CP for above ground tank

[vreijb]

We have subcontracted installation of CP system for 6 nos. above ground crude oil tanks. since 18 months 4 of the 6 tanks are completed and handed over to the client but we are facing a problem of final acceptance and partial completion of the project since the CP system is not giving proper results using the measurement of the Test posts. The system is impressed current CP system using mesh anode distribution below the tank bottom (60m dia.) and connected to rectifier 125A at 24Volt. design current is 90A and the system is not getting polarized to -ve 800mV requirement. the subcontractor is giving us the reason for that is because of the bottom plate is not painted. But it was a requirement for impressed current CP bottom to be bare and the design is based on that.
anyone has faced similar situation?

 

[ScottyUK]

Do you have any copper earthing material in the vicinity? A perimeter ring of shallow-buried copper cable around the tank would be bad news ; any substantial mass of buried copper between the remote anode (or anode array) and the tank is bad news too; a copper grid directly below the tank would be disastrous from a CP perspective.

What is the potential shift you are achieving? Does it indicate that you are close to the desired shift, or are you a long way short? Are you reaching suitable potential shift at any locations or is it a general shortfall?



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Sometimes I only open my mouth to swap feet...

 

[vreijb]

We are getting between 400mV to 650mV.
But at the begining when the system was first commissioned we were getting 900 to 1100mV.
Yes there is earthing system all around the Tank, the Tank dia. is 60m inside a recatangular bund wall of 120x100 meters. and the earthing is just running with the bund wall all over the perimeter with 1x70 sq mm insulated copper cable connected to 4 earth pits outside the bundwall and the earthing is connected to tank from the earth ring at 2 oposite sides. The bottom of the tank and all the area inside the bundwall are separated from ground through HDPE linning for environmental protection.

 

[ScottyUK]

I forgot to ask: are you running out of voltage, or current? If you are pushing out 125A and still not making the expected potential shift then you must be losing a lot of current to some buried structure which previously did not drain current away from the system. If you are at 24V output and still unable to circulate rated current then either you have a high resistance in the circuit or the groundbed is sick.

The earthing ring will be taking a relatively high current - has anything changed in respect of the earthing or the ground conditions, e.g. has the water table risen, has an additional bond been added, etc? Similarly if the water table has dropped this would account for an increase in resistance.

What are your connections in and out of the tank? Do you have isolation flanges to isolate the tank from above ground structures? Are these joints in good shape? Does a cable shield or armour introduce a connection to a remote earth, or does an impulse line or air line to an actuator breach the isolation joint and connect the rest of the plant to your tank?

What was the anode material and what was the intended design life at rated output?

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Sometimes I only open my mouth to swap feet...

 

[gopalsankar]

Please check whether all connected pipelines are positively isolated by insulating flange.Especially in the above ground ST CP System ,getting inadequate protection is mainly due to current drainage to the neary by structure.Painting is not a big issue and even some people are advising not to paint the surface due to holiday problems in the coating.

 

[vreijb]

The Isolation joints are good and newlly installed. We have fire and foam protection sydtem which are installed w/o isolation joints and we thought the problem could be there and for that we have installed flange isolation kit but without any change in the system. The only left issue is electric cable armours. we have Motor operated valves connected to power supply, lighting cables, and level gauges connected to power supply. Another issue is the high resistance of the ground. Water table has no effect I guess since there is HDPE linning which is separating the area from gound water.

 

[ScottyUK]

A DC clamp-on ammeter should pick up any current drain through the armour. For accuracy it would be nice if the cable was 'dead' but you should still see a DC component even with AC present.



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Sometimes I only open my mouth to swap feet...