Why insulate flange? 3

[yahoo123]

Why is it important to insulate flanges at transitions between below grade and above grade interface? What does it do?

Also why is it important to weld bond wires to pipe? What is the advantage of making the pipe conduct?

Thanks!

 

[LittleWheels]

2nd question - Cathodic protection probably.

 

[rcooper]

2nd question, electrical bonding, basically connecting all metal components to a common earth so no potential can build up between them. If they aren't bonded, an electrical fault could cause one section of pipe to be at a higher voltage than another, so anyone toughing it and the adjacent pipe would get an electric shock.

 

[coloeng]

1st question: You typically insulate a flange for cathodic protection reasons. Insulating the flange requires that no metal to metal contact occurs between the two flanges, this includes between the bolts. You need to use plastic sleeves and washers on the bolts that prevents this contact.

2nd question: Bond wires are typically provided between joints of metal pipe for cathodic protection purposes. A typical cathodic protection system either uses and impressed current on the pipe or sacrificial anodes. If you don't have a connection between the joints of pipe, you would have to provide a system for each individual joint.

 

[rcooper]

There was me reading question 1 as thermal insulation, and couldn't think of any reason to thermally insulate specific flanges.

Electrical insulation is for cathodic protection as coloeng states, typically where you have iron or steel potentially in contact with stainless steel.

 

[rconner]

I believe the answers to both your questions employ essentially the same logic. A designer employs an insulating flange ("kit" etc.) when they do not want an electric current to flow across a flanged joint, and a designer welds a wire (more normally I think across other types of joints, such as mechanical and push-on piping connections, but I guess could also be flanged) when they feel they have need dependable electrical continuity across said joints. Either or neither may be "important", depending on the specific application and exposures etc. at a location such as you describe.

 

[cvg]

above ground pipes are not exposed to the same corrosives (water, soil, stray current) as underground pipes are. so it makes sense to isolate the above ground portion with an insulating flange.

 

[SJones]

Just to refine some of the above points a little:

CP design will be based, essentially, upon the buried portion of the pipe. The pipe may come above grade and then enter a plant with substantial buried metal and/or earthing system. Since, without insulation, there will be electrical continuity, the efficacy of the protection on the buried pipe could be reduced or rendered ineffective. NACE Standard RP0286 will give you good background.

Bonding, as well as being required to overcome breaks in conductivity along the pipeline, may also be required between the pipeline and any adjacent structures to prevent interference. Again, NACE Standard SP0169 will give you good background.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[MTPipeliner]

An advantage of the insulated flange with a bonding wire (as opposed to an uninsulated flange) is that it can be disconnected to check the cathodic protection on the different sections of pipe instead of checking the system as a whole.

This is a pretty common setup on gas pipelines.