API 14 E equation for solid free services and which is based on liquid impingement is only restricted to metallic lines or can be extended to non metallic pipes such as GRE and RTP?
No, API RP 14E cannot be extended to non-metallics. Erosion performance will have to be evaluated by testing. There are some rule of thumb velocity limits bandied around, e.g. 4 m/s for GRP, with short term excursions to 10 m/s in solids free, liquid flow RR039. ISO 14692-3 used to have similar velocity limitation guidelines before it was updated to the current version.