Their high current output is capable of protecting large underground metal structures economically, is flexible to deal with varying conditions and less susceptible to soil resistivity. However, they rely on the continuity of their AC power source and can interfere with other nearby buried structures. The level of CP current that is applied from impressed current systems is important. Too little current will lead to corrosion damage; excessive current can lead to disbanding of the coating and hydrogen embrittlement.
For these reasons impressed current systems require regular monitoring. Pipe-to-Soil Potential ON Potential - The potential of a pipeline at a given location is commonly referred to as the pipe-to-soil potential. It results from the corrosive electrolytic reaction between the buried pipe and its surrounding soil the electrolyte. It is actually measured between the pipeline and a reference electrode most commonly copper sulphate , placed in the soil directly over the pipeline.
It is also known as the ON potential because the measurement is made while the CP system is energised. Instant OFF Potential - When a pipe-to-soil measurement is made, the pipeline potential will appear to be more negative then its true potential, due to IR drop errors. The instant OFF measurement corrects for these errors; the CP current is briefly interrupted to produce a "true" pipe-to-soil potential, free from undesirable IR drop effects and before any appreciable depolarisation has occurred.
This is a truer measure of the level of protection afforded to the pipeline. If it is not possible to disconnect the CP momentarily then an alternative approach is the use of a corrosion coupon see below.
Coupon Current - Corrosion coupons connected to cathodically-protected structures can be used to monitor the effectiveness of the CP system. A coupon is a representative sample of the pipeline material, buried close to the pipe so that it is subjected to the same environment. Connected to the pipeline via a test post, it simulates how the pipeline would react if there were a defect often referred to as a "holiday" in its coating.
It is especially useful when it is not possible to interrupt the CP system, since instant OFF potentials can conveniently be measured by interrupting the CP connection to the coupon. The surface area of the coupon allows the current density to be calculated. However, they are only representative of the pipeline at that point — and for a short length either side.
A direct connection is made to the pipeline and this trailing wire is unwound from a spool as the technician walks along its length. As he goes, the TR current output is interrupted to enable the technician to take a pipe-to-soil OFF potential measurement at approximately 1m intervals.
On pipelines with multiple TRs, all the outputs or at least those that influence the potential measurement at that point have to be interrupted synchronously. Interruption cycle times vary but the selected "on" period is longer than the "off" period to limit depolarisation of the pipeline during the survey.
DCVG is used for locating and sizing defects in the coating of the pipeline. Measurement of the voltage gradient at the surface above the pipeline enables even small flaws to be detected and positioned accurately.
Both CIPS and DCVG techniques are increasingly used — but can be time-consuming to set up in the field because of the requirement to synchronise transformer rectifier outputs.
Rectifiers can be configured to synchronise and interrupt their output simply by sending a message from a cellphone. Used in conjunction with specific MERLIN Transformer Rectifier Monitors , it enables interruption switching of the current output at a rectifier or solar station to be controlled remotely.
The Interrupter fails safe and switches the rectifier output loads encountered under the temperature, environmental and electrical conditions experienced in a rectifier cabinet. The solid state circuitry overcomes the limitations of electromechanical relays. All pipeline operators use CP extensively on their transmission pipelines. The big advantage of CP over other forms of corrosion treatment is that it is applied very simply by maintaining a DC circuit and its effectiveness can be monitored continuously.
Because of the importance of CP in protecting the pipe, operators are required to take and report regular measurements of CP data, both of the levels of protection applied to the pipe at source and the in situ levels measured along the pipe itself. CP2 - Cathodic Protection Technician. CP3 - Cathodic Protection Technologist. Close-Interval Potential Surveys. Cathodic Protection Survey Procedures, 3rd Edition. Cathodic Protection Remote Monitoring.
Cathodic protection systems are designed to control the corrosion of a metal surface. While localized damaged surfaces can be patched, ongoing degradation may weaken the integrity of the overall infrastructure. When designed correctly, CP systems can continuously protect infrastructure against corrosion, and techniques such as detailed equipment schematics, system specifications and designs, and field surveys are important when designing a CP system that will provide adequate corrosion protection.
Design of cathodic protection systems is essential to power plant operation. Protecting Concrete Wharves with Cathodic Protection.
Coatings play an important role in CP design. Cathodic protection of uncoated structures can be implemented, but CP of an uncoated structure is usually not cost effective. Coatings alone are not able to prevent corrosion of the metal underneath because they have a finite life, eventually allowing oxygen, water, and chemicals to reach the substrate. Typically coating and CP are used simultaneously, as together, they provide the best technical and economic protection. Coating inspection is also crucial because it is often desired to install CP to a structure with its coating in the best condition possible.
This will ensure the cathodic protection system performs as designed. Coatings in Conjunction with Cathodic Protection. Effects of Coating on Corrosion and Cathodic Protection. Pipeline Coatings. Interference is any electrical disturbance on a metallic structure caused by a stray current.
Stray current is defined as current flowing on a structure that is not part of the intended electrical circuit. Stray current can be produced by any system conducting an electric current, such as electrified train tracks of overhead power lines that has two or more points of contact with an electrolyte.
Cathodic protection can be designed to mitigate stray current problems. The advantage of CP is that it avoids the use of bonds and the installation and maintenance problems that may accompany them.
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My account. Log in. Home Cathodic protection explained Cathodic protection explained Cathodic protection is a method for preventing corrosion on submerged and underground metallic structures.
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