Welding C/S at high temperatures????

[MadDog88]

We have a refractory lined, carbon steel collection header which has sustained localised refractory damage and subsequent hot spots. We are unable to shut the process down to repair the refractory so I want to investigate an on-line “temporary” repair.

I am proposing to weld nozzles onto the vessel on-line in the areas of localised refractory damage (temps up to 930 OF when steam cooling is removed), hot tap into the vessel and inject liquid refractory into the void areas. To make it more interesting, the vessel contains hydrogen gas at approx 2000 kPa (290 psi).

Things I am wondering are;

· Will welding at these high temperatures cause excessive stresses in the joint when the system is eventually cooled down (hopefully in a year or so).
· Can welding be successfully carried out at these shell temperatures?
· Will the existing refractory be damaged by welding (shell is 16mm thick).

Any help on this subject will be appreciated.

Thanks

 

[Metalguy]

I think your biggest concern should be about what happens if the welding causes a "blowout" thru the weld area. *IF* you can perform an outside-surface, very low penetration weld with no danger of a blowout, the weld itself should be OK.

 

[Metalguy]

I think your biggest concern should be about what happens if the welding causes a "blowout" thru the weld area. *IF* you can perform an outside-surface, very low penetration weld with no danger of a blowout, the weld itself should be OK.

 

[stanweld]

Excessive stresses will not be induced by the proposed weld.
Assuming that the gas is diatomic and not monotomic hydrogen, welding can be successfully accomplished provided the interpass temperature is limited only slightly greater than the 930 F temp with low heat input GTAW/SMAW processes.
If refractory is presently damaged as you have indicated, welding may increase the damage area somewhat.
The weld will lack the toughness normally seen for such welds and may be unsuitable for your requirements. You should qualify your welding procedure to simulate the "preheat/interpass" conditions. You should also "qualify" your welder under similar simulated conditions including welding a nozzle to pressurized (with air) pipe to give him/her that "warm" feeling that all will be safe.

 

[unclesyd]

You had better shut the line down and repair the refractory as soon as possible.

I posted this in another thread.

H2 in line is dangerous no matter what state it's in. I think this is too dangerous to attempt a weld or use a hot tap machine, which is going to damage more refractory, on the line.


If you get a H2 leak and the resultant fire you will have a melt down of anything close.

H2 isn't interesting it is very dangerous.

 

[stanweld]

While I stand by my previous statements, I also agree with unclesyd. I think your on line repair idea is quite dangerous, especially if the actual design temperature of the steel vessel shell is much less than 930 F. You also never indicated the shell temperatures measured at the "hot spots" nor the size/area(s)affected.

 

[MadDog88]

Thanks for your responses. I agree that it is a very dangerous situation and I am just sounding out options at the moment. The 930F temperature was the hottest of the hot spots, this one on a 1.5Cr 0.5 Mo section of the vessel. The remaining hotspots are around 700F on carbon steel. These temperatures were measured by thermography when the steam cooling had been temporarily removed - with steam they are operating at much lower temperatures. All hotspots are around 6" in diameter or less.

We have had an experienced metallurgist give us alarm/high alarm/shutdown temperatures and we are monitoring them continuously. At the moment the temps have stabilised but I wanted a plan in place in case they got worse. Ideally the plan wouldn't involve shutting down but this may turn out to be unavoidable.

Thanks again.

 

[unclesyd]

I think you made a very wise decision.

We have actually used water sprays (mist) and steam on the same material at approximately your temperature. Just keep an eye on the line as H2 can make small hole very large quickly.

Your idea has does have merit as we do it all time on the plenums of an low pressure oxidizer system that operators at 3000°F. When we get a hot gas leak at break in the shell or a "stinger" in the shell we weld on encompassing 1/2" pipe nipples and blow a hole in shell and inject a phosphate based hydraulic refractory very carefully as not to push off the remaining internal refractory.
One other thing we have done in these plenums is the application of process know as refractory welding. This is repairing large areas of failed refractory and burn through without cooling below 2000°F. A little expensive but we avoided 3 days coming down and 3 days coming on stream. Since we didn't shut the unit down we suffered no recordable downtime.

Hope it last until you can have an outage.
Good luck.