second hydraulic tensioning device 110 used to join pipe flanges pf 1 and pf 2 of respective pipes p 1 and p 2 at a flange joint for ease of manufacture hydraulic tensioning device 110 has a device body in the shape of a ring formed of upper and lower annular discs 111 112 upper disc 111 has a plurality of downwardly convergent bores 113 through it to receive bolts 120 which extend above pipe flange pf 1 each conical bore 113 is shaped to receive a trifurcated nut cone 122 which engages screw threads 121 on bolt 120 cone 122 is prevented from escaping from conical bore 113 by spring clip 123 lower disc 112 has bores of larger diameter than bore 113 through it which form with upper disc 111 annular recess 114 which house thrust washers 117 so that upper 111 and lower 112 discs thrust washer 117 and bolt 120 form an annular chamber 130 to receive charging medium 150 each annular chamber 130 surrounding bolt 120 is interconnected by distribution galleries 151 extending around upper 111 and lower 112 discs by manufacturing the connector body as two discs distribution galleries 151 can be machined and upper 111 and lower 112 discs can be locked together by a plurality of joining bolts 119 as illustrated in fig6 charging medium 150 is injected under high pressure using media exchanger 160 which is screwed into passage 131 connecting to distribution gallery 151 passage 131 contains non return valve 141 which operates the.