back purging...hows it done?

[magnet]

I have been reading various posts about back purging . My question is how exactly is it done? Is it just a constant flow of shielding gas played on the back of the weld from an open pipe or is it some thing more sophisticated than that Cheers Magnet

 

[arthur]

Rubber glove with holes seal other and and feed gas....just as you say tbh....

 

[Rrunnach]

Bit more to it than above post. Flow rates are very important, especially when you come to closing up, fire proof tape, ecetera...........don't know how long rubber gloves would last, as not something I have ever seen used?

 

[backpurge]

Depends what you are doing, it can be as simple as flooding the reverse of the weld with inert gas or could involve ensuring residual oxygen levels are down to the parts per million

 

[chrismac]

Yes as above and also depending on what your doing a backing strip might be enough, for instance seam welds.

Chris

 

[hotrodder]

As said the specifics and small details of how exactly it's done vary a lot depending on what's being done and to what standard

Most important thing to remember is that argon is denser than air

Running the purge at positive pressure means that any leaks let argon out rather than the air in. If there's positive pressure in the purge then when you come to close the the weld the weldpool can 'blowout' if there's too much pressure
If you follow the simple approach of just plugging both ends then argon will escape from each joint until it's welded. If the fit is perfect then not much argon will escape. It rarely is though and often you want, or need, a root gap. If these aren't sealed up then the purge will fail and/or you'll use far more argon than necessary. 2 extremes...

Relatively small dia thinwall tube like say an exhaust. Typically be talking smallish lengths (slip joints for assembly) so purge volume is pretty small and all we want do is stop it coking/sugaring up. With all the joints taped up 2-3 litres a min will purge that in very little time. If you only remove the tape one joint at a time as you weld then the purge can be turned down to a trickle once established
If we we're talking large dia heavy wall pipe then 1) argon will pour out of each joint making it impossible to establish the purge without taping them shut, 2) peeling the tape off before striking an arc won't end well i.e. you need to peel it back as the weld progresses rather than one joint at a time and 3) purge volumes can go up a LOT. This is when setting up argon dams starts to make sense. They can be improvised or bought (not cheap though), basically it's a way of reducing the purge volume making it easier to eliminate the air and cheaper with regards to argon.

If flow rates are too high when setting up then you can get turbulance and mixing instead of the argon displacing the air, obviously there's a balance to be struck here as trickling it into something large will take forever. Rigging up something to diffuse the argon can help- think garden sprinkler versus the high pressure nozzle. Without an O2 meter a rough rule of thumb is to have at least 6 volume changes to get the O2 down nice and low. Remembering the argon is denser than air bit the purge exit obviously wants to ideally be at a high point with the entry at a low point- try to purge a tank by putting the argon in at the top and venting it at the bottom and you're doomed

 

[arthur]

It's like anything weld based, that's always the problem.... what you welding? An old exhaust or part of the USS Enterprise?


The rubber glove was the very basic method....wink wink.....

 

[pedrobedro]

Gaffer tape with a hole poked in it or party balloons in pipes, anything that blocks it really and leaves room for a gas pipe and a bit of through flow to let air out as it is replaced by shielding gas.

 

[magnet]

i'm not welding anything at the moment ....just curious,thanks for all the replies.......Magnet