the lower sealing backup housing 29 engages the lower sealing cone ring 19 with an intermediate annular lower sealing backup 30 similarly an annular upper sealing backup 31 is arranged between the upper cone ring 20 and a upper part 21 b of the lower sealing mandrel 21 functioning as an upper sealing backup housing the sealing backups 30 and 31 each comprises an annular row of overlapping arched sealing backup elements 30 a and 31 a respectively each backup element 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.