其它类型的设计则在中间有一个圆形的剪切块，使其适合井眼内管柱的直径以便当管柱位于井眼内时能够密封井眼。
Other designs have a circular cutout in the middle that corresponds to the diameter of the pipe in the hole to seal the well when pipe is in the hole.
其它类型的设计则在中间有一个圆形的剪切块，使其适合井眼内管柱的直径以便当管柱位于井眼内时能够密封井眼。
Other designs have a circular cutout in the middle that corresponds to the diameter of the pipe in the hole to seal the well when pipe is in the hole.
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