No matter where you are, if you are fabricating HDPE pipe spools make sure you are familiar with the ‘DVS Technical Codes on Plastics Joining Technologies’.
It’s a literal Bible of good practice and good ideas available in hard-copy and digital format, which will definitely add to your skills-base.
DVS 2207 includes butt, socket and extrusion welding of HDPE and other materials, while other sections cover tank production, sheet welding etc.
As the German Welding Society, DVS (Deutscher Verband für Schweissen und verwandte Verfahren) covers the jointing of most materials, so there is certain to be something in there for you.