Nozzle loads consideration for Flange rating selection. 4

[DK44]

My client asks to check rating flange against Nozzle loads equivalent pressure combined with equipment design pressure.If it fails check the flange as per ASME Sec VIII div 1 appendix 2.

For establishing equivalent pressure due to Nozzle loads Kellog's formula is generally used by pressure vessel designers. However this has not come into the codes?

In my experience when a rating flange is checked as per ASME Sec VIII div 1 app 2, it generally fails since allowable stress of ASME pressure vessel code is different from ASME Piping code.

However when a particular nozzle flange fails in the above conditions, can we design the flange as per ASME Sec VIII div 1 App 2, with allowable stress as per ASME B31.3 in consideration of para 304.5.1 where it allows such method. If it still fails, there is no other option except to enhance the pressure rating of the flange.

 

[mech_eng123]

This concept of checking Design Pressure + Equivalent Pressure (from external forces and moments) for comparison to only the Rating Pressure is extremely conservative and should absolutely not be done. Code Case 2901 provides one method for doing this (but it is not the only way, BTW).

Can I interprete this as "I can use other methods (Preferably DNV method) to check flange rating for U Stamp vessel." ?

 

[TGS4]

What is the DNV Method?

 

[mech_eng123]

DNV Method is one of the flange pressure rating method provided in PVElite alongwith Kellogg's method, PVP method and 50% Stress Method. Please check below abstracts from PVElite help.

Kellogg Method - The Kellogg method is well known and conservative. The axial load and moment are used to compute an equivalent pressure that is then deducted from the flange rating from the B16/47 table.

PVP Method - The PVP Method is based on the paper PVP 2013-97814 with some modification. PV Elite uses the Sustained load category with a factor of 32 on Me instead of 16 that is based on operating loads. Subsequent unpublished work based on this method uses the value of 32. Sustained forces and moments must be entered for those results to be meaningful. Otherwise, the computed flange rating is zero.

50% Stress Method - If the computed stress/allowable stress is < 0.5 on the pipe wall, then the allowable pressure is the full rating from the ANSI/ASME standard. If the stress ratio is >= 0.5, then the full equivalent pressure based on the Kellogg method is subtracted from the flange rating.

DNV Method - The DNV method is considered to be a bit unconservative. It is essentially 1.3 times the flange rating minus the equivalent pressure based on the Kellogg method. The idea is that because the flanges will be hydrotest at an elevated pressure and because there will loading applied (flanges in the piping system), then their rating can be elevated using the above equation. Most piping is tested to 1.5 times the design pressure, but we use a factor of 1.3 for conservatism and because 1.3 is the factor used in Division 1 for hydrotesting pressure vessels.

 

[TGS4]

Such a method would not be covered by the existing rules, and for ASME Section VIII, Division 1 construction, U-2(g) would apply. Subject to the provisions and requirements of that clause.