, clean and then well fluxed is something like Easyflow flux or some like equivalent, plently of heat from a good sized blowlamp in a well surrounded by fire brick boiler and you should have a good one.

 

Just remember never use brass for a boiler or bushes only use copper or bronze/gunmetal

 

Hope that helps.

 

Andy

[brian777999]

 

''Just remember never use brass for a boiler or bushes only use copper or bronze/gunmetal''

 

That is exactly what I was going to use ! Most boiler kits are brass like this one :

 

http://www.forest-classics.co.uk/steam_plant_kit1.htm

 

I do have a few books coming in the mail and I will be reading them before I make my first attempt at silver soldering. There are also a few useful videos on youtube.

.

It is very interesting that each time the solder is melted then its melting point increases slightly. I did not know that but it is very useful information.

[Beatty 139]

All Mamod boilers for many years were a deep drawing cartridge brass for occasional use in very low pressure boilers such as are used on steam toys your not going to get a lot of issues with the very occasional use they are going to get.

 

I note with the link that they are suggesting soft soldering I again can see an issue with this for a toy boiler.

 

Its all horses for course and is based on safety factors for materials, with a low pressure brass boiler any dezincifcation will result in a leek that the boiler will not be able to make steam against so you will just end up with a fizzle and a wet mess on the table!

 

There are a lot of urban myths around boilers and so called explosions, very rarely will any boiler explode but only have a joint or component fail that will result in a rapid depressurisation of the boiler.

 

Kind Regards

 

Andy

[PatB]

The problem with a true boiler explosion, as experienced with high pressure boilers, is the depressurisation. As the pressure in a closed vessel rises, so too does the boiling point of the water within it. You might have a boiler operating at a pressure where the boiling point of water is, say, 150^oC. As a result, in order to generate steam, the entire contents of the boiler must be at 150^oC. Whilst the boiler remains intact and holding pressure, this is fine. If, however, the boiler suffers a structural failure, the pressure within it will suddenly fall. In the case of a major failure, possibly to atmospheric. Suddenly you have a large volume of water at 50^o above its boiling point. This water will, inevitably, obey the laws of physics and, more or less instantaneously, flash into steam, rapidly increasing in volume by a factor of I forget what but something frighteningly big. This sudden rapid increase in volume will seek out the original failure and use it to rip a bloody great hole in your pressure vessel, releasing an expanding cloud of scalding vapour which will cook the flesh from anyone unfortunate enough to be standing in the way, to supplement their shrapnel wounds.

 

Suffice it to say that, as an engineer, the potential for failure of large, water/steam pressure vessels scares the pants off me far more than, say, simple compressed air tanks or the like.

 

That said, and as i implied in another thread, I don't see any of this being a major issue with a toy type boiler operating at maybe 1 atmosphere gauge pressure. History tells us that they just don't go seriously bang in anything more than invisibly small numbers.

[brian777999]

 

I note with the link that they are suggesting soft soldering I again can see an issue with this for a toy boiler.

 

 

Yes, I have sent them an email querying that. I think it may be a mistake. Anyway, I intend to hard solder (braze) it.

[PatB]

Mamod boilers were, (possibly still are) I believe, soft soldered. It's why boiling them dry tended to kill them. That and melting the plastic sight glasses that later ones had set into their ends.