Effectiveness of Tank Bottom Plate Soil Side Impressed Current Cathodic Protection System for Tanks Constructed on Asphalt lined Full Concrete Pad.

1??????INTRODUCTION

This study provides a cathodic protection system design review, current conditions of cathodic protection system components (transformer rectifier, anode bed, junction box ,cables connections/isolation ,ref electrodes and the),the associated facilities protection potential and any stray current interference on other facilities.

This study includes software modelling based on field test data of the system electrical circuits to simulating all possibilities to clarify the resistance of the system, stray current to asset , continuity and isolation of the associated asset from other facilities.

System data

(ICCP) Cathodic protection system with transformer rectifier (CPTR# X) rated 50V/50A and its deep well anode bed with two strings of anodes contain eight anodes.

The subject cathodic protection system found connected with pipe rack which carrying pipelines connected with 4 tanks (listed as below) and sharing many other pipelines

(Fire water lines, steam lines, service water lines, process pipelines---etc)

List of tanks with pipelines sitting on the pipe rack is listed as below:

·?????????V-1?(Demin Water Storage Tank) - External Tank Bottom Plate.

·?????????V-2?(Demin Water Storage Tank) - External Tank Bottom Plate.

·?????????V-3?(Tempered Water Storage tank) - External Tank Bottom Plate.

·?????????V-4?(Tempered Water Storage tank) - External Tank Bottom Plate.

2???Tanks foundation

Tank constructed on 13 mm asphalt lines concrete slap

3 Cathodic Protection System schematics

4 BACKGROUND

The above-mentioned tanks designed without cathodic protection as tanks sitting on 13 mm thickness asphalt lined concrete base.

Cathodic Protection System Transformer Rectifier Number (X) Manufacturing Data

potential measured outside the wall ring so readings influenced by the under ground fire water piping potential (fire water piping shorted with the pipe rack)

5??AS FOUND CONDITIONS

?From the measured conditions above

?CPTR#X operating current is 42 A and design current for the four tanks are 17.5 Amperes which giving an indication about 27.5 amperes additional current which may act as stray current to the plant by the cathodic protection system and collected by a concrete foundation of pipe rack, underground pipelines and any other third-party asset then returned back to the cathodic protection system through pipelines and pipe rack.

There are no cables connections between pipelines and pipe rack which meaning that the connection through pipeline to rack is only touch (contact area) which is a variable resistance and may cause a spark and fire ignition due to the high DC current passing through loose connection.

Resistance of tanks bottom plates to the concrete is very high as tanks already electrically isolated from the concrete base by 13 mm asphalt layer.(almost zero current collected by tank bottom plate)

·?Each tank to soil resistant is deferent value based on the conditions of the asphalt layer which make a big different in the current amount received by each tank and cause variation in the protection potential level of each tank.(in case there is any current passing through concrete slap and asphalt layer at damages /cracking locations)

·ON-OFF potential measurement showing tanks V-1, V-2 receiving current more than Tanks V-3, V-4 however, tanks V-1, V-2 are located more than 400 meters?away from the deep well anode bed which assure that this readings for the nearest fire water lines not tanks bottom plates .also all tanks got many patch repairs during T/A e.g. Tanks V-1 with 12 meters diameters which showing proper protection readings get about?21 patch repair on the bottom plate.

ON-OFF potential measurement for tanks V-3, V-4 affected by the deep well anode bed ,ground voltage raise as the anode bed is located near the tanks (less than 20 meters)

6 ADDITIONAL FIELD TEST FOR EVALUTION

Field tests had been carried out to evaluate the solution of connecting the transformer rectifier directly to tanks V-3 and V-4 and verify the interference from the stray current

Test?Procedure

·?Existing negative cable which welded to the pipe rack disconnected from the CPTR negative terminal

·?Suitable size test cable installed from CPTR negative terminal to Tank V-3 and CPTR voltage raised from zero till voltage of 20 V and provided current of 12A

Tank V-3 ON-OFF potential found 0.9V ON and 0.79V OFF which giving indication that most of current going to third part assets and returning to pipe rack and current received by tank is not enough to protect it, as the maximum ON reading was 0.9 voltages.

Test cable installed from CPTR negative terminal to Tank V-4 and CPTR voltage raised from zero till voltage of 20 V and provided of 11.5A

Tank V-40 ON-OFF potential found 1.4V ON?and 0.75V OFF which giving indication that most of current going to third party assets and returning to pipe rack?and?current received by tank is not enough to protect it?however the?maximum ON reading was 1.4 voltage which giving indication that tank can be partially protected.

?7 CATHODIC PROTECTION SYSTEM SOFTWARE MODELING .

Electrical Circuit Component Data for Software Modelling and Simulation?

Software Modelling and Simulation for the as Found Conditions

From software modelling the CPTR operation current is 32 A and about 95% of the current circulating in the plant, and picked up by rebar in the concrete foundation, underground pipelines and many third party items.

It is proofed from operation conditions that the operation current is almost 35 A and design current for all 4 tanks is 17.5 amperes and tanks found partially protected which proof the interference and stray current issue.

by providing soil access pipes and measuring the potential of tank bottom plate locating the ref electrode under the tank , it found that tank did not feel the on-off of TR

Software Modelling and Simulation for the actual test, which carried out in field.

Pipe rack cable disconnected and the transformer rectifier negative terminal connected with Tank V-4 and CPTR operation was 20V/12 A

8 FINDINGS.

1. Existing design of cathodic protection system, which connects the negative cable to steel structure pipe rack, is wrong and causing stray current inside the plant.
2. Deep well anode bed with 8 anodes rating is 35 A hoverer the rectifier rating is 50A
3. From actual test done , if rectifier connected directly to one tank the current will be 12 A for operation of 20V (rectifier rating resistance is less than the circuit resistance)
4. Tanks bottom plate found electrically isolated by 13 mm asphalt (tank drawings) and actual resistance is very high (based on the asphalt damages locations) and current will pass only through damages and cannot provide full protection.
5. Pipes sitting on the rack beams without jumper cables, this loose connection may cause sparks and be a source of fire.
6. Tanks bottom plate Cathodic protection design current is almost 17.5 A but the tank is electrically isolated from the anode bed by 13 mm asphalt layer ,so cathodic protection system operation current acting as stray current inside the plant, which may cause stray current corrosion and failure in the pipelines and equipment at isolation flanges for pipelines with wet service. (Cathodic Protection current can pass the isolation joint or any flange joint with high resistance through the conductive fluid and the pipe side where current leaving the metal to electrolyte will act as anode and will be be dissolved
7. Different tank bottom plate to soil resistance based on the asphalt layer conditions causing unbalance in the current distribution to each tank.?

9 Conclusion

• Distributed anodes and deep well anode bed cathodic protection system can not protect tanks sitting on high resistance concrete and/or asphalt layer ,
• For pre design drain current test , if tank pipes is not electrically isolated , the test result will be misleading as current will be picked up by under ground facilities which shorted with the tank pipes and current will return back through tank pipes and tank body to the negative cable and this current can change the potential of the this third party facility showing fake potential reading of the tank bottom plate.
• Deep well anode bed is not recommended inside the plant with congested under ground pipes ,as it may cause stray current interference.
• Tank foundation shall be considered before proposal and design of CP system