Joining the seam of a rolled boiler

[David KParticipant @davidk77137]

I believe there is a dove tail method but this would be  too difficult for me.

My preferred method would be to use a jointing strip to cover the butt join and solder in place.

My question is , is there a formula to say how wide the joint strip should be and how many

rivets should be used for a given length of tube.

 

[Thor 🇳🇴Participant @thor]

Hi David,

Is it something like this  or this you are looking for?

Thor

[bernard towersParticipant @bernardtowers37738]

I think the rivets are there to hold in place for soldering.

[JasonBModerator @jasonb]

Best to speak with your boiler inspector as to what he will accept due to the change in design. Also the strength of the barrel usually gets derated for a joint so check the material is still thick enough.

[David KParticipant @davidk77137]

My boiler is quite small 3 3/4” diameter and 9” long.  I am not a member of any club so have done

some searching and I will approach my local club and see if there is a boiler inspector.

[Thor 🇳🇴]

On Thor 🇳🇴 Said:

Hi David,

Is it something like this  or this you are looking for?

Thor

Thank you i will have a read through those

[duncan webster 1Participant @duncanwebster1]

Rule of thumb is lap 3 times plate thickness, and allow a factor of 0.8 for joint efficiency. Rivets are only to hold it together whilst silver soldering, as few as possible and don’t hammer the up tight, the solder has to bd able to wick through

[Brian Baker 2Participant @brianbaker2]

Greetings David,  speaking as a boiler inspector, it is a great joy to see someone making a boiler, rather than purchasing a finished one.  Well done for having a go.

I think your question has mostly been answered, Duncan has good suggestions.

However, I much prefer to see butt straps on the outside of a boiler when ever possible.  Then, there is no doubt they are there, and they are easier to produce, and inspect.

God luck with your efforts.

Regards

Brian B

[noel shelleyParticipant @noelshelley55608]

For a small boiler why are you using sheet copper – rather than drawn tube ? Do you have enough heat and equipment to properly silver solder this sort of size – many a potentially good boiler has been ruined by the lack of enough heat ?

The answer to your question depends on the plate thickness and also the working pressure. The calculations need to be done to arrive at the right plate thickness and there are many books that will give the formula – for a butt strap joint you will reduce the pressure by 0.8. I take it you are designing the boiler not using a published design ? Even for private use I would strongly advise you to join a club and speak to the boiler inspector, you will get all the help and advice you need and even for private use you will know it is safe ! Good Luck.  Noel.

[Nigel BennettParticipant @nigelbennett69913]

The FMES Website offers some useful advice on boiler design. You could do a lot worse than look at this:

https://fmes.org.uk/wp-content/uploads/2025/04/Boiler-design-calculations.pdf

[David KParticipant @davidk77137]

I am purchasing a boiler kit from a reputable dealer , they have advised me that tube of the size needed

is no longer available so it will be supplied as flat plate. They have offered to roll it for me but I will be doing the soldering.

I want to make the boiler myself , I may make mistakes , I may not , at the moment I am reading and watching

videos to try and build my confidence.

I am sure to be asking questions so thank you for the replies , appreciated.

[David KParticipant @davidk77137]

Thanks for the link Nigel , i will read through it .

[Author]

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