1. An installation for controlling down hole, control lineoperated equipment such as a safety valve of a petroleum well, comprising: a tubing head having a plurality of set screw ports of the sort normally provided for respective receipt of set screws to compress a pack-off down against a tubing hanger; a tubing hanger mounted in the tubing head for suspending a string of tubing in the well and pack-off means for sealing between the tubing head and the tubing; a control line extending up through the tubing hanger, up past the pack-off means and being exteriorly sealed with respect to the tubing head by the pack-off means; one of said set screw ports lacking a set screw and instead thereof being provided with: openable and closable valve means sealed therein and being operable from exteriorly of the tubing head; fluid-tight conduit means interconnecting the control line with said one set screw port down stream from said valve; and means exteriorly of the tubing hanger for connection of a pressure line to the one set screw port upstream from said valve.