31. The section of flange illustrated and indicated as «D» is a __________

The illustration depicts a pipe inserted into a recessed socket within the flange and secured with a fillet weld, which defines a socket weld flange. This design features a socket where the pipe seats against a shoulder, with a fillet weld at the top of the joint, differentiating it from threaded flanges (lacking threads and welds), van stone flanges (featuring flared pipe ends), and expansion flanges (incorporating sliding or bellows elements).