Pressure transmitter with flange rating below process design P/T

[Steve010]

I am purchasing a 600# 2 inch flanged pressure transmitter. The piping is 600# carbon steel. There is a process condition that could last for days with a pressure of 1250 psig and a temperature of 200 F. According to the ASME tables, a 600# carbon steel flange maximum pressure is 1350 psi at this temperature, so the piping is ok. The problem is that the pressure transmitter 2" flange should be 316 SS, and as per the ASME tables a 316 SS flange maximum pressure is 1240 psi at this temperature. Should i use a pressure transmitter with 900# instead of 600#? or is it acceptable to operate above the flange rating by a slight difference ? it seems weird to me to use a 900# rating instrument on a 600# piping. any advise ?

 

[MickMc]

I would use a higher rating flanged transmitter to meet the design conditions and note that it would have to be Class 1500 as Class 900 starts at NPS 3 and therefore your carbon steel flange also has to be Class 1500 to dimensionally match. Occasional Pressure/ Temperature variations are allowed under ASME B31.3 under certain criteria, if this is your code, but not I think for days at a time.