2 has a shape in cross section which corresponds to the shape illustrated in fig1 in other words the packing has rounded top and bottom faces 7 and 8 respectively as shown in fig1 or inclined top and bottom faces 9 and 10 respectively as shown in fig2 the angle of slope of the top and bottom faces of the packing 3 is smaller than or equal to the angle of repose natural angle of slope of the expanded graphite employed the angle of repose is the maximum angle such that the forces exerted by coupling flanges 11 on the graphite particles during the clamping operation remain oriented towards the interior of the packing without causing any displacement of particles towards the contacting faces 17 to 20 of the metallic rings on the coupling flanges it must in fact be ensured that decohesion of the graphite does not take place or in other words that the angle of slope is sufficiently small to prevent decohesion of the graphite in the case of fig1 the inner and outer rings 1 and 2 respectively are located at different levels the difference in level being equal to n 1 since the width of the packing is l 1.