1. One icon set for a computer program (same image but in different sizes). The general idea is a circular cross-section of a pipe with a fraction of the pipe filled with liquid, and the remaining fraction filled with gas (or empty-looking). The interface between the two should be simple (e.g. smooth). There can be a shiny star-like reflection coming off the interface surface, or, the whole thing can be lighted properly, or, a 3D-variant of the same basic idea - a circular pipe flowing liquid and gas.
2. Similar (though simpler) version of the icon above for a picture logo of the company that will offer this software product (in different sizes).