| 摘要 |
A valve assembly to be disposed between lower end of kelly and upper end of drill string comprises a spring loaded upwardly closing, pressure release, poppet valve, the poppet including an upwardly opening check valve with a frangible closure for the passage of well tools. A foraminous skirt on the downstream side of the poppet provides a strainer for the drilling fluid. An annular flange carried by the skirt engages and seals with the inside of the valve housing to provide increased area subject to upstream pressure when the poppet opens, the housing being ported to atmosphere downstream of the flange and the skirt sealing with the housing below the port to prevent loss of drilling fluid. The spring is housed in the annulus formed between the skirt and housing.
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