How PCRX can save time with DCVG Surveys
What is a DCVG Survey?
A direct current voltage gradient (DCVG) survey is a technique used to evaluate the effectiveness of corrosion protection systems on buried structures. This technique is used to analyze the coating condition and locate any coating “holidays” or coating defects on the buried pipeline.
In a DCVG survey, direct current (DC) is applied to the pipeline by using the pipeline’s impressed current rectifier or a portable power supply, and the voltage gradient is measured at intervals above the pipeline at the ground surface. The voltage gradient is measured by using two calibrated reference electrodes that are spaced a set distance apart. This measurement is the soil-to-soil potential difference and not the pipe-to-soil potential. Since most of the DC current will flow from the current source to a coating holiday, a voltage gradient will be present at a coating holiday and will appear as either current pick up or discharge.
Possible DCVG Survey Concerns with Standard Decouplers
During a DCVG survey, the impressed current rectifier or portable DC power supply is interrupted at a certain On/Off cycle similar to a Close Interval Survey (CIS). During the On cycle, the voltage gradient measured will be highest at the coating holiday. Conversely, during the Off cycle, the voltage gradient measured should not be present at that coating holiday as there is no DC current flow from the rectifier.
However, when standard decouplers (SSD, PCR, or PCRH) are installed nearby, this can affect the DCVG survey measurements and/or trigger a false reading with regards to coating holidays. This is due to the detection of a voltage gradient near standard decouplers and their connection to the grounding system. This may lead technicians to interpret this potential measurement as a large coating holiday and/or cause the operator to sway from the pipeline and follow the grounding system as it will be detected.
This voltage gradient is caused by the inherent capacitance of standard decouplers. These decouplers have a capacitor that is used to mitigate steady-state AC current from nearby HVAC lines. The capacitor holds a certain DC charge that looks like an uninterrupted DC source during the Off cycle of the survey. During the Off cycle, as the capacitor is discharged, it may still provide current to sustain the voltage gradient near the decoupler location and thus, may lead to an erroneous determination of a coating holiday.
The solution to this has been to temporarily disconnect the standard decouplers within the vicinity of the DCVG survey to remove this influence. However, this negatively affects the time to complete the survey and may increase the cost as well.
Solution For Faster DCVG Surveys
PCRX is Dairyland’s latest in solid-state decoupling technology. PCRX was introduced to the industry to virtually eliminate the capacitance issue found with standard decouplers. PCRX provides two main advantages over standard decouplers.
Firstly, it virtually eliminates the capacitive effects during the Off cycle, eliminating the voltage gradient and therefore, the possibility of an erroneous determination of a coating holiday. Secondly, there’s no need to disconnect the decoupler from the pipeline during the survey, always maintaining the connection to the mitigation system and keeping the pipeline grounded for over-voltage conditions. These benefits lead to safer and more efficient DCVG surveys.
This article is intended to bring awareness the reader on this complex issue with influences on interrupted surveys like DCVG, existing solutions and brief description on how PCRX provides value to solving key issues with interrupted surveys.
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