If the galvanized layer is not damaged or not seriously damaged, the natural potential of earthed pottery is close to that of zinc anode (the measured value is -I 03- 1. 08V CSE)。 When impressed current cathodic protection potential (charged potential 1 negative to -i.iov CSE), cathodic protection current began to leak.
Galvanized round steel and flat steel were installed in the valve chamber of the second phase oil transmission project in Kazakhstan as the grounding grid and connected with the block valve. The measured pipeline potential is-1.05VCSE (there are two magnesium anodes along the pipeline, and the potential is-1 ssv CSE). After the impressed current cathodic protection is put into use, the electrifying potential of the valve chamber of Yin Bao No.30 Station is L.1.95 vcse; The 3l# valve chamber, which is 24km away from Yin Bao Station, is powered by 1 129 V CSE potential. As can be seen from the potential curve, because the valve chamber is grounded, the potential value there is still the most positive, but it has little influence on the cathodic protection potential on both sides. Of course, whether the protection is sufficient can only be finally determined by measuring the power-off potential.
In the first phase of Kazakhstan-China oil pipeline project, graphite grounding module was adopted in the valve chamber along the line. The pipeline in room 36# min is short-circuited with the grounding grid, and the valve room is equipped with a potentiostat. The output voltage is 6 0V, the current is 2 0A, and the electrifying point potential is-0.94 VCSE: the valve chamber at 36 km is-1.64 V; Valve chamber at 50km-1. 42V。 After the short circuit is eliminated, the output voltage of potentiostat is 3. 19V, the output current is 0.76A, and the potential of valve chamber at 50km is-1.22r. This example shows that the short circuit of graphite grounding plate increases the protection distance. It may even lead to the pipeline leaving the electrification point being overprotected. 3 1# valve chamber has no potentiostat, and the pipeline is short-circuited with the grounding grid. The potential before short circuit elimination is -0.64V, and the potential after short circuit elimination is 1. 1 16V.
This fact also shows that graphite grounding module has a very adverse effect on cathodic protection, so it is forbidden to use graphite grounding module in future projects. It should be noted that the galvanized flat iron grounding electrode has little influence on cathodic protection at the initial stage of commissioning, but it does not mean that the influence on cathodic protection is still small after long-term operation. If the grounding grid is not fully protected for a long time, the root of the galvanized layer may be completely corroded and a large amount of cathodic protection current may leak. How to affect the cathodic protection can only be confirmed through follow-up investigation.