Analyzing bimetallic pipe per UG-27 and UG-28

[mengnr]

I am analyzing a pressure vessel design in accordance with the BPVC. It consists of a supply line, a section of pipe connecting the supply line to the pressure vessel, a return line and another section of pipe connecting the return line to the pressure vessel. To evaluate the pipe under internal or external pressure, I need a length from support to support (according to UG-28). However, each piece of pipe actually consists of a short section of one material welded to a section of a different material. So my options are (1) to treat the weld as a support and analyze one material from the PV to the weld and the other material from the weld to the supply/return line or (2) to treat the whole pipe as being the weaker material, which is conservative. Which seems more accurate (or neither)? Thank you.

 

[gr2vessels]

mengnr,
It's difficult to follow, please clarify the following:
1. Is your pipe the pressure vessel? Any flanges around? How is your pipe connected to the PV?
2. If you analyse the pipe as a PV, the second option is obviously the conservative option, because the circular weld is not a support.However, the pipe wall thickness is usually much larger then required by the BPVC.
3. The maximum allowable working pressure will decrease with the pipe length increase, but in the end you'll get the maximum unsupported length, which must be longer than your pipe.
4.I'm not sure if you need to apply a reducing factor for the circular weld between dissimilar materials, since that's the weakest link in your assessment(try 0.8)
Cheers

 

[prex]

I suppose that your problem is with external pressure only, as for internal pressure you don't need a length.
Coming back to external pressure, I see two possible options:
1) If the two materials are not very different in their buckling pressures (as they likely are), then you should take the full length between the actual end lines of support with the weaker
2) If they are very different, you could follow these steps:
-check first that the stronger material is OK when occupying the full length between lines of support
-determine the extra thickness available on the stronger pipe in the above configuration by calculating the minimum required thickness to resist external pressure
-check the weaker section with its actual length considering the weld to the stronger one as a line of support
-check per UG-29 the extra thickness on the stronger material as if it was a stiffening ring, by taking a contributing width equal to half the width specified in UG-29
Frankly don't think you'll go that far by using method 2.

prex

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[mengnr]

Thanks for the suggestions. To answer your questions:
1. Yes, the problem is just with external pressure. Sorry for being unclear.
2. The pipes are all internally pressurized under normal operating conditions, but could be subjected to a positive delta P on the outside during aberrant conditions.
3. There are no flanges in this problem. All joints are welded.