Induced Currents in Pipelines

[schell44]

We have many miles of gas transmission and distribution pipelines that share right of ways with electrical T&D lines. My concern is with induced currents on the pipelines caused by faults on the electrical lines and specifically, the shock hazards that personnel could be exposed to at regulating and metering stations. All of the pipelines are cathodically protected. I’m mainly interested in how others in the industry are addressing this problem. Do they perform grounding studies and calculate current contributions from each line? Do they use polarization replacement cells? Are there any papers written on the subject? Also, are they any organizations or groups of electrical engineers in the natural gas or pipeline industry? I haven’t found anything in NACE or AGA.

 

[alehman]

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4113689

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[RRaghunath]

I got the attached paper from internet search, find it quite informative.

 

[resqcapt19]

Here is a paper I found on this issue.

 

[xxjohnh]

Most oil companies ignore it, as long as the lines cross the pipe lines at 90 degrees. If you get some grounding expert he will design some complicated mitigation means. It is the stuff for papers not practical applications.If it ain't broke don't fix it.

 

[eamonkerrigan]

I would definitely not recommend ignoring this problem. Induced currents can be significant. Most new pipelines have a very good coating resistance which means that induced currents do not tend to just 'bleed off' by themselves. Under fault conditions, induced (and conductive) currents can cause significant pipeline damage as well as the obvious safety hazard. I've seen the coatings damage from faults which cause small pinholes in the pipeline walls. I've also seen steady state voltages above 50 volts measured from pipeline to ground.

There are hundreds of papers written on the subject. I would refer you to IEEE and also to NACE (national association of Corrosion engineers).

Polarization replacement cells are a pretty old technology. Dairyland Industries makes a nice product called a DC decoupling device which allows AC currents to be grounded while blocking DC current (for cathodic protection). You could also install a zinc ribbon anode system which can be directly connected to the pipeline (may require some extra CP studies especially if your using an impressed current system).

I used to perform these studies and they are a good way to cover yourself from liability...but I've also seen the trial and error method...install a bunch of zinc ribbon, measure your voltage, then if you need more, dig another trench.

 

[schell44]

We are not ignoring the issue. The reason I brought up the subject is that we are installing new metering, pressure limiting and regulating stations on existing pipelines that have active cathodic protection. Most of the older stations used zinc beds. As new electrical power is brought in for SCADA & controls, I've been specifying PRCs to isolate the pipeline from the 120 volt ground. However, most of the pipelines are old and we have lines that parallel electrical T&D lines some distance away from the station. For instance, one pipeline parallels a 21 kV line (If=5.5kA) for 3 miles starting 3 mile from the station and a 60 kA line for 1.5 miles starting 4.5 miles away. Two other piplines leaving the station are similar although with 12kV lines. My concern is the voltage rating of the PRCs.
Bill

 

[eamonkerrigan]

Bill,
I think I understand your issue a lot better now. To estimate the voltage ratings for your PRC's at the station is going to be difficult especially considering that these are older existing pipelines. The main reason is the resistivity of the coatings (for new FBE usually assume 1,000,000 Ohm*ft2 vs. Old Coal Tar could be only 100,000 Ohm*ft2 or less).

The usual thinking is that older (lower resitivity) coating provide an easier path for current to leave the pipeline. So you would have a lower voltage at the pipeline station. However, you would probably have to hire someone to do a thorough analysis because it's all based on geometry of the lines and how many pipelines...among many other things.

A good reference is from NACE Corrosion 2003, Paper #03698 entitled: Estimating AC Mitigation Requirements for Pipelines installed in High Voltage AC Corridors: Fault Conditions.

Hope this helps.

-Eamon

 

[niallmolloy]

This is also a serious issue in electric rail systems. As far as i know, corrosion is mainly a DC problem. Look up some IEEE papers on stray current for detailed anslysis.

Niall



Niall Molloy
Power Engineer - Railway Systems
Mott Macdonald