Cathodic Protection

A project in which I am in evolved in replacing the service and unit substations is reported to have had extensive corrosion of the domestic water piping system and is being replaced. The piping condition is being contributed to a large non-linear load (PC’s) distributed throughout the building. The building is of the 1960 vintage, has no oversize or dedicated neutral conductors, and utilizes conduit for the only grounding system. I would welcome any theories. Also would cathodic protection be a possible solution? Impressed current systems or protection with safrificial anodes?
Thanks for the comments

 

[GordS]

Corrosion is basically a DC phenomenon I think. Therefore, I don't think the corrosion seen is related to the non-linear loads. These loads would be expected to introduce neutral currents but these should be AC (multiples of base frequency). To have corrosion, there has to be a current flow in one direction that removed electrons from material (which corrodes away) and deposits the electrons on a different material. You can have corrosion if you impress a DC current between two materials. You can also have corrosion due to differences in galvanic potentials between two different materials in a conductive medium. If the latter is the case, than catodic protection or sacrificial electrodes are potential solutions.

 

[BJC]

Service entry grounds are and were required to be connect to the incomming water pipe of a building. That means the neutral ( star point of a three phase transformer or center tap of a single phase transformer )of your system is connected to the water pipe. The neutral in you service entry panel is connected to ground and the star point of the utility is also connected to ground. The water pipe is at least a parallel circuit witht this ground path.
It is possible for you system to generate harmonics with DC components. Draw you system and throw in some chopping power supplies. The problem is compounded with YY utility transformers.
I don't think you can analyze it on paper other than to know its possible. I would get an oscilloscope and set it up to see if there is a DC componenet in the ground/neurtal current flowing into the water pipe. IS ther or have there ever been battery systems with chargers in the place? Try the old engineering standby - draw the picture first.
Is the pipe iron or copper? what kind of joints? falling apart in places or all over? is it all burried. encased in concrete ( concrete with high flyash ?)?
The pipe could be falling apart from an existing galvanic coupling.
If the piping does have a voltage on it that is corroding it it may not be practical to use passive annodes on the system. The half cell votage of the iron and the annode would have to be greater than the imposed voltage. If you use a driven system ( use a low voltage source to "plate" the water pipe) the voltage may have to be to high. You may start having people get tingles when they touch facwetts etc.
In one case I read about the problems were similar and the local inspector insisted on a ground to water pipe connection. The engineer had a couple of feet of pipe put in with insulated joints on both sides and connected his ground to that and several driven ground rods.
Sorry that I can't be more helpful, just trying to give you a few things to think about.

 

[jbartos]

Suggestion: As indicated in the previous posting, the nonlinear load can create DC current to flow through the ground. The common mode currents are essentially AC currents; however, they can have a DC component, which is relatively small but sufficient to cause cathodic corrosion.

 

[cuky2000]

Two dissimilar buried metals such as copper and steel pipe form a galvanic cell causing an accelerated the corrosion process even without direct connection.

Recommendation from IEEE 80 and other technical references suggested the follow:

a. Insulating of sacrificial metal surfaces with a coating such as plastic tape, asphalt compound, or both.
b. Cathodic protection using sacrificial anodes or impressed current systems.
c. Use of nonmetallic pipes such as PVC, concrete or fiberglass.

Appears that the actual condition of the buried conductor require an assessment to determine the degree of decay in the material. If there is significant reduction in conductor section or there is evidence of deterioration in the ground grid integrity, reinforcement with new conductor is recommended particularly adding diagonal conductor in each corner of the substation under the crushed rock area.

Connection to the concrete structure rebar’s should be consider if the metallic pipe will be removed to compensate for the loss of metalic pipe.

The computer located in the building should be provided with low impedance Isolated ground (IG) to minimize the Electromagnetic Interference (EMI) or any undesirable electrical noise as suggested by the IEEE standard 142 and IEEE Std 1100.

The computer IG conductor should be as shortedv as possible connected to a triad of three ground rods arranged in triangular configuration separated at least by the dept of the rods.

On the other hand, the safety grounding system shall be tested using as a reference the standard IEEE 81. Caution should be observed to filter the circulating current and noise to advoid false readings. "Smart Multimiter" is recommended to determine all ground parameters such as Resistance, Step and touch potential, etc.

 

[jbartos]

Small encore: Visit

http://www.alliedcorrosion.com/

http://www.edi-cp.com/

http://www.farwst.com/intro/protect.htm

http://www.safetrack.se/

(railroad experience)

http://www.delweg.com/

for more info.

 

[WayneClark]

I am trying to determine if there is a cost effective method or device available for checking the condition of distribution utility anchor rods and ground rods. There are services available that utilise narrow been ultrasonic technology (basically ultrasound), but these are too expensive to use on distribution lines.