surface and the ball 20 comes back onto the sealing surface 21a a relatively high over pressure of for example 80 bars generates a sufficient force for overriding the safety element 28 shearing of the pin the force compresses the spring 27 the conjugate piece 24 reversing the direction of movement is pushed downwards as shown in fig2 b the blades 23 forming the sealing surface 21a move apart and permit the ball 20 to pass which engages in its new housing 33 fig2 zb inside the conjugate piece 24 stopping of the injection balances the pressures in the production tubing 6 overriding the force acting on the selective valve 18 whereby the spring 27 then resumes its initial position and ball 20 is imprisoned in housing 33 fig2 c after reversal of the flow of the fluid injection of the fluid through the annular space 9 the ball 20 comes into abutment on its new internal sealing surface 21b fig2 d stopping the passage of the fluid the rise of the pressure in pump 5 establishes a rising force which causes the pump 5 to be unlocked and to be raised in the production tubing 6 fig4 shows the detail of.