Circulating and pressure equalizing sub

A circulating sub for installation as a part of the tubing string above the packer sealing the tubing string with respect to the well casing, said circulating sub comprising:

• (a) body means adapted for threaded connection to joints of tubing, said body means defining a circulating passage in communication with the tubing and an equalizing port capable of communicating said circulating passage with the annulus between the tubing and casing;
• (b) equalizing valve means movably positioned within said circulating passage and normally closing said equalizing port;
• (c) fluid pressure differential responsive valve actuator means being passed through said tubing and moving into passage closing engagement with said equalizing valve means to permit opening of said equalizing valve means responsive to predetermined fluid pressure applied from said tubing to thus permit equalization of tubing and casing pressure and permit pumping of kill fluid through said tubing and through said equalizing port into the annulus between the tubing and casing to kill the well;
• and
• (d) yieldable lock means restraining said equalizing valve against movement relative to said body means below a predetermined pressure differential and yielding at said predetermined pressure differential to permit pressure energized movement of said equalizing valve to said open position, said yieldable lock means comprising:
• (1) a first lock element yielding at said preselected pressure differential to permit opening of said equalizing valve, and
• (2) a second lock element being movable into locking engagement with said equalizing valve upon movement of said equalizing valve to a preselected closed position, said second lock element also being yieldable at said preselected pressure differential to permit subsequent opening of said equalizing valve element.