Calculating surge loads is not a straight forward exercise unfortunately. Most times I have experienced this the approach has been very, very conservative. ie: P x A x DLF, which is basically assuming an instant valve closure and assuming the same pressure rise for the entire piping system regardless of the length or configuration.

It is worth speaking to an experienced process engineer who has practical piping and site knowledge. You need to know the surge rise, the valve closure time, the valve type (to determine effective closure time), acoustic velocity and some other things. A process engineer can tell you all those things, and they should be able to help work out realistic loads developed from elbow to elbow.

If you work with any sub-sea or pipelines engineers they should also be able to help you, as they would deal with substantial surge loads due to long straight lengths of pipe.