Leakage and inspection skills in pipelines
Leakage is mainly caused by corrosion, which accounts for about 60% of such incidents. Other reasons include misoperation and line collapse. If the pipeline is corroded or bumped, trachoma and cracks may occur. Following the impact, abrasion and corrosion, the occurrence of these trachoma and cracks will gradually expand, causing the pipeline to leak. The serious leakage on the pipeline can account for 2% to 3% of the flow. Leaks are just small leaks. Leakage can be divided into three phases: leakage leakage transient phase, leakage leakage stabilization phase, closing valve shutdown phase.
As shown in Figure 7-17, the abscissa is the time after the leakage begins, and the ordinate is the percentage leakage flow with a steady leakage flow rate of 100%. In the first 1-2s of the leakage transient phase, the leakage flow quickly increased to the maximum value, and a relatively obvious decompression wave occurred. This wave was transmitted up and down, causing hydraulic transients in the cement pipeline . In the future, the effects such as frictional restraint and active conditioning will be attenuated, and the leakage flow will be gradually reduced. A new and stable transportation condition will be gradually established across the entire line. The third stage is to manually or actively close the line valve to prevent leakage. The first and third stages are transient conditions. Whether there is stability or not depends on whether the valve is closed in time.
In the pipeline leak detection system, the detection sensitivity, leakage direction and leak detection time are the primary goals. Leak detection sensitivity is the smallest leak detection capability, and it is a function of all operating parameters (pipeline fluid, pipe standard, pipe length, stability relative to instantaneous operating conditions, and pressure, temperature, and accuracy of the flow meter), and the pipe manager must rely on Go through the correct analysis of leak detection data. The leak detection system should have a minimum leak detection capability of 0.5% -1% during the change period of 2-24h. Leakages below this level of sensitivity are generally undetectable, and can only be located and confirmed by hydraulic testing when the pipeline is stopped. The leak detection sensitivity of some pipeline systems can reach 0.05% -0.1%, or it can be designed for a certain number of barrels, and these leak points can be detected in much less time. However, these improved sensitivities are only suitable for specific uses, and not guaranteed for all pipelines.
In large-scale natural gas pipelines, the common method for pipeline break detection has been to actively control the closing and opening of the shut-off valve through the pneumatic or electronic detector installed on the shut-off valve of the pipeline. On natural gas pipelines, a shut-off valve is usually installed every 20km.
When the natural gas cement pipeline cracks (that is, natural gas leaks out of the pipeline 100% from the leak hole), as the natural gas leaks into the surrounding environment, the pipeline will immediately have a pressure drop. The leakage hole is like an orifice plate flow meter, and the pressure drop in the pipeline drops to a certain value of the flow meter. The natural gas flow in the upstream line of the leakage hole increases, while the air flow in the downstream line reverses. This increase in flow rate and reverse flow rate increases with the increase in the diameter of the pipeline and the increase in flow rate. The amount of gas leaking through the leakage hole is proportional to the pressure of the pipeline on both sides of the orifice plate. With the separation of the pipeline itself, the pressure of natural gas pipelines has gradually reduced. The orientation of the nearby shut-off valve is a key factor in determining leak detection equipment and detecting leakage points. If the pipeline cracks near the shut-off valve, the detector of pressure drop and pressure change rate will operate immediately; however, if the leak point is 32.18km away, the time to detect the pressure drop and pressure change rate is longer. 10min,
Leak detection of cement pipes is a practical skill. There are many specific methods, which have been improved rapidly over the years. Especially in recent years, with the development of computer skills and communication skills, the relevant data was collected according to the hosting line SCADA system, and transmitted to the host computer to analyze using pre-programmed application software, identify leaking sections, and operate corresponding shut-off valves.