Hey Chiara, this is looking really nice, I like the addition of the earth in the background!
When it comes to that pipe, I think what makes it look 'awkward' are those intersecting lines. The way you have them right now makes the form look like its almost semi-circular rather than a cylinder. Try drawing through the object, make those intersecting lines a complete circle (or ellipse for this example) and then draw your final clean lines over the top. You'll see that those intersecting lines should look like they continue around the form and not finish at the boundaries of the viewer.
Here is what I mean


Also something else I noticed while doing the drawing that wasn't imediately obvious at first, was that we are looking at the pipe from below so therefore we see the intersection from underneath. So we see more of the underside and less of the top.
Hope this helps 