Hot Tapping For PSV Installation

[Djack63]

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?

 

[Bill3752]

Djack, I may be doing this incorrectly, but I had historically assumed that as long as the nominal piping was = nominal valve inlet that all was ok. That said, I always use full port valves. I am interested in what others might post.

 

[The Obturator]

What you need to bear in mind is that the actual minimum bore area of the nozzle of the PSV is much smaller than the actual inlet nominal diameter of the PSV (well, in API full nozzle designs anyway). You may note that these designs have a tapering reducing bore from the inlet. This does not mean that you can have any inlet piping size smaller than the inlet ND. All PSV manufacturers will tell you that this inlet tapered (slope) is critical to the PSV design and has bearing on the established coefficient of discharge. Since the inlet size is a "nominal bore" the actual inlet size may differ between manufacturers. I would therefore identify the PSV OEM and ask the question.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[chengg29]

Yes I faced a similar situation few years ago with an operating company i worked with. However it was understood that at alltimes the mating relief valve inlet pipe ND would match the relief valve inlet ND..irrespective of what the internal diameter of the relief valve inlet or the nozzle would be (avalveman is right). Hence your hot tapping connection should be adjusted accordingly to suit the said pipng configuration.
That said however the operating company's policy was not to do hot tapping on any relief valve connections in the plant since for them it was a safety issue (for example there could be a process upset at the time of the actual hot tapping work on the very relief valve on which a relief demand is placed).