| Cathodic protection technology, as an important technical method of pipeline anticorrosion, has been widely used. Even if the cathodic protection system is operateing, the cathodic protection also can lose efficacy because of certain disturbance factosr. The disturbance factors embrace the third cathodic protection system which lies nearby the pipelines, and the strike-parallel AC high voltage transmission line, and electrified railway and so on. All of these factors form stray current with different intensity and direction.The stray current is flowing underground outside the shielding systems and corrosion comes into being. The stray current is divided into direct stray current and alternative stray current. Generally, the stray current is heavy and can lead to electrolysis corrosion. To a great extent, the stray current can speed up the electrochemistry process and cause the metal pipelines’intense corrosion.It was difficult to find the corrosion of stray current because pipelines are underground. So the stray current can cause tremendous risks.In order to protect pipelines’safety, it must to evaluate the effection caused by stray current and investigate the results because of stray current. It is very necessary to study the technology of the stray current’s testing and draining.Specification of cathodic protection for underground steel pipelines has pointed out: when designing compulsion electric current cathodic protection system, it should pay attention to prevent interference between the protection system’s pipeline and other metal structure. Specification also pointed out:when the pipeline was investigated periodically, it need to test the relationship between pipeline and environment. For the region has the disturbance of stray current, a supplemental test should be added and it should take useful method to drain the stray current. Howerer, the sensitivity of the existing test method, the continuity of sampling time, as well as the analysis and treatment of testing data make it difficult to evaluate and explain the frequency, the size of stray current and when and where stray current comes into the pipelines.In the past, the test and discharge technology of stray current in metal pipeline has not been recognized sufficiently. And, a practical technology to test and discharge a stray current has not been formed. This is a new task for pipeline maintainace and it is also the goal of the theis.This dissertation have studied the information and method, both at home and abroad, about testing and draining the stray current, and the influence of stray current to gas pipeline cathodic protection system was investigated.By analysing all kinds of testing devices’ technology principle both at home and abroad and comparing their advantage and disadvantage, the paper studied the the testing devices and apparatus from their technology index and performance.Morever, the paper found out the problems which the stray current test system used in Si Chuan and Chong Qing have.On basis of it,the paper have put forward the performance requirements of the testing system and pointed out it is necessary to develop a new testing system of stray current. Later, the paper have developed a new test system of the stray current.And it have used the new system for testing gas pipeline in complicated areas such as the pipelines intersecting to electrified railway, paralleling or intersect to high voltage transmission line and so on. It evaluated the effect of draining to the direct current and alternative current. The result shows that the system’s testing data is exact and reliable. It also shows that the system’s functions satisfy the claim of the field testing and it has fine performance. The achievement worked out by this dissertation makes the cathodic protection system to be more perfect and effective, and it makes the existing pipeline to display function to a maximum limit. This achievement has provided a scientific basis and a important technical method for instructing a pipeline protection. |
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