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Stray Current Corrosion ControlStray Current Corrosion ControlFor DC Rail Transit SystemsFor DC Rail Transit Systems
Dale Lindemuth, P.E.Director of Engineering
Houston, Texas
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For TodayFor TodayFactors that influence stray current corrosion generated from DC transit systemsThe consequences of transit stray currentControlling transit stray current at the sourceMonitoring for transit stray currentMitigating transit stray currentQ & A
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Stray Current Corrosion – Electrically Continuous Pipe
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Stray Current Corrosion – Electrically Discontinuous Pipe
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Different Metals Corrode At Different Rates (Faraday’s Law)
Consumption RateMetal Pounds/Ampere-Year Steel 20 Lead 74
Copper 45 Zinc 23
Magnesium 8.8 Aluminum 6.4
Assumption: Constant, continuous electrolytic stray current discAssumption: Constant, continuous electrolytic stray current dischargeharge
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Transit stray current varies over time, 24/7/365Transit stray current varies over time, 24/7/365
24 Hours
Track-to-Earth Potential (Volts)Rails discharging stray current ~100% of the time
TimeTime--Weighted AverageWeighted Average
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Very Dynamic Track To Earth Potentials!Very Dynamic Track To Earth Potentials!
-120
-100
-80
-60
-40
-20
0
20
40
12:00 AM 6:00 AM 12:00 PM 6:00 PM 12:00 AM
TRACK TO EARTH
POTE
NTIAL (VOLTS)
-120
-100
-80
-60
-40
-20
0
20
4012:00 AM 6:00 AM 12:00 PM 6:00 PM 12:00 AM
TRACK TO EARTH
POTE
NTIAL (VOLTS)
July 2007July 2007
May 2008May 2008
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Yard/Shop Stray Current When Yard Is Connected To MainlineYard/Shop Stray Current When Yard Is Connected To Mainline
-600
-400
-200
0
200
400
600
10:00 AM 10:15 AM 10:30 AM 10:45 AM
AM
PER
ES
Stray current Stray current ““spikesspikes”” are not zero time durationare not zero time duration
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Stray Current Corrosion EquivalenciesStray Current Corrosion Equivalencies
0
5
10
15
20
25
0 20 40 60 80 100 120 140 160 180
Time (seconds)
Voltage
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100 120 140 160 180
Time (seconds)
Volta
ge
Am
peresA
mperes
Corrosion = Current x TimeCorrosion = Current x Time
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STEEL PIPE STEEL REINFORCING BAR
1 Square Inch Having 0.25-Inch Wall Thickness, Complete
Perforation
6-Inch Long Section of #4 Bar, 0.5-Inch Diameter, 50% Loss
1 Ampere 4.3 Days 10 Days
0.1 Ampere 43 Days 100 Days
0.01 Ampere 1.2 Years 2.8 Years
0.001 Ampere 12 Years 28 Years
0.0001 Ampere 120 Years 280 Years
CORROSION DETERIORATION
Time-Varying Stray Current Discharged From Structure To Earth, 30% Duty Cycle
How Much Corrosive Stray Current Is Too Much?How Much Corrosive Stray Current Is Too Much?
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New Orleans New Orleans –– Saint Charles LineSaint Charles Line
0
100
200
300
400
500
600
700
3:00 PM 6:00 PM 9:00 PM 12:00 AM 3:00 AM 6:00 AM 9:00 AM 12:00 PM 3:00 PM
AM
PER
ES
1985: 700 amps stray current = 47% of total propulsion current
2002: 500 amps stray current = 33% of total propulsion current
1985 – after ~9 years service
2002
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PhiladelphiaPhiladelphia
STRAY CURRENT DRAINAGE CURRENT (AMPERES)
0
2000
4000
6000
6/ 18/ 01 12:00 AM 6/ 28/ 01 12:00 AM 7/ 8/ 01 12:00 AM 7/ 18/ 01 12:00 AM 7/ 28/ 01 12:00 AM 8/ 7/ 01 12:00 AM 8/ 17/ 01 12:00 AM 8/ 27/ 01 12:00 AM 9/ 6/ 01 12:00 AM
Weekdays Weekends
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Valley Metro Valley Metro –– 20 Miles 20 Miles –– Opening December 2008Opening December 2008*** Designed for Less Than 1 Ampere Stray Current Per Mile ****** Designed for Less Than 1 Ampere Stray Current Per Mile ***
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Other Consequences of DC Stray Current Other Consequences of DC Stray Current (Besides Corrosion of Rails, Pipelines & Other Structures)(Besides Corrosion of Rails, Pipelines & Other Structures)
“Free” cathodic protection (“consequence” or “benefit”?)Hydrogen embrittlement of pipelines & reinforced concrete structuresReduced cathodic protection system life and effectivenessNot in compliance with company SOPs and regulations, e.g. 49-CFR-192Spalling and softening of concrete (acid formation)Loss of electrical groundingElectrical shockFire, e.g. “slow-burn” 3rd rail electrical leakage (heavy-rail transit)Saturation of electric company transformer coresMalfunction of transit, freight & commuter rail signals & crossing gatesP-I-C (?)
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D = distance between substation and loadRN = resistance thru negative conductors (rails)
RL = negative system to earth resistance at load endRS = negative system to earth resistance at substation end
IT = train operating currentIN = current returning thru negative conductorsIS = current returning thru earth
TPSSTPSSLoadLoad IITT IINN
Positive CircuitPositive Circuit
Negative CircuitNegative Circuit
RRLL IISS RRSS
DD
RRNNIISS
Simplified Transit Electrical ModelSimplified Transit Electrical Model
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Transit System ModelingTransit System Modeling
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Computer Simulations & Predictive ModelingComputer Simulations & Predictive Modeling
-125
-75
-25
25
75
125
9500 12000 14500 17000 19500 22000 24500 27000
Station Number
Str
ay C
urre
nt L
evel
s (m
A/1,
000
feet
)
-30
-20
-10
0
10
20
30
9500 12000 14500 17000 19500 22000 24500 27000
Station Number
Trac
k-to
-Ear
th P
oten
tial (
V)
Stray Current Levels - Multi-Loads
All Trackwork @ 250 Ohms-1,000 Feet
Track-to-Earth Potentials
Multi-Loads
Simulation: 6,610 ampere loads (trains) at 8 Stations
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√ “Floating” negative return (ungrounded)
√ Traction substations close to passenger stations
√ Traction substations reasonably spaced
√ Continuously welded rail√ Track slab rebar electrically
continuous (“stray current collector”) and or epoxy coated
√ High resistivity concrete for track slab, e.g. silica fume additive
√ Yards electrically isolated from mainline
√ Relocated utilities designed with stray current control provisions
√ PRIMARY DEFENSE = High track to earth resistance - Valley Metro design/construction criteria = 250 ohms/1000 ft., minimum
√ O&M is essential
Controlling Transit Stray Current At The SourceControlling Transit Stray Current At The Source
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Typical Insulated Track Designs
Boot Tie & Ballast
Direct Fixation
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Stray Current On Pipelines
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Transit Stray Current: What’s a Pipeline Corrosion Engineer To Do?
Include specific stray current control language and action levels in master utility relocation agreement with transit agencyDocument pipe potentials and currents before transit operations commenceRoutine (reasonable) surveillance once transit line is in service:
Thorough baseline survey shortly after transit operations beginInclude data collection during rain eventsBase testing locations on track-to-earth resistance data reported by transit agency; include crossovers and street crossingsEvaluate remote monitoring vs. manual data collectionParticipate in corrosion control coordinating committee
Easiest & Best Solution (my opinion) = Hire Corrpro!
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Current/Potential DataloggersCurrent/Potential Dataloggers
2”
X/Y X/Y PlotterPlotter
22--Channel Channel Logger w/ X/Y Logger w/ X/Y
CapabilityCapability
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Track-to-Earth Potential (Volts)Rails discharging stray current ~100% of the time
Pipe-to-Soil Potential (Volts) Pipe accumulating stray current ~100% of the time
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Correlations Correlations –– ““Cause and EffectCause and Effect””
Vga Vgb VgcVgx Vgd+ +
+ ++Vga Vgb Vgc
Vgx Vgd+ +
+ ++
Ea Eb Ec+++ Ea Eb Ec+++
Vg1 = -0.0984EM – 0.4964
M 1 -++ -
Rails pickRails pick--up stray current, pipeline discharges stray currentup stray current, pipeline discharges stray current
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Correlations Correlations –– ““Cause and EffectCause and Effect””
Vga Vgb VgcVgx Vgd+ +
+ ++Vga Vgb Vgc
Vgx Vgd+ +
+ ++
Ea Eb Ec+++ Ea Eb Ec+++
M+ - 5 -+
Vg5 = +0.0953EM – 0.4958
Rails and pipeline pickRails and pipeline pick--up stray currentup stray current
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Stray Current Stray Current ““BetaBeta”” ((ββ) Profile) Profile
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TimeTime--Weighted AveragingWeighted Averaging
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
9:00 AM 11:00 AM 1:00 PM 3:00 PM 5:00 PM
PIPE
TO
SO
IL P
OTE
NTI
AL
VAR
IATI
ON
(VO
LT)
Main at University (400S), at TRAX Switch Track, Northeast Corner
Time Weighted Average Corrosive (+) Potential Variation +0.034 Volt
Time Weighted Average Protective (-) Potential Variation -0.042 Volt
Time-Weighted Average Change = 0.034 Volt, <20% of Maximum Variation
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PipePipe--ToTo--Soil Potentials Soil Potentials –– Effective Monitoring/Detection Is EssentialEffective Monitoring/Detection Is Essential
-10
-8
-6
-4
-2
0
2
4
6
8
106:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM 12:00 AM
PIPE
TO SOIL POTE
NTIAL (VOLT
S)
-10
-8
-6
-4
-2
0
2
4
6
8
10
6:00 PM 7:00 PM 8:00 PM 9:00 PM 10:00 PM 11:00 PM 12:00 AM
PIPE
TO SOIL POTE
NTIAL (VOLT
S)
March 2008March 2008
May 2008May 2008
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““BiBi--Polar Stray CurrentsPolar Stray Currents”” 10+ Miles From Transit Line10+ Miles From Transit Line
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
012:00 AM 6:00 AM 12:00 PM 6:00 PM 12:00 AM
PIPE
TO
SO
IL P
OTE
NTI
AL
(VO
LTS)
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Probe & Coupon TechnologiesProbe & Coupon Technologies
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Remote MonitoringRemote MonitoringProbe-Tag: 5500 K11A-1 Probe Type: PA-0.4-10-0.1-6 Probe serial No.: PA04270025Test initiated: 20-12-2005
0
100
200
300
400
500
600
700
800
900
1000
17/03/06 18/03/06 19/03/06 20/03/06 21/03/06 22/03/06 23/03/06 24/03/06 25/03/06
Date
AC c
urre
nt -
A/m
2C
orro
sion
rate
mic
ron/
y
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
JDC
A/m2
Uac (V)
Corrosion rate Uac Jdc
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WhatWhat’’s a tolerable pipes a tolerable pipe--toto--soil potential shift?soil potential shift?
Typical corrosion engineer’s response:It all depends
Dale’s rules-of-thumb:< 0.05 volt – typically not a concern0.05 – 0.1 volt – could turn into a problem, increased monitoring required0.1 – 0.3 volt – further evaluation warranted to determine corrosion significance> 0.3 volt – likely a problem, mitigation appropriate
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Water Pipe
Transformer - Rectifier
+ -
Anode Junction Box
Impressed Current Anodes
Stray Current Corrosion ControlStray Current Corrosion Control
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V A+ -
RI
Stray Current Control using Galvanic AnodesStray Current Control using Galvanic Anodes
Install diode only when needed based on field testsInstall diode only when needed based on field tests
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Stray Current Control using Potential Controlled RectifierStray Current Control using Potential Controlled Rectifier
-+
--+
1 -+
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Transit System
Structure 1 Structure 2
Stray Current Stray Current Drainage Drainage
BondsBonds
Stray current drainage bonds can create an avalanching effect and
should typically only be used as a last resort
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SummarySummaryLeft unchecked, transit generated stray currents can be horrendousStray current control at the source is the best long-term solution…proven technology existsStray current control maintenance and monitoring are essential by the transit agency and by neighboring facility operatorsThe level of monitoring by neighboring facilities (and related cost) is usually inversely proportional to the level of O&M by the transit agencyCorrosion control coordinating committees are a good forum for information exchange and resolution of problems (hopefully before they occur)
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Thank YouQ & A
Dale [email protected]
(713)460-6040www.corrpro.com