In a lot of place in the actuel code B31.1 and B31.3 you can read the following:
That means if you know that there are more applicable data ,you have to use them!
In some cases the current Code rules are not sufficiently conservative (common error is 2). In some cases the current Code rules are grossly over-conservative. more applicable data is less conservative than the current code, and in some cases the more applicable data is grossly over conservative, generally in the vicinity of 0.05(R/T)2/3. These gross overconservatisms can cause project delays, unnecessary pipe rerouting, oversupporting, and an oveall waste of the clients time and money.
In the previous study from WRC 329,it was recognized that some corrections have to be done
1)Corrects well known SIF Errors
2)Provides k-factors that can be significant when D/T > 50
3)Provides SSI values which can be significant when D/T > 40
a)SSI x (M/Z) < Sh (sustained load analysis)
the ASME Committee hired PRG (Paulin Research Group) to align the Code SIFs and flexibility factors. The result of this work was published in the ASME ST-LLC document: STP-PT-073..
Tony Paulin is the author of FE / Pipe and NozzlePRO, and he has developed the first versions of CAESAR II for COADE Inc.. In cooperation with Intergraph, he is now responsible for the development of FEATools.
This consulting job has been published by ASME and can be used in the stress Analysis of piping systems as more applicable data. STP-PT-073 is also the basis for most of the SIF and flexibility rule changes outlined in B31.J.
What does it means for the stress engineer ?
a-In the previous code you had SIF(i) and SIF(o) which were valid for both Expansion Stress and Sustained Stress.
b-Now you can find the following.Not only the SIF are different for the primary and secondary stress but in addition you have to consider SIF for Tortion and axial effect!
SIF is valid for Expansion Stress (SIF)and Index(SSI) is valid for Sustained Stress.
The k factor was previoulsly =1 by default and now you can see the difference!
When the design of Heat Exchanger is very tight ,the k value used in the stress analysis is even more important as the SIF value.
You can save the design by using the proper k factor.
By using the modification shown,some design which were overstressed, could be OK and the nozzle Loads are in this case acceptable.
Tony Paulin is visiting us on the 28th and 29th of September 2015 and he is going to talk about:
Intoduction into using FEA for piping, the status of STP-PT-073 and B31J, Errors in Bend Calculations, Bends with Trunions, and SIFs, SSIs, and k-Factors for Heat Exchangers, Vessels and Rectangular Header Boxes. Allowable loads for pipe shoes and vessel saddles are also read in from NozzlePRO results and evaluated in FEATools for a given CAESAR model. The use of laser scanning can be used to validate pipe stress models and aid in predicting high stress locations for inspection. The new version of FEAToolsV3 with CAESAR reads in laser scanned results and validates the operating position of the piping system against the calculated results using the latest single value decomposition technology.
Please send us your comments!!!