Abstract: A mechanical seal for closing the gap between adjacent ends of rigid tubes which must remain in fixed position makes use of two complementary semicylindrical shells, each of which carries a molded sealing gasket provided with a recess adapted to fit around the tubes and around the space between them. A clamp attached to one shell has opposite ends which reach around corresponding protuberances on the other shell. The clamp operates to draw the shells and the molded sealing gaskets into sealing engagement with the rigid tube ends and with each other, thereby to close the gap with a dependable, replaceable seal.