How conservative is ASME B31.8 Reinforcement Area Calculation.

[WM2RF]

Hi,

I am quite new to Eng-Tips and let me start off by stating that the wealth of information available in this forum is truly amazing and beneficial to a young engineers as myself.

Now unto the question at hand. I designed a tee according to B31.8 and obtained the reinforcement area needed to satisfy the design requirements (I used the very minimum thickness needed per code). I then performed a linearized analysis per ASME Sec VIII Div 2 for that same fitting using FEA (ANSYS), and noticed how low the stresses were. Moreover, I reduced the reinforcement area and it still passed the analysis per ASME Sec VIII Div 2.

I have posed this question to several co-workers, but I haven't been able to get a concrete answer. Any help in clarifying this issue is greatly appreciated.

Also, please correct me if my comparison of the two methods is invalid given the different design criteria applicable for both codes.

I do apologize if this question has been addressed before.

Thanks,

Ben

 

[ColonelSanders83]

I do not have a copy of ASME B31.8, i do however use B31.1, B31.3, B31.5 and B31.9. The reinforcment rules you are quoting are conservative in the piping code. It is based on the area compensation method. This will generally give you a conservative result that you do not need to worry about. This is done in the interest of maintaining cost effective engineering.

The experimental stress analysis you performed is comparativly a very time intensive thing to do. You must perform and properly document the analysis in order to be able to justify the geometry used at a later date (in case of future trouble). In the end what is the benefit? You may be able to have a 3/8" repad instead on a 3/4" repad but the differance in cost between the two options is miniscule compared to the cost of the engineering hours you put in, not to mention the fact that this is now a nonstandard joint. Generally a company caries between 50-75$ per hour to keep and engineer. If I do a standard area compensation calc, It might take me half an hour and I'm done, I never have to worry about the joint again. If I perform an experimental FEA it might take a day or two to get everything bottoned up, I may have to revisit the joint if the loads I used in the FEA analysis changed. were talking between 8-20 hours or $400 to $1000 per joint. instead of $25 and the cost of a thicker repad.

Just my two cents worth.

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