A wellbore pump comprises a plunger having a passage which may be open or closed by a valve operated by a piston which is biased in the open position by a spring of selected strength such that the wellbore pressure may overcome the spring to close the valve when the accumulated hydrostatic pressure reaches...http://www.google.es/patents/US6637510?utm_source=gb-gplus-sharePatente US6637510 - Wellbore mechanism for liquid and gas discharge
