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wmrazani
08-29-2006, 09:49 AM
Hi, I am constructing a new 10" SS304L pipeline to jetty. The piperack also contains many flammable chemicals and hydrocarbon. Portable diesel welding generator (Miller Big Blue 500) and TIG weld process is use for welding pipes. There is also a hot dip galvanised carbon steel cable ladder with earthing bonds on each connections. In the middle of cable ladder run, there is one earthing cable connected to earthing pit located on the shore. Resistance of this ground earthing pit was checked and found to be good. Continuity check on cable ladder bondings, between piperack structure and piperack structure to cable ladder are found to be good, ie. < 5ohm.

They are using piperack structure to connect the earthing cable from the welding machine. But somehow, at certain section of jetty, there is arcings produced when they connected the machine's earthing to piperack structure using clamps. There are 3 groups of welders welding on the same piperack but farther away. There is only sea around no ground to drive copper rods and no earthing connection from structure to jetty's piles. Because of concern with danger of arc to flammables, they tried to connect direct to cable ladder but still arcings. I have suggested to dump the cooper rods inside sea but many not convinced. I sometimes found welder hook machine's earthing to dummy structure to continue welding.

My inquiries are:
1. Why arcings happened at certain section of my piperack?
When arcing happened, welders complaint felt electricity on adjacent pipes or structures.

2. Is the method of earthing of welding machine onto same piperack structure (workpiece) or cable ladders (bonded to piperack structure) where welding take place is correct or safe practice?

3. What is the proposed best alternative or practice for grounding the welding machine in this situation?

4. Is the current from weldings can create hazards to adjacent flammable pipeline, i.e. LPG, etcs?

5. Earthing welding machine using copper rod dipped into seawater is acceptable or not?

6: There is also a habit for welders to open or sometimes remove welding machine door panel when running the machine. Why they do this and Is this a safe practice?

BillDaCatt
08-30-2006, 01:02 AM
Hmmm??
OK here goes. Guys feel free to jump in here and correct me.

First off, Not sure what a "jetty" is but it sounds like you are on a ship or an oil rig or something like that. Cool job!

Try tacking a bolt onto one of the pipes to be welded and the clamp your weld ground to that. When you are done, grind the tacks off and remove the bolt. Should cure your arcing between unwelded sections problem.

Are you all using the same welder/generator or is each group using a different one? Either way, it sounds like when one arc is struck the electicity finds all of the grounding leads or a distant one. The current will always follow the shortest path to the grounding lead. Keep that in mind when making your connections. Keep that loop as small as possible. If you have guys between the torch and the ground lead they could get shocked.

Don't try to fix your earth ground by dropping a copper rod over the side. I have seen guys in the Navy die from doing stuff like that. :eek: Talk to your electricial dept if you are having ground issues.

The welding arc can ignite nearby flamables pretty easy. Sounds like you may be outside though, so follow your safety procedures for your job site and you should be OK. You should not have any current going through any flamable pipes. :eek:

As far as the others leaving the cover open on the welder. I'm going to guess it's to allow for more cooling. It can get pretty hot under those covers. Keep your hands out of there and you should be fine.
Dave

whateg0
08-30-2006, 11:12 AM
You should not have any current going through any flamable pipes. :eek:

Not that I am going to try it, but I have heard of people welding on LPG lines and others with the substance flowing through it. Something about displacing enough O2 to keep it from burning.

Just a thought.

Dave

diesel
08-30-2006, 01:11 PM
Hmmm??
OK here goes. Guys feel free to jump in here and correct me.

OK


First off, Not sure what a "jetty" is but it sounds like you are on a ship or an oil rig or something like that. Cool job!

A jetty is a riprap **** on both sides of a river that channels the river out beyond the breaking surf. As a side note never go out to the end of one to enjoy the "spray" from the waves. This will just end up as a valuable lesson and a head injury.

I'm guessing that the runs are too long to get back to the machine and, like always, the current is searching for a shorter path. Check and clean the connections at the machine. This isn't an easy thing to diagnose. How far away from the machine are the welders working?
d.

wmrazani
08-30-2006, 02:20 PM
Thanks for replys,

I wud like to clarify that "jetty" or pier in my context is an extended concrete bridge over the sea for cargo ship to berth/landing and load/unload liquid cargoes using pipelines. Jetty will have access road for vehicle and pipeline run on its sides.

Piperack is a steel structure similar to steel bridges where pipes are place/rack inside it.

Pipes that I weld is new pipes and empty thus risk for fires are minimum. No requirement for O2 free. What concerns me is effects of weld current to adjacents live pipes with distance from 200mm to 6000mm that contain flammable fluids. These live pipes experiencing current flowing on its pipe body or may be inside the fluid itself.

Each group of welders use their own machine and they are separated about 300 meters away. Each welder is not far from their machine, ie. < 50 meters. The generator is sitting directly on concrete surface.

The arcing is happened at the contact point when we try to clamp cable from the generator's frame to steel piperack in order to ensure earth connection. On jetty, there is no soil to drive grounding rod for earth connections. Without grounding generator frame, I still can welding because this is not work lead. The objective is to keep the frame of generator's frame at zero (or earth) voltage. In case power circuit has fault condition (such as short cause by bare wire), current will flow to ground.

My intention to ground generator's frame are for:
- prevent build up of voltage which is dangerous to person or equipment
- to make sure no voltage difference exist between grounded generator's frame and earth so that no electric can flow
- avoid current flow through person body
- requirement by NEC and manufacturer and of course safety procedure

Hope the above explanation will give better picture to you all. Thks

Sberry
08-30-2006, 03:11 PM
Unless there is a short to generator connected equipment there should not be arcing more than static level when you make the frame connection. Doing seperate earth grounding wont help a thing I dont think, seems to me anywhere there is an arc there shouldnt be means there is a bond failure or there is a fault basically. Are the racks structurally bonded, interconnected, or do the pipes make the connection? Someone putting work leads in wrong location causing ground loops

diesel
08-31-2006, 12:52 AM
Something isn't working right. What kind of machine are you using? Here in the states we have thousands of welders operating from 1 ton trucks like a Chevy or Dodge. These units are insulated from the ground by the trucks tires. Many times you will have 2 or 3 welders passing current through the same pipe and this is standard practice with no stray current that I have ever heard of. It would be hard to match the salty conductive environment of a jetty but a good welder should have no problem completing its own circuit unless there is some kind of faulty connection. Why not measure the electrical potential between pipes and racks and the ground and whatever else that you can think of without any welders running. If you measure a lot of potential then maybe the people welding are just more aware of the problem because they are working with electricty. Whenever you have fluid or gas that flows through a pipe you generate electricty. If the cathodic protection isn't working then you will get charge build up. Even if the protection is working the generated charge will need to travel to the nearest cathode and if you bridge a gap to a closer cathode then you become the path. Are the other pipe racks flowing? If they are look for the current to be comming from them.
d.