At the risk of asking something that has been asked (and answered) many times before..... we intend to place an UNrestrained expansion joint directly on a nozzle of an atmospheric FRP vessel. Pressure thrust will be considered and accounted for as follows: the nozzle will constitute an anchor on one side of the exjt - the pipe on the opposite side will be anchored close to the exjt. My question: is the pressure thrust the nozzle sees the line pressure times the area associated with the exjt equivalent diameter - OR can one assume it's the force that arises from the line pressure times the "annular" area - the area associated with the exjt eq. diameter less the internal area of the pipe. If the latter then (perhaps) the Eq. ID field in the CAESAR II input file would have to be modified to acccount for this?

CAESAR II cannot "look" up and down the line to locate the actual surfaces where the pressure will affect your line. We place those pressure thrust loads on either side of the XJ when an effective ID is specified with the XJ.
If you wish to develop a more "correct" model, you will have to locate those forces by hand. If you do, be sure to include that force set in the appropriate load cases.

As you pointed out, this topic has been discussed many times. However, we can look at it again. Technically, the longitudinal force on a nozzle is the annular area from the bellows mean diameter to the ID of the pipe. The balance of the force will develop on the backside of your vessel. Also, there will be an equal and opposite full thrust (PxA) force on the elbow on the opposite side of the EJ. Having said this, you should check the vessel and it’s anchor for the full pressure thrust.
Also, there is a special concern about using an unrestrained expansion joint in FRP pipe. FRP is not a homogenous material but a composite material. It relays on the fiberglass to restrain both the hoop and longitudinal forces it does this on a diagonal bias. Think of the Chinese finger puzzle where you put a finger in each end and try to pull your fingers out and can’t. If you eliminate the longitudinal force, the hoop stress becomes unbalance and the pipe will break down. Unfortunately, since an EJ in FRP is so unusual, I can’t cite any documents on this, but you need to be very careful and when in doubt, don’t use a joint.

Dave and Dan - thanks for your help!

Rod