Ceasar,
DCVG is not normally done on sacrificial systems, the reason being is for a DCVG to be meaningful it is required to achieve a swing between the on potential (energised) and the off potential (polarised) in excess of 300mVDC (see NACE and ICORR specs). This is rarely achievable with a sacrificial system, as with 6 test posts, which would suggest 6 sets of anodes, all would need to be switched with syncronised switches at the same time, even if the swing at the test posts were to achieve a swing of 300mVDC it is almost certain ( due to low current throw from sacrificial anode) that the swing in potentials at mid-points between the anodes would not achieve 300mVDC. Under these circumstances the data gathered from a DCVG survey would at best be inconclusive and at worst rather pointless. Also the purpose of a DCVG survey is to identify coating defects and to give an indication of the magnitude of any defects found, it does not indicate the level of Cathodic Protection being supplied by the installed CP system. To achieve the necessay 300mVDC it is normally required to impress current onto the pipe structure at the test points, by connecting a DC powersource negative to sacrificial anodes and positive to the pipe structure at each anode installation. Even then the current attenuation between the test post will be high due to the close proximity of the sacrificial anodes to the pipe. This would mean the swing potential at the midpoint between test post could well be below the 300mVDC swing required. Even then to confirm the 300mVDC swing in potential a CIPS potentials on and off would need to be taken at mid points between test posts. It is more common to utilise a temporary DC power source connected to a remote temporary ground bed with the sacrificial anodes disconnected to achieve a 300mVDC swing in potential..
A CIPS survey is however, specifically intended to verify the level of corrosion protection being supplied to the structure along the whole length of the structure. Although the CIPS survey will indicate areas of poor protection or protection below defined criterion, there are many reason for poor levels, not necessarily coating defects.
As for, is a CIPS survey required I would answer as follows, if local legislation requires the pipe to be protected from corrosion, due to the nature of the product being transmitted through the pipe which would look at risk of fire, explosion, toxicity and/or environmental impact (not an exhaustive list), then it is most likely that it is also a legal requirement to monitor and maintain the corrosion protection system. Then a CIPS survey would be required on a periodic basis to confirm the whole structure is being protected from corrosion by the CP system. Where the period between CIPS surveys maybe be 12 months or more, there would be other lower level monitoring surveys carried out, as specified by a competant CP engineer in accordance with local regulations.
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