Interrupted Surveys and Dairyland Decouplers
In some instances, decouplers such as the PCR and SSD, when installed on well-coated pipelines can affect OFF-potential readings during interrupted survey testing. The survey crew may witness irregular OFF-potential waveforms at or near where decouplers are installed. Most notably, the OFF values will be excessively electronegative as shown in the figure below.
It could be incorrectly concluded that the pipeline is well protected, however personnel should be aware that such OFF potential values may be in error. In some cases, such polarized values are not actually possible on protected steel. Instead of clean ON and OFF potential waveforms, personnel may observe curved and slowly changing waveforms that do not allow for a clear OFF potential reading. The reasons for this deviance have been investigated and are discussed in this video.
The focus of this article is to help evaluate and resolve survey readings where OFF-potential waveforms are not reliable and decouplers are installed.
Following are steps that can be taken for possible resolution of response times that are unacceptable. This assumes that the user has established coordinated interruption of all other current sources and has captured waveforms of on and off response. Where present voltage capture settings are inadequate, consider the following alternative actions.
- For the same OFF period duration, capture the reading later in the cycle. For example, for a one second OFF cycle, take a reading at 700ms instead of 300ms.
- If the waveform is still decaying at the end of the OFF cycle, without the possibility of a valid potential measurement, set the ON/OFF interruption cycle to longer periods. The longer OFF period may allow valid voltage capture. Looking at the waveform after lengthening the cycle duration will show if this will be an effective option. Capture the pipeline potential at the earliest point in the OFF waveform that still provides an acceptable OFF value. Note: The use of a remote monitor at such locations is most helpful because the ON/OFF cycle and the off potential delay can be set remotely. Check with the monitor manufacturer for help to remotely set the ON and OFF interruption cycles, and how to extend the OFF potential delay read.
- If increasing the ON/OFF cycle time and increasing the OFF potential delay time has not provided acceptable data, then another option is to remove the decoupler from the circuit and perform the survey. The Dairyland Isolation Switch, an optional device that can be permanently installed in series with the decoupler, offers a safe and convenient way to remove the decoupler for interrupted surveys or any other purpose. Warning:For AC mitigation and certain other applications, removing the Dairyland decoupler from the circuit will leave the structure as an open circuit and increase the steady state AC voltage on the pipeline. The induced voltage on the pipeline may rise above the 15V AC safe level established by NACE guidelines. If the steady-state AC voltage rise is unacceptable, this may not be an option for the user. Once the decoupler is disconnected safely, proceed with the survey. You should now see OFF-potential waveforms that are more consistent with that of the other portions of the structure, as the decoupler and its effect have been completely removed from the circuit. When finished, it is important to reinstall the Dairyland decoupler into the circuit by placing the Dairyland Isolation Switch back into the normal operating position according to its instructions. Failure to do so will leave elevated AC voltage on the pipeline and at contact points.
- Should the potentials still not fall within an expected range then we suggest installing coupons to help achieve accurate potential readings. IR free coupons represent a low-cost option for obtaining polarized potentials because they avoid measurement errors due to current flow in the soil and are independent of the pipeline readings. Coupons could be used for any application where an independent reading is desired.
To address this interference issue that some users may experience, Dairyland has also developed the PCRX. Using sophisticated design techniques that represent the work of numerous leading experts around the world, the PCRX’s new camouflage technology renders it virtually invisible to interrupted survey testing, addressing the interference issue for users.
Like our other decouplers, the PCRX is a solid-state, maintenance-free device designed to simultaneously provide DC decoupling and AC continuity/grounding when used with cathodically protected structures, such as pipelines, tanks, and grounding systems. Commonly used DC decoupling of equipment from grounding systems, for AC mitigation projects, or other isolation and grounding applications, the PCRX provides safety grounding while also isolating your cathodically protected structure, improving CP system performance. Learn more about the here.
Should any questions remain about these procedures, please contact Dairyland Technical Support.