Just wondering ............

[Mick B]

While making my little log and axe today I drilled 2 X 3mm holes in the bottom of it before I started laying all the beads on it because the ends are capped off and I didn't want it to turn into a bomb with all the heat inside.
So lets say you cap both ends of a piece of 50mm pipe (with 3mm wall thickness) and don't have any holes drilled anywhere therefore making it air tight. With lots of weld beads there is going to be a lot of heat inside the pipe. Could this build up enough pressure to blow the pipe apart or would that only happen on say 1mm wall pipe?

 

[HMB]

Expanding gas gotta go somewhere

 

[rtbcomp]

I don't think there'd be a problem. P = pressure, V = volume, T = temperature degK

P1 * V1 / T1 = P2 * V2 / T2

The volume stays the same

P1 / T1 = P2 / T2

P2 = P1 * T2 / T1

P1 = 1 bar, T1 = 20 degC = 293 degK, heat up to 1000 degC = 1273 degK

P2 = 1 * 1273 / 293 = 4.3 bar

3mm is about the thickness of a CO2 cylinder which holds about 50 bar, so I think you'd be safe.

The above assumes you heat the whole pipe upto 1000 degC, in practice you're only heating a small area so bags of safety margin.

Expanding gas gotta go somewhere

Heated gas in a closed container doesn't expand (unless the container bursts), it just increases in pressure.

 

[Shox Dr]

If you are concerned, drill a very small hole and cap once finished.

 

[colnerov]

If you get pressure in there towards the end of the job and you have the molten pool going on a weld could that 'pop'?

 

[rtbcomp]

That would release the pressure, so it should solidify OK, needs watching though.

 

[wyn]

I sometimes have a job that requires welding quite a few bits onto a closed 25x2 tube.

It quite often builds up enough pressure to blow the weld out, usually on the last couple of bits to be stuck on......it's no problem though wait a second and weld up the hole.

 

[Mick B]

I don't think there'd be a problem. P = pressure, V = volume, T = temperature degK

P1 * V1 / T1 = P2 * V2 / T2

The volume stays the same

P1 / T1 = P2 / T2

P2 = P1 * T2 / T1

P1 = 1 bar, T1 = 20 degC = 293 degK, heat up to 1000 degC = 1273 degK

P2 = 1 * 1273 / 293 = 4.3 bar

3mm is about the thickness of a CO2 cylinder which holds about 50 bar, so I think you'd be safe.

The above assumes you heat the whole pipe upto 1000 degC, in practice you're only heating a small area so bags of safety margin.

Heated gas in a closed container doesn't expand (unless the container bursts), it just increases in pressure.

When I first saw this message I thought either your computer or my computer was playing up with all those P1 s and slashes and = signs everywhere until I got to the bottom and started reading some words. I know I'm pretty smart, (and good looking !) but I'm afraid I was lost there for a while !! Now I'm still lost, but in the future I will continue to drill a small hole and then I KNOW the pressure will not build up too much.

 

[rtbcomp]

^^ Good plan

 

[jweaver]

Sorry all. I need to ask.. Whats a "log and axe"?

 

[Flathead_Dave]

Sorry all. I need to ask.. Whats a "log and axe"?

http://www.mig-welding.co.uk/forum/threads/log-and-axe-project.59126/

 

[Kent]

Done a full fill in loads just let things cool inside, small vessels can be an issue but mainly getting the final weld to close

Yes stuff can explode ask a galvanising plant supervisor but that heats the gases trapped very much quicker than a welder might at work. Just go for a brew and come back to it usually works
Remember volume matters

 

[rtbcomp]

For a given amount of heat the temperature rise will be less for a large volume, so the pressure will not rise so much.