I have recently encounted an interesting problem involving determining the stresses in a FRP piping system with a structural liner. Usually when modelling FRP pipe to BS 7159, the liner is assumed to be a non-structural entity, contributing only weight to the piping system. However, I am currently working on a problem where the liner (PVDF) is a structural entity and can itself withstand the design pressure. We have been told that the FRP has only been added to increase the support distances (the thickness of the FRP pipe has been determined from BS 7159 according to the design pressure). The client is concerned that because the liner is structural, it will impose a high stress on the FRP pipe when exposed to high temperatures, owing to the large differences in the coefficients of thermal expansion. I have got a few ideas of how to tackle this problem, but they are quite complicated and "un-tested". Is there a simple way to solve this problem, or a solution that has been tested and proved to be successfull?

I would appreciate any usefull comments.

Best regards

Jason

The only way I can think of to tackle this is to treat this system as a "jacketed pipe" system. The core pipe would be your liner, and the jacket would be your FRP.

There would be differential expansion as you noted. You would also have to ensure that you specify a pressure of 0 for the FRP elements, to avoid Bourdon effects and hoop stress consequences, since the liner is handling the pressure containment.

Also, the liner and FRP would only be connected at the locations you associate with CNODEs. The big problem here is that you're probably continuously connected between liner and FRP, whereas this is not the case in a jacketed system. There is no way to model the slippage or friction between the two (liner and FRP).

This is related to FRP pipe coding..
FRP pipe thickness is always a vendor dependent issue, the minimum pipe thickness should be considered what the vendor is supplying.

Secondly
UKOOA code is more conservative then BS 7159,
and some FRP vendors recomends UKOOA for stress.

BS 7159 calculates a "combined stress", which is an equivalent stress calculated using simplified 2-D Mohr's circles to find the greater maximum shear stress between 1) the circumferential tensile/shear plane and the radial (which is always assumed as zero) plane and 2) the longitudinal tensile/shear plane and the radial plane (par 7.3.3.3).

This is a typical steel approach. UKOOA code uses the latest design approaches regarding GRE piping systems (for example: allowable stress design envelope).