Mimentzos,
There are ASME Standards that tell you how to make pipe, fittings, flanges and valves. There are Codes that tell you the minimum requirements for building pipelines and different types of process plants. But, there is no "Code" (to my knowledge) that addresses the subject of how to create isometrics even to the limited degree that I did above. Sorry!
There are some text books that would help you but I do not think you are looking to go back to school.
Example:
Piping Drafting and Design
(Using manual, AutoCAD and Pro-Pipe Applications)
Roy A. Parisher and Robert A. Rhea
Gulf Publishing Company
ISBN 0-88415-657-5
Also available by the same author
Piping Drafting and Design “Workbook”
And
Piping Drafting and Design “Instructors Guide”
Process Piping Drafting (this book is out of print but you may be able to find someone who has a copy)
Rip Weaver
Gulf Publishing Company
ISBN 0-87201-761-3
Also available by the same author
Process Piping Drafting “Workbook”
I also suggest you visit
Look at the "Training", "Tips", "Tools" and "Forum" links, you will find a lot of information about piping design.