Operation and Maintenance of Cathodic Protection in India

Operation and Maintenance of Cathodic Protection in India

Operation and Maintenance of Cathodic Protection in India

Operation and Maintenance of Cathodic Protection as per Standards and Statutory Requirement for a range of sectors, including oil and petrochemical and industrial structure etc.

Schedule of monitoring and maintenance as per standards and statutory requirement

Recommended value of CP monitoring parameters and troubleshooting

Standard Operating Procedures
  • SCADA connection: Parameters transmitted are DC output voltage, DC output current, Selected reference cell potential, under-protection and over-protection, reference cell failure. To check transducers and compare data with field measurement
  • Non-SCADA connection: Fortnightly monitoring of DC output voltage, DC output current, reference cell potential, under-protection and over-protection, reference cell failure
  • The minimum potential is set at -0.85 V and maximum potential at -1.2 V both OFF. OFF potential through reference cell at drain point should be maximum - 1.2 V and at midpoint minimum - 0.85 V
  • Individual reference cell can be put as a controlling electrode to match reference cell potential with a target potential
  • Rectifier is operated on constant potential mode or constant current mode depending upon variation in potential readings.
Measurement of PSP (ON-OFF)
  • TRU/CPPSM is kept in manual/AVCC mode and the output current is set to obtain PSP(OFF) potential between -0.85 V to -1.20 that is drain point potential is maximum -1.2 V and midpoint potential is minimum -0.85 V.
  • Monitoring is carried out between IJ to IJ and all power sources are interrupted synchronously by GPS based current interrupter in 4:1 ratio that is the ON-OFF cycle
  • All temporary and permanent connections to the pipeline are disconnected such as zinc anode, bonding etc. Only zinc anodes placed at HT line crossings for grounding purpose is not disconnected.
  • High impedance digital voltmeter should be used to measure PSP and ON-OFF potential should be recorded after 300 ms after the interruption to avoid a potential spike
  • Sign of potential is important. If the positive terminal of a voltmeter is connected to a pipe and negative to portable reference cell then sign should be negative in voltmeter that is pipe and reference cell polarity is reversed. Similarly, if a pipe is connected to a negative terminal and
  • Reference cell contact with earth should be proper to avoid potential error in the electropositive side.
  • Reference cell should be placed on top of the pipe to avoid potential error in electropositive side
  • The cable from voltmeter to reference cell should be short, otherwise, due to insulation damage or touching with the ground will cause a potential error in the electropositive side.
  • The Cu-CuSo4 solution in the half-cell should be concentrated, otherwise, potential error in the electropositive side will occur.
  • The potential criteria is applicable for temperature @ 25?C. Temperature correction is required for temperature different from 25?C
  • Either resistance bonding should be interrupted or power sources of both the pipelines should be interrupted simultaneously to arrive at correct ON-OFF potential.

Factors that can affect the accuracy of measurement

  • Directly connected galvanic anodes
  • Bonding between own pipeline and foreign structures
  • Bonding between own pipeline and traction system
  • Interference from HVAC and HVDC electric power systems
  • Interference from telluric currents
  • DC mining equipment
  • Uninterrupted power sources
  • Stray current interference from unintended earthed structures or CP shielding
Calibration of stationary reference electrodes (SRE)
  • Stationary reference electrodes (SRE) along with coupons are installed at CP stations, multiple pipelines, interference zones etc to measure pipe to soil potential at that location without interrupting rectifier unit
  • The potential is measured between coupon and SRE and between coupon and portable reference electrode both in ON and OFF condition and compared for computing the error.
  • This process can be repeated quarterly and TRU output being adjusted
 
OFF PSP monitoring through SCADA
 
PSP ON-OFF Graph 1
 
Interpretation of PSP ON-OFF Graph
  • The Off potential measurement is taken annually at each test stations to get an idea of protection level and analyze data for checking the integrity of insulating joint, healthiness of cased crossing, Stray current interference, coating anomalies and other factors for CP shielding
  • In fig 1, the PSP is maximum at drain point and minimum at midpoint but this is an ideal condition without any variation of soil conditions, coating anomalies, interference or any short. The current distribution is uniform
  • In graph 1, as can be seen, overprotection and under protection both are indicated considering -0.85 V to -1.2 V as criteria but so far as 100 mV protection criteria is concerned, the measurement meets the criteria. 100 mV criteria is not applicable at certain conditions and even -750 mV is applicable at high resistivity soil and old pipelines.

Over polarization and its effects

  • The higher value of pH of soil around pipeline together with oxygen and moisture causes deposition of OH?ions at the metal-electrolyte interface and increases the tendency for steel surface to form passive film. This polarizes pipe in an electronegative direction as CP current is received
  • Good protective coating with minor holidays also increase polarization
  • The higher polarization causes coating disbondment due to higher OH?ions between pipe surface and coating. Polarized potential reading on the pipeline will indicate protection level for the section where CP current passes through holidays but CP current will not reach beyond 10 cm of holidays and this section will be corroded in absence of CP current.
  • Due to a constraint in the CIPL survey result in case of coating disbondment and limitations of coating surveys in detecting small holidays, corrosion under disbonded coating cannot be predicted.
  • Precaution should be taken to restrict polarized potential below maximum limit of -1.2 V and also where soil sample indicates higher pH values. Direct assessment should be done at these locations.
Corrosion under disbonded coating

Monitoring of Anode Ground Bed
  • Individual anode output current is measured by shunt resistance in the anode circuit
  • Shunt rating is chosen to measure voltage drop in voltmeter corresponding to current flow in the shunt. Supposing 50 mA to measure, a voltage drop of minimum 5 mV is required. The shunt rating of 0.5 A, 50 mV will be required to measure 5 mV drop which will indicate 50 mA.
  • The anode current is required to be equally distributed. Anode current can be adjusted by variable resistor in the circuit. If any anode output current is zero then it is anticipated that either anode cable is cut or anode has consumed.
  • Total anode ground bed resistance is measured through Wenner 3 pin method. It should be less than 1 ohm
  • Remoteness of anode ground bed can be checked by calculation of rising in voltage and measuring remote earth
External corrosion monitoring by ER Probes
  • ER probes work on the principle that as corrosion occurs, its area reduces and its resistance increases. A portable corrosion meter measures directly metal loss in mils and corrosion rate in mils/year
  • External resistance probes are probes of similar metals as of pipeline and placed at the bottom of the pipe to measure corrosion activity along the same metal as pipeline.
 
Insulating joint & Surge diverter
  • The purpose of insulating joint is to provide electrical isolation from underground protected pipeline from above ground station piping for improving the effectiveness of CP system
  • Surge diverter is provided across insulation joint to protect insulation joint against lightening through grounding by zinc grounding cell

Checking integrity of insulating joint and surge diverter
  • Integrity test of IJ is carried out annually along with PSP (ON-OFF) monitoring. A healthy IJ will indicate no variation in PSP of un-protected side in ON and OFF condition as PSP of protected side varies in On and OFF condition.
  • Any resemblance in the PSP values of both protected and un-protected side in ON and OFF condition indicates IJ short or surge diverter in closed condition. In that case PSP of unprotected side in ON and OFF condition can be checked by disconnecting the surge diverter.
  • Any shorted IJ will cause current drainage through zinc grounding cell or earthing device and earthed structures and decreased PSP values on the pipeline,
  • Surge diverter can be checked by measuring the resistance between two terminals which should indicate open condition. Any closed condition indicates shorted terminal. In that case it should be replaced

Cased crossing

Casings are installed to lay the pipelines across roads/railway lines/canals without resorting to open cut trenching. Casings prevent potential stress damage to pipelines caused from transient heavy loads at road/rail rossings. It also allows easy removal and replacement of pipelines at these locations without disrupting the other utilities at crossings.
 
Cased crossing-introduction
  • Insertion of carrier pipe inside the casing is carried out by supporting the carrier pipe with casing insulators suitably spaced apart – typically 1 meter to ensure that carrier pipe coating does not get damaged during insertion and the carrier pipe is electrically isolated from the casing pipe.
  • The carrier and the casing pipe annulus environment is then sealed with casing end-seal seals to prevent entry of moisture / water.
  • The casing pipe should be two sizes larger than the pipeline being protected.
  • The casing pipe may be coated or uncoated. In case of coating, sacrificial anode is provided for casing protection. Coated
  • casing will shield CP current to reach carrier pipe and protect in case of holidays on carrier pipe with ingress of water inside annular gap. Zinc ribbon anodes are provided on the periphery of carrier pipe to protect itin case of coated casting.
Cased crossing-introduction
  • If casing is uncoated then no sacrificial anode is required. CP current will flow to the carrier pipe through uncoated casing and protect it.
  • The cased crossings have vent and drain pipes on either side to flush the moisture and mud in the annulus space by blowing in dry air during regular maintenance schedule.
  • the pipeline integrity is maintained as long as the carrier pipe is isolated from the metallic casing pipe and the annulus space between the carrier and the casing pipe is free of moisture / water and there are no damages to the carrier pipeline coating inside the casing pipe.
  • If water/moisture enters the annulus area, then corrosion on the external surface of pipeline takes place if not protected by CP current

Atmospheric Corrosion

  • The increased humidity inside annulus space creates a depression in the annulus as a result of which more humid air is drawn into the annulus space, sustaining further atmospheric corrosion.
  • In case of failure of the casing end seals, ground water enters the annulus between casing and carrier causing relative humidity of the air rising to 100 % and initiating an atmospheric corrosion attack at the coating holidays on the carrier pipe.
Electrical short between carrier and casing pipe
A metallic short occurs when casing is in direct contact with carrier pipe either through metallic bolts or through zinc ribbon anodes or insulators between casing and carrier pipe.
Implications of shorted casing
  • Direct short prevents CP current to reach holidays on carrier pipe and in presence of electrolyte between casing and carrier pipe, corrosion takes place
  • Further casing will draw huge CP current and due to this, carrier pipe adjacent to casing end both upstream and downstream will not get adequate current and achieve protection level
  • The ON-OFF potential measurement on both casing and carrier pipe indicates same potential or less than 10 mV differential as both are shorted but the potential of carrier pipe is not true potential as casing shields protective current. Reference cell is required to be placed between annular gap near pipe surface.
  • There is always a likelihood of corrosion in shorted casing as CP current will not protect any holidays on carrier pipe.    
Electrolytic coupling
An electrolytic couple occurs when electrolyte is present in the annulus between casing and carrier pipe
Implications of electrolytic couple
  • Here, electrolyte in the form of water or mud enters inside the annular space through end seals and if coating defect is present will cause corrosion if not protected by CP
  • External wall of Casing is protected by CP current but CP current is discharged from inside of casing wall and will be corroded. Current through electrolyte will protect the carrier pipe inside the casing.
  • Again, casing draws a large amount of CP current and carrier pipe adjacent to casing end both upstream and downstream will not get adequate current to protect the pipe.
  • The PSP (ON-OFF) reading of casing and carrier pipe differ from each by more than 10 mV up to 100 mV. True PSP can be measured only if reference cell is placed in the annular gap near pipe.
  • The holiday on carrier pipe will be protected by CP Current.
Measures to avoid implications of direct short or electrolytic coupling
  • The casing should be avoided at first instant. Instead pipeline with higher thickness can be used along with HDD at road and rail crossing.
  • The end caps or seals should be in good conditions to stop ingress of water inside the annular gap. If water is present in the gap then problem in end seals is suspected. Need to be checked and replaced.
  • If the coated casing is used, it should be protected by sacrificial anode and annular gap should be filled by Petrolatum wax or petroleum based compound to isolate any electrolyte between casing and carrier pipe.
  • Zink ribbon anode is not required to be placed on the periphery of carrier pipe.
  • Casing isolators should be non metallic.
Monitoring of interference bond, polarization cell and zinc grounding
  • Interference bond: Resistance bonding and diode bonding should be checked to ensure that there is less than 50 mV positive swing on interfered with structure when rectifier of an interfering structure is switched ON from OFF condition and stray current is flowing from interfered with structure to interfering structure.
  • Polarization cell: Polarization cell function is to block DC current flow to ground and AC induced current to ground through zinc grounding electrode. In addition, it also protects the pipeline from lightning and line faults. At site, its function of blocking DC current and grounding AC current can be checked by measuring DC potential and AC potential as per design specification of polarization cell
  • Zinc grounding electrode: Zinc grounding electrode resistance should be checked by Wenner 3 pin method. Its resistance should
  • be less than 5 ?.
Live examples and lessons learnt
  • In one of the spur lines, protection level was not available despite high current density of more than 100 μA/m². On inspection, it was found that coating at the above ground portion had damaged and exposed portion was touching earthed fencing of receiving station.Coating was repaired and contact was removed. Protection level was achieved and current density came down to design level
  • During PSP (ON-OFF) monitoring on one of the pipelines, overprotection was observed at a stretch. On inspection, it was found that one TRU from other side of IJ was also feeding current and it was not isolated. Further at railway crossing magnesium anode for casing pipe was connected to carrier pipe. TRU was isolated and Mg anode was disconnected. Now PSP (OFF) was in a protected zone.
  • AC corrosion was observed at several locations at pipelines running parallel to HT transmission lines. AC voltage was 12V and current density was more than 100 A/m². Zinc anode resistance was 6-7 ?. Polarization cell was replaced and copper conductor earthing was provided. After that AC voltage was 1-2 V and current density was 40-50 A/m².
  • In one of the new pipelines, at SV station, tapping for instrument tubing were taken from protected side. PT and TT cable were earthed. Due to huge current drainage, current density was 100 μA/m². Dielectric coupling was provided isolating protected and unprotected side and current density was found to be less than 50 μA/m².
Monitoring and maintenance of cathodic protection system of plant piping, underground bullets and storage tank bottoms
  • Monitoring and maintenance of process industries piping and tank bottoms cannot be done as of cross country pipelines due to very congested structures profile and interference from other structures. However, process of monitoring and maintenance is same as that of cross country pipeline.
  • Since simultaneous interruption of power sources is not possible for measurement of ON-OFF potential, hence CIPL and DCurvey cannot be done on these structures. Reference cells are placed closed to tank bottoms at the time of construction and measurement points are placed at approaching locations. The potential measurement indicates OFF potential at that point. If polarization coupon is provided, then interrupting it momentarily by magnetic switch and measuring potential with respect to closed reference cells and will indicate polarized potential at holiday.
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