Use of isolation for UG to AG pipeline 1

[brpillai]

Dear Experts,

I am working on a lquid pipeline project (over 2100km long), which is having piping and pipeline going from UG to AG for locations such as below:

1. Pipe crossings at road crossings within the station. these include product pipes, fire water pipes and drain pipes.

2. At block valve stations pie branching for bypass to the block valve stations as shown in the attcahed sketch. Please note that this pipe is not connected to any underground servies, only a by pass in the atmosphere.
My queries are as below:
a. At road crossings, if in the vicinity there are no underground steel pipes/ structures/ earthing, do I need an isolation joint/ isolation flange.
b. What are the criteia for deciding where Iso joints are used and where iso flanges can be used?
c. For the pipeline valve stations, as per the sketch, is it necessary to have isolation joint or can it be done wioth isolation flange?
d. Is it neccessary that isolation flanges to be used for all 31.4 and 31.8 pipelines?

Your valuable advice is sought.
Thanks and regards

 

[BigInch]

You do not need isolation flanges or monolithic isolation joints at any of the locations you have mentioned above.

Isolation joints are used to isolate pipelines from other pipelines, such as where two pipelines are joined together, or where multiple pipelines are being joined at a common header.

Isolation is not required simply because components of an underground pipeline rise above the earth's surface, as you have shown in your diagram. Coat the underground pipe consistant with your corrosion coating scheme and the pipe going above ground for a distance of 0.5 meters above grade and paint the rest of the above ground pipe consistant with your scheme for corrosion protection for above ground pipe.

Isolation joints are often used to connect pipelines to stations, so the cathodic protection system for the pump station can operate as independently as possible from the cathodic protection system used on the pipeline. Isolation flanges and monolithic isolation joints can be short circuited by misalignments caused by excessive (especially bending) stresses, so it is prudent to NOT locate these in areas where stresses can be high, such as between pipeline and pig launchers where the pipeline comes above ground. That is typically one of the highest stress locations to be found on a pipeline. Treat pig launchers and attached kicker lines as part of the pipeline and do any isolations that may be required at pump stations at one point on the line between the launcher and the station. Isolation of instrument tubings connecting pig launchers back to the station piping, if any, will also be required.

Isolation is also required for offshore pipeline systems between pipeline and platform, so that the cathodic protection systems for each can function independently.

 

[brpillai]

@BigInch, Thanks for a very clear explanation.

 

[SJones]

The need for isolation is usually generated in the cathodic protection design report - see, for example, ISO 15589-1. Type selection will be driven by such things as: location, transported fluid, maintainability, reliability, and process safety amongst others.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[brpillai]

@Steve Jones, Thank you for the information.

 

[BigInch]

Ideally that's true, but it would be a first for me if I were to get a CP report before I had to complete the pipe design. A lot of the in place cp potentials and interferences that will require isolation aren't apparent until a year or more after the pipeline is installed. Plus the CP guys don't know the low stress locations on the pipeline.

 

[SJones]

A lot of the in place cp potentials and interferences that will require isolation aren't apparent until a year or more after the pipeline is installed.

That would become apparent from a monitoring report. An upfront CP design report should be able to identify the immediate need for isolation. An offshore pipeline generally has to go in with the CP - can't be doing the design when it's on the sea bed. Interference is more likely to be addressed by cross-bonding and other methods rather than isolation. In some cases, it could actually be the isolation that contributes to the onset of interference. The 'CP guys' will tell you the need for isolation and the options for type and location; you as the pipeline engineer have to work out what form it will take and where exactly it can go.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[BigInch]

Ah. Good point. I over-blended interferences with isolation.