Welding new nozzle on existing weld seam. Possible? 2

[indimech]

Hi All,
I am working on modification of SS 316L atmospheric tank. The design internal pressure is 10 psi, design external pressure is 2 psi and design temp is 250 deg F.

As per the drawing the vessel is manufactured as per ASME Code section VIII Div 1 but it is not inspected and stamped.

My problem is I have to locate the nozzle and at the most preferred location there is weld seam of shell. The nozzle is a 6" pipe which will be welded to make tangential entry of liquid.
My question is can we weld on weld seam if not then what is the minimum distance another welding should start from one weld?

What are the problems of welding on weld seam.

Please let me know if you need any other information.

Thanks in advance.

 

[metengr]

My question is can we weld on weld seam if not then what is the minimum distance another welding should start from one weld?

Yes, you can penetrate the weld seam and weld a new nozzle for this vessel. After drilling the hole for the new nozzle, I would break the corners on the ID and OD surfaces and perform a liquid PT before installing the new nozzle.

What are the problems of welding on weld seam.

The only problems are overlapping weld heat affected zones and more exposure to sensitization of the austenitic stainless steel base material, but sometimes this is unavoidable.

 

[indimech]

Thanks metengr for quick reply.
Whether overlapping HAZs will make the weld weak or what will be the disadvantage of overlapping HAZs?

 

[metengr]

indimech;
Anytime welding is performed you have base material which surrounds the weld region exposed to thermal gradients that results in grain coarsening and alteration of the original base metal microstructure. For austenitic stainless steels, the key to having optimum corrosion resistance is to avoid any unnecessary heat input that results in sensitization. Sensitization occurs when austenitic stainless steel is exposed to the temperature range of 900-1400 deg F. What happens is the chromium in solution wants to join with carbon atoms in solution resulting in precipitates. The loss of chromium from the austenite solid solution results in lower corrosion resistance.

Since you have 316L (low carbon) for your application the effects of sensitization will be less. So, as I mentioned above you should be ok with the second attachment weld. If this was a standard 316 or 304 base materials, the heat from welding a second nozzle would locally expose the seam weld region to potentially 900-1400 deg F sensitization.

For carbon and low alloy steel, overlapping heat affected zones could result in lower fatigue strength.

 

[indimech]

So am I right in saying that first welding alters the properties of heat affected zone and second welding temperature at the same place may deteriorate it further which can decrease the fatigue strength?
If I want to determine whether after welding on a seam weld the tank will be in operating condition for intended service what options do I have? Any kind of test may confirm this? Can I know before hand that second welding will not make the tank useless?

 

[metengr]

So am I right in saying that first welding alters the properties of heat affected zone and second welding temperature at the same place may deteriorate it further which can decrease the fatigue strength?

No, not for this grade of stainless steel that was orginally supplied in a solution annealed condition. I cited the example for carbon and low alloy steels where you have a more altered microstructure due to the nature of the alloy.

As I mentioned sbove, your only concern would be decreased corrosion resistance where the new weld overlaps the original seam weld. What is inside the tank?

 

[indimech]

Tank will store perfume microcapsules. As per the piping specification sheet no corrosion allowance recommended so I guess by nature its not corrosive.

 

[deco0404]

Indimech,

at the temperature and pressure you are talking about I don't think you will have too much trouble with this. We do a lot of hot-tap work in the mainline gas distribution business and quite often we need to weld over weld. It is not an ideal situation, and is best avoided, but it is something that is done quite frequently (Transco have a branch dedicated to this type of work). We pressure test at around 120Bar, and as yet have had no disasters.

As Meteng stated you should do NDT on the existing weld. I would go a little further back than just the face of the welds. If you can do x-Rays or ultrasonic tests, go back approx 500mm either side of your cuts because no mention has been made of the quality of the existing weld. If there is a defect in the existing weld and you weld over it, this could seriously compromise the integrity of the new one...

Declan