the outlet end flange 22 of the housing the upstream assembly retainer 36 comprises a depending annular flange 58 depending from a main body portion 60 and may be molded including the velocity reducing plate 34 in its construction the flange 58 is sized to fit into the upstream end of the downstream retainer 52 with the main body portion 60 of the upstream retainer 36 seated against the upstream edge 62 of the downstream retainer 52 thus the upstream assembly 24 seats snugly into the downstream assembly 26 the distributor plate 46 extends across the flow passage 18 as best seen in fig7 the distributor plate 46 has a plurality of orifices 64 arranged in a plurality of groups in the version shown in fig7 there are three groups of orifices namely a first group of orifices 64a a second group of orifices 64b and a third group of orifices 64c the first group of orifices 64a are furthest away from the center of the distributor plate 46 the third group of orifices 64c are closest to the center of the plate 46 and the second group of orifices 64b are therebetween the orifices of each group are of the same size.