Repair Welding on Water-filled Pipe

[TechnoZA]

Greetings all. We are doing some maintenance on a major water pipeline in our Agri-business. This requires welding a steel plate to "patch" a leak. As the repairs are being done, water is being transferred through the pipe.

Will this seriously affect the weld penetration? Will the weld be less reliable due to the heat transfer with the water?

 

[MikeHalloran]

Yes. Yes.

The welds will be ugly, too.
That will make the welders unhappy.

BTDT. At night, in winter, on a tower.
Thankfully, there was no wind.
There was no wind because it was well below 0F.
Okay, I was holding the cable so it wouldn't slither back down the tower.



Mike Halloran
Pembroke Pines, FL, USA

 

[metengr]

This is called water-backed welding. Yes, it can be done for carbon steel base materials that are considered weldable and provided you qualify a weld procedure specification for this practice. Obviously, you did not do this because you asked the obvious question on the effects.

 

[TechnoZA]

Metengr: Can you direct me to where I could find a sample of such a procedure - or guidelines to get an idea of the matters to keep in mind.

I assume one critical factor is safety of the welder from electrical shock. A leaking pipe means that the ground on which he is working is damp - probably an electrical shock hazard as well?

Thanks for the help.

 

[metengr]

TechnoZA;
One piece of information that you need to know is that the water leak MUST be stopped before welding both for welder safety and weld quality. There are various ways to do this -peening or use of freeze plugs to isolate the line for weld repair.

At our Power Plants we have on occasion needed to repair low pressure service water piping systems. A weld procedure for water-backing was qualified in accordance with ASME Boiler and Pressure Vessel Code, Section IX.

 

[EdDanzer]

To do this procedure you will need to use 6010 stick rod and some preheat for the first pass. A whipping motion should allow the last section to be sealed over if the pressure is not too high. The second pass will need to be with 6010, and then a cover pass with 7018 if the pressure is very high. Although I have never done this with water, I have welded hydraulic tanks and cylinders with oil spraying out (less than 10 psi). The flames make this task difficult. Anyone trying to do this should have rubber boots, rain gear and rubber gloves on. They should use an electronic hood so they won’t get sprayed with hot water or steam. It would be prudent to test the process and procedure with sample parts at the same pressure to determine feasibility and weld quality.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Compositepro]

There are also alternative repair techniques. Clamp on patches and fiberglass/epoxy.

 

[EdStainless]

The only time that I saw this done they used a special epoxy patch to seal the leak and then welded a large steel patch over it.
One serious concern is the condition of the pipe. If one area failed there could be other thin or corroded areas near by. The weld patch will increase local stresses. I have heard stories of entire patches blowing out due to thinned pipe walls.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[corrosionman]

We once patched a 24 inch steam line containing 15 psi steam with good surface preparation and a suitable High Temperature resin. The emergency patch was still there 6 months later. Such techniques would be easier than welded patch on a water pipe, but if you really want a weld repair method, weld on a sizeable flanged branch which encircles the hole - - the welding will be distant from the corrosion of the hole and then you can blank flange the branch.
good luck
CM

 

[MTPipeliner]

The water in the pipe will tend to pull heat away from the weld, so penetration is definitely affected. The quick cooling rate will also lead to hydrogen induced cracking. Because of these two issues you need to have a higher heat input than when welding empty pipe. If the heat input becomes too high you risk having a burn through, so it’s a balancing act to stay between the two extremes. As was mentioned above you also need to make sure that you are welding onto base metal that is in good condition. You need to inspect the outside and inside (UT or other method) to ensure you have full wall thickness and no laminations where you are welding.

There is quite a bit of information about in-service welding for pipelines (mostly natural gas and oil from what I’ve seen). There are some fairly expensive computer models to help select the welding parameters.

If this is a onetime thing you don’t want to go to the work of researching and qualifying a procedure for future use, you may want to contact a pipeline contractor who has experience doing this type of work. They can probably help you out with a procedure as well as provide welders who have experience with in-service welding. Look for contractors who do work for oil and gas companies, or who specialize in hot tap work. Sleeving over leaks and damage is fairly common for some pipeline companies.

 

[racookpe1978]

How big is the hole?

What location around the pipe? (Top, side, upper 45, low 30 degree point, bottom, on an elbow or bend?)

Can you get to the break? (Is it in mid-air, near the ground waist high, or under the pipe in a water-filled ditch?)

Is the (current) outside of the pipe nice and smoothly prepped? Did the break cause deformations and raised or cracked or broken-out parts of the pipe where the new patch can't sit smoothly?

 

[dik]

and try not to use low hydrogen electrodes <G>...