technical seminar for cathodic protection to gogc design unit …. interferenceacdc_technical... ·...
TRANSCRIPT
Technical Seminar for Cathodic Protection
to GOGC Design Unit Specialists
Dr. Nick Kioupis, Cathodic & Lightning Protection
Section Head, DESFA
Interference
• DC
–Constant (CP systems of third
parties)
–Fluctuating (e.g. railways, dc
sources, mines, welding machines
–Tellurics
2
Sources of dc stray current
Trolley-buses and dc traction
systems
HVDC transmission
systems
DC sources
Industrial equipment (e.g.
welding machines)
DC telephone network
Cathodic protection systems
Traffic light equipment - dc operated
3
Stray current corrosion from dc railways interference
Identification and measurement of dc stray currents
• Recording of potential
• Voltage gradients along the pipeline and perpendicular to it
• DC line current measurement/recordings
• Line current measurement by current clamp method
• Current to polarisation probe or corrosion coupon
• Interference between CP systems
• Stray Current Mapper
5
Criteria according to ΕΝ 50162
• Criteria for non cathodically protected pipelines
• Criterion for cathodically protected pipeline
– IR free potential to be within acceptable limits (e.g. -0,85V, cf. EN
12954)
– Criteria related to how long an anodic current exits from a
probe/coupon
– Cathodic interference >500mV on (IR included) or EiRfree<El
6
Practical example of a real case of potential dip due to
crossing with foreign pipeline
IFO method to detect coating defects
13
Typical pattern of voltage gradients around a coating defect
14
2 and 3 electrodes methods
15
Adding-up method
16
Mathematically expressed:
[1] UA1 - UA2 = 0
[2] UA1 = UA2
(considering remote
ground)
Thus:
[3] UB1 - UA1 = UB1 - UA2
[4] UB2 - UA2 = UB2 - UA1
Assumption:
[5] ΔU1 = UB1 - UA1
[6] ΔU2 = UB2 - UA1
resulting in equation (for UA1):
[7] ΔU1 - UB1 = ΔU2 - UB2
[8] 0 = ΔU1 + (UB2 - UB1) - ΔU2
Thus:
ΔU2 = UB2 - UB1 + ΔU1
Adding-up
17
Pipe locators and coating defects detection with A-
frame
18
Investigation with Stray Current Mapper
19
DCVG and Pearson Tecniques
20
• ΑC
–Short-term
during fault currents
–Long-term
during
normal operation
of the powerline
21
Interference
• ΑC
–Capacitive
–Conductive
– Inductive
22
Interference
AC corrosion from powerlines interference
• History
• End of 1980s: First cases of a.c. corrosion in several european countries
• 2001: Publishing of the CEOCOR a.c. corrosion booklet that summarized all
relevant European experiences
• 2006: Acceptance of CEN/XP TS 15280 based on activities within
CEOCOR
• 2013: Issue of EN 15280 Evaluation of a.c. corrosion likelihood of buried
pipelines applicable to cathodically protected pipelines
• Conclusion
• The current standard represents the European experience collected within
the last 25 years
• there is no fundamental additional information that was not already known
by 2004
• There is vast field experience for all of the used thresholds
AC corrosion
Practical examples from pipelines in Switzerland
Limit length Lgr or spacing of a pipeline when laid
parallel to a 50Hz three phase high voltage overhead line
Interference distance according to NPR 2760
46
Analysis of induced overvoltages on a distribution pipeline by
a nearby lightning strike
47
Electrical Interferences measurements
• Cathodic protection potentials recordings at least for 24h
• Current magnitude and direction measured on coupons,
• Line current recording,
• Comparison of patterns of recordings on the stray current
source and on the pipeline itself,
• Corrosion rate recording on ER probes.
– Other measurements referenced in ΕΝ 50443.
48
Interference distances (prΕΝ 50443:2011)
49