Introduction NACE approves 4 tests for determining Coating Condition AC Current Attenuation AC Voltage Gradient DC Voltage Gradient Pearson Method old school Note CIS is NOT one of them Does not find small defects.
Introduction Before joining the Pipeline / Corrosion industry, my schooling was Electronics Engineering Technology. This ppt focuses on 2 of the tests ACCA/VG Both these inspection techniques are very electrical in nature and are well explained by simple Ohms law.
ACCA Introduction AC Current Attenuation Sometimes called the EM method BUT it doesnt do anything really tricky with electromagnetics. I personally think RP0502 should have this reworded Simply uses Gauss & Faradays 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.
ACCA Application Apply a known and consistent I. Measure the current at points along the line. Calculate a loss/distance ratio Alternately factor in diameter for loss/*area* ratio to compare different diameters More loss/distance = worse coating. Current loss can also indicate unknown connections, insulator and casing problems.
dB Current Expressing current loss as a ratio to current before the loss is more accurate. deciBels are a ratio Next 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 (Ts to similar service mains) are the same size/length.
Current Attenuation Graph 3 steps are different looking in mA but nearly identical in dB
Common 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 ~100m Current fluctuates down the line * Cant GAIN Current walking away from Source We are detecting EM fields and they can be distorted by large metallic objects and other EM fields Reduce bonds / bleed-over Some places you just cant take a reading. Deep >10-18 pipes can sometimes be a problem.
Using TrendLine function in Excel
Bad Example #1 Example of current that goes reading #47 Also, survey is using <20mA, small errors look big, better to use 100mA+ Survey done with locate frequency, not low (ie:4Hz)
Bad Example #2 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 Current cant go up by any amount, never mind double
AC Frequency Even through a perfect coating, an AC signal will bleed off due to capacitive coupling to the ground outside the coating. Higher f = higher attenuation.
Bad 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).
Shielded defects ….. Higher frequency I will bleed off faster. Capacitive impedance drops as f Remembering 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 isnt grounded)
Distribution of current. 1kHz vs. 4Hz 1A (1KHz) 400mA 600mA 200mA 400mA 1A (4Hz) 40mA 960mA 900mA 60mA fault 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.
Low frequencies go to the short
Troubleshooting Loops Current Direction can be seen above ground without contact.
Benefits and Strengths Easy to operate by one single operator No direct contact to ground necessary Near DC fault detection signal makes capacitive effects insignificant Low frequency for pipe detection: long distances Current Direction facility to enable short identification With addition of earth-frame, often same hardware can be used for an AC Voltage Gradient survey.
Benefits and Strengths II Previously Difficult situations: Casings – Now, just check if here is a current loss Insulators – 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.
Difficult 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 EM Older equipment was subject to 50/60 Hz interference Newer circuitry sees PCM current through other EM noise.
The Effect of Insulators (i.e.: Dressers, ZapLock, Bell/Spigots in some conditions.) When low resistance connection is in effect, current distribution is uniform PCM shows 50/50 split of current When joint goes high resistance, current splits unevenly PCM (& CP) current drops to undetectable level ahead of open (Is still protected – potential is maintained, just takes less current)
Mg Anodes Anodes 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 one Any 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.
Underground Gas Distribution Short to Water Quite prevalent, especially in warmer climates (water lines more shallow) Hard to find without excavation PCM method walked technician right to it. Picture of site on next slide
Underground Gas Short
CASE I N DE Insulator Mainline InsulatorDE 100 ma 94 ma 400 ma No Signal 475 ma 600 ma 950 ma 805 ma 700 ma 500 ma 2.50 A 100 ma No Signal 98 ma 78 ma 1.60 A DE 375 ma450 ma 2.60 A Typical Mag. anode - connected Area Rectifier - Anodes used as PCM ground & PCM 3 amperes output MSA not insulated ma found on gas serviceline 875 ma 578 ma 110 ma
2 nd Distribution Example
Transmission 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 transmitter Perform survey towards transmitter, find 2 areas of higher than average loss Found 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.
Transmission Coating Survey
ACCA Questions ?
ACVG AC Voltage Gradient Like DCVG / Pin-to-pin / similar but uses a transmitter, not the CP DC Sizing similar to DCVG Empirical testing now underway
ACVG An improvement though is to tightly tune the voltmeter to the transmitter frequency A Fluke ® on DC can find other sources: Stray current, sacrificial anodes, dissimilar metals A Fluke ® on AC isnt tuned: Any AC frequency from ~5 Hz - 25kHz 60 Hz.AC currents/faults, telephone/scada noise
Pool of Potential Is AC, but at any instant in time, there is a direction.
ACVG Sometimes part of an ACCA tool Is a big AC voltmeter Voltmeter Leads are the probes Results in dB gives very wide dynamic range Calibrated amplification reduces a variable Consistent lead spacing (a-frame) removes one other variable
ACVG Receiver Theory
ACVG Faults (on an otherwise good cable) have been found exceeding 6.8 MΩ Very sensitive as it has an extremely high signal-noise ratio
AC Voltage Gradient Can be part of Current tools Becoming very popular Extreme sensitivity Rejection of interference Very accurate location of faults typically better then 6" Sometimes part of Current Attenuation equipment This method deserves to be considered as a solid tool for integrity and the ECDA process.
ACVG in Operation Both signal strength and direction arrows lead user to holiday. Fault value is proportional to holiday size and soil resistivity.
In 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..
ACVG Tuning Older systems used a simple DMM Does not tune to any one frequency 60 Hz, cable earth faults, telecom noise Rxd Very tight tuning in the signal generator and receiver effectively increases sensitivity as it ignores current from other sources SNR improves
ACVG Find Transversal A-frame dB readings: 80 (right) and 85 (l) dB (from PCM) before fault= 49.85,after= Distance between readings = 15 meters 37mB/15m (actually just fault) Pipe dia.= 12 /PE in two layers Soil Ohm x m Fault size = 60 mm length x 2 mm wide Note the higher trans dB reading on left side that is the same side where valve station is located
Jordanian 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.
Sarvjit Singh Corrpro Companies Middle East L.L.C Client : Jordan Petroleum Refinery Company
Sarvjit Singh Corrpro Companies Middle East L.L.C Client : Jordan Petroleum Refinery Company
My Favorite ACCA/VG Story 2 mile long distribution main, 25 yr old For last 4 years, potentials have been dropping Several surveys showed no shorts ACCA: in each 100 span, current was dropping like a rock but no single point of trouble ACVG: 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.