If you have a need for a simple 2-D CAD drafting package, download DraftSight® by Dassault Systemes, for FREE.  Create Piping and Instrumentation Diagrams and Process Flow Diagrams as you would with any high priced drafting software on the market.  Dassault Systemes’ core business is 3-D, so they are giving this 2-D system away for free.  DraftSight® Professional and Enterprise versions are also available.  Click on the image below for your free version.