| 摘要 |
A method of cementing oil well casings and a cementing tool used when performing the method are provided. A novel top cementing plug function s in a conventional manner during proper oil well casing cementing operations, but if inserted into the oil well casing in error, allows cement to be pumped throu gh it. The top plug includes an outer, hard rubber casing having vertically spaced fins formed about its circumference. A central passage is provided through the outer casing and receives a cylindrical, metal sleeve. In one embodiment, a bore is formed in a portion of the metal sleeve to define a shoulder within the sleeve. The bore receives a cylindrical housing which carries a spring loaded, ball valve mechanism. T he valve mechanism includes an annular spring, retainer which rests on the shoulder. One end of a helical spring within the housing contacts the retainer while the other end of the spring biases a ball against an annular flange at the top of the housing to seal the passage. The ball moves into the housing and compresses the spring when an increased pressure is applied to the top of the ball thereby opening the passage. In a second embodiment, the bore formed in the cylindrical sleeve is reduced in depth but still defines a shoulder. A burst plate sits on the shoulder to block th epassage in normal operation. The burst plate is designed to rupture under increased pressure to open the passage, but maintains intact in normal operation.
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