30 a and 31 a comprises an outer sealing part 30 b and 31 b and an inner leg 30 c and 31 c the sealing backups 30 and 31 are attached to the lower sealing backup housing 29 and the upper sealing backup housing 21 b respectively with ten bolts 32 each each spring bolt 32 is carrying a set of dish springs 33 the head 32 a of each bolt 32 has threads for engaging radial holes in the respective housings being locked by radial locking screws 328 the shanks of the bolts 32 are sliding in an opening in the leg 30 c and 31 c respectively of the associated backup element 30 a and 31 a the expandable sealing element 22 is activated by the relative axial movement of the lower sealing backup housing 29 and the upper sealing backup housing 21 b at the setting of the device according to the invention the upper sealing assembly 16 is corresponding generally to the lower sealing assembly 15 having an expandable sealing element 34 fig1 each sealing backup assembly 30 and 31 comprises an annular chain of ten sealing backup elements 30 a and 31 a respectively each sealing backup element comprises an outer arched sealing backup part 30 b and 31 b respectively with a circumferential wing 44 on one side and a correspond slot 45 on the other to allow overlapping of neighboring sealing backup elements thus the sealing backup elements will have a closing annular face against the outer rim of the expandable sealing element 22.