Welding new pipe to corroded pipe 1

[sentrifice]

Hi,

I am working on a piping modification project that involves hot corrosive fluids (>200000 ppm chlorides, 200 F). The current piping is 1.5" sch 10, and the material is 6% moly (254SMO). Design pressure is 15 psig.

As part of the project, we will need to tie in new piping to the old piping. I am worried about the tie in process because I am assuming that the existing piping will be at least partially corroded, and it would not be a good idea to buttweld to corroded pipe. I thought to demo the piping back to the nearest flange to avoid welding new pipe to old, but the nearest flange is pretty far away so that's a lot of expensive piping that would need to be replaced.

My thoughts are that I have 3 options:
1) Demo pipe back to nearest flange and have an expensive design
2) Perform test or examination on the pipe to confirm pressure design and estimate remaining life
3) If client doesn't want to do the first 2, they need to somehow absolve the engineer and contractor from responsibility for the existing piping. I'm not a fan of this option since it doesn't seem possible to absolve anybody from responsibility, and the fluid can be considered hazardous which is even worse.

I would like to go with option 2, but I have no idea what that testing process looks like. While I wait for input I will do some research on the topic.

 

[1503-44]

Hazardous fluid.
Corroded pipe.
200°F
Client already knows new pipe is expensive.
It's a cost of doing business.
It's still cheaper than an accident.
You are already worried about liability.
You are not there to take on your client's risks.
It's not your money.
Best technical solution is #1
Get some sleep.
#1

Always propose the best technical solution.
If client objects, let him propose other inferior options (in writing).

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TugboatEng]

This is very thin walled pipe to begin with. It obviously doesn't need strength. It will be difficult to control the HAZ in the pitted areas due to the varying thickness. Is this material sensitive to corrosion in the HAZ? What is the risk associated with failure? Is it tolerable? Or, can you qualify the repair through some form of NDT? Will this qualification cost less than replacement? Insurance will likely want a 3rd party involved.

 

[LittleInch]

This is the key here though for me "because I am assuming that the existing piping will be at least partially corroded, "

Why are you assuming this? Are you guessing or do you actually have data? If not go and get the data such as some 360 degree UT scans of the pipe.

OIs this a new pipe connection or you replacing a broken bit?

There is always a trade off here when connecting to an existing system about how much, if any, do you replace.

Usually you scan the tie in point and if its corroded, you start moving back until you find an uncorroded bit. The existing pipe is then no worse that it is at present. That's the key point - don't do anything with your new pipe which can cause higher stress / pressure / temperature tan before. If you do then you need to test the whole thing or don't do it.

Then you hydro it all as if it was new pipe and if if fails then you replace it. If it doesn't it has passed the test.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

Assuming that the old pipeline is corroded and after measuring the thicknesses, there is a feeling that it can still be used. Under such situation I propose the following for additional safety(for review by experts)

Basically, a fabricated reinforcement is shown for strengthening of existing pipe at weld location. This reinforcement can be welded to the new pipe by using a fabricated transition piece.
A sample piece can be cut from the existing pipe to be fabricated like the above, and testing can be done to determine its strength and durability.


Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[sentrifice]

LittleInch: It is a new connection, not repairing a broken piece. But I think your approach is a good one either way - it seems unnecessary to assume complete replacement from the get-go if we can properly qualify the system. Like you said, pressure testing would qualify the existing pipe, and testing/examination will qualify the weld/tie-in point.

So for my design, I will choose a suitable tie-in point and assume that the existing pipe is good (I haven't heard of too many failures from the client). Then in the contractors scope I will have them qualify the tie-in point with scans or visual inspection. If the tie-in point is too corroded, then it will be a change order to replace more pipe and that's just the risk of business. If the hydro springs a leak, then it would be the same outcome. I will obviously include the client on this decision.

And to Tugboats point, I'll have to think through the long-term effects of the weld on pitted pipe.

Thanks all for the input.

And Goutam: I appreciate the suggestion but if I did that, I think I'd be laughed back to the office.

 

[goutam_freelance]

@sentrifice

Could you give some comments on the sketch as a token of appreciation, Such that I do not regret the time spent?

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[LittleInch]

Tieing in new stuff to existing always brings up these issues so the best you can do is require a site inspection to find a suitably non corroded section of pipe. Ditto re testing the existing is also a potential risk in terms of finding new leaks / creating damage, but at a design pressure of 15psi something the thickness of a piece of paper will hold that pressure.

If the client wants to reduce risk, then suggest they do it (a pipe survey) now, not when the contractor is on the meter for any changes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[sentrifice]

@LI
Good point, thanks for the wisdom. I've never gone through the pipe survey process, so I'll need to do some research.