Research On Double-layered Pressure Pipes And Their On-line Safety Monitoring Technology

Piping systems, as a reliable, economical, convenient, pollution free means of transportation, have been widely used in the transportation of petroleum, natural gas, coal liquid etc. The safety of piping is of high important. However, accidents occur frequently due to the corrosive nature of the medium, variation of geography and climate, natural wear, ageing and man-made destruction.Piping accidents could be reduced to some extents by traditional leakage detection methods, while the safeties of pipelines have not been changed radically, for they are only correction maintenance methods. The risk-evaluation technology played an important role overseas in recent years, while it is very difficult to be used in China for insufficient basic data of in-service pipelines at present.Double-layered tube, which suits long-distance transportation pipelines especially, has carried a radical reformation on structure of piping. Compared with single-layered tube, double-layered tube is superior in many aspects, such as security, reliability, resisting explosion, detection online and so on.Supported by the Specialized Research Fund for the Doctoral Program of HigherEducation "Key Technical Research on Strength about Numerical Simulation ofPressure Equipment" (No. 20010335032) and the Excellent Talent of New Century forEducation Ministry and the project "Research on Large Double-layered PressurePipes and their On-line Safety Monitoring Technology", a series of studies have beencarried out on technologies of fabrication, application, design and safety monitoringof double-layered tubes. The main achievements are summarized as follows:1. Based on elastic-plastic theory, the stress and strain for the inner and outer tubesare analyzed during the process of hydro-forming, the formulae of residual contactpressure related to hydro-expanded pressure, contact pressure between layersrelated to hydro-expanded pressure and the variable range of hydro-expandedpressure are derived. Further, the hydraulic expansion criterion is also put forward,which give the suitable conditions the geometry dimensions and materialproperties of inner and outer tubes need to meet. These formulae are strict in mechanics and applicable to cases where the inner tube is both thin-walled and thick-walled.2. A concept of Equivalent Yield Strength (EYS) is put forward, and a method to get EYS of actual material is also given. With the strain hardening effect being taken into account, the method is simple to be applied which only needs several basic material parameters such as yield strength, tensile strength and elongation percentage, not to measure the factual stress-strain curves. The results based on this method are perfectly consistent with experiment and FEA data.3. A set of hydro-expanded equipment of double-layered tubes is designed, which is especially fit for the conditions of big dimensions and high pressure. With a unique structure of multi bolts, the problem of axial force is solved and work efficiency is also improved with simple operation. A Y-type seal ring is also designed, which add a new function of self-tightening seal in radial direction. In this type of seal ring, the seal faces of lip-type seal ring are changed into three ones, and the maximum radial displacement could reach 11 mm for its excellent capability of elastic deformation.4. The data acquisition function of gas pressure between layers of inner and outer tubes could be realized through the special designed safety-monitor system, and on-line alarm and real-time detection could also be carried out at the same time. It is convenient to display the varying pressure of every channel with curves, and the time as well as the channel number could be captured when the pressure exceeds threshold value. The acquired and overrun data could be stored in a designated formation for later use.