Papers & Case Studies
Stop Stealing Our Cathodic Protection!
This article was first featured in the October 2019 issue of World Pipelines magazine.
When protecting pipelines with cathodic protection (CP), one of the issues that commonly occurs is that the CP is shorted out to the electric grounding grid or is absorbed by another unintended neighboring structure. This can cause a technician to struggle with keeping the intended pipeline in compliance as per PHMSA 192.463, Appendix D, pertaining to external corrosion control and cathodic protection levels. Usually, the initial attempt at a solution is to turn up the rectifier current output.
Recently, an example of this scenario was presented to Dairyland Electrical Industries by a pipeline operator. The operator had an electrical substation positioned between the pipeline station and the impressed current cathodic protection (ICCP) ground bed system protecting it. This included a fence in common between the two facilities. (See Figure 1)
The ICCP was intended to protect not only the station, but also the pipeline from the north and south of the station. In an attempt to have the station and pipeline in compliance, the operator had the rectifier output at 55 amps. Readings showed that 33 amps were provided to the station and of those, 28 amps were protecting the electrical substation grounding grid. This reading was taken at the grounding connection at the fence that separated the two entities from one another, as can be seen in Figure 2. This value would shift to near zero when the rectifier was turned off, proving that it was not coming from some other source. Readings for the pipeline from the North side were at 8 amps and from the South side were 14 amps. The remaining current was protecting the station grounding and some of the piping.
With a substantial amount of the ICCP current protecting the utility grounding grid, both pipelines to the north and south of the station did not have adequate CP, with the south pipeline being out of compliance just a short distance away with only a -0.609 volt reading to a reference cell – well below the -0.850 volt industry criteria. The operator had a decision to make in order to improve the cathodic protection readings on the pipeline to make it compliant with PHMSA regulations.
However, what happens when turning up the rectifier simply won’t achieve the desired outcome in order to maintain compliance?
The first option considered was to install an additional ground bed. This would be costly and would not address the issue of the existing ground bed shorting out and protecting the electrical grounding grid. Moreover, depending on the connections, the operator could be faced with the same issue as the existing ground bed and have a significant portion of the amperage displaced onto the electrical substation grounding grid.
A more desirable option is to decouple the facility from the offending substation. Doing so would isolate the DC current to the intended pipeline while still providing AC continuity for safety grounding purposes. Decoupling allowed the pipeline operator to eliminate the negative impact the substation was having on their CP system while retaining an effective ground path and electrical isolation from other underground metallic structures – a compliance requirement of CFR 192.467 (PHMSA).
When choosing to decouple, determining the location(s) to install a decoupler is critical in order to achieve sufficient DC isolation to enable compliance to be achieved on all of the structures the ICCP system was designed to protect.
Critical to proper product selection, one of the first questions to ask when decoupling a complex facility is whether or not there is the possibility of AC being induced on to the system?
In this case, yes. Due to the fact that the structure which is absorbing the CP current is the electrical substation grounding grid, fed by adjacent high voltage power lines, there is the possibility of AC being induced on the piping.
Dairyland manufactures two rugged and reliable devices that are commonly used for this application; the Solid State Decoupler (SSD) and the Polarization Cell Replacement (PCR). These are solid state devices that require no maintenance, have an indefinite lifetime, and are designed to handle multiple fault-current events and even lightning strikes. Due to the possible fault current available from the electrical substation, the operator needed to choose the PCR, as this has a higher fault current capacity than the SSD.
An additional challenge of having an unnecessarily large amount of CP current applied is the increased chance of interference with other structures. Foreign pipelines, for example, may be shifted to a more positive potential due to excess current interaction from the subject CP system. An opportunity to decrease that current output, while still achieving cathodic protection criteria is desirable.
There are multiple areas a decoupler could be installed in order to isolate DC current in this application from the facility electrical grounding system:
- Utility decoupling at the transformer
- Station electrical panel between the neutral bus and the grounding bus
- Motor Operated Valve (MOV) grounding conductor – one decoupler for each
- Station grounding grid bonded to the electrical substation grounding grid – one decoupler for each point of connection
To achieve the best possible result, the pipeline operator looked at each installation in order to determine the best long-term outcome.
Utility Decoupling at the Transformer
For example, by isolating at the utility transformer, the CP current will be isolated from reaching the utility substation. However, everything within the operator’s station will receive cathodic protection. The decoupler would be installed by the utility company and would not be owned by the operator. With this installation, everything within the operator’s station would receive cathodic protection including piping, grounding grids, conduits, buildings, gradient control mats, etc.
Decoupling at the Electrical Panel
If installed at the operator’s electrical panel, a similar result would occur. Everything within the station would be protected by the ICCP system. However, in this scenario, there is no utility involvement, and the operator can have an electrician install the decoupler. With this installation, everything within the station would receive cathodic protection including piping, grounding grids, conduits, buildings, gradient control mats, etc.
Decoupling at the MOV
A Motor Operated Valve with AC power has a grounding conductor that bonds the cathodically protected pipeline to the operator’s grounding grid. Placing a certified Dairyland decoupler in series in the grounding conductor blocks CP current but meets all electrical safety grounding codes. For this scenario, one decoupler would be needed for each MOV in the station. Next, possible bypasses of the decoupler would need to be mitigated. These include metallic conduit, communication cabling, and measurement tubing as possible sources that could bypass the decoupler. The bond between the operator’s grounding grid and the electrical substation would need an additional decoupler installed in series at that connection to further optimize CP performance. By performing this installation, the only structures that receive CP current should only be the intended structures. The station grounding system and the electrical substation grounding grid would no longer receive CP current.
Decoupling the Grounding Grid
Installing a decoupler between the operator’s grounding grid and the electrical substation grounding grid would require a decoupler installed in series in each connection. This would only address this connection and may require further decoupling of the MOVs or other bonds that could be potential bypasses.
In each of these situations, there still would have been an issue with the grounding grid connection that is located at the fence. Even though these solutions would have prevented the CP current from flowing to the utility grounding grid and typically would have been a good solution for this scenario, this would not have addressed the direct connection at the fence. This connection would have bypassed the previous options and would have thwarted any attempt to isolate the DC current.
In this case the operator installed a decoupler in series in the bare copper wires that bonded the electrical grounding grids of the two facilities located at the fence that separated them. A Dairyland decoupler can be installed at this location due to the fact that the device is UL listed as an effective ground fault path as defined in NEC 250.4(A)(5). With this installation, the connection between the two facilities grounding grids are still AC continuous, providing the necessary safety grounding to protect personnel and equipment, however the DC current is isolated at that point of connection.
The result of installing the decoupler at this location was that the CP current was now effectively blocked from flowing to the electrical substation grounding grid. CP current remained on the operator’s station and piping as it was originally intended to do and designed for.
As is evidenced in the readings in Table 1, there was a significant change in the CP potential readings and distribution of current before and after the Dairyland decoupler was installed. After installing the decoupler, the section of pipeline located in the ROW just south of the station improved in electro-negative value from -0.609 volts to -1.412 volts, bringing it back into compliance. All other test points improved in value, meaning that the operator could now turn the rectifier down to compensate for these improved values and efficiencies gained through installing a Dairyland decoupler. The CP current was shifted to the intended structure – the pipeline – as compared to the electrical grounding grid. By turning down the rectifier output, the monthly costs to operate it will be reduced, while simultaneously increasing the life of the ground bed itself.
As for the small amount of amperage that was still going to the electric substation, this was not found to be flowing through the decoupler, but had bypassed the device and was flowing on to the electrical substation grounding grid through another path. The operator didn’t believe this to be a significant amount of current to warrant further investigation at this time and was pleased with the results that were currently being achieved.
The biggest take-away from this case is that the operator could cancel plans for a deep well ICCP system – which was the intended Plan B if the decoupler did not fix the issue. This ended up saving the company $80,000 USD of installing a second ground bed, along with the additional annual costs associated with monitoring and maintenance of the bed. Even though the installation of a second ground bed would have brought the CP readings to an acceptable level, that would not have fixed the problem of the operator losing CP current onto the neighboring structure. This solution would have been the same as having a house with a broken window in the middle of winter, which would create the need for the furnace to be turned up in order to compensate for this source of leakage. In this scenario, would you choose to fix the window to keep the heat from escaping, or not fix the window and place a second heater in that room in order to bring the temperature back up? When put in these terms, it makes it much easier to understand the importance of addressing the issue of CP current being drained by an unintended structure.
If an operator is experiencing similar issues with their CP current flowing onto unintended structures, installing more CP is not the optimal solution. Instead, these structures should be decoupled at the bonds that connect them. This will keep a pipeline in compliance with PHMSA 192.467 and increases the ability to achieve the criteria required in PHMSA 192.463. By incorporating the decoupling of unintended structures into an existing corrosion prevention program, an improved CP system, reduced in maintenance costs, and a life extension of both the intended structure and the CP system itself, can all be achieved.