The results of a recent PSV study at one of our plants indicate that we will need to install some new PSVs on piping and Section VIII vessels. We plan to do the installations online which will require hot taps. API 520 and ASME Sec. VIII are fairly clear that the inlet piping to the PSV SHALL not have a cross sectional area less than the inlet connection of the PSV. B31.3 is less clear on the subject. Talking to the hot tap company and looking at the std ID of a ANSI 300 class flange, it doesn't appear that we will be able meet the cross sectional area requirement in API and ASME. For example the inlet of the 4" 300 Cl flange is 4.03"; with Sch 40 pipe the largest bit they can use for the hot tap is 3.938". So, in this case, to meet the API/ASME requirement, we will have to use a 1 size larger hot tap and swedge down to the PSV inlet. Unfortunately, in some cases we will be doing 4" x 4" hot taps so a 1 size larger is not an option. Granted the difference is not large and most of any increased pressure drop will be recovered, but strictly speaking we will be in violation of the code. Has anyone else run into the problem? Thoughts?
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