Here's a current rule in CAESAR II: If Bourdon Effect is not active in your model and if there is no "Effective Diameter" specified for expansion joints, there will be no structural response (i.e., no load & no movement) associated with a pressure load.
Pressure causes little response in terms of system movement in most metallic piping systems. For many years, the Codes were quiet on the formulation of the stress due to sustained loads and engineers typically threw in a "PD/4t" for the pressure stress component. Here's where your GRP (FRP) designation comes into play.
GRP/FRP pipe is quite flexible in the axial direction and pressure, alone, may cause system distortion. So, if GRP/FRP material is specified in your CAESAR II model, this "Bourdon effect" is automatically included in the analysis.
BUT, our Bourdon term has two components - 1) the axial extension due to pressure and 2) the "Poisson shrinkage" in the axial direction due to hoop pressure. Note that I am calling these strains rather than loads. That's how they are applied in CAESAR II - they look more like a thermal expansion rather than end loads on each element.
You cannot unscramble these eggs. You cannot just use the Effective Diameter to adjust the calculated flange load due to pressure.
What I would do: 1) run an analysis without pressure to get the flange load excluding the pressure term, and 2) model the pressure distribution between the XJ and flange by hand to get the pressure term, and 3) add 1) & 2) to get the total flange load. To model the pressure load distribution by hand I would specify the P*A(in) load on the back of the elbow beyond the XJ from the nozzle (the balancing force is pushing the impellers/back of the pump) and the P*[A(XJ)-A(in)] on either end of the XJ, pointing away from the XJ; do not specify an XJ effective diameter. These loads would be defined as a force set (e.g., F1). Run this force set alone to see how the pressure loads are distributed through your assembly.