| 摘要 |
Downhole apparatus in the form of a valve (140) comprises a body (148) defining a bore (168) with a curved disc closure member (142) positioned in the bore. First and second retaining members (144, 146) are positioned on respective sides of the disc (142) for retaining the disc in a closed position and to hold pressure from both sides. One of the retaining members (144) is retractable to permit opening of the valve disc (142). The disc (142) may be locked closed by locking the retractable retaining member (144) in position. On release of the member (144), application of fluid pressure to portions of the member (144) will retract the member (144).
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