a fixed body 1 of parallelepipedal shape having two opposite large faces 2 3 two opposite small faces 4 5 and two opposite end faces 6 the body 1 is made of steel an oblong chamber 7 of axis 8 is made in the body 1 from one end face 6 to the other both end faces 6 have connectors not shown forming an inlet communicating with a pressurized water source which is not illustrated the chamber 7 contains a cylindrical filter 9 of the same shape as the chamber 7 but of smaller diameter the face 5 of the body consists of two clamps 10 screwed to the rest of the body 1 by screws 11 a channel 12 in the form of a slit extending along one generatrix or two adjacent generatrices of the chamber 7 leads from the chamber 7 to an opening 13 in the form of a slit formed in the face 5 and communicating with the exterior the clamps 10 define a cavity housing a seal holder 14 made of metal plastic or a composite material having a 5 mm thick web 15 and two side arms 16 17 forming guide parts extending parallel to the axis 8 the seal holder includes a longitudinal slit 18 aligned with the channel 12 and with the outlet 13 on either side of the slit 18 the seal holder 14 comprises a groove 19 which faces in towards the chamber 7 and which houses an annular seal.