2Introduction NACE approves 4 tests for determining Coating Condition AC Current AttenuationAC Voltage GradientDC Voltage GradientPearson Method‘old school’Note CIS is NOT one of themDoes not find small defects.
3IntroductionBefore joining the Pipeline / Corrosion industry, my schooling was Electronics Engineering Technology.This ppt focuses on 2 of the tests ACCA/VGBoth these inspection techniques are very electrical in nature and are well explained by simple Ohm’s law.
4ACCA Introduction AC Current Attenuation Sometimes called the EM method BUT it doesn’t do anything really tricky with electromagnetics.I personally think RP0502 should have this rewordedSimply uses Gauss & Faraday’s well proven laws that the strength of an EM field is proportional to the current through a conductor.Working backwards, knowing the EM strength & conductor depth, we can calculate the current.
5ACCA Application Apply a known and consistent I. Measure the current at points along the line.Calculate a loss/distance ratioAlternately factor in diameter for loss/*area* ratio to compare different diametersMore loss/distance = worse coating.Current loss can also indicate unknown connections, insulator and casing problems.
6dB CurrentExpressing current loss as a ratio to current before the loss is more accurate.deciBels are a ratioNext chart: Loss 1 = ~1000 mA, 2~400, and #3~150.Loss 1 *looks* much worse but expressed in dB all are the same. Hence all three features (T’s to similar service mains) are the same size/length.
7Current Attenuation Graph 3 steps are different looking in mA but nearly identical in dBNote dB mA
8Common Problems Current goes UP as you walk away from Tx Current returning to ground point is sufficiently concentrated near Tx ground stake to create an EM field that cancels out some of the EM field on the pipe.Returns to normal attenuation after ~100mCurrent fluctuates down the line* Can’t GAIN Current walking away from SourceWe are detecting EM fields and they can be distorted by large metallic objects and other EM fieldsReduce bonds / bleed-overSome places you just can’t take a reading.Deep >10-18’ pipes can sometimes be a problem.If you can’t take a reading, the graph will show more loss across more distance, ratio will still be accurate.
12Bad Example #1 Example of current that goes up @ reading #47 Also, survey is using <20mA, small errors look big, better to use 100mA+Survey done with locate frequency, not low (ie:4Hz)
13Bad Example #2 Current can’t go up by any amount, never mind double Same data but shown on Log vert. axis.See how first 60% now looks more even compared to linear graph previously.Segments 0-10 are worse then 10-40
14AC FrequencyEven through a perfect coating, an AC signal will bleed off due to capacitive coupling to the ground outside the coating.Higher f = higher attenuation.
15Bad Example #2 Same graph as previously. In my opinion, the high attenuation near the beginning is only due to the higher frequency (remember this was done at a locate freq, not 4Hz).
16Shielded defects ….. Higher frequency I will bleed off faster. Capacitive impedance drops as fRemembering we are detecting current loss, does disbonded coating have a lower impedance?i.e.: does the pipe-coating-dirt model lose current faster then the pipe-electrolyte-coating-dirt ? Why ? (the electrolyte isn’t grounded)
17Distribution of current. 1kHz vs. 4Hz Because of an overall lower impedance at higher frequencies, current will travel down stubs and short mains (and attenuate faster on all runs).It also bleeds through what are actually good (DC) insulators.Surveying at a lower frequency will more accurately mimic where your CP current is going.1A (4Hz)1A (1KHz)40mA400mA600mA960mA900mA200mAfault60mA400mA
18Low frequencies go to the short Avoid distorted readings
19Troubleshooting Loops Current Direction can be seen above ground without contact.
20Benefits and Strengths Easy to operate by one single operatorNo direct contact to ground necessaryNear DC fault detection signal makes capacitive effects insignificantLow frequency for pipe detection: long distancesCurrent Direction facility to enable short identificationWith addition of earth-frame, often same hardware can be used for an AC Voltage Gradient survey.Most of these benefits all boil down to cost savings.
21Benefits and Strengths II Previously Difficult situations:Casings – Now, just check if here is a current lossInsulators – If both zones have similar potential, there could be a question as to if insulator is shorted. But in Current Mapping, if there is current getting through to far side piping, there is a short.Bonds – large current loss and/or direction reversal show below grade bonds very easily.Most of these benefits all boil down to cost savings.
22Difficult Applications We are locating current flow and current does not travel into an open circuit.Insulating flanges and fittings (ZapLock, Bell&Spigot, Dresser) will limit current flow along a length of pipe.This is still valuable information to locate these and verifies their proper operation.Congested areas – confusing EMOlder equipment was subject to 50/60 Hz interferenceNewer circuitry ‘sees’ PCM current through other EM noise.
23The Effect of Insulators (i. e The Effect of Insulators (i.e.: Dressers™, ZapLock, Bell/Spigots in some conditions.)When low resistance connection is in effect, current distribution is uniformPCM shows 50/50 split of currentWhen joint goes high resistance, current splits unevenlyPCM (& CP) current drops to undetectable level ahead of open(Is still protected – potential is maintained, just takes less current)
24Mg AnodesAnodes will provide a path to ground for the AC signals we are detecting (4Hz and locate freqs.).It may look like a loss but likely a big oneAny current this big is a short (casing, other utility, insulator), so look for 4 Hz somewhere else as current has to go somewhere. If nowhere else, is a Mg.Amount of signal loss will be proportional to anode quality / life expectancy.
26Underground Gas Distribution Short to Water Quite prevalent, especially in warmer climates (water lines more shallow)Hard to find without excavationPCM method walked technician right to it.Picture of site on next slide
27Underground Gas ShortBlack pipe is gas line, copper is poorly installed water line.Current flowed down gas to water, then direction showed current up both sides of water line.
28CASE I N MSA not insulated - 875 ma found on gas serviceline Area Rectifier - Anodes used as PCM ground & PCM 3 amperes outputDEInsulator875 ma110 maN450 ma375 ma94 ma578 maDE475 maDE700 ma100 maDE805 maDE400 ma500 ma950 ma1.60 A2.60 ANo Signal2.50 A98 ma600 maNo Signal100 ma78 maTypical Mag. anode - connectedMainline InsulatorDE
30Transmission Coating Survey (and bad insulator) Started as just a demonstration, no known problems on line.Upon connecting transmitter, 85% of signal going in one direction !?!Drive to next road crossing (only ~1200m) and had already lost 90% of remaining current, thus problem is between road and transmitterPerform survey towards transmitter, find 2 areas of higher than average lossFound major loss across one point, investigation found a shorted insulator in underground T connection to foreign compressor site.If both systems had similar protection levels, a CIPS may not have shown any defect.If short had gone unnoticed, stray currents could develop causing problems and/or rectifier power bills /current rating could be higher than necessary.Several important points on this page.
31Transmission Coating Survey Perhaps I shouldn’t have included all the info here, this looks like a scary graph.Includes mA, dB, depth and loss/distance ratio.Loss/distance is most important trace and depth is handy.
33ACVG AC Voltage Gradient Like DCVG / Pin-to-pin / similar but uses a transmitter, not the CP DCSizing similar to DCVGEmpirical testing now underway
34ACVGAn improvement though is to tightly tune the voltmeter to the transmitter frequencyA Fluke® on DC can find other sources:Stray current, sacrificial anodes, dissimilar metalsA Fluke® on AC isn’t tuned:Any AC frequency from ~5 Hz - 25kHz60 Hz.AC currents/faults, telephone/scada noise
35Is AC, but at any instant in time, there is a direction. Pool of PotentialIs AC, but at any instant in time, there is a direction.
36ACVG Sometimes part of an ACCA tool Is a big AC voltmeter Voltmeter Leads are the probesResults in dB gives very wide dynamic rangeCalibrated amplification reduces a variableConsistent lead spacing (a-frame) removes one other variable
38ACVGFaults (on an otherwise good cable) have been found exceeding 6.8 MΩVery sensitive as it has an extremely high signal-noise ratio
39AC Voltage Gradient Can be part of Current tools Becoming very popular Extreme sensitivityRejection of interferenceVery accurate location of faultstypically better then 6"Sometimes part of Current Attenuation equipmentThis method deserves to be considered as a solid tool for integrity and the ECDA process.
40ACVG in OperationBoth signal strength and direction arrows lead user to holiday.Fault value is proportional to holiday size and soil resistivity.
41In this case the next fault was quite close (20-30m) which is why the left side of graph climbs quickly.Other cases may show 100s of meters of signal at 30 and under..
42ACVG Tuning Older systems used a simple DMM Does not tune to any one frequency60 Hz, cable earth faults, telecom noise Rx’dVery tight tuning in the signal generator and receiver effectively increases sensitivity as it ignores current from other sourcesSNR improves
43ACVG Find Transversal A-frame dB readings: 80 (right) and 85 (l) dB (from PCM) before fault= 49.85,after= 49.48Distance between readings = 15 meters37mB/15m (actually just fault)Pipe dia.= 12”/PE in two layersSoil Ohm x mFault size = 60 mm length x 2 mm wideNote the higher trans dB reading on left side that is the same side where valve station is located
45Jordanian Fire Water Line 16”/30yr old “We are using this equipment for locating coating defects on the coal tar epoxy coated pipelines. While doing survey on the pipeline we got 63 dB microvolt with A-Frame. The epicenter of the defect was located by taking readings above and in line with the pipeline and perpendicular to the pipeline. From all four sides direction of arrow was indicated towards the defect. The defect location was excavated.“Upon excavation, we could not find bare pipe at this location. What we found was that the coating has degraded badly and it has become permeable. This has happened at two locations.“Please advise if the current is flowing through the permeable coating or there are other reasons.”
46Sarvjit SinghCorrpro Companies Middle East L.L.CClient : Jordan Petroleum Refinery Company
47Sarvjit SinghCorrpro Companies Middle East L.L.CClient : Jordan Petroleum Refinery Company
48My Favorite ACCA/VG Story 2 mile long distribution main, 25 yr oldFor last 4 years, potentials have been droppingSeveral surveys showed no shortsACCA: in each 100’ span, current was dropping like a rock but no single point of troubleACVG: BAM… BAM… every 40’Had spent 40+ person-hours with no resolution. In 45 minutes we found and quantified the problem. The customer could budget digging up 6’ of every 40’ to replace the joint coating.Do the math, 40 hours of a techs time or 40 minutes? This one application paid for a kit.In addition, reduced power consumption and likelihood of further coating damage from high current density.