The research on the DDV is less at home and most of them only buy the foreign products or imitate them, and hasn't mastered the design theory and the design method.
而国内对套管阀的研究工作较少，仅有的套管阀研究工作只是对国外产品的结构仿制，完全没有掌握套管阀的设计理论和设计方法。
In summary, this research work improve the performance of DDV and provide a new theoretical basis and research tools for researching the performance of it and designing of the new DDV.
总之本文的研究工作提高了套管阀的综合性能，为研究套管阀工作性能和开发高性能的套管阀提供新的理论依据和方法。
In summary, this research work improve the performance of DDV and provide a new theoretical basis and research tools for researching the performance of it and designing of the new DDV.
总之本文的研究工作提高了套管阀的综合性能，为研究套管阀工作性能和开发高性能的套管阀提供新的理论依据和方法。
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