overline assessment pipelines. figures. jan 2003.ppt [read...
Post on 30-Apr-2020
4 Views
Preview:
TRANSCRIPT
V = Measured potentialM
V = Ohmic component ( IR error ) IR
V = Polarised Potential P
V = V + VM IR
Fig 1I.R. Drop Error Component of Potential Measurement
VM
AnodeVIR
P
+
-
+ -
Pipe
D.CSupply
I.R Error
Fig 2Idealised Polarised Potential Measurement
0 500 1000 1500 2000
- 0.8
- 1.0
- 1.2 Polarised or InstantaneousOFF Potential
Time - Milli- seconds
Depolarisation
C.P.ON C.P. OFF
Switch OFF
Fig 3Close Interval Overline Potential Survey
Distance andcontact wire
TestPost
ReferenceElectrode
Recording Voltmeter( Data logger )
Defect
DefectDefect
Soil Variance MetallicContact
DISTANCE
-1.5
-1.0
-0.85
-0.5
0Pipe
/ So
il Po
tent
ial
wrt
Cu/
CuS
Ovo
lts4
Fig 4Close Interval Overline Polarised Potential Survey
Distance andcontact wire
TestPost
ReferenceElectrode
Recording Voltmeter( Data logger )
Defect
DefectDefect
Soil Variance MetallicContact
DISTANCE
-1.5
-1.0
-0.85
-0.5
0Pipe
/ So
il Po
tent
ial
wrt
Cu/
CuS
Ovo
lts4
TransformerRectifier
Timer/Contactor
Groundbed
Fence
Test Point Fence
Pipe /SoilPotential ‘ON’
I.R. Drop FreePolarised ‘OFF’Potential
0 10mS
- 0.6- 0.7- 0.8- 0.9- 1.0- 1.1- 1.2
Switch Off
C.P. ON C.P. OFF
0.1 - 10 seconds
Time not to scale
PipelineReactance effectplus switchingspike
Depolarisation
This period significantlyshortened in the presence of sulphate reducingbacteria
Can extend to50mS in some circumstances
Pipe
/ So
il Po
tent
ial
wrt
Cu
/ CuS
O v
olts
4
Fig 5Polarised Potential Measurementwith Pipeline Reactance and Switching Spike Effects
6 7 8 9 10 16 17 18 19 20
* * * * ON OFF ON OFF
Time 100x mS
Measurements
Distance Metres
Pipe
/ So
il Po
tent
ial
wrt
Cu/
CuS
O v
olts
4
Survey Characteristics
Nominal Speed 1 metre / second ( 3.6km / hour ) ON : OFF Ratio 4:1 OFF Period 0.2 second ON Period 0.8 second Measurement spacing. 1 metre nominal Measure both ‘ON’ and ‘INSTANT OFF’ every 1 metre
Fig 6 Typical Combination of ‘ON’ and ‘OFF’ PotentialMeasurements During Fast Survey
0 1 2
- 0.6- 0.7
- 0.8
- 0.9- 1.0- 1.1
Pipe
/ So
il Po
tent
ial
wrt
Cu
/ CuS
O4
volts
Fig 7Polarised Potential Measurement Errorsdue to Inaccurate Timing
1 2
1 21 2
1 2
Indicated Polarised Potential
- 0.95v optimum timing ( accurate)
- 0.88v 0 - 100mS early- 1.10v > 100mS early
- 0.94 late
1 ‘Wait period’2 ‘Measure period’
Survey CharacteristicsOFF period 1 SecondWait period 0.1 SecondMeasurement period 0.1 Second
0 200 400 600 800 1000Milli - Seconds
Fig 8Close Interval Overline Polarised Potential SurveyMobile + Static + Switching Device
Mobile Data logger
Defect
DefectDefect
Soil Variance MetallicContact
TransformerRectifier
Timer/SwitchingDevice
StaticDataCollector
StrayCurrent
Mob
ileSt
atic
Precise Time Linkage Mobile and Static
Fig 9Close Interval Overline Polarised Potential Survey and DCVGMobile Data logger + Static + Switching Device
Defect
DefectDefect
Soil Variance MetallicContact
TransformerRectifier
Timer SwitchingDevice
StaticDataCollector
StrayCurrent
Precise Time Linkage Mobile , DCGV and Static
DC
VG 0
+200
-200
Approaching a Defect Over a Defect Past a Defect
Current FlowThrough the Soil
Current FlowThrough the Soil
EquipotentialGradientLines
DefectDefect
Soil Variance MetallicContact
TransformerRectifier
Slow Non PrecisionSynchronisedTimer/SwitchingDevice
Survey Progress
No RecordingAnalogue Indication OnlyNo Local Pipe / Soil PotentialNo Distance MeasurementPeg / Mark and Manually Record
Pipe / SoilPotential at Testpoints only
Pipe
Repeat Measurements1 2 3 4
15mV 8mV 3mV 1mV
= 27mV
Defect
Analogue DCVG Defect Characterisation
1. “DEFECT IR” : “Overline to Remote Earth” = 27mV.
2. “Pipe / Soil Potential shift ( ON - OFF ) : Applied Signal” at Defect is as straight line attenuation between test points, say 300mV
3. Defect Size ( % IR ) = “ Defect IR” mV
say = 9% 300
ESTIMATED
27
“Applied Signal” mV
Fig 12Non Recorded Analogue DCVG Defect Characterisation
For Example
Combined Enhanced CIPS & Recorded DCVG to Indicate Defect Size.
Defect
DefectDefect
Soil Variance MetallicContact
TransformerRectifier
PrecisionTimer /SwitchingDevice
OptionalStaticDataCollector
StrayCurrent
Precise Time Linkage Mobile , DCGV and Static
DC
VG 0
+200
-200
Fig. 13
0
-200
Static Data on anothergraph
top related