| 摘要 |
<p>A downhole non return valve (10) , comprising: a housing (12, 14, 16) defining a valve inlet (18) and a valve outlet (20); a plug (22) moveable between an open position and a fully sealed position; and a a biasing member (24) urging the plug towards the fully sealed position wherein the urging force of the biasing member is sufficient to move the plug to a partially sealed position but is selected to be insufficient to move the plug to a fully sealed position.</p> |
