fluid an alternative means of detecting leakage is illustrated in fig3 and 5 as in the previously described embodiment the gasket 18 is accommodated within the aperture 22 of the annular element 30 the annular element 30 has a projection means 32 which extends radially outwards onto the flanges 12 however the projection means 32 terminates within the flanges and does not protrude into the exterior thus when the bolts 12 are tightened and the joint is sealed the projection means 32 is not open to or visible form the exterior the annular element 30 is provided with a barrier 34 of molded plastics e g ptfe the barrier 34 surrounds the outer peripheral edge 36 of the annular element 30 as in the previous embodiment the barrier 34 acts to direct any leaked fluid round the annular element 30 into the projection means 32 for detection it also serves to prevent any leaked fluid contaminating the bolt and the outer periphery of the flanges as shown in fig5 the projection means 32 is provided with an indentation 38 through which the upper and lower faces of the annular element 30 are in fluid communication the indentation 38 extends longitudinally inwards from the outer peripheral edge 40 of the projection means 32 to a point radially inside the flange bolts 14 a tube 42 of.