Duplex / Super Duplex are different than SS316. DSS / SDSS are selected because of service fluid requirement. Hence you can not go for SS316 instead of DSS/ SDSS just because of unavailability in market. However you can go for higher material orifice plate like 254SMO, Inconel625, HastelloyC276...
Recently i have done project with FRP/RTR lines upto 36 inches with pressure ranges from 10 bar to 27 bar. You shall follow only FRP/RTR flange manufacturer procedure for bolt tightening. For each flange size, manufacturer provides Torque increment & final torque value. You can get this table...
