soft or silver solder it

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soft or silver solder it

This topic has 13 replies, 8 voices, and was last updated 3 February 2015 at 14:15 by JasonB.

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4 November 2012 at 17:57 #102770
Bob Perkins
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@bobperkins67044
I

I'm making a small low pressure boiler for my next project. its diameter is 70mm and it's height is 75mm. From my research the original model ran on @ 5psi, which seems bit low to me, so I'm planning for a bit more. My initial intention was to soft solder the joints. I've been reading Tubal Cain's Simple Model Steam Engines, and he advises the following "Can soft solder be used? NO. The hazard arises not from the pressure of the steam, but rather the risk of the boiler running dry whilst under fire".

So my question(s)

Should I worry about this?

Is this a risk I need to consider?

Should I soft or silver solder it?

Thanks in advance

Bob..
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4 November 2012 at 17:57 #6420
Bob Perkins
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@bobperkins67044
4 November 2012 at 19:02 #102776
Jeff Dayman
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@jeffdayman43397
Yes,Yes,silver solder.

When you say you are planning to use more than 5 psi, how much more ? Have you done some basic calculations to determine the max safe pressure? Tubal Cain's "model engineering handbook" and other books have the calculations just FYI.

If there is any doubt, I suggest you keep it around the 5 psi mark. If you use a tapered cork as a safety valve you will probably not go far wrong.

Any other safety valve,any higher pressure and you should do the calcs first, and hydrostatic test it totally full of water, no heat, with a hand pump, to be sure before steaming it.

We don't want to see you in the casualty reports.

JD
4 November 2012 at 20:20 #102781
nigel jones 5
Participant
@nigeljones5
thats like super tiny ! It shouldnt be soft soldered as it might just run dry one day and you would soon have a projectile on your hands! You only need a b&Q type blowlamp and a stick of SS so cost is minimal. Dont forget flux…
5 November 2012 at 09:13 #102821
Bob Perkins
Participant
@bobperkins67044
Thanks for the feedback it was most welcome and taken on board. I kind of new the answer I would get before I asked the question, so I WILL silver solder. The model will have a counter weighted tapered cork type safety valve. I was considering running this @ 20 PSI max, but that depends. If it will run on the 5 PSI that the origial model then there will be no need. It is Tiny, but built to scale.

A bit of dig through previous ME forums, and some number crunching has provides a max safe pressure of 152 psi. I would be interested if anybody has any web links re boiler calculations or Tubal cains boiler calculations.

Thanks all

Bob..
5 November 2012 at 10:19 #102829
JasonB
Moderator
@jasonb
150psi on that unstayed bottom surface sounds very high and you also seem to have quite shallow flanges.

I would also go for soft solder.
5 November 2012 at 11:01 #102834
FMES
Participant
@fmes
Definately Silver Solder even at the low pressure you are looking at, but I would add that Fizzy's point is not quite true, If the boiler runs dry, there will be no steam left to make pressure and you will just have a lump of hot metal. (It will however suffer severe joint failure if soft soldered).

I remember a case with a 3.5" Maisie, it came into the station with the safety lifting hard, and the water off the gauge, the fire was dropped immediately but by that time the safety had ceased to blow and it was ascertained that there was actually no water left in the boiler as the pressure gauge read zero.

A slight touch on the hand pump and the pressure immediately came back up to 80 psi so the boiler was flash steaming.

Subsequently the boiler was removed from the frames, fully inspected and subjected to the full 'new boiler' hydraulic test (we guessed there was a possibility that the crown may have become annealed).

That was about 20 years ago and the boiler is still in current use, fully certified and regularly tested.

Lofty
5 November 2012 at 11:03 #102836
Steambuff
Participant
@steambuff
JasonB

Do you mean Silver Solder ?

Dave
5 November 2012 at 11:23 #102838
JasonB
Moderator
@jasonb
I do don't know what made me say soft
5 November 2012 at 18:26 #102889
nigel jones 5
Participant
@nigeljones5
To defend my point, once all the water has turned to steam the heat from the fire will melt the solder very very quickly, there is very likely still pressure in the boiler at this time. Regarding boiler strength, I recon it would be the same shape as an orange at 150psi….I built one a long time ago using ultra thin copper, on steaming both ends bellowed out, other than that it was fine! But I dont support this method of construction these days!! Send me the dims and I will give you a safe working pressure.
5 November 2012 at 19:21 #102892
Bob Perkins
Participant
@bobperkins67044
Fizzy

Thanks for your interest, and time. I've been a member of the forum for about a year now and remain amazed at the support, encouragement and expertise available to a newbie.

The boiler shell is a piece of 2 3/4 inch diameter 16swg copper tube, with 20swg copper sheet for the crown sheet and boiler head and brass sleeves for the openings . With the crown sheet fitted the Internal height is 2 1/2 inches.

The bolier has two bands of 1/32" rivets, 45 in each (or visual effect only). The shell was drilled with a 0.79mm hole for each. Each rivet has been peined over on the inside. I plan to silver solder these on the inside to seal and strengthen them. I've added a photo to hopefully clarify this.

If you need any more info please let me know.

Regards

Bob..
8 November 2012 at 03:52 #103117
JC Uknz 1
Participant
@jcuknz1
My first post here but I have been interested for several decades and over the years read most of LBSC's articles thanks to club library of MEs

Here is the K.N Harris formulae from his book on boiler making.

T=PxD/2t and P=2Txt/D

Where D = Internal diameter of shell in inches

Where T =thickness of shell in inches

Where P= working pressure in lb/inch squared

Where t= Maximum safe stress of material in lb/inch square

Copper is 25,000/8 where 8 is the safety factor [ so 25,000/8 ]

We assume that it is a silver soldered boiler … only Mamod and the like soft soldered as I understand it … not responsible Model Engineers.

I gather the Australasian limits for boilers not requiring testing is diameter up to 50mm, Working Pressure no more than 35 Lb/in and capacity of boiler less than one litre … there may be other limits I was not told about.

Hope this helps
3 February 2015 at 14:02 #178319
Russell Eberhardt
Participant
@russelleberhardt48058
Posted by JC Uknz 1 on 08/11/2012 03:52:58:

I gather the Australasian limits for boilers not requiring testing is diameter up to 50mm, Working Pressure no more than 35 Lb/in and capacity of boiler less than one litre … there may be other limits I was not told about.

Yes, but in the UK and throughout Europe the rules are different and governed by EU Directives and, if you want insurance, rules agreed by the insurance companies.

Russell
3 February 2015 at 14:15 #178321
JasonB
Moderator
@jasonb
Russell this is an old dead post that was resurected by a new member who's post I have moved to here rather than bring this one back to life.

J
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