Pipeline Active Cathodic Protection - PCR, SSD ?

[TwoBlocked]

Had a conversation with a guy that services a customer's gas pipeline cathodic protection system. He said the system cannot handle the DC current coming from a particular gas compression site (that I am doing some install work on) and is recommending either a PCR (Polarized Cell Replacement) or a SSD (Solid State Decoupler). This would be installed between the ground and the neutral in the system, becoming the MBJ (Main Bonding Jumper). This site's MBJ is in the meter box where the neutral bus is bolted directly to the enclosure.

That's the situation, but I don't understand a whole lot about it. Seems the only info on these types of systems are from the manufacturer's own websites. I prefer info from independent sources. We would most likely be the outfit doing or arranging the installation of the device(s).

So any of you fine folks been involved in such a system? As soon as I understood that this is messing with the MBJ all kinds of alarms went off in my head. And considering that the MBJ is the meter box itself, am thinking this might require a different meter box.

 

[TwoBlocked]

Hmmm, guess there is not much experience here on these devices. OK, but there are other things that someone might be able to explain to me. We took clamp-on amp readings on the EGC (Electrode Grounding Conductor). There was 0.3 amps AC and 0.8 amps DC. When the guy bumped up the CP (cathodic protection), the DC amps went up to 1.3 amps. And with the CP off, the DC amps went down to 0.4. The CP unit is installed at another compressor station miles away. The pipeline at this site is electrically insulated before it goes underground.

The site is the only facility on the pole mounted 120V/240V split phase transformer. The meter is a Fluke 376C and was zeroed before each DC amp reading. The clamp meter's showed "positive" current flow during all readings with the "-" mark facing toward the MBJ. I take this to mean the grounding grid is the anode, and the site's neutral is the cathode. but am not sure.

So here is what I hope you fine folks can help me with:

What could be powering the DC current loop even when the CP is off?

Is the site's grounding grid the cathode or the anode?

 

[synchro]

First of all I don't have experience with cathodic protection of pipelines, but I have a few questions.

Is the "Electrode Grounding Conductor" you put a clamp meter on connected between the CP unit and the buried anode used for cathodic protection? Or is it on a conductor between the grounding grid and the MBJ of the service, or somewhere else?

Also, is this CP unit the only one on an electrically isolated segment of the pipeline, or could there be another CP unit connected to the pipeline some distance away that could be contributing current,?

 

[TwoBlocked]

First of all I don't have experience with cathodic protection of pipelines, but I have a few questions.

Is the "Electrode Grounding Conductor" you put a clamp meter on connected between the CP unit and the buried anode used for cathodic protection? Or is it on a conductor between the grounding grid and the MBJ of the service, or somewhere else?

Also, is this CP unit the only one on an electrically isolated segment of the pipeline, or could there be another CP unit connected to the pipeline some distance away that could be contributing current,?

The EGC starts at the main service disconnect where it is connected to the enclosure and continues into the dirt where it connects to the grounding grid. The current measurements were taken on this grounding cable between the main service disconnect and the dirt. The EGC is electrically connected to the neutral through the main disconnect switch, a piece of metal conduit, the meter enclosure. and the terminal strip in the meter enclosure, which is permanently fastened to the enclosure. There is no MBJ in the main service disconnect as required by the NEC.

My understanding is there is only the one CP on the section of pipeline leaving the site, and it is miles away. I guess the buried electrode follows along the pipeline. There are some current limiting resistor in the system, one at this site, that the existence of was a "Clue" to the CP guy that there has been a problem. I guess this resistor would connect the site's piping with the piping going underground.

 

[Dsg319]

I work on pipeline valve sites where we install SSD from the field devices EGC and back to our RTU ground bar.

I don’t know much about them but I have reached out to dairyland and they were fairly quick about emailing me back.

I know we use them to keep dc current from the cathodic system off of our electrical system. I was curious if it would still allow 24VDC fault current to activate a OCPD.

I’m still curious a lot about them and havnt done any field test yet to test my idea. But would like to learn more about them. I’m thinking it’s a low threshold voltage that they can keep away but still allow a 24VDC system to clear a fault?

 

[winnie]

I don't work on these, but was curious. It looks like a potential cell is simply lots stainless steel plates in a potassium hydroxide solution.

Think battery but with same materials for each electrode, thus no natural DC voltage.

I'd expect it to act a bit like a very leaky capacitor, and to flow ion current if the voltage exceeds a threshold value.

I presume the solid state replacements are designed to act like the chemical cells.

 

[TwoBlocked]

I understand that what they do is block DC up to 2V, but pass AC. At 2VDC it breaks away, allowing the charge to dissipate, I guess. The idea of putting this in place of the MBJ, I don't know. The idea I got from the guy was that the PCR or SSD is so the CP system wouldn't have to work as hard. If they want this installed, I'm going to refer it to our company engineer out in Colorado.

 

[synchro]

I think the PCR or SSD would be intended to go between the underground pipeline and buried anodes or other grounding means. And there would be insulating couplings to electrically isolate the underground pipeline from the above ground piping so that the CP system can apply a negative DC voltage of something like -1.2V to the underground pipleline relative to the earth. And so from what I've read, the PCR or SSD would not be intended to replace the MBJ of the electrical system.

By the way are there any HVDC power lines or rail traction systems along or crossing the pipeline? They could possibly couple some DC leakage current into the pipeline system.

 

[TwoBlocked]

I think the PCR or SSD would be intended to go between the underground pipeline and buried anodes or other grounding means. And there would be insulating couplings to electrically isolate the underground pipeline from the above ground piping so that the CP system can apply a negative DC voltage of something like -1.2V to the underground pipleline relative to the earth. And so from what I've read, the PCR or SSD would not be intended to replace the MBJ of the electrical system.

By the way are there any HVDC power lines or rail traction systems along or crossing the pipeline? They could possibly couple some DC leakage current into the pipeline system.

There is a very rural setting, just trees.

I am seeing these devices as high pass filters, allowing AC but not DC, with a DC voltage limiting function at 2 volts.

As far as where the DC current on the GEC is coming from, there is always inductance where there is AC. So suppose a heat trace feeder runs along a pipe or ladderway, inducing AC current which is then rectified where it comes in contact with the soil. Don't know if this is a stretch or not.

Not thinking too clearly lately. Been working in a lift running cable inside a gas compressor building - a bit of heat stress.