| 摘要 |
The phase of this process is done by the functionality of the perforated lower stem, which due to the fact that its internal conduit is able to connect the lower bellow in an alternated way, first to the tubing and after, to the casing. This is possible due to the fact that it is assisted by: a) a window that communicates the casing with the tubing when the lower bellow is open, and b) sealing packing installed in the lower stem to isolate the casing pressure, in order that the lower bellow can register only the tubing pressure when it is in closing position.
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