When is a flange isolation kit recommended? 2

[cpcuriosity]

When it is recommended to use a flange isolation kit? I have two scenarios that I think might call for a flange kit but I don't have the expertise to determine if they are needed and why.

First scenario:
DIP flange adapter that will connect to PVC pipe. DIP flange will be bolted to a steel flange connected to steel piping. Since there will be dissimilar metals it seems to me that a flange isolation kit would be a good idea at that flange connection to eliminate galvanic corrosion. However I have been told that isolation here will not be an issue because the DIP flange will be connected to PVC piping… any thoughts on this?

Second scenario:
Steel flanges connected to steel piping. Since there are no dissimilar metals, no chance of galvanic corrosion. However I still see the possible need to electrically isolate to prevent current from traveling beyond the flange connection. When is flange isolation used in a CP system? At what intervals?

Thank you in advance.

 

[bimr]

There are a number of factors to be considered, but the first scenario is most likely not an application. The reason for this is the position of the metals in the galvanic series. Ductile iron and steel are considered to be close galvanically. Corrosion is more of a concern when the metals are far apart galvanically. PVC also does not conduct electricity.

Factors that will come into play will be: geometric factors, fluid electrolyte properties, flow rate, surface condition of the pipe metals, buried piping, etc.. The question is, will there be an expectation of significant metal loss. The answer may come from operating experience.

The second scenario is probably not an application either.

Isolation flange kits are for isolating pipelines that have Cathodic protection. An insulating flange arrangement may be necessary if the fluid conductivity warranted it or you had dissimiliar metals. Reliability and maintenance issues should also be evaluated.

Application of Flange Insulation kits
•To limit the extent and cost of cathodic protection current to only those pipes that need to be protected from the main cathodic protection system.
•To electrically "split up" long pipelines into distinctive cathodic protection systems.
•To isolate a pipeline to ensure that cathodic protection or stray electricity currents do not cause increased corrosion, or cause a hazard.
•To isolate piping systems where dissimilar metals are present.
•To eliminate static loading charge transfer in discharge or loading operations at oil storage terminals.

http://www.piping-world.com/flginsul_01.html

https://www.farwestcorrosion.com/fw/downloads/WorldPipe-April2012.pdf

 

[LittleInch]

first scenario I agree with Bimr

Second - CP systems are only effective on buried pipe. Hence isolation from other pipe which is earthed or can be touched is at the first point possible after coming above ground.

Insulating flanges have a poor track record in operation and often fail in service leading to poor CP protection and potential for corrosion elsewhere. Isolation Joints are normally preferred.

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

 

[cpcuriosity]

bimr, thank you for the response, very informative. Question, what criteria is used to determine placement of flange kits to split up a pipeline to isolate when creating cp zones?

LittleInch, thank you too. Also very informative. In your experience of insulating flange failures, and realizing there are many possible reasons that an isolated flange can fail, is there a common factor that stands out time and again?

Thank you both for your time and input, it is kindly appreciated.

 

[LittleInch]

The bolt sleeves are often the cause of failure and with their number it only takes one to short the flange.

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

 

[cpcuriosity]

Thank you LittleInch. That makes sense.

 

[bimr]

For a tape wrapped welded steel pipe - The coating efficiency is 98% with a cathodic protection station required every 20,000 feet. Test stations are every 1,000 feet.

 

[cpcuriosity]

Thanks bimr, appreciate the great information and the link!