Inconsistent DCVG Results

[sannay]

Hello everyone,
I am conducting a DCVG survey on our plant's gas lines. The coating is very old (~30 years), and has been rarely inspected. In the last DCVG survey in 2019, a moderate severity fault was detected at a location with %IR of ~16%. This time around, at that exact same location, only a small fault with %IR of ~4% was detected, and hence not of a concern. We decided to excavate and assess the coating visually at that location and
the following pictures were taken. As you can see, the coating is clearly damaged. This raises a doubt in my doubt about my current survey's reliability. But this looks like a small severity fault and not a moderate one. A lump of soil was found stuck at the coating damage location as well. In the pictures below, the breaks in the coating are exposing the metal underneath.

Is there any way I can visually discern if a coating fault is of medium or small severity. Note that there are also some points of medium and large severity in this survey which were not identified in 2019.




Thanks.

 

[EdStainless]

Changes in soil conditions will impact the results.
Did you take conductivity measurements of the soil?

 

[sannay]

Changes in soil conditions will impact the results.
Did you take conductivity measurements of the soil?

Thanks. Thats what I was suspecting too. Unfortunately dont have 2019 resistivity values.
Hence, I am conducting CIPS on the lines now. Would that be enough to give me more confidence as to the condition of the coating?

 

[GarryB1957]

Changes in soil resistivity will have an effect on all over ground surveys. It would be of interest to know what was your on/off swing during the DCVG survey. How was it measured? As it is very often difficult to achieve accurate on/off potentials without specialist equipment, or without a very good DVM. Possibly the biggest reason for variations in DCVG results is more to do with recent rainfall and operator experience levels. Even with CP technicians some are good with this type of survey other are not so good.

As you have already excavated the area this will give you much information as the the severity of the coating damage and the overall effectiveness of the CP system. As you stated there are clearly witnessed coating defects at the location the DCVG indicated coating defects however, you don't indicate if there was any metal loss at the damage site?

The main purpose of a CIPS survey is to establish the levels of CP protection being applied to the underground structure. This may indicate a defect in the coating, however, there may be various other reasons for changes in the on/off potential of any underground structure that are not indicative of a coating defect.

As you mentioned this is a site gas supply a Pearson survey may not be of any great value, as on many sites there are other underground structure that may interfere with the Pearson survey.