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Cathodic Protection
- 1. IEEE PES Cathodic Protection Use On Tank Bottoms & Underground Piping In Power Generation Plants PG&E Office San Francisco January 18, 2007 Craig K. Meier Corrosion Control Incorporated
- 2. IEEE PES • Corrosion process for tanks and piping • Principles of cathodic protection • Effects of coatings • Cathodic protection design requirements • Selecting the proper system • Construction implementation issues • Compliance testing • Continued monitoring and maintenance
- 4. IEEE PES
- 6. IEEE PES Fe++ Iron ion Cathode Anode e- Electron flow H2O + 1/2O2 + 2e- → 2OH- Feº→ Fe++ + 2e- Conventional Current Flow
- 7. IEEE PES Rate of Corrosion • Soil Corrosivity – Resistivity – pH – Salts • Bi-metallic effect • Mixed soils • Differences in oxygen
- 9. IEEE PES Corrosion Bare steel pipe -500 mV Copper ground -200 mV Corrosion Current
- 11. IEEE PES Corrosion Tank floor Clay lump Copper ground Corrosion
- 12. IEEE PES
- 13. IEEE PES • API recommends against the use of asphalt and concrete tank pads. Concrete or asphalt Tank floor Corrosion
- 14. IEEE PES
- 15. IEEE PES Corrosion Losses • Will occur in all plants • Rate of loss dependent on soil and materials used • Coatings alone do not stop soil corrosion • Care during construction can significantly reduce losses
- 16. IEEE PES Corrosion Control • Material selection • Bedding • Coatings • Cathodic Protection
- 17. IEEE PES • Reduces amount of exposed metal • Coatings are not perfect Steel Coating flaw Coating
- 18. IEEE PES Cathodic Protection H2O + 1/2O2 + 2e- → 2OH- Pipe Anode Ground
- 19. IEEE PES Cathodic Protection • Galvanic • Impressed Current
- 20. IEEE PES Galvanic Anode Pipe -500mV Magnesium anode -1700mV Exothermic weld
- 21. IEEE PES Galvanic Cathodic Protection • Galvanic anodes have a limited current output, 10 to 30 milliamperes per anode. • Due to the limited current output, the use of galvanic anodes is limited to coated and electrically isolated underground piping and underground tanks.
- 22. IEEE PES Galvanic Cathodic Protection Magnesium anode Coating damage Pipe Coating Dielectric flange
- 23. IEEE PES Galvanic anode Protects only outer floor Bare steel tank bottom
- 24. IEEE PES Impressed Current AC Power Rectifier Anode Structure (+) (-)
- 25. IEEE PES Impressed Current Unit 1 Unit 3Unit 2 Rectifier Anode
- 26. IEEE PES
- 28. IEEE PES Impressed Current • Anodes produce high amounts of current, 1000 to 5000 milliamperes per anode. • High current output allows protection of bare steel structures. • Structures do not necessarily need to be electrically isolated.
- 29. IEEE PES New Power Plant Design • Involve corrosion engineer from the start • Obtain site soil resistivitiy data • Select piping materials • Select coating types and quality control procedures • Decide which structures require cathodic protection
- 30. IEEE PES Cathodic Protection Design - Piping • Soil resistivity data • Clearly identify piping to receive cathodic protection • Yard plan showing piping layout, isometric drawings are difficult to use • Location of all risers • Type of coating • Responsibility of isolation gaskets • Desired service life
- 31. IEEE PES Cathodic Protection Design - Tanks • Site soil resistivity data • Tank diameters and layout • Equipment access • Type of foundation • Containment • Monitoring capability • Desired service life
- 32. IEEE PES Design Options • Design documents issued in construction plans • Contract responsible for cathodic design
- 33. IEEE PES Design Submittal • Qualifications of person responsible for design, supervision during installation and testing – Can not mail order • Detailed calculations for each tank and pipeline showing current required, number of anodes, circuit resistance and life • Material list providing descriptions, models and quantities • Product data sheets • Detailed installation plans – not diagrams
- 34. IEEE PES Implementation • The cathodic design submittals must be approved before construction of structures • A site meeting must be held to discuss how the cathodic protection system will be installed, and special concerns during construction • Mechanical and electrical foremen should be involved in cathodic discussions
- 35. IEEE PES Avoid Costly Mistakes • Grounding cables must be at least 12 inches from underground pipes • Temporary pipe supports must be removed • Piping must be inspected for coating quality and separation from grounding before burial
- 36. IEEE PES Grounded Pipeline Pipe Ground wire Anode
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- 53. IEEE PES Coating Standard Weld Weld Pipe wall Post Hydro Weld Clip Coating Pipe wall
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- 55. IEEE PES Anode Rectifier Tank bottom Liner Anode array 12” sand
- 56. IEEE PES Completed Before Testing • All pipe flanges connected and isolated • Pipes back filled and cathodic protection is complete • AC power to rectifier unit • Storage tanks contain product • Schedule for testing and training issued
- 57. IEEE PES Compliance Testing - Specialist • Inspect cathodic systems to ensure they are properly installed • Test each dielectric flange for isolation • Obtain native potentials on pipes and tanks • Connect galvanic anodes at test stations and/or energize rectifier(s) • Cathodic protection systems on isolated coated structures should operate at least 4 hours before testing • Cathodic protection systems on bare steel or grounded structures must operate between 12 hours and 7 days before testing
- 58. IEEE PES Soil Set to DC volts -.892 Pipe Reference cell 6”
- 59. IEEE PES Galvanic Anode Current Shunt Set to DC millivolts
- 60. IEEE PES Impressed Current Rectifier Unit Shunt Meters Transformer adjustments AmpsVolts (-) (+) Breaker Output lug
- 61. IEEE PES Impressed anode Voltage gradient Cell Potential plus gradient Potential of steel Meter
- 62. IEEE PES Emeasured = Ptank + I R • If the current is momentarily interrupted, I=O, then; Emeasured = Ptank + (O) (R) • “Instant Off” potential
- 63. IEEE PES Cathodic Protection Criteria • Galvanic anodes – -850mV or more negative with anodes connected • Impressed current – -850mV or more negative “instant-off” -or- – 100mV polarization shift
- 64. IEEE PES 100mV Polarization Criteria -525 “Native” -1104 “On” Polarization 167 “Instant off” -692
- 65. IEEE PES Compliance Report • Tabulated Data • Narrative analysis of the data • Compliance statement • Calculated anode life • Test procedures and instrumentation • As-built drawings • O&M Manual
- 66. IEEE PES O&M Manual • Test Procedures • Instrument requirements • Qualification requirements • Frequency • Criteria • Trouble shooting • Spare parts • Wiring diagrams
- 67. IEEE PES Conclusions • Cathodic protection is viable and necessary to preserve power plant structures • Proper planning and coordination are essential through out the design and construction phases, to ensure cathodic protection systems work properly • Properly implemented and monitored cathodic protection will extend the service life of storage tanks and yard piping more than 30 years
- 68. IEEE PES Owner Monitoring & Maintenance • Record rectifier output levels monthly and compare output to target values • Repair inoperable rectifiers within 30 days • Inspect test stations and dielectric flanges every six (6) months for damage and removal • Instruct maintenance staff to replace and test removed dielectric gaskets • Annually, obtain potential measurements and ensure effective cathodic protection levels