Cathodic Protection for water pipeline 3

[WaterGIS]

Hi all
we have a 600mm Dia. ductile water pipe, that is buried 2 at least 2 m, a Gas line 600mm will be buried over it and it has Cathodic Protection , our water line dose not has any Cathodic Protection , will that will be harmful to the water line.
Regards,

 

[waterworker]

http://www.dipra.org/pdf/strayCurrent.pdf

note page 4/5

I would say "probably", based on the explanation refered to in the above document.

I'm sure there is someone here more knowledgable than I in corrosion control, and I have had (only) ancedotal evidence of an impressed current on a gasline above a (fifty plus year old?) cast iron Universal joint ( 4 bolt flanged joints) water main seemingly causing more corrosion leaks ;and no water leaks on sections of the same line where they are not one on top of the other..

For three quarters of the length of one long block, a six ( or eight, I forget) gas main (at depth + or- 4 feet )cathodicaly protected with an impressed current exactly parrallels , and is directly above the above mentioned 4 inch diameter water main, (depth + or - eight feet).

There were three or four corrosion caused leaks ( in a four year period) on the water main in that section ( four hundred feet or so) with the gasline right above it. The water main then bends with two forty fives and crosses from close to the South curb to close to the North curb, and in this north curb length, about thirty feet horizonal seperation from the gas main, there have been no leaks. Age of pipe , soil, etc. seem to be the same in these pipes-My GUESS is that it's the impressed current from the gasline causing accelerated corrosion on the cast water main.

 

[WaterGIS]

Thank you a lot Waterworker , this was very helpful to me, but how could I protect my water line then? Any recommended solutions for the intersected area? We have also had a GRP line that intersects that Cathodic Protected GAS line. Is it affected by this or not.

 

[Fizzhead]

WaterGIS:

The answer to your question is "Yes, most likely there will be some effect." It is possible, but not likely, that there is no effect. The person/company doing the gas line cathodic protection study and design should take this into account and at the least, provide you with recommendations on how to prevent enhanced corrosion of your installation. I have been involved where the gas company would be responsible for providing whatever protective measures are required for you system and they have done so willingly, asking only that you grant permission and approve of the methods employed. Barring that solution, you should find someone to investigate for you and determine any effects and recommend solution(s).

 

[HB2U]

When my company has to place water line within 50 feet of a gas line that is cathodically protected we go with either a polyethylen wrap or C-900 pipe in that section. Since your line is already there, shouldn't the onus of protectioj of the water line be on the gas utility? They can replace the line outright with the c-900 or install new, wrapped ductile.

The gas company could also use plastic pipe in that area.
I personally wouldn;t let them tell me that the annode beds that they place along that line will protect the water line unless they sign an agreement to warantee that fact.

Here's the spec that I've used in my past encounters with gas lines. The appurtenance wrapping section can just be used outright if you want to use ductile instead of plastic pipe.

"Water line shall be constructed of PVC pipe in accordance with AWWA Standard C-900 where it is located within 100 feet of either the xyz or abc Gas Corp. pipeline. The PVC pipe shall be installed in such a manner that a full length of pipe is bisected at the point of intersection with the petroleum pipeline. All appurtenances and fittings located within this area shall be ductile iron and shall be wrapped in high density cross laminated (HDCL) polyethylene film with a minimum thickness of 4 mil. Minimum sheet width for wrapping appurtenances shall be as follows – 6” diameter requires 32”, 8” diameter requires 40”, and 12” diameter requires 54”. HDCL wrap shall extend a minimum of 3 feet beyond the centerline of any appurtenance to be wrapped. All HDCL polyethylene wrapping shall be done in accordance with AWWA Standard C- 105."

Hope this helps.

HB