I have done quite a handful GRP stress calculations so far in my career, just recently I ran into one sample calculation based on ISO 14692 in which, it was suggested to do extra stress check for Bends/Tees. I wonder if any one could clarify.

thanks

Dear Pooria,

Generally, critical items should be considered in design of GRP joints adjacent to bends/tees:

1- GRP bends are far more stiffer than steel bends (typ. Kgrp=5*Ksteel)
2- Limited contribution of bends in absorbtion of expansion
3- Highest Stresses near Bends and (Tees) (typically)
4- Prediction of flexibilities
5- most of the failures accure at joints

I hope it helps.

Dear Farhad,

thanks for the explanation although it brought up even more questions for me:

1- do you mean, the bend flexibility which is calculated by CII for a GRP piping is not conservative enough?
2- I would always put the SIF values before and after the bends/tees manually in the calculations (right at the red rings in the screen shot attached to your text) is there more cautions I should take?
3- the sample calculation which I talked about, had extra excel sheets in which new allowable stresses were calculated for bends/tees in addition, SIF for hoop stress had been calculated and were implemented in the results of the calculation. I was a bit confused about the excel sheets and new allowables.

Regards,

Dear Pooria,

Your approach to GRP analysis is correct. As far as you follow the code rules and manufacturers' recommendations and values, you may have a "fit & forget" design. But there are unpredictable problems in GRP piping systems during testing and lifetime operating. That's why you are being asked for extra stress check. I mention some of common problems below:

1- Small part of the failures occurs during installation or operation
2- Most of the failures occur during testing (pressure testing)

Due to:

1- Material defect
2- Defective installation
3- Overloading of the material due to shortcomings in design
4- Overestimation of joint capability
5- Pressure surge (excess pressure, vacuum, liquid hammer, etc.)

So, generally, in order to have a safe GRP piping system, you should do:

1- Stress analysis to limit the stress and loads
2- Surge analysis to limit the pressure
3- Fully control and supervision on installation during construction to check as built vs. design.