Corrosion on a split tee after hot tap

[wagga]

Hi All

I am doing a line stop on a 10" low pressure steam line. Steam pressure of 200 - 300 kPa. Temperature of 150 degC.

I will need to weld in a split tee to accommodate the line stop equipment. It will be an encasement of the pipe by the split tee. It will be hot tap to allow for insertion of the stopple. After the work, the stopple will be removed.

My concern : there is now a small gap between the tee and the pipe where hot condensate might crept in and will be a source of corrosion. The only bit holding the steam in is the fillet weld around the pipe and seal weld along the lateral.

Question: Has anyone had any experience with this repair. Would that split tee expected to be a permanent feature, or expected to be removed totally in the future.

 

[wagga]

Hi All

I;ve attached a schematic of it.

 

[EdStainless]

If you get liquid in the gap and then hit it with steam and flash it you will either split the tee or buckle the line.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Correct. You will get some leakage between the outer pipe and inner tee. The only way out is to have a bigger tee than the hole you are going to drill and seal weld the gap before you tap the pipe. Then you need to allow a weep hole in the tee for any trapped water to escape which also will prove your weld under operating conditions.

Or grout up the inside from the bottom before you hot tap

Given that the tee will have a valve on to be able to hot tap you end up with a big arrangement.

Having said all that I don't believe there will be much corrosion and you can do UT checks on a regular basis. If it is becoming a problem, include it's cut out in the next shutdown.

Ed - There will be a gap for the water to flash off back into the main pipe

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[StoneCold]

Why not weld the branch directly to the side of the pipe and add a reinforcement collar around the pipe stub. Then you won't have to cut it out later and you will have avoided this problem

Regards
StoneCold