Insulation joints on tank nozzle for CP isolation 1

[svi]

Referencing the thread 172892.

Does not the HDPE lining provided under the tank pad make the CP galvanic cell independent of the rest of the ground, thereby not requiring the piping connection isloations. A tank has a static grounding also provided against static buildup, this ground connection may be going deeper than the HDPE lining.

 

[svi]

Was it not in error that the tank bottom was not coated?

 

[SJones]

In the original thread, the close coupled, distributed anode mesh will be inside the HDPE liner, thus current will flow onto the tank bottom and to connected structures. The potential measurements need to be made in accordance with NACE TM0101. If the tank pad material had been selected and installed correctly, it could be argued that coating would be superfluous. However, it would help in cases where the protection current needed to be minimised to prevent issues in attached systems.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[svi]

The connected structure is the pipe supports for the piping connected to the tank shell nozzles and this will be outside the HDPE liner area. The HDPE liner creates a seperate galavanic cell comprising the anode mesh, the tank bootom and the tank pad of compacted soil. My query is whether there is likely to be CP (impressed current) loss to the outside ground through the piping (without any insulation joints in place at the nozzles).

 

[SJones]

There will be current "loss," but since the anode is close coupled, there should not be an issue with achieving the correct protection potentials on the tank bottom.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[svi]

Thanks Steve,
There is also the Tank equipment ground electrical earthing connection provided. This will be a leakage path. The CP vendor has proposed the use of a polarization device that will stop the DC CP impressed current from leaking to the outside ground through the earthing ground connection. With the use of this, I think it makes sense to have the insulation joints on the piping connection. The piping connections being the only leakage path. Though the insulation joints don't make effective sealing at the joints, they are vulnerable to failure, especially bolt sleeve of bakelite or other brittle material. Despite the cons on the inulation joint, I think I have to accept the use. I would not want the CP contractor to say that he has a problem make the system effective because of the leakage through the pipe flange.